What does Grounding have to do with Arc Welding Safety? Grounding of electrical circuits is a safety practice that is documented in various codes and standards. A typical arc welding setup may consist of several electrical circuits. Applying and maintaining proper grounding methods within the welding area is important to promote electrical safety in the workplace. Associated processes such as plasma cutting will also benefit from proper grounding. The important grounding topics in a typical welding environment are discussed. Welding Machine Ground Welding machines that utilize a flexible cord and plug arrangement or those that are permanently wired into an electrical supply system contain a grounding conductor. The grounding conductor connects the metal enclosure of the welding machine to ground. If we could trace the grounding wire back
Transcript
What does Grounding have to do with Arc Welding Safety?
Grounding of electrical circuits is a safety practice that is documented in various codes and standards. A typical arc welding setup may consist of several electrical circuits. Applying and maintaining proper grounding methods within the welding area is important to promote electrical safety in the workplace. Associated processes such as plasma cutting will also benefit from proper grounding. The important grounding topics in a typical welding environment are discussed.
Welding Machine GroundWelding machines that utilize a flexible cord and plug arrangement or those that are permanently wired into an electrical supply system contain a grounding conductor. The grounding conductor connects the metal enclosure of the welding machine to ground. If we could trace the grounding wire back through the electrical power distribution system we would find that it is connected to earth, and usually through a metal rod driven into the earth.
The purpose of connecting the equipment enclosure to ground is to ensure that the metal enclosure of the welding machine and ground is at the same potential. When they are at the same potential, a person will not experience an electrical shock when touching the two points. Grounding the enclosure also limits the voltage on the enclosure in the event that insulation should fail within the equipment.
The current carrying capability of the grounding conductor is coordinated with the overcurrent device of the electrical supply system. The coordination of ampacity allows the grounding conductor to remain intact even if there is an electrical fault within the welding machine.
Some welding machines may have a double insulated design. In this case, a grounding conductor connection is not required. This type of welding machine relies on extra insulation to protect the user from shock. When double insulation is present it is identified by a "box within a box" symbol on the rating plate.
For small welding machines that utilize a plug on the end of a power cord, the grounding conductor connection is made automatically when the welding machine is plugged into the receptacle. The grounding pin of the plug makes a connection within the receptacle. The use of adapters that effectively remove the grounding pin connection at the plug is not recommended. Furthermore, do not cut off or remove the grounding pin from the plug. All safety benefit of the grounding conductor is lost without the connection.
Receptacle circuit testers will easily check the continuity of the grounding conductor. Receptacle circuit testers for 120-volt circuits are available at electrical supply or hardware stores; these inexpensive test devices plug into an electrical outlet. Indicator lights show whether the grounding circuit is available at the outlet, as well as other circuit tests. If the test device shows the absence of a ground connection or other circuit problem, call a qualified electrician for assistance. This is a simple test and should be done periodically. Consult with a qualified electrician to test circuits greater than 120 volts.
Workpiece GroundThe welding circuit consists of all conductive material through which the welding current is intended to flow. Welding current flows through the welding machine terminals, welding cables, workpiece connection, gun,
torch, electrode holder and workpiece. The welding circuit is not connected to ground within the welding machine, but is isolated from ground. How do we ground the welding circuit?
According to ANSI Z49.1, "Safety in Welding, Cutting and Allied Processes," the workpiece or the metal table that the workpiece rests upon must be grounded. We must connect the workpiece or work table to a suitable ground, such as a metal building frame. The ground connection should be independent or separate from the welding circuit connection.
Grounding the workpiece has similar benefit to grounding the welding machine enclosure. When the workpiece is grounded, it is at the same potential as other grounded objects in the area. In the event of insulation failure in the arc welding machine or other equipment, the voltage between the workpiece and ground will be limited. Note that it is possible to have an ungrounded workpiece, but this requires the approval of a qualified person.
The Workpiece Connection is not a Ground Clamp"Ground clamp" and "ground lead" are common terms used by many welders. The workpiece is connected to a welding cable typically by means of a spring loaded clamp or screw clamp. Unfortunately, a workpiece connection is often incorrectly called a "ground clamp" by many welders and the workpiece lead is incorrectly called "ground lead." The welding cable does not bring a ground connection to the workpiece. The ground connection is separate from the workpiece connection.
High Grequency GroundSome welding machines utilize starting and stabilizing circuits that contain a high frequency voltage. This is common on Tungsten Inert Gas (TIG) welding machines. The high frequency voltage may have frequency components that extend into the megahertz region. In contrast, the welding voltage may be as low as 60 Hertz.
High frequency signals have a tendency to radiate away from the welding area. These signals may cause interference with nearby radio and television reception or other electrical equipment. One method to minimize the radiation of high frequency signals is to ground the welding circuit. The welding machine instruction manual will have specific instructions on how to ground the welding circuit and components in the surrounding area to minimize the radiation effect.
Portable and Vehicle Mounted Welding Generator GroundingPortable and vehicle mounted arc welding generators often have the capability to supply 120 and 240 volt auxiliary power. These generators are used in remote locations away from an electrical power distribution system. A convenient earth ground is not usually available for connection. Should the generator frame be grounded?
The rules for grounding depend on the specific use and design of the auxiliary power generator. Most applications fall into one of the two categories summarized below:
1. If all of these requirements are met, then it is not required to ground the generator frame:
The generator is mounted to truck or trailer The auxiliary power is taken from receptacles on the generator using a cord and plug
arrangement The receptacles have a grounding pin The frame of the generator is bonded, or electrically connected, to the truck or trailer
frame
2. If either of these conditions are met, then the generator frame must be grounded:
The generator is connected to a premises wiring system. For example, to supply power to a house during a blackout.
The auxiliary power is hard wired into the generator without the use of cords and plugs.
The summary above does not go into details and the reader is urged to consult their local electrical codes and ANSI/NFPA 70, "National Electrical Code" for the specifics.
Extension Cord GroundsExtension cords should be periodically tested for ground continuity. Extension cords lead a rough life while lying on the ground; they are under foot and prone to damage. The use of a receptacle circuit tester will confirm that all of the connections are intact within the cord, plug and receptacle.
Welding Circuit Shock HazardsUtilizing proper grounding in the welding environment is a good practice, but it does not remove all possibility of electrical shock. The welding circuit is energized by welding voltage. A person will receive a shock if they become the electrical path across the welding circuit. Precautions must be taken to insulate the welder from the welding circuit. Use dry insulating gloves and other insulating means. Also maintain insulation on weld cables, electrode holders, guns and torches to provide protection.
Similarly, electric shock originating from the electrical supply system can be prevented. Proper maintenance of electrical equipment and extension cords will insulate the welder from electrical sources.
Information Sources
American Welding Society, ANSI Z49.1:2005 "Safety in Welding, Cutting, and Allied Processes."
National Fire Protection Association, NFPA 70, "National Electrical Code", 2005. American Welding Society, Safety and Health Fact Sheet No. 29, "Grounding of Portable
and Vehicle Mounted Welding Generators", July 2004. American Welding Society, AWS A3.0-2001, "Standard Welding Terms and
Definitions."
GMAW: Common MIG Problems and RemediesReprinted with permission from the September/October, 1997 issue of Practical Welding Today magazine, copyright 1997 by The Croydon Group, Ltd., Rockford, IL
In much the same way that the automatic transmission has simplified the process of driving, Gas Metal Arc Welding (GMAW) has simplified the process of welding. Of all welding methods, GMAW is said to be one of the easiest to learn and perform. The main reason is because the power source does virtually all the work as it adjusts welding parameters to handle differing conditions; much like the sophisticated electronics of an automatic transmission.
Because less skill is required, many operators are able to GMA weld at an acceptable level with limited training. These same operators run into trouble, however, when they begin creating inferior welds and are unable to diagnose and correct their own problems. The guidelines listed below will help even inexperienced operators create high quality welds as well as offering tips for those who have been using the GMAW process for a number of years.
Most common welding problems fall into four categories:
1. Weld porosity 2. Improper weld bead profile
3. Lack of fusion4. Faulty wire delivery related to equipment set-up and maintenance.
1. Weld Metal Porosity
Porosity Problem #1: Improper Surface ConditionsThe most common cause of weld porosity is an improper surface condition of the metal. For example, oil, rust, paint or grease on the base metal may prevent proper weld penetration and hence lead to porosity. Welding processes that generate a slag such as Shielded Metal Arc Welding (SMAW) or Flux-Cored Arc Welding (FCAW) tend to tolerate surface contaminates better than GMAW since components found within the slag help to clean the metal’s surface. In GMAW, the only contamination protection is provided by the elements which are alloyed into the wire.
RemediesTo control porosity, use a deoxidizer within the wire such as silicon, manganese or trace amounts of aluminum, zirconium or titanium. Wire chemistry can be determined by referring to the American Welding Society (AWS) wire classification system.
Test the various types of wire available to find the right chemistry for a given application. To start, try the most common wire type, ER70S-3 (Lincoln L50) which contains 0.9-1.4 percent manganese and 0.45-0.75 percent silicon. If porosity is still present in the finished weld, increase the amount of silicon and manganese found in the wire by switching to an ER70S-4 (Lincoln L54) or an ER70S-6 which has the highest levels of silicon (0.8 -1.15 percent) and manganese (1.4-1.8 percent). Some operators prefer to use a triple deoxidizer such as ER70S-2 (Lincoln L52) which contains aluminum, zirconium or titanium in addition to the silicon and manganese.
In addition to changing the wire, further prevent porosity by cleaning the surface of the metal with a grinder or chemical solvents (such as a degreaser.) A word of caution though if using
solvents, be certain not to use a chlorinated degreaser such as trichlorethylene near the welding arc -- the fume may react with the arc and produce toxic gases.
Porosity Problem #2: Gas Coverage The second leading cause of porosity in welds is a problem with the shielding gas coverage. The GMAW process relies on the shielding gas to physically protect the weld puddle from the air and to act as an arc stabilizer. If the shielding gas is disturbed, there is a potential that air could contaminate the weld puddle and lead to porosity.
Remedies Shielding gas flow varies depending on wire size, amperage, transfer mode and wind speed. Typical gas flow should be approximately 30-40 cubic feet per hour. Using a flow meter, check that the shielding gas flow is set properly. There are a variety of flow meters on the market today ranging from simple dial gauges to ball flows all the way up to sophisticated, computerized models. Some operators mistakenly think that a pressure regulator is all that is needed, but the pressure meter will not set flow.
A pure carbon dioxide shielding gas requires the use of special flow meters designed specifically for carbon dioxide. These special flow meters are not affected by the frosting that may occur as the carbon dioxide changes from liquid form to a gas.
If high winds are blowing the shielding gas away from the puddle, it may be necessary to erect wind screens. According to the AWS Structural Welding Code, it is advisable not to GMA weld when wind speeds are greater than 5 mph. Indoors, ventilation systems may hamper gas coverage. In this case, redirect air flow away from the puddle. If fume extraction is necessary, use equipment designed specifically for this purpose such as MAGNUM™ Extraction Guns from Lincoln Electric -- they will remove the fume, but not disturb the shielding gas.
A turbulent flow of gas as it exits the gun may also lead to porosity problems. Ideally, the gas will lay over the weld puddle much like a blanket. Turbulent gas flow can be caused by too high a flow, an excessive amount of spatter inside the gun nozzle, or spatter build-up in the gas diffuser.
Other possible causes of insufficient gas flow may be damaged guns, cables, gas lines, hoses or loose gas fittings. These damaged accessories may create what is referred to as a “venturi effect” where air is sucked in through these openings and flow is reduced.
Lastly, welding with a drag or backhand technique can lead to gas coverage problems. Try to weld with a push or forehand technique which lays the gas blanket out ahead of the arc and lets the gas settle into the joint.
Porosity Problem#3: Base Metal PropertiesAnother cause of weld porosity may be attributed simply to the chemistry of the base metal. For instance, the base metal may be extremely high in sulfur content.
Remedy Unfortunately, if the problem with porosity lies within the base metal properties, there is not much that can be done. The best solution is to use a different grade of steel or switch to a slag-generating welding process.
2. Improper Weld Bead ProfileIf operators are experiencing a convex-shaped or concave-shaped bead, this may indicate a problem with heat input or technique.
Improper Bead Problem #1: Insufficient Heat InputA convex or “ropy” bead indicates that the settings being used are too cold for the thickness of the material being welded. In other words, there is insufficient heat in the weld to enable it to penetrate into the base metal.
RemediesTo correct a problem with running “too cold,” an operator must first determine if the amperage is proper for the thickness of the material. Charts are available from the major manufacturers, including Lincoln Electric, that provide guidelines on amperage use under varying conditions.
If the amperage is determined to be high enough, check the voltage. Voltage that is too low usually is accompanied by another telltale sign of a problem: a high amount of spatter. On the other hand, if voltage is too high, the operator will have problems controlling the process and the weld will have a tendency to undercut.
One way to check if the voltage is set properly is to test it by listening. A properly running arc will have a certain sound. For instance, in short arc transfer at low amperages, an arc should have a steady buzz. At high amperages using spray arc transfer, the arc will make a crackling sound. The arc sound can also indicate problems -- a steady hiss will indicate that voltage is too high and the operator is prone to undercut; while a loud, raspy sound may indicate voltage that is too low.
Improper Bead Problem #2: TechniqueA concave or convex-shaped bead may also be caused by using an improper welding technique. For example, a push or forehand technique tends to create a flatter bead shape than a pull or backhand technique.
RemedyFor best bead shapes, it is recommended to use a push angle of 5-10 degrees.
Improper Bead Problem #3: Inadequate Work CableProblems with the work cable can result in inadequate voltage available at the arc. Evidence of a work cable problem would be improper bead shape or a hot work cable.
RemedyWork cables have a tendency to overheat if they are too small or excessively worn. In replacing the cable, consult a chart to determine size based on length and current being used. The higher the current and longer the distance, the larger the cable needed.
3. Lack of FusionIf the consumable has improperly adhered to the base metal, a lack of fusion may occur. Improper fusion creates a weak, low quality weld and may ultimately lead to structural problems in the finished product.
Lack of Fusion Problem: Cold Lapping in the Short Arc Transfer Process
In short arc transfer, the wire directly touches the weld pool and a short circuit in the system causes the end of the wire to melt and detach a droplet. This shorting happens 40 to 200 times per second. Fusion problems may occur when the metal in the weld pool is melted, but there is not enough energy left to fuse it to the base plate. In these cases, the weld will have a good appearance, but none of the metal has actually been joined together. Since lack of fusion is difficult to detect visually, it must be checked by dye-penetrant, ultrasonic or bend testing.
RemediesTo guarantee correct fusion, ensure that voltage and amperage are set correctly. If the operator is still having problems after making those adjustments, it may require a change in the welding technique. For example, changing to a flux-cored wire or using the spray arc transfer method instead. In spray arc transfer, the arc never goes out so cold lapping and lack of fusion are not issues. Spray arc welding takes place at amperages high enough to melt the end of the wire and propel the droplet across the arc into the weld puddle.
4. Faulty Wire DeliveryIf the wire is not feeding smoothly or if the operator is experiencing a chattering sound within the gun cable, there may be a problem with the wire delivery system. Most of the problems related to wire delivery are attributed to equipment set-up and maintenance.
Faulty Wire Delivery Problem #1: Contact Tip There is a tendency among operators to use oversized tips, which can lead to contact problems, inconsistencies in the arc, porosity and poor bead shape.
RemedyMake sure that the contact tip in the gun is in working order and sized appropriately to the wire being used. Visually inspect the tip and if it is wearing out (becoming egg-shaped), it will need to be replaced.
Faulty Wire Delivery Problem #2: Gun Liner A gun liner, like the contact tip, must be sized to the wire being fed through it. It also needs to be cleaned or replaced when wire is not being fed smoothly.
RemedyTo clean the liner, blow it out with low-pressure compressed air from the contact tip end, or replace the liner.
Faulty Wire Delivery Problem #3: Worn Out GunInside the gun are very fine strands of copper wire that will eventually break and wear out with time.
RemedyIf the gun becomes extremely hot during use in one particular area, that is an indication that there is internal damage and it will need to be replaced. In addition, be certain that the gun is large enough for the application. Operators like to use small guns since they are easy on the hand, but if the gun is too small for the application, it will overheat.
Faulty Wire Delivery Problem #4: Drive Roll Drive rolls on the wire feeder periodically wear out and need to be replaced.
RemediesThere are usually visual indications of wear on the grooves of the rolls if replacement is necessary. Also, make sure that the drive roll tension is set properly. To check tension, disconnect the welding input cable from the feeder or switch to the cold feed option. Feed the wire and pinch it as it exits the gun with the thumb and forefinger. If the wire can be stopped by pinching, more drive roll tension is needed. The optimum tension will be indicated by feeding that is not stopped while pinching the wire. If the drive roll tension is too high, it may deform the wire leading to birdnesting (tangling) and a burn back (when the arc climbs the wire and fuses the wire to the contact tip).
Make sure that the drive rolls and the guide tube are as close together as possible. Next, check the path from where the wire leaves the reel to where it enters the drive rolls. The wire must line up with the incoming guide tubes so there is no scrapping of the wire as it goes through the tube. On some wire feeders, the wire spool position is adjustable -- align it so that it makes a straight path into the tube.
Faulty Wire Delivery Problem #5: Wire Coming Off Reel and TanglingSome wire feeding problems occur because the inertia from the wire reel causes it to coast after the gun trigger is released.
RemedyIf the reel continues to coast, the wire on the reel will loosen and the wire may come off or become tangled. Most wire feeding systems have an adjustable brake on the wire reel. The brake tension should be set so that the reel does not coast.
By following these four guidelines, a GMAW operator new to the world of welding or even someone more experienced should have an easier time diagnosing problems before they affect the quality of the work.
The world is changing. That's no surprise to anyone who is even remotely conscious of their surroundings. However, it's tempting to look at long- established technologies, such as welding, and believe that there is little or no technology development taking place at this late date. However, the person who took that view would be wrong. In fact, the design and capabilities of welding power supplies has changed and is continuing to change, rapidly. One of the technologies driving this change is the development and popularization of power supplies based on inverter technology. This technology is particularly well suited to welding aluminum alloys, especially thin aluminum alloys.
What's New?In the past, welding power supplies have been based on transformers. The power supply took in 60 Hertz 230, 460 or 575 volt power. A metallic transformer changed it from the relatively high input voltage to 60 Hertz current at a lower voltage. This low voltage current was then rectified by some sort of rectifier bridge to get direct current (DC) welding output. Control of this output was usually performed by some sort of relatively slow magnetic amplifiers.
Transformer based TIG welders are typically heavy and large. Transformers are relatively inefficient operating at 50 or 60 Hertz. A lot of heat is generated in the transformer, and the transformer must be relatively large and heavy. A significant part of the power cost goes into heating the transformer and the surrounding air. Most such welding power supplies weigh around 400 pounds and have a shape something like a 32 inch cube. Additionally, if 60 Hertz is used, control signals are limited to being issued at no more than 120 per second, so it's impossible to pulse the welding current any faster than this.
In inverter controlled power supplies, the same incoming 60 Hertz power is used. However, instead of being fed directly into a transformer, it is first rectified to 60 Hertz DC. Then it is fed into the inverter section of the power supply where it is switched on and off by solid state switches at frequencies as high as 20,000 Hertz. This pulsed, high voltage , high frequency DC is then fed to the main power transformer, where it is transformed into low voltage 20,000 Hertz DC suitable for welding. Finally it is put through a filtering and rectifying circuit . Output control is performed by solid state controls which modulate the switching rate of the switching transistors.
What advantages does this new inverter controlled design offer? First, the main power transformer, which operates at 20,000Hertz is vastly more efficient than 60Hertz transformers, which means it can be much smaller. Remember, transformer - based machines typically weigh 400 pounds plus and are a 32 inch cube. The accompanying photo shows the Lincoln line of inverter - based gas tungsten arc welding (GTAW) power supplies. The machine in the center, the V205, weighs 33 pounds and is 9 inches wide, 19 inches deep and 15 inches high. The other two machines are DC only inverters and are even lighter and smaller. So there is a huge advantage in weight and portability in favor of the inverter - based machines.
There is another advantage of the inverter power supplies - power cost. The inverter equipment is much more efficient than transformer equipment. For instance, the current draw at 205 amperes for the Lincoln V205 is 29 amperes on 230Volt single phase power. The current draw of an older
transformer welder is typically 50 to 60 amperes on 230 Volt single phase power when welding at similar currents. While the cost savings in switching to inverters is often overstated, under normal circumstances, it is safe to say that annual power savings are approximately 10% of the power supply purchase price.
The other significant advantage of inverter power supplies is that, by "choppingup" the incoming AC so finely, we end up with a very steady DC, without the typical 60 Hertz ripple. This results in a much smoother, more stable DC welding arc.
So far, we've only discussed inverters that supply direct current. For quite a few years, this was all that was available. Inverters that supplied AC output simply did not exist. Then, someone had the idea of packaging two inverters inside one case. By having them run at different polarities and alternately switching them on and off, a pseudo AC output was generated. Some inverters still generate AC in this manner. There are also more sophisticated methods of generating AC today, but for the purposes of this article, it's easier to think of generating the AC from two inverters at opposite polarities.
The ability to generate AC is what really makes the inverter shine for welding aluminum using GTAW. The fact that the arc voltage never truly goes through zero means that the AC arc is much more stable than previously. Most inverter - based GTAW power supplies do not need the high frequency to be on continuously for stability. In fact, the Lincoln V205 has no provision for using continuous high frequency. It will automatically be extinguished as soon as the arc starts.. The elimination of continuous high frequency drastically reduces the amount of RFI generated by the power supply.
Second, the fact that we can send control signals at 20 kilohertz means that we can vary the frequency of the AC welding output. Older machines were 60 Hertz AC output only. The V205 can put out AC at anywhere 20 and 150 Hertz. Higher frequencies can be beneficial in welding thin materials. As the frequency is raised, the arc cone, and the weld, become narrower, resulting in deeper penetration.
It was realized many years ago that in GTAW, weld penetration comes from the electrode negative part of the AC cycle. During the part of the cycle when the electrode is positive, weld penetration is reduced and more heat goes into the tungsten electrode. However, during the electrode positive part of the cycle, the arc actually acts to remove the oxides from the surface of the aluminum, making welding easier. It is for this reason that, although most other materials are GTA welded using direct current, aluminum is usually welded using AC. Very early GTAW power supplies supplied a simple sine wave output where equal amounts of electrode positive and electrode negative were generated. However, this was inefficient. We didn't need that much electrode positive to get adequate cleaning. Later power supplies allowed us to vary the proportion of electrode negative to electrode positive. It was found that approximately 65% electrode negative and 35% electrode positive gave adequate arc cleaning and good penetration. However, a lot of the arc energy was still going to heat the tungsten electrode., so that large diameter tungsten electrodes were required.
The inverter power supplies provide adequate arc cleaning with as little as 15% electrode positive. Reducing the amount of electrode positive makes the process more efficient, increases weld penetration, and reduces the amount of heat going into the tungsten electrode, which means smaller diameter, pointed electrodes can be used. This further concentrates and narrows the weld.
Finally, the newer inverter power supplies are software programmable. This makes it much easier to change power supply characteristics. The accompanying photo shows another Lincoln power supply, the Invertec® V350 Pro. This power supply is primarily designed as an inverter - based machine for gas metal arc welding (GMAW). It contains quite a number of different programs for steady state, pulsed GMAW and non - traditional control algorithms for GMAW. A good number of the pulsed GMAW programs where the pulsing parameters are optimized for specific filler materials and wire sizes. However, because of the software programming, it is also ready to use as a power supply for shielded metal arc welding or gas tungsten arc welding. It can also be reprogrammed in the field in a short time. Along with all of this, the power supply weighs 79 pounds and can put out as much as 425 amperes.
The future is here.
AbstractToday there are numerous ways to weld high yield pipe in the field. It is necessary to understand these various processes to insure that the process selected with meet the quality and productivity requirements of a pipeline project. Several prcocesses are discussed, with emphasis on shielded metal arc welding with cellulosic electrodes and self-shielded flux cored arc welding.
IntroductionIn today’s world cross country transmission pipelines have to address many issues including higher service pressures, sour products, new high strength steels, more severe operating environments, tighter governing codes, and a host of environmental concerns. These conditions must be balanced by the needs of the pipeline contractor to control costs and complete the project in a timely manner while still meeting more stringent quality requirements. A knowledge of welding processes can help the contractor meet his needs and deliver the required quality. This
same knowledge can help the specifying engineer understand that there are numerous ways to meet his quality and design needs without imposing unnecessary costs on the contractor.
Several processes and combinations of processes currently used for the field welding of cross country line pipe. These include shielded metal arc welding (SMAW), self shielded flux cored arc welding (FCAW-S), and gas metal arc welding (GMAW). With GMAW transfer mode must also be consider, short arc, controlled short arc as in Surface Tension Transfer®, spray, and globular. Attention will be placed on those processes which lend themselves to high quality and high productivity field welding with conservative capital investment.
Review of Pipeline SteelsToday’s pipe steels are higher strength than those used previously and are today designed with weldability in mind. The most common steels used for oil and gas cross country pipelines conform to API 5LX or similar such standards.
Table 1. Summary API 5L Strength Requirements
X42 X46 X52 X56 X60 X65 X70 X80 Tensile
(ksi) 60 63 66 71 75 77 82 90-120
Yield (ksi) 42 46 52 56 60 65 70 80
Strength levels can be achieved by several methods including gross chemistry, micro-alloying, and cold expansion of the pipe when produced at the pipe mill. In higher strength grades the trend is to use cold expansion and micro-alloying so that carbon and manganese can be kept at relatively low levels, thus reducing heat affected zone hardness and helping reduce, though not eliminate concerns about weld metal hydrogen. For example, it is typical to see carbon contents of less than 0.05% in modern X70 and X80 steels with some X80 steels having Pcm values of less than 0.20.
Welding ProcessesObviously the first step in the welding of pipe is to run the root pass. This is perhaps the most critical pass on a pipe weld for several reasons. First, this is the most difficult pass to make on a pipe weld, requiring good operator skill for manually applied processes, with good process control combined with good alignment. Automatically applied processes require operators with high degrees of technical skill combined with good alignment and backing systems. The automated process of choice today is gas metal arc welding and is generally used with either an internal copper backup ring, or, if the diameter is large enough, an internal welding system. Both of these approaches add complexity to field welding and impose certain restrictions the use of traditional GMAW transfer modes.
With backup rings there is the possibility of unacceptable copper pick up in the root pass. With internal welding systems there is a minimum pipe diameter below which the systems are not
practical. The ideal welding process would allow welding of a root bead without backup rings and internal systems and would have a root bead with sound weld metal and just enough buildup to insure a full thickness weld. This weld would also have no internal undercut, no lack of fusion, no porosity, and good mechanical properties.
Welding speed must also be considered when looking at the welding of the root pass. The pace of pipe laying is determined by how quickly the root pass can be done. While some time can be gained by putting more operators on this pass, there is a practical limit to this approach. Therefore, high travel speeds are essential. Speed is needed to maintain schedules and control equipment leasing costs.
Much of the pipeline welding done today is in the emerging economies of the world, often in remote inhospitable climates and must draw on local labor pools for welders. Tthis means that the process used must cope with adverse conditions of weather including wind, temperature extremes, and moisture. The necessary skills need either to exist in the local labor pool, or be easily learned. The required welding equipment must also be rugged, reliable, and durable.
When all of the above factors are considered, two welding processes emerge as the leading processes, shielded metal arc welding and self shielded flux cored arc welding. In the case of shielded metal arc welding, Figure 1, there are advantages to using cellulosic electrodes run in the vertical down direction instead of using low hydrogen electrodes, even on higher strength steels. Because cellulosic electrodes generate a significant amount of shielding gases in use and have a focused forceful arc, these electrodes tend to have better root pass properties and better root pass control. The high arc force helps to maintain puddle and slag control in vertical down progression, while also having high travel speeds. Low hydrogen electrodes primarily use slag to protect the weld pool and this can lead to contamination of the weld pool from the back side of the bead, reducing weld properties and increasing the chances for porosity. The relatively low penetration of low hydrogen electrodes when compared to cellulosic electrodes also means that wider root gaps must be used which increase welding time and slow down the welding operation. Cellulosic electrodes can put in root passes at speeds that exceed 14 inches per minute (356 mm per minute) and with consistent inside buildups of under 1/16 inch (1.6 mm).
Cracking concerns with cellulosic electrodes are addressed with proper preheat and interpass temperature control, and by using procedures which insure adequate ligament in the root pass. Preheat and interpass temperatures are dictated by steel chemistries which today are more forgiving than previously. Use of the correct electrode size run in the middle to lower portion the range for that electrode helps insure a proper ligament. Root bead cracking can also be minimized by not moving the lineup clamp until the second pass has been completed.
Self shielded flux cored arc welding, Figure 2, has the advantages of shielded metal arc welding with cellulosic electrodes including high arc force, high penetration and excellent puddle control when welding with a vertical down progression. In addition, this process has the advantages of automated processes including high deposition rates, high travel speeds, high arc on times, and controlled hydrogen levels. Frequently self shielded is used over root passes made with shielded metal arc welding. This is one approach to the welding of X80, where hydrogen cracking in the
parent steel is not a concern for the root pass, but weld metal hydrogen cracking could be a concern on subsequent passes.
In shielded metal arc welding shielding is generated by the decomposition of the flux at the arc. In self shielded flux cored arc welding a continuous tubular electrode contains are stabilizers and core materials which will generate shielding when they reach the arc. Both processes work outside under severe weather conditions including temperature extremes and high winds. Likewise, both shielded metal arc welding with cellulosic electrodes and self shielded flux cored
arc welding are easily learned by welding operators already skilled in other forms of shielded metal arc welding. For example, one instructor was recently able to train and qualify to API 1104 over ninety welding operators unfamiliar with self shielded flux cored arc welding.
Table 2. Vertical Down Pipe Welding ElectrodesAWS CLASS API 5L Strength Levels
X42 X46 X52 X56 X60 X65 X70 X80 ROOT PASS E6010 X X X
E7010G X X X X E8010G X X X X X
E71T-13H8 X X X X X X X X HOT FILL AND CAP PASSES
E6010 X X X E7010G X X X X E7010G X X X X X
E71T8-K6 X X X X X X X E91T8-G X
Notice in the above table that only the self shielded arc welding process is recommended for the welding of X80 once the root and hot passes are completed.
Both processes are capable of delivering properties which meet or exceed the minimum properties specified for the parent steels, which is all that most governing codes require. Here are a few test results for two pipe grades run with the typical pipe joint detail shown in Figure 3.
Welding EconomicsThe one issue not yet discussed is that of economics. Many things affect the cost of welding including material costs, equipment cost, labor rates and a host of others outside the scope of this paper. For purposes of comparison time to complete a welded joint will be used for a relative indicator of cost. The basic assumption is that if equipment costs and labor rates are similar, the time to complete a weld joint will be indicative of cost, less time translating into lower costs and higher productivity. All comparisons will be done using the typical joint detail used above to simplify the results. In reality compound preparations may reduce the total time on heavier wall pipe. Welding comparisons will use .750 in (19 mm) wall, 48 in (1219 mm) diameter pipe.
Table 5. Typical Procedures All Progression is down hill, 5G Position
These times represent man-minutes of welding. The joint done with all self shielded flux cored arc welding has the lowest total time, but the combination of shielded metal arc welding with self shielded flux cored arc welding will result in the greatest amount of pipe laid on a given day because of the time savings in the root pass. Tthis combination will result in the best overall compromise of reduced total time and maximum pipe laid in a given period of time.
ConclusionsAs can be seen, shielded metal arc welding and self shielded flux cored arc welding present cost effective ways to produce quality welds under field conditions. Also, the best solution to field welding of cross country pipelines is often is to use a combination of welding processes.
ReferencesWelding Handbook, 8th Edition, (1991) American Welding Society, Miami
The Procedure Handbook of Arc Welding, 13th Edition, (1994), Lincoln Electric Company, Cleveland
INVERTOR BASED POWER SOURCE
The advantages of inverter welding units over traditional transformer-rectifiers are many. Inverters are more portable and lighter weight, making them easier to maneuver around the job site. In addition, inverters offer high-quality, multi-process welding capabilities so that one machine can handle Stick, MIG, TIG, FCAW, arc gouging and even pulsing. And even more importantly, inverters take advantage of Lincoln's Waveform Control Technology™ to offer greater control of arc variables and automatically fine tune the arc to create the best possible weld, controlling problems such as burnthrough.
But did you know that using an inverter can also save you money in energy costs over a traditional-type power source? Each year about $15 million worth of electricity is consumed in the U.S. and $99 million worldwide for welding. To increase efficiencies and cut down on the money your company is spending on electricity related to welding, an inverter is an attractive option. In fact, because of their efficiency, these machines can provide substantial savings in utility costs.
But how can a switch to an inverter save in energy consumption? In the design of inverter welders, such as Lincoln's Invertec® V350 Pro, the transformer cores, transformer windings and power electronic switching components are all carefully chosen to minimize operating losses. Here are some other reasons that inverters save in energy costs:
Greater transformer efficiencies are realized through the use of ferrite cores in the inverter's power transformer. This reduces the current losses resulting in lower idle currents in the supply conductors
The inverter transformer coils are physically smaller than common transformers. A smaller coil translates to less wire wrapping around the core - less wire means fewer losses and greater efficiency
The inverter's power electronic components have been carefully designed to reduce losses and extend operating life
Many inverters, such as Lincoln's Invertec V350 Pro, uses a copper conductor. Copper has higher thermal and electrical conductivity compared to aluminum, which will minimize losses and maximize efficiency
Operating at higher frequencies than conventional welders, inverters require less output inductance for smooth operation. The energy needed for stick welding or for globular transfer welding processes is stored in capacitors allowing for smaller output chokes
The compact design and relatively small physical size of an inverter welder means shorter leads and cables (or even direct connections) between power components. Shorter current paths translate to lower resistances and better efficiencies
Because the inverter is designed to inherently have low losses, smaller cooling fans are required. This means less power is needed for moving cooling air and, again, greater efficiency
The smaller size of the components inside the inverter machine translate into less heat to dissipate and again, greater efficiency
How can you calculate how an inverter can save you money over a traditional transformer- rectifier and which inverter is the best in creating energy efficiencies? Use the worksheet below to make that assessment.
Step #1 - Calculate Output PowerFirst look at your machine to determine the output voltage (Vout) which is given as volts on your machine. In our example this is 32v. Then multiply that by the output current (Iout), found on your machine in amps. In this case the amps are given as 300.
Vout x Iout = Output Power (Wout) in watts32v x 300 amps = 9,600 watts OR 9.6 KW (1,000 watts = 1 KW)
Step #2 - Calculate Input PowerNow take the output power from above (KWout) and divide by the efficiency (Eff). The
efficiency is given by the machine's manufacturer. Calculating this will give you input power in kilowatts.
KWout ÷ Eff = Input power in kilowatts (KWin)9.6 KW ÷ 88.2% (or 0.882) = 10.88 KW
Step #3 - Calculate Operating Costs During WeldingA) Next, you will calculate the kilowatt hours used in one day (KWh1/day) by taking the input power calculated in Step #2 (KWin) multiplied by the hours per day that the machine runs (for our example, we will assume welding is performed four hours per day.)
KWin x #Hrs/Day = Kilowatt hours used in one day (KWh1/day)10.88 KW x 4 Hrs. = 43.52 KWHrs/day
B) Now take your input power calculated (KWin) multiplied by the number of hours per day the machine runs multiplied by the price per KW hour of the power. Note: the price of the power is calculated at $0.12578 which is the industry average.
KWin x #Hrs/Day x Price per KWHrs ($/KWh) = Daily Operating Costs Welding10.88 x 4 x $0.12578 = $5.47
Step #4 - Calculate the Operating Costs During IdleA) You will now calculate the idle consumption per day (KWh2). To do this, take the input power (KWIdle) multiplied by the idle hours per day. (We are assuming that in an eight-hour day, if welding is performed four hours, idle hours will be four as well.)
KWIdle x Idle Hrs. = Idle Consumption Per Day (KWh2)0.4 KW x 4 Hrs. = 1.6 KW hrs
B) Now take the input power idle (KWIdle) which is given on the power transformer in watts - in this case 400 watts (or 0.4 KW) - multiplied by the idle hours x the price per kilowatt hour of power.
KWidle x IdleHrs x Price per KW Hrs = Daily Operating Costs Idle0.4 KW x 4 Hrs. x $0.12578 = $0.20
Step #5 - Calculate the Total Operating CostsNow take the daily operating costs welding calculated in Step #3 and add the daily operating costs idle from Step #4 above equals the daily operating costs in dollars.
By comparing this number against a traditional transformer-rectifier or another competitive inverter, you can easily tell which machine will provide the cost savings.
An inverter with a list price of $3,200 and efficiency of 87 percent compared to a traditional transformer rectifier that has a list price of $2800 and a 67 percent efficiency rate would save approximately $300 in utility cost on an annual basis. The payback for the difference in price would then be in one to one-and-a-half years.
What is Arc Welding?Arc welding is a method of joining two pieces of metal into one solid piece. To do this, the heat of an electric arc is concentrated on the edges of two pieces of metal to be joined. The metal melts, while the edges are still molten, additional melted metal is added. This molten mass then cools and solidifies into one solid piece.
Welding Consumables
Stick Electrode A short stick of welding filler metal consisting of a core of bare electrode covered by chemical or metallic materials that provide shielding of the welding arc against the surrounding air. It also completes the electrical circuit, thereby creating the arc. (Also known as SMAW, or Stick Metal Arc Welding.)
MIG Wire Like a stick electrode, MIG wire completes the electrical circuit creating the arc, but it is continually fed through a welding gun from a spool or drum. MIG wire is a solid, non-coated wire and receives shielding from a mixture of gases. (Process is also known as GMAW, or Gas Metal Arc Welding.)Cored Wire (Flux-Cored Wire) Cored wire is similar to MIG wire in that it is spooled filler metal for continuous welding. However, Cored wire is not solid, but contains flux internally (chemical & metallic materials) that provides shielding. Gas is often not required for shielding. (Process is also known as FCAW, or Flux-Cored Arc Welding.)Submerged Arc A bare metal wire is used in conjunction with a separate flux. Flux is a granular composition of chemical and metallic materials that shields the arc. The actual point of metal fusion, and the arc, is submerged within the flux. (Process is also known as SAW, or Submerged Arc Welding.)
Stainless SteelStainless steel electrodes and wire are used for welding applications where corrosion resistance is required. Stainless steel consumables are designed to match the composition of stainless steel base metals.
HardfacingA stick of electrode or cored wire that is designed not to fuse two pieces of metal together, but to add a layer of surface metal to a work-piece in order to reduce wear. An example of this is the shovel on an excavator.
Welding Equipment
Stick Welders Heating the coated stick electrode and the base metal with an arc creates fusion of metals. An AC and/or DC electrical current is produced by this machine to create the heat needed. An electrode holder handles stick electrodes and a ground clamp completes the circuit.TIG Welders A less intense current produces a finer, more aesthetically pleasing weld appearance. A tungsten electrode (non-consumable) is used to carry the arc to the workpiece. Filler metals are sometimes supplied with a separate electrode. Gas is used for shielding. (Process is also known as GTAW, or Gas Tungsten Arc Welding.)
MIG Welders and Multi-Process WeldersConstant Voltage and Constant Current welders are used for MIG welding and are a semi-automated process when used in conjunction with a wire feeder. Wire is fed through a gun to the weld-joint as long as the trigger is depressed. This process is easier to operate than stick welding and provides higher productivity levels. CC/CV welders operate similarily to CC (MIG) welders except that they possess multi-process capabilities - meaning that they are capable of performing flux-cored, stick and even TIG processes as well as MIG.Engine Driven WeldersLarge stick or multi-process welders are able to operate independent of input power and are powered by a gasoline, diesel, or LPG engine instead. Ideal for construction sites and places where power is unavailable.
Wire Feeder / WeldersFor MIG welding or Flux-Cored wire welding, wire feeder welders are usually complete and portable welding kits. A small built in wire feeder guides wire through the gun to the piece.
Semiautomatic Wire Feeders For MIG welding or Flux-Cored welding, semiautomatic wire feeders are connected to a welding power source and are used to feed a spool of wire through the welding gun. Wire is only fed when the trigger is depressed. These units are portable.
Automatic Wire Feeders For MIG, Flux-Cored, or submerged arc welding, automatic wire feeders feed a spool of wire at a constant rate to the weld joint. They are usually mounted onto a fixture in a factory/industrial setting and are used in conjunction with a separate power source.Magnum Guns / TorchesMIG welding guns and TIG welding torches are hand-held welding application tools connected to both the wire feeder and power source. They direct the welding wire to the weld joint and control the wire feed with the use of a trigger mechanism.
Cutting
Plasma CuttersA constricted cutting arc is created by this machine, which easily slices through metals. A high velocity jet of ionized gas removes molten material from the application.
Oxyfuel Gas CuttingOxyfuel gas cutting process involves preheating the base metal to a bright cherry red, then introducing a stream of cutting oxygen which will ignite and burn the metal.
Welding Automation / Robotic Welding
Robotic Welding Systems The combination of a robotic arm, a welding power source and a wire feeder produces welds automatically using various programs, welding fixtures and accessories.
Environmental SystemsAlso known as fume extraction, these systems are often incorporated into a robotic fixture to remove welding fumes natural to the process from the welding environment. Usually a vacuum unit, they can be portable or mounted onto a wall.n arc welding, an arc is established from the electrode to the workpiece. To do this, a smooth flow of electricity needs to complete the electrical circuit, hence the need for good electrical connections. Not only will good work lead connections, commonly incorrectly called ground connections, affect the welding arc and the quality of the finished weld, having good work lead connections are important to minimize electrical shock hazards. This article will explain some of the ways to achieve a good work lead connection.
Attaching the Work Lead to the WorkpieceA high production automotive part may benefit from more than one connection.
A high production automotive part may benefit from more than one connection.
To be effective, the work lead must make good electrical contact with the workpiece. This is usually performed by using a work lead clamp (also, commonly incorrectly called ground clamp), or through copper-graphite brushes, or sliding or rotating shoes. Also, keep in mind that aluminum is poorly suited for this purpose, as it quickly forms an oxide that is poor conductor of welding current.
A work lead clamp is perhaps the common method to connect the work lead cable to the workpiece. The work lead clamp is bolted to the lug at the end of the work lead cable and has brass or copper jaws to ensure good electrical contact to the workpiece. The work lead clamp may be attached directly to the workpiece or to the fixture holding the workpiece. Remember that the work lead cables or cables should be neatly organized, not strung about haphazardly.
The work lead connection may vary with the welding process, amperage, or application. Automatic welding installations commonly use a permanent stationary connection. Automatic circumferential welds usually use a work lead connection of brushes or rotating or sliding shoes with an electrically conductive lubricant.
Be careful not to allow the welding current to flow through sensitive electronic components.
When grounding is through a sliding shoe, two or more shoes should always be used. This will prevent interruptions of current in case one shoe loses contact by an unexpected surface protrusion, such as the reinforcement of another weld, a piece of weld spatter, or granular flux. Be careful not to allow the weldingcurrent to flow through sensitive electronic components.
An improvement on sliding brushes, preloaded, tapered roller bearings are excellent for rotating grounds. To assure trouble-free performance, the contact area of the bearing should be sufficient
to carry the current capacity of the welding cable used in the installation. Since most mechanized welding installations use two 4/0 cables in parallel, the tapered bearings are usually fairly large.
Regardless of how the work lead connection is made, it should be a secure positive connection, properly placed to minimize any welding interference or arc blow. The experienced welder knows that good work lead connections are essential for good welds and should not be overlooked.
Using Steel Bars or Reinforcing RodsIn some welding applications, a steel bar or a steel reinforcing rod may be used as a work lead connection or between two or more weldments. When using steel bars, care must be taken to assure that the bar has adequate cross-sectional area to match the copper welding cable in total electrical conductivity. Since the conductivity of copper is almost seven times that of mild steel, the cross-sectional area of the steel bar should be at least seven times the cross section of the welding cable conductor. A bar that is inadequate in cross section may result in an overheated connection and result in poor welding performance.
The arc itself is a very complex phenomenon. In-depth understanding of the physics of the arc is of little value to the welder, but some knowledge of its general characteristics can be useful.
Cleaning the Work Lead Connection AreaA rotating piece needs a special "moving" connection. A point to bear in mind is that the work lead clamping area should be at least equal to the cross-sectional area of the conductor. This means that the area of contact must be free from any scale, rust, oil, grease, oxides, or dirt that may act as points of insulation. Cleaning the area of contact with sandpaper or a wire brush before making the connection is good practice.
A rotating piece needs a special "moving" connection.
Testing the CircuitA simple way to test the soundness of the circuit is to run a hand over the length of the cable from the power source to the electrode. This should be performed immediately after an hour or more of welding after the power source is disconnected. For safety, always make sure the power is off before performing this test. If a "hot" section of the cable is felt, this is a potential problem as it's an area of increased electrical resistance. If the hot section is near a terminal, the connection at the terminal is suspect; if any place along the cable; the cause is probably damaged strands within the cable. If the entire cable is hot to touch, it is probably undersized for the welding current being used.
Also, this is good time to inspect the electrical cables connected to the welding power source and determine if the cables can be safely used or needs to repaired or replaced. For everyone's safety, always keep your cables in good condition.
TroubleshootingPoor work lead connections may arc and weld the connection to the workpiece. In addition, poor connections reduce the voltage at the welding arc. This may cause poor arc starting, excessive spatter, poor bead shape, and reduced weld quality. If any of these conditions occur, inspect your work lead connections immediately.
SafetyThe hazard of electrical shock is one of the most serious and immediate risks facing personnel working in the welding area. Contact with metal parts that are "electrically hot" can cause injury or death. With this in mind, always unplug the input power cord or disconnect the main power before attempting to inspect or service electrical problems. When in doubt, refer to the equipment's operation manual or call a qualified electrician.
IntroductionOperators are exposed to fume and gases when welding, and exposures vary depending upon the process and specific working conditions. Fabricators are under continual pressure to reduce worker exposure to potentially harmful substances in the workplace, including welding fume. This article will address the following:
How welding fume is generated Coordinating factors that affect fume generation and exposure to fume such as welding design, process, equipment, consumables, gases, work management and ventilation Highlights of fume extraction technology The current U.S. regulatory climate with regard to welding fume Current published exposure limits for typical components of fume
What Is Welding Fume?Although many people think of gases and vapors from gasoline or other chemicals as "fume," technically, fume is comprised of very small, solid particles. Since Arc welding usually produces only small concentrations of gases, exposure to gases is seldom a concern except in confined areas. Therefore, the issue of secondary gas production will not be specifically discussed here.
Arc welding creates fume as some of the metal boils from the tip of the electrode and from the surface of molten droplets as they cross the arc. This metal vapor combines with oxygen in the air and solidifies to form tiny fume particles. These particles are visible because of their quantity, but each particle is only between 0.2 and 1.0 micron in size. Since fume primarily comes from
the electrode, it consists of oxides of its metals, alloys and flux compounds. In steel welding, therefore, fume is primarily iron oxide and oxides of alloys such as manganese and chromium. With plated or coated metals, some of the fume comes from the weld pool as well. This adds oxides of metals from the base material into the fume such as zinc oxide from welding galvanized steels.
A Total Systems ApproachThere are many ways to reduce exposure to welding fume. Each solution addresses part of the welding system. Each solution, however, has its advantages and disadvantages and should be considered in the context of the total system. Likewise, a solution cannot work without proper implementation. The most successful solutions rely on a coordinated effort between managers, engineers, welding supervisors, vendors and especially welders themselves.
Although "fume extraction" may be the first solution that comes to mind, other options should be considered as well. Approaches to controlling welding fume actually fall into two broad categories:
Reducing fume generation Limiting operator exposure to fume
Reducing Fume GenerationWelding Design ConsiderationsLimiting the generation of welding fume begins at the design stage. All other things being equal, a properly sized weld will result in the lowest amount of welding fume for a given process and set of procedures. Overwelding, on the other hand, unnecessarily increases welding fume. As the amount of weld metal increases, the amount of fume also increases. The welding engineer should be aware of the role that weld size plays in the creation of fume.
Welding Process Selection
Significant reductions in fume creation can come with a change in the welding process. Therefore, fabricators and welding supervisors should be aware of the impact process selection will have on fume generation. They must also remember, however, that each process offers specific advantages and disadvantages for a given application and a given situation.
Submerged Arc Welding (SAW) contains the majority of the fume (and the arc) under a bed of flux, making it an excellent choice when reducing fume generation is a primary concern. This process has certain limitations, however. SAW requires flat or horizontal positioning, slag cleaning, maintenance of the granular flux, and is most commonly used for mechanized welding of relatively thick steel plate.
Gas Tungsten Arc Welding (GTAW) also produces very little fume, since the filler metal does not carry the welding current, and the arc is very stable. However, manual GTAW is a low deposition rate process requiring highly skilled operators. As such, it is often the process of choice for precision welding or certain special applications. Using GTAW to weld heavy plate would not be practical.
Flux Cored Arc Welding (FCAW) processes are usually considered the largest fume producers due to typically high deposition rates. However, many applications are best served by FCAW precisely because of its high deposition rates, especially in out-of-position applications. Fume generation rates vary widely, depending upon the electrode type, grade, and design. The design of the electrode can have a major impact on the amount of fume that will be generated. Several manufacturers offer reduced fume flux cored electrodes. Research indicates that some metal cored electrodes used with a pulsed current power source can yield low fume generation rates as well.
Gas Metal Arc Welding (GMAW) is a practical option for many applications, from thin sheet metal to heavy plate. Fume generation in GMAW depends upon procedures, droplet transfer, shielding gas, and the grade of electrode used. ER70S-6, for instance, has higher levels of manganese than ER70S-3. Since manganese levels are often a key factor in determining regulatory compliance, this can be a significant issue.
Shielding GasThe shielding gas also affects arc physics and fume generation. The energy levels required to dissociate and ionize the various gases relates to the excess energy available to boil metal from the electrode and molten droplet. In practice, using 100% CO2 will require a procedure increase of 1-2 volts compared to Argon blends. This adds energy to the arc, boiling off more metal and creating more fume. Although some active gas is needed in GMAW and FCAW of steels, higher percentages of argon in blended gases tend to reduce fume generation. These blends tend to be more expensive than 100% CO2, especially in Europe and Asia.
Waveform Control Technology™Another way to reduce fume generation is to use one of the various waveform controlling power sources. With pulsed GMAW, for example, less fume is typically produced than with a conventional constant voltage power source. In this mode, the arc is controlled by pulsing the current from a background level to a peak level at a specified frequency. This reduces the total
arc energy and decreases the amount of metal that is vaporized, which leads to reduced fume generation.
Invertec STT II - pipe welding, low fume applicationLincoln Electric's new STT® inverter is another waveform controlled power source that has led to the creation a new transfer mode: Surface Tension Transfer® (STT) welding. In conventional short-circuit transfer, the current rises to high levels immediately before the droplet detaches from the electrode causing some of the electrode to vaporize. This causes violent droplet detachment and creation of spatter and fume. The STT power source is able to control the current during droplet transfer. When the droplet is about to detach, the current level decreases, and the droplet is pulled into the puddle by surface tension forces resulting is reduced spatter. After detachment, current is then controlled to prevent overheating the tip of the electrode. This control significantly reduces droplet temperatures and increases arc stability. Spatter can be decreased by 90% and fume generation by 50%, compared to conventional short-arc transfer. STT, however, is limited to applications appropriate for short-circuit transfer.
Limiting Operator Exposure to FumeThe second broad category of controlling welding fume covers methods of limiting personnel exposure to the fume. Management will be responsible for initiating the decisions in this category, while employees at various levels of the organization will need cooperate to ensure their success.
Job SharingThe most direct approach to limiting personnel exposure is simply to limit the amount of time an operator spends welding. This can often be accomplished via job sharing. For example, an operator could spend half a day welding an SAW application, and the remainder of the day welding an FCAW application. Or, the second half of the day might be spent driving a forklift. It is not a cost-free method; after all, twice as many individuals must be trained and qualified as welding operators for any given application. However, it can yield dividends in terms of higher
productivity, greater job satisfaction resulting from mastering a variety of tasks, and a more versatile, cross-trained workforce. This simple approach deserves thoughtful consideration by management.
Automated Welding SystemsRobotics and other automated welding systems provide another route to limiting employee exposure to welding fume. Automation can be a viable alternative if the initial capital expense can be justified by higher productivity and improved quality. However, automated welding cells commonly operate at high duty cycles, and employee exposure to fume must still be evaluated.
Fume Extraction TechnologyThe one method of fume control effective for almost any welding process is ventilation. Since the operator's breathing zone is the critical area, localized ventilation, usually called "fume extraction," is the preferred method. Fume extraction technology falls into two categories: low vacuum/high volume, or high vacuum/low volume.
Low vacuum/high volume system (mobile unit) Low Vacuum/High VolumeRegular building ventilation systems are low vacuum, high volume systems, sometimes called "low static, high flow." When industry needed better ventilation solutions, many companies modified low vacuum systems for localized ventilation. Hoses 6 to 9 inches (160 - 200 mm) in diameter were added for flexibility and eventually structures were designed to support the hoses and make it easier to position them. Manufacturers began to make these arms with different designs and features, and they are still used in many industries, including the welding industry.
The articulated arms generally move between 600 and 900 cubic feet (900 - 1500 m3/hr) per minute (CFM) of air, but use low vacuum levels (3 to 5 inches water gauge [750 - 1250 Pa]) to minimize power requirements. Water gauge (WG) is a measure of negative pressure: higher numbers mean more negative pressure (more "suction"). With this volume of airflow, the end of
the arm can be generally 10 to 15 inches (250 - 375 mm) away from the arc and still capture the fume. Articulated fume extraction arms are produced by a wide range of manufacturers, using 6 inch or 8 inch hose, or hose and tubing combinations. Lengths are typically 7, 10, or 13 feet (2, 3 or 4 m), with boom extensions available. The arms may be wall mounted, attached to mobile units, or incorporated into a centralized system.
For greater capture distances, a larger volume of air is required to achieve the necessary "capture velocity" and capture the fume. In practice, however, longer capture distances may mean that breathing zone exposure is compromised. Overhead hoods, for example, capture most of the fume, but only after it has passed through the breathing zone of the operator.
Cross draft ventilation is a variation of overhead hood technology. These systems use a plenum with openings to the side of the work space, rather than above it. Therefore, the fume moves sideways, away from the operator's breathing zone. These systems can be effective for small booths when small parts are being welded. The CFM required for effectiveness varies depending upon the installation design, but frequently can be 1,000 CFM or higher.
There are, however, certain disadvantages associated with the low vacuum systems. For example, in systems incorporating articulated fume extraction arms, the operator must stop to reposition the arm over each weld area, which diminishes productivity. These arms also have limited reach, commonly 10 to 13 feet. The high volume of air flow requires large hoses, and ductwork ranging from 8 to 36 inches in diameter or more, depending upon the installation. Exhausting air outside often requires make-up air systems and make-up air heaters. Filtration systems are large due to the high air volume being processed.
High Vacuum/Low VolumeHigh vacuum/low volume fume extraction systems are much more specific to point-source applications such as welding. Their chief advantage: they remove the fume directly at the source, within inches of the arc. This means that fume is captured before it can reach the operator's breathing zone or disperse into the room. Because of the close proximity to the source, fume extraction can be achieved with lower airflow rates, typically 80 to 100 CFM for suction nozzles, depending upon the design, and 35 to 60 CFM for integrated fume extraction guns. The vacuum level is high (40 to 70 inches WG), permitting the use of hose featuring longer lengths (10 to 25 feet) and smaller diameters (1.25 to 1.75 inches). High vacuum equipment ranges from small, portable units to mobile three-phase systems, to large, centralized systems.
There are two methods of high vacuum extraction: welding guns with built-in extraction, or separate suction nozzles of various designs. (Photo.) Suction nozzles are positioned near the weld, typically with magnets, and commonly use capture distances of less than four inches. Fume extraction guns use fume capture nozzles built into the gun tube and handle. Therefore, no repositioning is required, since the suction automatically follows the arc.
High vacuum extraction, like other solutions, has its limitations. Although manufacturers have greatly improved designs, fume extraction guns are larger than regular welding guns. Furthermore, fume guns do not control residual fume and smoke, since the gun is moved away immediately after welding is completed. Finally, unless they are set in weld fixtures, high vacuum suction nozzles also require repositioning.
Nevertheless, high vacuum/low volume methods of fume extraction offer significant advantages to welding fabricators. Of chief importance is the removal of fume right at its source, before it can reach the operator's breathing zone. Since fume guns eliminate the repositioning required by articulated arms or suction nozzles, productivity is not directly reduced.
Many other advantages come from reducing the total amount of airflow required. A lower volume of air means smaller ductwork, smaller hoses, much smaller filter systems, and less strain on make-up air systems if the air is exhausted outside. This translates into lower material, installation and maintenance costs. A typical low vacuum system for twenty stations, for instance, might require an airflow rate of 12,000 CFM, whereas a high vacuum system serving the same facility could require an airflow rate as low as 1,200 CFM.
After fume is removed from the source, it is either exhausted directly to the atmosphere or is passed through an electrostatic or cartridge filter. Because electrostatic filters lose efficiency if they are not frequently washed, the welding industry primarily uses more easily maintained cartridge filters. Most cartridge filters have a high efficiency level, usually 98% or higher. Although cartridges classified as "HEPA" have extremely high efficiency when new, they are expensive and have shorter life. HEPA filters are normally not necessary in fume extraction equipment, since capture efficiency has a much greater impact on breathing zone exposure than filtration efficiency.
Regulatory Bodies
Two major types of organizations study and regulate exposure to welding fume and other particulates in the workplace: industrial health organizations, and government regulatory agencies. In the U.S., two major industrial health organizations are the American Conference of Governmental and Industrial Hygienists (ACGIH) and the National Institute of Occupational Safety and Health (NIOSH). They set exposure limits for a variety of materials, including those found in welding fume. The ACGIH calls their limit the Threshold Limit Value (TLV). The TLV is influential in industry and is a standard followed by most insurance companies. As important as the TLV is, however, it is not enforceable by law. The Occupational Safety and Health Administration (OSHA) is the only organization that can establish legally enforceable limits for exposure to chemicals in the workplace. At both state and federal levels, OSHA's mandatory Permissible Exposure Limits (PEL) place tough demands on the welding industry.
Exposure LimitsThe limits for fume exposure set by OSHA and others are measured in milligrams of particulate per cubic meter of air (mg/m3). The total amount of fume produced is not limited, but rather the concentration of fume is limited. During facility testing, a sampling device is placed in the breathing zone of the operator (e.g., the welding hood, not on the lapel). At the end of the operator's shift, a number is calculated that reflects an 8-hour Time Weighted Average (TWA) of the fume concentration in the operator's breathing zone, in mg/m3.
Since this method focuses on breathing zone exposure, the results are highly unpredictable, even when the process, procedure and other influences are consistent. Therefore, to ensure compliance with exposure limits, companies should test their own operators while they are welding in everyday applications to obtain an accurate concentration value. The results can then be compared to benchmarks such as the TLV or PEL. If the number is higher than the standard, then that company is out of compliance.
Listed in Table 1 are the current welding fume exposure limits as specified by OSHA and ACGIH. Note that the table does not contain a PEL for total welding fume. The PEL of 5 mg/m3 established in 1989 was challenged in a lawsuit, and is no longer enforced.
Table 1. Exposure Guidelines for Materials Sometimes Found in Welding Fume
ACGIH(1)TLV (mg/m3)
OSHA(2)PEL (mg/m3)
Welding Fume 5.0 Iron Oxide, as Fe 5.0 10.0Manganese (all forms) 0.2 1.0(3) 5.0 (c)Chromium III compounds
0.5 0.5
Chromium VI compounds, sol
0.05 0.05 (c)
Chromium VI compounds, insol
0.010.5 (c) NIC.0005 - .005 (both forms)
Nickel, insol compounds, as N
(1.0) 0.5 NIC
1.0
Aluminum, Welding Fumes, as Al
5.0
Zinc Oxide, fume 5.0 10.0 (c) 5.0Barium compounds, sol, as Ba
0.5 0.5
Beryllium & compounds, as Be
0.002 .01(c) 0.002 .005(c)
Cadmium Oxide, as Cd 0.002 0.005Cobalt oxide, as Co 0.02 0.1Copper fume, as Cu 0.2 0.1Flourides, as F 2.5 2.5Magnesium oxide fume
10.0 15.0 total particulate
Molybdenum, insol compounds, as Mo
10.0 15.0 total particulate
Tin oxide 2.0 2.0 Vanadium pentoxide, as V2O5
0.05 0.1(c)
(1) Threshold Limit Value set by ACGIH (American Conference of Governmental Industrial Hygenists) based upon 8 hour TWA (Time Weighted Average), as of 9/98.(2) OSHA Permissable Exposure Limit based upon 8 hour TWA, as of 9/98.(3) Short Term Exposure Limit (STEL) for Manganese, based on a 15 minute TWA, is 3 mg/m3(c) Maximum Exposure Concentration: not to be exceeded at any time (not a TWA).NIC - Notice of intended changes
Manganese and chromium are two examples of materials which have strict time exposure limits as well. When limits are measured on an 8-hour TWA, an operator may be exposed to high concentrations in the morning, but the facility may still be in compliance if concentrations are lower in the afternoon. The limits for certain forms of chromium are "ceilings," meaning that any overexposure during the day will cause the facility to fail compliance.
Since the U.S. regulatory climate regarding welding fume depends greatly upon the specific state, local regulators should always be contacted for relevant information. Companies should check the Material Safety Data Sheet (MSDS) for the welding electrode they use. The MSDS report will show not only the composition of the electrode, but also the components of welding fume that can be created by the welding process. The report also shows the TLV and PEL for each item, and gives valuable information concerning health risks and other reference data. The only way to get a clear picture of where a company stands, however, is testing operators while they are welding in the company's actual facilities.
ConclusionWhile exposure to fume can be an issue in any welding application, no one solution is the best for all of them. Each solution only addresses part of the welding system and has its advantages and disadvantages. The best solution will be found when managers, engineers, welding
supervisors, vendors and welders work together to meet the needs of the company with a total systems approach
Basics of Arc welding
Arc welding is one of several fusion processes for joining metals. By applying intense heat, metal at the joint between two parts is melted and caused to intermix - directly, or more commonly, with an intermediate molten filler metal. Upon cooling and solidification, a metallurgical bond is created. Since the joining is an intermixture of metals, the final weldment potentially has the same strength properties as the metal of the parts. This is in sharp contrast to non-fusion processes of joining (i.e. soldering, brazing etc.) in which the mechanical and physical properties of the base materials cannot be duplicated at the joint.
Fig. 1 The basic arc-welding circuit
In arc welding, the intense heat needed to melt metal is produced by an electric arc. The arc is formed between the actual work and an electrode (stick or wire) that is manually or mechanically guided along the joint. The electrode can either be a rod with the purpose of simply carrying the current between the tip and the work. Or, it may be a specially prepared rod or wire that not only conducts the current but also melts and supplies filler metal to the joint. Most welding in the manufacture of steel products uses the second type of electrode.
Basic Welding CircuitThe basic arc-welding circuit is illustrated in Fig. 1. An AC or DC power source, fitted with whatever controls may be needed, is connected by a work cable to the workpiece and by a "hot" cable to an electrode holder of some type, which makes an electrical contact with the welding electrode.
An arc is created across the gap when the energized circuit and the electrode tip touches the workpiece and is withdrawn, yet still with in close contact.
The arc produces a temperature of about 6500ºF at the tip. This heat melts both the base metal and the electrode, producing a pool of molten metal sometimes called a "crater." The crater solidifies behind the electrode as it is moved along the joint. The result is a fusion bond.
Arc ShieldingHowever, joining metals requires more than moving an electrode along a joint. Metals at high temperatures tend to react chemically with elements in the air - oxygen and nitrogen. When metal in the molten pool comes into contact with air, oxides and nitrides form which destroy the strength and toughness of the weld joint. Therefore, many arc-welding processes provide some means of covering the arc and the molten pool with a protective shield of gas, vapor, or slag. This is called arc shielding. This shielding prevents or minimizes contact of the molten metal with air. Shielding also may improve the weld. An example is a granular flux, which actually adds deoxidizers to the weld.
Fig. 2 This shows how the coating on a coated (stick) electrode provides a gaseous shield around the arc and a slag covering on the hot weld deposit.
Figure 2 illustrates the shielding of the welding arc and molten pool with a Stick electrode. The extruded covering on the filler metal rod, provides a shielding gas at the point of contact while the slag protects the fresh weld from the air.
The arc itself is a very complex phenomenon. In-depth understanding of the physics of the arc is of little value to the welder, but some knowledge of its general characteristics can be useful.
Nature of the ArcAn arc is an electric current flowing between two electrodes through an ionized column of gas. A negatively charged cathode and a positively charged anode create the intense heat of the welding arc. Negative and positive ions are bounced off of each other in the plasma column at an accelerated rate.
In welding, the arc not only provides the heat needed to melt the electrode and the base metal, but under certain conditions must also supply the means to transport the molten metal from the tip of the electrode to the work. Several mechanisms for metal transfer exist. Two (of many) examples include:
1. Surface Tension Transfer® - a drop of molten metal touches the molten metal pool and is drawn into it by surface tension
2. Spray Arc - the drop is ejected from the molten metal at the electrode tip by an electric pinch propelling it to the molten pool (great for overhead welding)
If an electrode is consumable, the tip melts under the heat of the arc and molten droplets are detached and transported to the work through the arc column. Any arc welding system in which the electrode is melted off to become part of the weld is described as metal-arc. In carbon or tungsten (TIG) welding there are no molten droplets to be forced across the gap and onto the work. Filler metal is melted into the joint from a separate rod or wire.
More of the heat developed by the arc is transferred to the weld pool with consumable electrodes. This produces higher thermal efficiencies and narrower heat-affected zones.
Since there must be an ionized path to conduct electricity across a gap, the mere switching on of the welding current with an electrically cold electrode posed over it will not start the arc. The arc must be ignited. This is caused by either supplying an initial voltage high enough to cause a discharge or by touching the electrode to the work and then withdrawing it as the contact area becomes heated.
Arc welding may be done with direct current (DC) with the electrode either positive or negative or alternating current (AC). The choice of current and polarity depends on the process, the type of electrode, the arc atmosphere, and the metal being welded.
PREHEATING
Preheating involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding. Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source. The interpass temperature, defined as the base metal temperature between the first and last welding passes, cannot fall below the preheat temperature. Interpass temperature will not be discussed further here. Preheating can produce many beneficial effects; however, without a working knowledge of the fundamentals involved, one risks wasting money, or even worse, degrading the integrity of the weldment.
Why Preheat?There are four primary reasons to utilize preheat: (1) it lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking (2) the slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking (3) it reduces the shrinkage stresses in
the weld and adjacent base metal, which is especially important in highly restrained joints and (4) it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch toughness.
When Should Preheat be Used?In determining whether or not to preheat, the following array of factors should be considered: code requirements, section thickness, base metal chemistry, restraint, ambient temperature, filler metal hydrogen content and previous cracking problems. If a welding code must be followed, then the code generally will specify the minimum preheat temperature for a given base metal, welding process and section thickness. This minimum value must be attained regardless of the restraint or variation in base metal chemistry; however, the minimum value may be increased if necessary. An example is illustrated in the next section.
When there are no codes governing the welding, one must determine whether preheat is required, and if so, what preheat temperature will be appropriate. In general, preheat usually is not required on low carbon steels less than 1 in, (25 mm) thick. However, as the chemistry, diffusible hydrogen level of the weld metal, restraint or section thickness increases, the demand for preheat also increases. There are several methods to determine the required preheat temperature for a given base metal and section thickness that will be discussed in the next section.
What Preheat Temperature is Required?Welding codes generally specify minimum values for the preheat temperature, which may or may not be adequate to prohibit cracking in every application. For example, if a beam-to-column connection is to be fabricated with a low-hydrogen electrode made of ASTM A572-Gr50 and A36 jumbo sections (thickness ranging from 4 to 5 in.), then a minimum prequalified preheat of 225°F (107°C) is required (AWS D1.1-96, Table 3.2). However, for making butt splices in jumbo sections, it is advisable to increase the preheat temperate beyond the minimum prequalified level to that required by AISC for making butt splices in jumbo sections, namely 350°F (175°C) (AISC LRFD J2.8). This conservative recommendation acknowledges that the minimum preheat requirements prescribed by AWS D1.1 may not be adequate for these highly restrained connections.
When no welding codes are specified, and the need for preheat has been established, how does one determine an appropriate preheat temperature? As a basis for discussion, consider AWS D1.1-96, Annex XI: "Guideline on Alternative Methods for Determining Preheat'' which presents two procedures for establishing a preheat temperature developed primarily from laboratory cracking tests. These techniques are beneficial when the risk of cracking is increased due to composition, restraint, hydrogen level or lower welding heat input.
The two methods outlined in Annex XI of AWS D1.1-96 are: (1) heat affected zone (HAZ) hardness control and (2) hydrogen control. The HAZ hardness control method, which is restricted to fillet welds, is based on the assumption that cracking will not occur if the hardness of the HAZ is kept below some critical value. This is achieved by controlling the cooling rate. The critical cooling rate for a given hardness can be related to the carbon equivalent of the steel, which is defined as:
CE = C + ((Mn + Si)/6) + ((Cr + Mo + V)/5) + ((Ni + Cu)/15)
From the critical cooling rate, a minimum preheat temperature can then be calculated. (Blodgett's paper entitled "Calculating Cooling Rates by Computer Programming'' outlines a calculation procedure based on cooling rate, heat input, plate thickness, temperature at which cooling rate is critical, preheat temperature, thermal conductivity and specific heat.) It should be pointed out, however, that "although the method can be used to determine a preheat level, its main value is in determining the minimum heat input (and hence minimum weld size) that prevents excessive hardening'' (Annex XI, paragraph 3.4,AWS D1.1-96.)
The hydrogen control method is based on the assumption that cracking will not occur if the amount of hydrogen remaining in the joint after it has cooled down to about 120°F (50°C) does not exceed a critical value dependent on the composition of the steel and the restraint. This procedure is extremely useful for high strength, low-alloy steels that have high hardenability. However, the calculated preheat may be too conservative for carbon steels.
The three basic steps of the hydrogen control method are: (1) Calculate a composition parameter similar to the carbon equivalent; (2) Calculate a susceptibility index as a function of the composition parameter and the filler metal diffusible hydrogen content; and (3) Determine the minimum preheat temperature from the restraint level, material thickness, and susceptibility index.
How is Preheat Applied?The material thickness, size of the weldment and available heating equipment should be considered when choosing a method for applying preheat. For example, small production assemblies may be heated most effectively in a furnace. However, large structural components often require banks of heating torches, electrical strip heaters, or induction or radiant heaters.
A high level of accuracy generally is not required for preheating carbon steels. Although it is important that the work be heated to a minimum temperate, it is acceptable to exceed that temperature by approximately 100°F (40°C). However, this is not the case for quenched and tempered (Q&T) steels, since welding on overheated Q&T steels may be detrimental in the heat affected zone. Therefore, Q&T steels require that maximum and minimum preheat temperatures be established and closely followed.
When heating the joint to be welded, the AWS D1.1 code requires that the minimum preheat temperature be established at a distance that is at least equal to the thickness of the thickest member, but not less than 3 in. (75 mm) in all directions from the point of welding. To ensure that the full material volume surrounding the joint is heated, it is recommended practice to heat the side opposite of that which is to be welded and to measure the surface temperature adjacent to the joint. Finally, the steel temperature should be checked to verify that the minimum preheat temperature has been established just prior to initiating the arc for each pass.
SummaryPreheat can prevent cracking and/or ensure specific mechanical properties such as notch toughness.
Preheat must be used whenever applicable codes so specify; when no codes apply to a given situation, the welding engineer must determine whether or not preheat is needed, and what temperature will be required for a given base metal and section thickness.
Annex XI of AWS D1.1-96 provides guidelines for alternative methods of determining proper amounts of preheat: the HAZ hardness control method, or the hydrogen control method.
Preheat may be applied in a furnace, or by using heating torches, electrical strip heaters, or induction or radiant heaters. Carbon steels do not require precise temperature accuracy, but induction or radium heaters, maximum and minimum preheat temperatures must be followed closely for quenched and tempered steels.
BibliographyANSI/AWS D1.1-96 Structural Welding Code: Steel. The American Welding Society, 1996.
Bailey, N. Weldability of Ferritic Steels. The Welding Institute, 1995.
Bailey, N. et al, Welding Steels Without Hydrogen Cracking. The Welding Institute, 1973.
Blodgett, 0. "Calculating Cooling Rates by Computer Programming," Welding Journal. March 1984.
Graville, B.A. The Principles of Cold Cracking Control in Welds. Dominion Bridge Company Ltd., 1975.
Irving, B. "Preheat: The Main Defense against Hydrogen Cracking." Welding Journal. July 1992.
Stout, R.D. and Doty, W.D., Weldability of Steels. Welding Research Council, 1971.
The Procedure Handbook of Arc Welding. The James F. Lincoln Arc Welding Foundation, 1994.
Beginning welders and even those that are more experienced commonly struggle with the problem of weld distortion, (warping of the base plate caused by heat from the welding arc). Distortion is troublesome for a number of reasons, but one of the most critical is the potential creation of a weld that is not structurally sound. This article will help to define what weld distortion is and then provide a practical understanding of the causes of distortion, effects of shrinkage in various types of welded assemblies and how to control it, and finally look at methods for distortion control.
What is Weld Distortion?Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion, such as physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase (Fig. 3-1). These changes, in turn, affect heat flow and uniformity of heat distribution.
Fig. 3-1 Changes in the properties of steel with increases in temperature complicate analysis of what happens during the welding cycle - and, thus, understanding of the factors contributing to weldment distortion.
Reasons for DistortionTo understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. 3-2. As the bar is uniformly heated, it expands in all directions, as shown in Fig. 3-2(a). As the metal cools to room temperature it contracts uniformly to its original dimensions.
Fig. 3-2 If a steel bar is
uniformly heated while unrestrained, as in (a), it will expand in all directions and return to its original dimentions on cooling. If restrained, as in (b), during heating, it can expand only in the vertical direction - become thicker. On cooling, the deformed bar contracts uniformly, as shown in (c), and, thus, is permanently deformed. This is a simplified explanation of basic cause of distortion in welding assemblies.
But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. 3-2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. 3-2 (c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted. (For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)
In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. By the time the weld reaches room temperature - assuming complete restraint of the base metal so that it cannot move - the weld will contain locked-in tensile stresses approximately equal to the yield strength of the metal. If the restraints (clamps that hold the workpiece, or an opposing shrinkage force) are removed, the residual stresses are partially relieved as they cause the base metal to move, thus distorting the weldment.
Shrinkage Control - What You Can Do to Minimize DistortionTo prevent or minimize weld distortion, methods must be used both in design and during
welding to overcome the effects of the heating and cooling cycle. Shrinkage cannot be prevented, but it can be controlled. Several ways can be used to minimize distortion caused by shrinkage:
1. Do not overweld The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. The amount of weld metal in a fillet weld can be minimized by the use of a flat or slightly convex bead, and in a butt joint by proper edge preparation and fitup. The excess weld metal in a highly convex bead does not increase the allowable strength in code work, but it does increase shrinkage forces.
When welding heavy plate (over 1 inch thick) bevelling or even double bevelling can save a substantial amount of weld metal which translates into much less distortion automatically.
In general, if distortion is not a problem, select the most economical joint. If distortion is a problem, select either a joint in which the weld stresses balance each other or a joint requiring the least amount of weld metal.
2. Use intermittent weldingAnother way to minimize weld metal is to use intermittent rather than continuous welds where possible, as in Fig. 3-7(c). For attaching stiffeners to plate, for example, intermittent welds can reduce the weld metal by as much as 75 percent yet provide the needed strength.
Fig. 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle.
3. Use as few weld passes as possibleFewer passes with large electrodes, Fig. 3-7(d), are preferable to a greater number of passes with
small electrodes when transverse distortion could be a problem. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used.
4. Place welds near the neutral axisDistortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. Figure 3-7(e) illustrates this. Both design of the weldment and welding sequence can be used effectively to control distortion.
Fig. 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle.
5. Balance welds around the neutral axisThis practice, shown in Fig. 3-7(f), offsets one shrinkage force with another to effectively minimize distortion of the weldment. Here, too, design of the assembly and proper sequence of welding are important factors.
6. Use backstep weldingIn the backstep technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left as in Fig. 3-7(g). As each bead segment is placed, the heated edges expand, which temporarily separates the plates at B. But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. This separation is most pronounced as the first bead is laid. With successive beads, the plates expand less and less because of the restraint of prior welds. Backstepping may not be effective in all applications, and it cannot be used economically in automatic welding.
Fig. 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle.
7. Anticipate the shrinkage forcesPresetting parts (at first glance, I thought that this was referring to overhead or vertical welding positions, which is not the case) before welding can make shrinkage perform constructive work. Several assemblies, preset in this manner, are shown in Fig. 3-7(h). The required amount of preset for shrinkage to pull the plates into alignment can be determined from a few trial welds.
Prebending, presetting or prespringing the parts to be welded, Fig. 3-7(i), is a simple example of the use of opposing mechanical forces to counteract distortion due to welding. The top of the weld groove - which will contain the bulk of the weld metal - is lengthened when the plates are preset. Thus the completed weld is slightly longer than it would be if it had been made on the flat plate. When the clamps are released after welding, the plates return to the flat shape, allowing the weld to relieve its longitudinal shrinkage stresses by shortening to a straight line. The two actions coincide, and the welded plates assume the desired flatness.
Another common practice for balancing shrinkage forces is to position identical weldments back to back, Fig. 3-7(j), clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.
In heavy weldments, particularly, the rigidity of the members and their arrangement relative to each other may provide the balancing forces needed. If these natural balancing forces are not present, it is necessary to use other means to counteract the shrinkage forces in the weld metal. This can be accomplished by balancing one shrinkage force against another or by creating an opposing force through the fixturing. The opposing forces may be: other shrinkage forces; restraining forces imposed by clamps, jigs, or fixtures; restraining forces arising from the arrangement of members in the assembly; or the force from the sag in a member due to gravity.
8. Plan the welding sequenceA well-planned welding sequence involves placing weld metal at different points of the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a complete joint penetration groove weld in a butt joint, as in Fig. 3-7(k). Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. 3-7(l). In these examples, the shrinkage in weld No. 1 is balanced by the shrinkage in weld No. 2.
Fig. 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle.
Clamps, jigs, and fixtures that lock parts into a desired position and hold them until welding is finished are probably the most widely used means for controlling distortion in small assemblies or components. It was mentioned earlier in this section that the restraining force provided by clamps increases internal stresses in the weldment until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this stress level would approximate 45,000 psi. One might expect this stress to cause considerable movement or distortion after the welded part is removed from the jig or clamps. This does not occur, however, since the strain (unit contraction) from this stress is very low compared to the amount of movement that would occur if no restraint were used during welding.
9. Remove shrinkage forces after weldingPeening is one way to counteract the shrinkage forces of a weld bead as it cools. Essentially, peening the bead stretches it and makes it thinner, thus relieving (by plastic deformation) the stresses induced by contraction as the metal cools. But this method must be used with care. For
example, a root bead should never be peened, because of the danger of either concealing a crack or causing one. Generally, peening is not permitted on the final pass, because of the possibility of covering a crack and interfering with inspection, and because of the undesirable work-hardening effect. Thus, the utility of the technique is limited, even though there have been instances where between-pass peening proved to be the only solution for a distortion or cracking problem. Before peening is used on a job, engineering approval should be obtained.
Another method for removing shrinkage forces is by thermal stress relieving - controlled heating of the weldment to an elevated temperature, followed by controlled cooling. Sometimes two identical weldments are clamped back to back, welded, and then stress-relieved while being held in this straight condition. The residual stresses that would tend to distort the weldments are thus minimized.
10. Minimize welding timeSince complex cycles of heating and cooling take place during welding, and since time is required for heat transmission, the time factor affects distortion. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands. The welding process used, type and size of electrode, welding current, and speed of travel, thus, affect the degree of shrinkage and distortion of a weldment. The use of mechanized welding equipment reduces welding time and the amount of metal affected by heat and, consequently, distortion. For example, depositing a given-size weld on thick plate with a process operating at 175 amp, 25 volts, and 3 ipm requires 87,500 joules of energy per linear inch of weld (also known as heat input). A weld with approximately the same size produced with a process operating at 310 amp, 35 volts, and 8 ipm requires 81,400 joules per linear inch. The weld made with the higher heat input generally results in a greater amount of distortion. (note: I don't want to use the words "excessive" and "more than necessary" because the weld size is, in fact, tied to the heat input. In general, the fillet weld size (in inches) is equal to the square root of the quantity of the heat input (kJ/in) divided by 500. Thus these two welds are most likely not the same size.
Other Techniques for Distortion Control
Water-Cooled Jig Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig (Fig. 3-33) is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.
Fig. 3-33 A water-cooled jig for rapid removal of heat when welding sheet meta.
StrongbackThe "strongback" is another useful technique for distortion control during butt welding of plates, as in Fig. 3-34(a). Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.
Fig. 3-34 Various strongback arrangements to control distortion during butt-welding.
Thermal Stress RelievingExcept in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.
Summary: A Checklist to Minimize DistortionFollow this checklist in order to minimize distortion in the design and fabrication of weldments:
Do not overweldControl fitupUse intermittent welds where possible and consistent with design requirementsUse the smallest leg size permissible when fillet weldingFor groove welds, use joints that will minimize the volume of weld metal. Consider double-sided joints instead of single-sided jointsWeld alternately on either side of the joint when possible with multiple-pass weldsUse minimal number of weld passesUse low heat input procedures. This generally means high deposition rates and higher travel speedsUse welding positioners to achieve the maximum amount of flat-position welding. The flat position permits the use of large-diameter electrodes and high- deposition-rate welding proceduresBalance welds about the neutral axis of the memberDistribute the welding heat as evenly as possible through a planned welding sequence and weldment positioningWeld toward the unrestrained part of the memberUse clamps, fixtures, and strongbacks to maintain fitup and alignmentPrebend the members or preset the joints to let shrinkage pull them back into alignmentSequence subassemblies and final assemblies so that the welds being made continually balance each other around the neutral axis of the section
Following these techniques will help minimize the effects of distortion and residual stresses.
Many companies strive to get the best possible price on welding equipment and consumables. Although this is an admirable goal, these companies may be overlooking the big picture which says that rather than aim for a savings based on a one-time purchase price, look for ways to get productivity savings. By reducing overall welding costs, the productivity savings that are realized multiply year after year. Productivity savings will allow a company to keep saving even when the price of equipment, consumables or welding accessories goes up.
Looking at the typical work cell model, you will notice that only 20 percent of the cost of welding is related to materials, while the bulk of the costs - more than 80 percent - are attributed to labor and overhead. Hence, if a company saves 10 percent on the material costs of welding, the company is only saving two percent of the total welding costs. But, if a company can save 10 percent on the costs associated with labor and overhead, the company will achieve an eight percent savings on the total welding costs in the work cell model. The work cell information is valid for manual or semi-automatic welding process mild steel application.
Outlined below are 10 steps that companies can take to reduce welding costs and realize productivity savings in the cost of doing business. These are some of the most common items that Lincoln Electric examines when auditing a company.
1. Analyze the delivery of consumables and accessories to the welding points
In many shops, the operator has to go to a tool room or supply area for a new contact tip, coil of wire or other welding accessory. This takes valuable time away from the welding cell and slows down overall productivity. To improve the operating efficiency and minimize wasted time, companies should stock at least a limited supply of all necessary items near the welding station - this includes shielding gas, flux and wire. Another helpful productivity enhancing tip is to switch to larger spools of wire such as from 25 lb. spools to 44 or 60 lb. spools to even larger packages of 1,000 lb. reels or 1,000 lb. drums. A simple switch like this means less changeover time, which adds up over the weeks, months and years.
Shops should also be on the lookout for shielding gas waste. A simple device called a surge turbine can be placed at the end of the gun to provide a digital readout of the gas surge and flow rate. If the surge rate is high, investing in a surge guard can reduce the pressure, eliminating gas surges and waste.
Leaks in the gas delivery system can also create a potential loss of money. By looking at the amount of consumables purchased each year and then examining the total gas purchased, a company can determine if there is a significant loss. Welding manufacturers and distributors
should be able to provide average utilization figures so that loss can be detected. If there is a loss suspected, one of the easiest ways to check for leaks is to shut off the gas delivery system over the weekend. Check the level on Friday evening and then again on Monday morning to determine if gas was used while the system was in shut down mode.
2. Analyze whether material handling is effective
Delivery of parts to the welding station in an organized and logical fashion is also a way to reduce welding costs. For example, one company was manufacturing concrete mixing drums. In the fabrication process, the company produced 10 parts for one section, then went on to make 10 parts of another drum section, etc. As pieces came off the line, they were put onto the floor of the shop. When it was time to weld, the operator had to hunt for the pieces needed and sort through them. When the outside welding expert pointed out the amount of time being wasted in this process, the company started to batch each one on a cart. In this way, the pieces needed to weld one drum were stored together and could easily be moved to the welding area.
This type of scenario is also true for companies that may outsource parts to a vendor. Though it may cost more to have parts delivered in batches, it may save more in time than having to organize and search through parts to be able to get to the welding stage.
How many times each piece is handled in the shop may be an eye-opener to reducing wasted time. To measure such an intangible as this, operators are asked to put a soapstone mark on the piece each time it is touched - some companies are surprised to find out how many times a part is picked up, transported and laid down in the manufacturing process. In the case of one company, moving the welding shop closer to the heat treatment station eliminated four extra times that the
part was handled. Basically, handling a part as few times as possible and creating a more efficient production line or work cell will reduce overall costs.
3. Look for ways to correct overwelding
One of the "cardinal sins" that almost every shop does is overweld. This means that if the drawing calls for a 1/4" fillet weld, most shops will put down a 5/16" weld. The reasons? Either they don't have a fillet gauge and are not exactly sure of the size of the weld they are producing or they put in some extra to "cover" themselves and make sure there is enough weld metal in place.
But, overwelding leads to tremendous consumable waste. Let's look again at our example. For a 1/4" fillet weld, the typical operator will use .129 lbs. per foot of weld metal. The 5/16" weld requires .201 lbs. per foot of weld metal - a 56 percent increase in weld volume compared to what is really needed. Plus, you must take into account the additional labor necessary to put down a larger weld. Not only is the company paying for extra, wasted consumable material, a weld with more weld metal is more likely to have warpage and distortion because of the added heat input. It is recommended that every operator be given a fillet gauge to accurately produce the weld specified - and nothing more. In addition, changes in wire diameter may be used to eliminate overwelding.
4. Enhance current welding processes and procedures
Look for ways to create more efficiencies in the welding process. This includes examining such things as wire diameter, wire feed speed, voltage, travel speed, gas type, transfer mode, etc. For instance, if the shop is currently welding with a short arc process and a 75/25 blend of shielding gas, it may be more effective to switch to a different gas and a spray mode of transfer. Or, a change in process may be warranted based on the condition of the part. If there is oxide on the part, it may be easier to change to a process that will overcome contamination problems rather than try to clean each part before welding. Your welding supplier should be up to date on the
latest technology and be able to advise you on new processes, machinery and consumables that can optimize welding at the shop.
5. Optimize joint preparation
In some cases, it may be better to double bevel a joint to prepare it for welding rather than single bevel it. It is recommended to double bevel any material that is more than 3/4" in thickness. Just this simple change in procedure can save quite a bit in weld metal. On a 3/4" thick piece, a double bevel will use 1.45 lbs. per foot of weld metal while a single bevel will use 1.95 lbs. per foot.
6. Eliminate any extra welds from the design
Look for ways to modify product designs to eliminate unnecessary welds. For example, one company that manufactured boxes originally had a design that called for welded lift handles on each side of the box. By simply changing the design of the box to cut out lifting slots, it eliminated the need for welding the handles - saving time and money. In another instance, rather than making a part with an open corner, the design was changed to accommodate a closed corner, which meant 1/3 less metal required to fill the corner.
7. Look for items that can be welded rather than cast
We've already discussed ways to eliminate welds to create efficiencies, but what about adding welds? In some cases, it may be more cost effective to weld metal pieces to a part rather than cast the entire component in a costly alloy or exotic metal. For example, a company that originally used a part cast in a high-nickel alloy found that 50 percent of the part could be composed of standard, structural steel which allowed a savings in material and thus a savings in total cost. Also, the company was further able to redesign the part so that it was more efficient.
8. Look for ways to eliminate costly record keeping
Many companies get completely "bogged down" in the paperwork required to run a business. But with today's latest technological advances, there are items that can be a great help. For instance, Lincoln Electric offers something called ArcWorks software which can document procedures, create drawings everyone in the shop can access, keep track of welding operator's qualifications, and many other things. Software such as this can be tailored to the individual company's needs and provide great efficiencies and also eliminate mistakes.
9. Adding robotics or hard automation to the operation
Today's technological advances offer many options. Robotics can be justified when the volume of parts a company produces is so great that it can offset the monies spent on a robot. Robotics can also be considered if there are a number of different parts that are similar enough in nature to be able to be handled by the same robot.
If robots are not justified, a company might determine that fixturing or hard automation could be used to increase efficiency or quality. One company incorporated fixturing and clamps to hold down a tank while the seam was being welded. In another case, an automotive manufacturer decided that automation was necessary because of the amount of parts and intricate angles and welding positions.
10. Examine safety concerns
Although it may not lead to immediate welding cost reductions, operating under proper safety techniques will save money in the long run by reducing employee accidents. Safety items to consider may include chaining gas cylinders so they can't fall, installing flash arrestors to eliminate blow back when oxyfuel cutting or labeling piping to avoid mishaps.
Conclusion
These are just some of the items that are considered when The Lincoln Electric Company performs its Guaranteed Cost Reduction program. Under this program, a team of Lincoln welding experts visits a facility and performs an audit. A menu of cost reduction ideas is then presented to the company from which they choose and prioritize. Lincoln will calculate the savings and actually guarantee a certain amount of savings if the ideas presented are implemented. If those savings are not realized, Lincoln will write a check for the difference.
As the old saying goes, "don't be penny wise and pound foolish" - look for ways to decrease welding costs, increase efficiencies and improve productivity, these are the savings items that will reap benefits time and time again.
The ABC's of Nondestructive Weld ExaminationReprinted courtesy of Welding Journal magazine
An understanding of the benefits and drawbacks of each form of nondestructive examination can help you choose the best method for your application.
The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree required by the application. A standard should be established based on the service requirements.
Standards designed to impart weld quality may differ from job to job, but the use of appropriate weld techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking after his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.
Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultra-sonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows real-time
or near real-time viewing, comparative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.
Visual Inspection (VT)Visual inspection is often the most cost-effective method, but it must take place prior to, during and after welding. Many standards require its use before other methods, because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. The ANSI/AWS D1.1, Structural Welding Code - Steel, states, "Welds subject to nondestructive examination shall have been found acceptable by visual inspection." Visual inspection requires little equipment. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity.
Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used.
During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld defects that can be recognized visually are cracking, surface slag inclusions, surface porosity and undercut.
On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of metal filler is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass is most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.
Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.
After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.
Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.
Radiographic InspectionRadiography (X-ray) is one of the most important, versatile and widely accepted of all the nondestructive examination methods - Fig. 1. X-ray is used to determine internal soundness of the welds. The term "X-ray quality," widely used to indicate high quality in welds, arises from this inspection method.
Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness and be determined.
X-rays are produced by high-voltage generators. As the high voltage applied to an X-ray tube is increased, the wavelength of the emitted X-ray becomes shorter , providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the longer intensity.
When X-rays or gamma rays are directed at a section of weldment , not all of the radiation passes are through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy.
The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. Thicker areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter - Fig 2.
Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that a radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph.
IQI's used to determine radiographic quality are also called penetrameters. A standard hole-type penetrameter is a rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece of metal is a percentage of the thickness of the specimen being radiographed. The diameter of each hole is different and is a given multiple of the penetrameter thickness. Wire-type penetrameters are also widely used, especially outside the United States. They consist of several pieces of wire, each of a different diameter. Sensitivity is determined by the smallest diameter of wire that can be clearly seen on the radiograph.
A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the minimum detectable flaw size. It is an indicator of the quality of the radiographic technique.
Radiographic images are not always easy to interpret. Film handling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities.
Surface defects will show up on the film and must be recognized. Because the angle of exposure will also influence the radiograph, it is difficult or impossible to analyze fillet welds by this method. Because a radiograph compresses all the defects that occur throughout the thickness of the weld into one plane, it tends to give an exaggerated impression of scattered type defects such as porosity or inclusions.
An X-ray image of the interior of the weld may be viewed on a fluorescent screen, as well as on developed film. This makes it possible to inspect parts faster and at a lower cost, but the image definition is poorer. Computerization has made it possible to overcome many of the shortcomings of radiographic imaging by linking the fluorescent screen with a video camera. Instead of waiting for film to be developed, the images can be viewed in real time. This can improve quality and reduce costs on production applications such as pipe welding, where a problem can be identified and corrected quickly.
By digitizing the image and loading it into a computer, the image can be enhanced and analyzed to a degree never before possible. Multiple images can be superimposed. Pixel values can be adjusted to change shading and contrast, bringing out small flaws and discontinuities that would not show up on film. Colors can be assigned to the various shades of gray to further enhance the image and make flaws stand out better. The process of digitizing an image taken from the fluorescent screen - having that image computer enhanced and transferred to a viewing monitor - takes only a few seconds. However, because there is a time delay, we can no longer consider this "real time." It is called "radioscopy imagery."
Existing films can be digitized to achieve the same results and improve the analysis process. Another advantage is the ability to archive images on laser optical disks, which take up far less space than vaults of old films and are much easier to recall when needed.
Industrial radiography, then, is an inspection method using X-rays and gamma rays as a penetrating medium, and densitized film as a recording medium, to obtain a photographic record of internal quality. Generally, defects in welds consist either of a void in the weld metal itself or an inclusion that differs in density from the surrounding weld metal.
Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial radiographers should be permitted to do radiographic testing.
Magnetic Particle Inspection (MT)Magnetic particle inspection is a method of locating and defining discontinuities in magnetic materials. It is excellent for detecting surface defects in welds, including discontinuities that are too small to be seen with the naked eye, and those that are slightly subsurface.
This method may be used to inspect plate edges prior to welding, in process inspection of each weld pass or layer, postweld evaluation and to inspect repairs - Fig. 3.
It is a good method for detecting surface cracks of all sizes in both the weld and adjacent base metal, subsurface cracks, incomplete fusion, undercut and inadequate penetration in the weld, as well as defects on the repaired edges of the base metal. Although magnetic particle testing should not be a substitute for radiography or ultrasonics for subsurface evaluations, it may present an advantage over their methods in detecting tight cracks and surface discontinuities.
With this method, probes are usually placed on each side of the area to be inspected, and a high amperage is passed through the workplace between them. A magnetic flux is produced at right angles to the flow of current - Fig. 3. When these lines of force encounter a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more tenaciously than elsewhere, forming an indication of the discontinuity.
For this indication to develop, the discontinuity must be angled against the magnetic lines of force. Thus, when current is passed longitudinally through a workpiece, only longitudinal flaws will show. Putting the workpiece inside a solenoid coil will create longitudinal lines of force (Fig. 3) that cause transverse and angular cracks to become visible when the magnetic powder is applied.
Although much simpler to use than radiographic inspection, the magnetic particle method is limited to use with ferromagnetic materials and cannot be used with austenitic steels. A joint between a base metal and a weld of different magnetic characteristics will create magnetic discontinuities that may falsely be interpreted as unsound. On the other hand, a true defect can be obscured by the powder clinging over the harmless magnetic discontinuity. Sensitivity decreases with the size of the defect and is also less with round cracks such as gas pockets. It is best with elongated forms, such as cracks, and is limited to surface flaws and some subsurface flaws, mostly on thinner materials.
Because the field must be distorted sufficiently to create the external leakage required to identify flaws, the fine, elongated discontinuities, such as hairline cracks, seams or inclusions that are parallel to the magnetic field, will not show up. They can be developed by changing the direction of the field, and it is advisable to apply the field from two directions, preferably at right angles to each other.
Magnetic powders maybe applied dry or wet. The dry powder method is popular for inspecting heavy weldments, while the wet method is often used in inspecting aircraft components. Dry powder is dusted uniformly over the work with a spray gun, dusting bag or atomizer. The finely divided magnetic particles are coated to increase their mobility and are available in gray, black and red colors to improve visibility. In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. This can be flowed or sprayed on, or the part may be dipped into the liquid. The wet method is more sensitive than the dry method, because it allows the use of finer particles that can detect exceedingly fine defects. Fluorescent powders may be used for further sensitivity and are especially useful for locating discontinuities in corners, keyways, splines and deep holes.
Liquid Penetrant Inspection (PT)Surface cracks and pinholes that are not visible to the naked eye can be located by the liquid penetrant inspection. It is widely used to locate leaks in welds and can be applied with austentic steels and nonferrous materials where magnetic particle inspection would be useless.
Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D.1. Code, say that "welds subject to liquid penetrant testing�shall be evaluated on the basis of the requirements for visual inspection."
Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed. A "developer" is used to draw the penetrant to the surface, and
the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects.
Dye penetrant inspection is similar, except that vividly colored dyes visible under ordinary light are used - Fig. 4. Normally, a white developer is used with the dye penetrants that creates a sharply contrasting background to the vivid dye color. This allows greater portability by eliminating the need for ultraviolet light.
The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the penetrant. Penetrants can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the liquid to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work.
Liquid penetrant inspection is widely used for leak detection. A common procedure is to apply fluorescent material to one side of a joint, wait an adequate time for capillary action to take place, and then view the other side with ultraviolet light. In thin-walled vessels, this technique will identify leaks that ordinarily would not be located by the usual air test with pressures of 5-20 lb/in.2 When wall thickness exceeds � in., however, sensitivity of the leak test decreases.
Ultrasonic Inspection (UT)Ultrasonic inspection is a method of detecting discontinuities by directing a high-frequency sound beam through the base plate and weld on a predictable path. When the sound beam's plate path strikes an interruption in the material continuity, some of the sound is reflected back. The sound is collected by the instrument, amplified and displayed as a vertical trance on a video screen - Fig. 5.
Both surface and subsurface detects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods.
The ultrasonic unit contains a crystal of quartz or other piezoelectric material encapsulated in a transducer or probe. When a voltage is applied, the crystal vibrates rapidly. As an ultrasonic transducer is held against the metal to be inspected, it imparts mechanical vibrations of the same frequency as the crystal through a couplet material into the base metal and weld. These
vibrational waves are propagated through the material until they reach a discontinuity or change in density. At these points, some of the vibrational energy is reflected back. As the current that causes the vibration is shut off and on at 60-1000 times per second, the quartz crystal intermittently acts as a receiver to pick up the reflected vibrations. These cause pressure on the crystal and generate an electrical current. Fed to a video screen, this current produces vertical deflections on the horizontal base line. The resulting pattern on the face of the tube represents the reflected signal and the discontinuity. Compact portable ultrasonic equipment is available for field inspection and is commonly used on bridge and structural work.
Ultrasonic testing is less suitable than other NDE methods for determining porosity in welds, because round gas pores respond to ultrasonic tests as a series of single point reflectors. This results in low-amplitude responses that are easily confused with "base-line noise" inherent with testing parameters. However, it is the preferred test method for detecting plainer-type discontinuities and lamination.
Portable ultrasonic equipment is available with digital operation and microprocessor controls. These instruments may have built-in memory and can provide hard-copy printouts or video monitoring and recording. They can be interfaced with computers, which allows further analysis, documentation and archiving, much as with radiographic data. Ultrasonic examination requires expert interpretation from highly skilled and extensively trained personnel.
Choices Control QualityA good NDE inspection program must recognize the inherent limitations of each process. For example, both radiography and ultrasound have distinct orientation factors that may guide the choice of which process to use for a particular job. Their strengths and weaknesses tend to compliment each other. While radiography is unable to reliably detect lamination-like defects, ultrasound is much better at it. On the other hand, ultrasound is poorly suited to detecting scattered porosity, while radiography is very good.
Whatever inspection techniques are used, paying attention to the "Five P's" of weld quality will help reduce subsequent inspection to a routine checking activity. Then, the proper use of NDE methods will serve as a check to keep variables in line and weld quality within standards.
The Five P's are:1. Process Selection - the process must be right for the job.2. Preparation - the joint configuration must be right and compatible with the welding process.3. Procedures - the procedures must be spelled out in detail and followed religiously during welding.4. Pretesting - full-scale mockups or simulated specimens should be used to prove that the process and procedures give the desired standard of quality.5. Personnel - qualified people must be assigned to the job.
Q: Why does one use a shielding gas when using a flux-cored wire such as Outershield® 71M? What are the advantages? I have only welded with an
Innershield® wire, which did not use any shielding gas.
A: I would like to answer your question as a general discussion of flux-cored welding. The American Welding Society (AWS) classifies all tubular electrodes having a flux on the inside as "flux-cored" wires, and calls it the Flux Cored Arc Welding (FCAW) process. All flux-cored wires have some similar characteristics. These include forming a protective slag over the weld, use a drag angle technique, have the ability to weld out-of-position or flat and horizontal only at higher deposition rates (depending on type of wire), ability to handle contaminants on the plate, etc. However, there are two fundamentally different types of flux-cored wires. One type is self-shielded and the other type is gas-shielded. These two types are often subcategorized as the FCAW-S process (self-shielded, flux-cored) and FCAW-G process (gas-shielded, flux-cored).
Figure 1: FCAW-S Process
Self-shielded, flux-cored wires, commonly referred to as Innershield® wires, are often described as "a stick electrode that is inside out". Just like covered or stick electrodes, they rely solely on their slag system and the gases produced from chemical reactions in the arc to protect the molten metal from the atmosphere (see Figure 1). The flux ingredients in the core perform multiple functions, which include: 1) They deoxidize and denitrify the molten metal. 2) Forms a protective slag, which also shapes the bead and can hold molten metal out-of-position. 3) Adds alloying elements to the weld metal to produce desired mechanical properties. 4) Affects welding characteristics (i.e. deep penetration characteristics and high deposition rates).
You can think of the FCAW-S process as a productivity extension of stick (i.e., manual) welding, providing much higher deposition rate capabilities with a semi-automatic process for almost all the same applications as stick electrodes are used. As an example, they are very popular for outdoor welding, as there is no need for an external shielding gas (where the gas can easily be blown away by wind and result in porosity with gas-shielded
processes).
Operationally, most self-shielded wire types run on DC- polarity. They have a globular arc transfer, ranging from fine droplets to large droplets of metal. While some have very smooth arc characteristics, most tend to have a little harsher arc and more spatter than FCAW-G electrodes. The metallurgy and design of self-shielded, flux-cored wires are unique compared to electrodes for other arc welding processes. As an example, they uniquely use aluminum with most wires to actively react with the atmosphere to produce a sound weld deposit.
By comparison, gas-shielded, flux-cored wires (Outershield®, UltraCore®) use both a slag system and an external shielding gas to protect the arc from the atmosphere (see Figure 2). The two most common types of shielding gas used are 100% carbon dioxide (CO2), or a 75 – 85% argon (Ar) / balance CO2 mix (with 75%Ar / 25% CO2 the most popular). These wires are often described as “double shielded” electrodes. Like self-shielded wires, the core ingredients produce a slag, add the desired alloying elements to the weld metal and affect the welding characteristics. However, they do not provide any protection from the atmosphere, but rather the FCAW-G process relies completely on an external shielding gas around the arc to do this. In addition, the use of shielding gas greatly improves the operator appeal and usability of these wires.
Most types of wires have a small droplet arc transfer with a smooth, spray arc. The recommended polarity is DC+ for all types of wires. They are generally preferred for shop (i.e. inside) welding,
Figure 2: FCAW-G Process
as they have smoother arc characteristics. They can be used outside, but require extra precautions to prevent the wind from blowing away the shielding gas. Without the shielding gas, weld porosity will result.Here are some of the most frequently asked questions that The Lincoln Electric Company receives regarding general MIG welding issues.
Does my choice of MIG welding wire really affect the quality of the weld? Does shielding gas affect the quality of the finished weld? Are there any other tips you can provide for higher quality MIG welding? How important is a good electrical ground in MIG welding? How important is the Contact Tip in MIG welding?
Q: Does my choice of MIG welding wire really affect the quality of the weld?
A: While there are many options on the market today for mild steel welding wire, we will concentrate on the two most popular for small shops or hobbyists.
Lincoln Electric offers several types of its copper-coated SuperArc® MIG wire - including the popular L-50™ and L-56™. Although both are 70,000 lb. tensile strength wires designed for welding mild or carbon steels, it is the amount of “deoxidizers” found in the wires that sets them apart.
SuperArc L-50 (AWS classification ER70S-3) is a great general fabrication MIG wire and it usually allows you to make quality welds on clean steel. For production work, .035", and .045" are the most common diameters.
However, you may want consider SuperArc L-56 when you need to weld steels that have less than perfect surface conditions. In the same way you can upgrade gasoline for your automobile from regular to premium for enhanced performance, you can do the same for welding wire.
For this reason, SuperArc L-56 wire (AWS classification of ER70S-6) carries more deoxidizers in its chemistry. This means that it has more built-in “cleaning action” to handle contaminants of welding such as surface rust, oil, paint and dirt. With L-56, you may not be required to do as much cleaning of the steel before welding. This higher quality of cleaning offered by the deoxidizers usually translates into a higher quality weld materials with less than stellar surface conditions. Most automotive manufacturers now mandate this type of wire for any automotive repairs. In addition, this wire is available in diameters ranging from .025” to 1/16” which meet the welding performance demands of thin sheet metal (24 gage) to heavy plate welding.
Q: Does shielding gas affect the quality of the finished weld?
A: For most mild steel applications, CO2 will provide adequate shielding, but when you must have a flatter bead profile, less spatter or better wetting action, you may want to consider adding 75 to 90% argon to your CO2 shielding gas mix.
Why? Argon is essentially inert to the molten weld metal and therefore will not react with the molten weld metal. When CO2 is mixed with Argon, the reactivity of the gas is reduced and the arc becomes more stable. But, Argon is more expensive. In production welding, selecting the perfect shielding gas can be a science of its own. Attributes such as material thickness, welding position, electrode diameter, surface condition, welding procedures and others can affect results.
Common gas mixes for the home hobbyist and small fabricator:
100% CO2 -Lowest price, generally greatest penetration, and higher levels of spatter. Limited to short circuit and globular transfer.
75% Argon - 25% CO2 -Higher price, most commonly used by home hobbyist and light fabricator, lower levels of spatter and flatter weld bead than 100% CO2. Limited to short circuit and globular transfer
85% Argon - 15% CO2-Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and excellent penetration for heavier plate applications and with steels that have more mill scale. Can be used in short circuit, globular, pulse and spray transfer
90% Argon - 10% CO2- Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and good penetration for a wide variety of steel plate applications. Can be used in short circuit, globular, pulse and spray transfer
Q: Are there any other tips you can provide for higher quality MIG welding?
A: Try a smaller diameter wire. Although the most common diameters of welding wire are .035" and .045", a smaller diameter wire usually will make it easier to create a good weld. Try an .025" wire diameter, which is especially useful on thin materials of 1/8" or less. The reason? Most welders tend to make a weld that is too big - leading to potential burnthrough problems. A smaller diameter wire welds more stable at a lower current which gives less arc force and less tendency to burn through. If you keep your weld current lower, you will have a greater chance of success on thinner materials. This is a good recommendation for thinner materials; but be careful using this approach on thicker materials (>3/16”) because there may be a risk of lack of fusion. Whenever a change like this is made, always verify the quality of the weld meets its intended application.
Q: How important is a good electrical ground in MIG welding?
A: In arc welding, an arc is established from the electrode to the workpiece. To do this properly, the arc requires a smooth flow of electricity through the complete electrical circuit, with minimum resistance. If you crimp a garden hose while watering the lawn, the flow at the sprinkler head is much reduced. Beginning welders often make the mistake of attaching the work clamp (or electrical ground) to a painted panel or a rusty surface. Both of these surfaces are electrical insulators and do not allow the welding current to flow properly. The resulting welding arc will be difficult to establish and not very stable. Other telltale signs of an improper electrical connection are a work clamp that is hot to the touch or cables that generate heat. Another key point to consider when attaching the welding ground is to place the welding ground on the piece being welded. Welding current will seek the path of least resistance so if care is not taken to place the welding ground close to the arc, the welding current may find a path unknown to the operator and destroy components unintended to be in the welding circuit.
Q: How important is the Contact Tip in MIG welding?
A: Very important. Make sure the gun tip isn’t worn out or that weld spatter is not on the tip near the exit hole. The contact tip in the gun should be perfectly round and just a few thousandths larger than the wire itself. Worn tips are typically oval and can cause an erratic arc from the random electrical connection and physical movement of the wire inside the worn tip. Genuine Lincoln contact tips are precisely made from a wear-resistant copper alloy for superior welding performance. If the contact tip enters the molten weld pool, it should be immediately replaced. For most casual welders, a good rule of thumb to assure high quality welding is to change the tip after ever 100 lbs. of wire. Another point to remember about contact tips is that they should always be threaded completely into the gas diffuser and tightened prior to welding to give a smooth flow of welding current.
If Contact Tip looks questionable, get a new Lincoln Tip, thread it completelty into the gas diffuser and tighten.
Conclusion
The Lincoln Electric Company offers a full range of MIG solutions – Take a look at our equipment like the mid-sized PowerMIG® 255, the Waveform Control Technology™ tour de force named the PowerWave® 455, and rugged, adaptive Series 10 wire feeders capable of MIG pulsing. Even more importantly, try for yourself the consistent quality and feedability of our SuperArc® copper-coated and SuperGlide® bare mild steel wires, the carefully crafted Blue Max® Stainless MIG wires and the wide range of aluminum SuperGlaze® MIG wires now available.
Health Effects: Fumes Q: What compounds are found in common welding fume?
A: The most common compounds in arc welding fume mild steel are iron, manganese and silicon although other compounds in the electrode or on the base metal may be in the welding fume. Q: What types of electrode products are likely to have chromium or nickel in the welding fume?A: Fumes from the use of stainless steel and hardfacing products contain chromium or nickel. Q: What are the potential health effects that may result from long-term overexposure to chromium or nickel?A: Asthma has been reported and some forms of these metals are known or suspected to cause lung cancer in processes other than welding. Therefore, it is recommended that precautions be taken to keep exposures as low as possible. Q: What are the potential health effects that may result from sustained overexposure to manganese?A: Manganese overexposure may affect the central nervous system, resulting in poor coordination, difficulty in speaking and tremor of arms or legs. This condition is considered irreversible. Q: What are the long-term health effects associated with exposure to welding fume?A: Check an LH70 MSDS sheet, including comments on siderosis and irritation of nose and throat. Q: What are the potential health effects that may result from overexposure to zinc?A: Overexposure to zinc may cause fume fever with symptoms similar to the common flu. Q: What is a common source of zinc in welding fume?A: Zinc in welding fume usually comes from welding on galvanized steel. Warnings Q: Where can you find safety instructions regarding welding products that you use?A: Each welding power source and container of consumable product has a warning label which contains specific safety instructions regarding the arc welding product you have chosen to use. Q: What information is contained on a material safety data sheet (MSDS)? A: An MSDS contains additional information including a summary of the Hazardous Materials used to manufacture the product, a summary of Fire and Explosion Hazard Data, Health Hazard Data and Reactivity Data, and information on the precautions to observe for the Safe Handling and Use of the product. Q: Where can you find the MSDS for the consumable product you are using?A: Inside each Lincoln Electric consumable container. It can also be found on the Lincoln Electric website, on the AWS website and from your supervisor. Q: Since fumes and gases can be dangerous to your health, what three steps should you take to
protect yourself?A: 1) Keep fumes and gases from your breathing zone and general area 2) Keep your head out of the fumes 3) Use enough ventilation or exhaust at the arc, or both, to keep fumes and gases from your breathing zone and general area. Q: What additional precautions should be followed for products that require special ventilation?A: If special ventilation products are used indoors, use local exhaust. If special ventilation products are used outdoors, a respirator may be required. Q: What types of products generally require special ventilation?A: Hardfacing and stainless products. Health Effects: Gases Q: What are the potential health hazards related to shielding gases used in arc welding?A: Most of the shielding gases (argon, helium and carbon dioxide) are non-toxic, but they can displace oxygen in your breathing air causing dizziness, unconsciousness and possible death. Carbon monoxide can also be present and may pose a hazard if levels are excessive. Adequate Ventilation Q: What is the one of the most basic safety precautions that a welder can take to protect themselves from overexposure to welding fume?A: Keep your head out of the fume plume! Q: Where is the concentration of fumes and gases greatest?A: Concentration of fumes and gases is greatest in the plume. Q: How can you keep fumes and gases away from your breathing zone?A: Keep fumes and gases from your breathing zone and general area using natural ventilation, mechanical ventilation, fixed or moveable exhaust hoods, or local exhaust at the arc. Q: What precautions must be taken if adequate ventilation cannot be provided?A: It may be necessary to wear an NIOSH approved respirator if adequate ventilation cannot be provided. Q: Does OSHA require engineering or workplace controls be installed before respirators can be used?A: OSHA requires that engineering and workplace controls be installed first and if the controls alone do not keep exposures below applicable limits, use respirators. Q: How can a welder determine if there is adequate ventilation?A: As a practical rule of thumb for welders, for many mild steel electrodes, if the welder is comfortable and the air is visibly clear, the welder has adequate ventilation.
Q: What method is used to accurately measure a welder's exposure to welding fume?A: A welder's exposure can only be determined by having a qualified professional take a sample of the welder's breathing air during the workday. Q: When is it most important to measure a welder's exposure to welding fume? A: Measuring a welder‘s exposure to welding fume is essential if you are welding with stainless, hardfacing or other special ventilation products (see the product label). Q: What precautions should be taken when welding a base metal which is plated or painted?A: If the base metal cannot be cleaned before welding, the composition of the coating should be evaluated. Q: What should you do if you feel overexposed to welding fume?A: Stop welding and get some fresh air immediately. If you continue to feel the symptoms, see your doctor. Notify your supervisor and co-workers so the situation can be corrected and other workers are aware of and can avoid the hazard. Be sure you are following safe practices, as stated upon the consumable labeling and MSDS, and improve the ventilation in your area. Do not continue welding until the situation has been corrected. Q: What does adequate ventilation mean?A: Your work area has adequate ventilation when there is enough ventilation and exhaust to control worker exposure to the hazardous materials in the welding fumes and gases (so the applicable exposure limit for those materials is not exceeded). Q: What are the most commonly used exposure limits?A: The two most common U.S. exposure limits are established by OSHA in the form of permissible exposure limits or PEL and by the ACGIH in the form of Threshold Limit Values or TLV. Q: What exposure limit is mandatory in the United States?A: Your employer must keep exposures below the PEL. Q: Where can you find the applicable limits for the PEL and TLV for substances in welding fume?A: The PEL and TLV are listed on the first page of the MSDS for compounds in each electrode or flux. Evaluating the Welding Environment Q: What steps can you, the welder, take to identify hazardous substances?A: There are also steps that you should take to identify hazardous substances in your welding environment. Read the product label to review the warnings, safety precautions and to determine if special ventilation is needed. Obtain and review the material safety data sheet (MSDS) for the electrode which your employer or supervisor has posted in the work place or that you find inside
the electrode or flux container. You should review the complete MSDS to determine specifically what compounds you may be exposed to when using the product. Q: Where can the welder find information about materials in the base metal or any coating on the base metal?A: Obtain a copy of the supplier's MSDS for the base metal being welded, as this should be reviewed as well. Welding Fume Control Q: What is natural ventilation?A: Natural ventilation is the movement of air through the workplace caused by natural forces. Outside, this is usually the wind. Inside, this may be the flow of air through open windows and doors. Q: What is mechanical ventilation?A: Mechanical ventilation is the movement of air through the workplace caused by an electrical device such as a portable fan or permanently mounted fan in the ceiling or wall. Q: What is local exhaust?A: Local exhaust is a mechanical device used to capture welding fume at or near the arc and remove contaminants from the air. Q: What factors need to be considered when determining the exhaust requirements for your application?A: The ventilation or exhaust needed for your application depends upon factors such as:
Workspace volumeWorkspace configurationNumber of weldersWelding process and currentConsumables used (mild steel, hardfacing, stainless, etc.)Allowable levels (TLV, PEL, etc.)Material welded (including paint or plating)Natural airflow Q: Name several types of local exhaust that can be used to control exposure to welding fume?A: Local exhaust of welding fumes can be provided by any of the following: adjustable "elephant trunk" exhaust systems, fume extraction guns or fixed enclosures, or booths with exhaust hoods. Q: Which system is more effective and economical: general ventilation or local exhaust systems?A: Local exhaust systems are more effective and economical to operate than a general ventilation system, particularly in the winter, because they require less replacement air to be brought into the room and heated.
Q: What is the minimum air velocity (speed) required near the welding arc? A: Minimum required air velocity at the welding arc is 100 fpm. Q: When should an employee's exposure to welding fume be obtained?A: Exposure should be checked when new ventilation equipment is installed, when the process is modified or when the welder feels uncomfortable. Periodically, exposure should be re-checked to be sure it is still working properly and is adequate. Special Ventilation Reminder Q: What must be done to insure that there is adequate ventilation when welding with electrodes that require special ventilation (such as stainless or hardfacing, or other products which require special ventilation - see instructions on container or MSDS) or on lead or cadmium plated steel and other metals or coatings like galvanized steel, which produce hazardous fumes?A: Keep exposure as low as possible and below exposure limit values (PEL and TLV) for materials in the fume using local exhaust. Q: When should a respirator be used?A: In confined spaces or in some circumstances, for example outdoors, a respirator may be required if exposure cannot be controlled to the PEL or TLV (see MSDS). Q: When does OSHA consider natural ventilation sufficient? A: According to OSHA regulations, when welding and cutting (mild steels), natural ventilation is usually considered sufficient to meet requirements, provided that:
The room or welding area contains at least 10,000 cubic feet (about 22' x 22' x 22') for each welderThe ceiling height is not less than 16 feetCross ventilation is not blocked by partitions, equipment or other structural barriersWelding is not done in a confined spaceRegardless of the whether the ventilation meets these requirements, the welder's exposure must be controlled to below the PEL or TLV (if applicable) exposure limit to be adequate
