Giao Trinh Han-BASIC_welding

7/29/2019 Giao Trinh Han-BASIC_welding

1/66

BASIC

WELDING FILLER METAL

TECHNOLOGY

A Correspondence Course

LESSON I THE BASICS OF ARC WELDING

! An Introduction to Metals ! Electricity for Welding

COPYRIGHT 2000 THE ESAB GROUP, INC.


2/66

TABLE OF CONTENTS

LESSON I THE BASICS OF

ARC WELDING

PART A. AN INTRODUCTION TO METALS

Section Mr. Section Title Page

1.1 Source and Manufacturing ................ 11.1.1 Rimmed Steel ................................ ........ 21.1.2 Capped Steel ................................ ......... 21.1.3 Killed Steel ................................ ............. 31.1.4 Semi-Killed Steel ................................ .. 31.1.5 Vacuum Deoxidized Steel ..................... 31.2 Classification of Steels ..................... 31.2.1 Carbon Stee l................................ .......... 31.2.2 Low Alloy Steel ................................ ...... 3

1.2.3 High Alloy Steel ................................ ..... 4

1.3 Specifications ................................ ..... 51.4 Crystalline Structure of Metals .......... 61.4.1 Grains and Grain Boundaries ................ 71.5 Heat Treatment ................................ .... 81.5.1 Preheat ................................ .................. 81.5.2 Stress Relieving ................................ .... 91.5.3 Hardening ................................ .............. 91.5.4 Tempering................................ .............. 91.5.5 Annealing................................ ............... 91.5.6 Normalizing................................ ............ 10

1.5.7 Heat Treatment Trade-Off..................... 101.6 Properties of Metals ............................ 101.6.1 Tensile Strength ................................ .... 101.6.2 Yield Strength ................................ ........ 111.6.3 Ultimate Tensile Strength ...................... 111.6.4 Percentage of Elongation .................... 11

COPYRIGHT1998THEESABGROUP.INI


3/66

TABLE OF CONTENTS LESSON I

- Con't.

Section Nr. Section Title Page

1.6.5 Reduction of Area ................................ .. 11

1.6.6 Charpy Impacts ................................ ..... 111.6.7 Fatigue Strength ................................ .... 121.6.8 Creep Strength ................................ ..... 131.6.9 Oxidation Resistance ............................ 131.6.10 Hardness Test ................................ ...... 131.6.11 Coefficient of Expansion........................ 141.6.12 Thermal Conductivity ............................ 141.7 Effects of Alloying Elements .............. 141.7.1 Carbon ................................ ................... 141.7.2 Sulphur ................................ ................. 141.7.3 Manganese................................ ............ 151.7.4 Chromium ................................ ............. 15

1.7.5 Nickel ................................ ..................... 151.7.6 Molybdenum ................................ ......... 151.7.7 Silicon ................................ .................... 151.7.8 Phosphorus ................................ .......... 151.7.9 Aluminum ................................ .............. 151.7.10 Copper................................ ................... 151.7.11 Columbium................................ ............ 161.7.12 Tungsten................................ ................ 161.7.13 Vanadium................................ ............... 161.7.14 Nitrogen ................................ ................. 161.7.15 Alloying Elements summary ................. 16

PART B.ELECTRICITY FOR WELDINGSection Nr. Section Title Page

1.8 Electricity for Welding .......................... 171.8.1 Principles of Electricity .......................... 171.8.2 Ohm's Law ................................ ............ 181.8.3 Electrical Power................................ ..... 191.8.4 Power Generation ................................ .. 20

COPYRIGHT 1998 THE ESAB GROUP, INJ


4/66

TABLE OF CONTENTS LESSON I

- Con't.

Section Nr. Section Title Page

1.8.5 Transformers ................................ ......... 22

1.8.6 Power Requirements ............................. 241.8.7 Rectifying AC to DC .............................. 251.9 Constant Current or Constant Voltage ... 261.9.1 Constant Current Characteristics .......... 261.9.2 Constant Voltage Characteristics ......... 261.9.3 Types of Welding Power Sources ......... 271.9.4 Power Source Controls ......................... 28

Appendix A Glossary of Terms ...................... 29

COPYRIGHT 1998 THE ESAB GROUP, INC


5/66

LESSON I, PART A

AN INTRODUCTION TO METALS M

SOURCE AND MANUFACTURING

Metals come from natural deposits of ore in the earth's crust.Most ores are contaminated with impurities that must beremoved by mechanical and chemical means. Metal extractedfrom the purified ore is known as primary or virgin metal, andmetal that comes from scrap is called secondary metal. Mostmining of metal bearing ores is done by either open pit orunderground methods. The two methods of mining employedare known as "se/ect;Ve"in which small veins or beds of highgrade ore are worked, and "bulk"in which large quantities of lowgrade ore are mined to extract a high grade po rtion.

1.1.0.1 There are two types of ores, ferrous andnonferrous. The term ferrous comes from the Latin word

"ferrum"meaning iron, and a ferrous metal is one that has ahigh iron content. Nonferrous metals, such as copper andaluminum, are those that contain little or no iron. There isapproximately 20 times the tonnage of iron in the earth's crustcompared to all other nonferrous products combined; therefore,it is the most important and widely used metal.

1.1.0.2 Aluminum, because of its attractive appearance,light weight and strength, is the next most widely used metal.Commercial aluminum ore, known as bauxite, is a residualdeposit formed at or near the earth's surface.

1.1.0.3 Some of the chemical processes that occur during

steel making are repeated during the welding operation and anunderstanding of welding metallurgy can be gained by imaginingthe welding arc as a miniature steel mill.

1.1.0.4 The largest percentage of commercially producediron comes from the blast furnace process. A typical blastfurnace is a circular shaft approximately 90 to 100 feet in heightwith an internal diameter of approximately 28 feet. The steelshell of the furnace is lined with a refractory material, usually ahard, dense clay firebrick.

1.1.0.5 The iron blast furnace utilizes the chemical reactionbetween a solid fuel charge and the resulting rising column ofgas in the furnace. The three different materials used for thecharge are ore, flux and coke. The ore consists of iron oxid eabout four inches in diameter. The flux is limestone thatdecomposes into calcium oxide and carbon dioxide. The limereacts with impurities in the ore and floats them to the surface inthe form of a slag. Coke, which is primarily carbon, is the idealfuel for blast furnaces because it produces carbon monoxidegas, the main agent for reducing iron ore into iron metal.


6/66

COPYRIGHT1999THEESABGROUP.INC |


7/66

LESSON I, PART A

1.1.0.6 The basic operation of the blast furnace is to reduceiron oxide to iron metal and to remove impurities from the metal.Reduced elements pass into the iron and oxidized elementsdissolve into the slag. The metal that comes from the blastfurnace is called pig iron and is used as a starting material for

further purification processes.

1.1.0.7 Pig iron contains excessive amounts of elementsthat must be reduced before steel can be produced. Differenttypes of furnaces, most notably the open hearth, electric andbasic oxygen, are used to continue this refining process. Eac hfurnace performs the task of removing or reducing elementssuch as carbon, silicon, phosphorus, sulfur and nitrogen bysaturating the molten metal with oxygen and slag formingingredients. The oxygen reduces elements by forming gasesthat are blown away and the slag attracts impurities as itseparates from the molten metal.

1.1.0.8 Depending upon the type of slag that is used,refining furnaces are classed as either acid or basic. Largeamounts of lime are contained in basic slags and high quantit iesof silica are present in acid slags. This differential between acidand basic slags is also present in welding electrodes for muchof the same refining process occurs in the welding operation.

1.1.0.9 After passing through the refining furnace, the metalis poured into cast iron ingot molds. The ingot produced is arather large square column of steel. At this point, the metal issaturated with oxygen. To avoid the formation of large gaspockets in the cast metal, a substantial portion of the oxyg en

must be removed. This process is known as deoxidation, and itis accomplished through additives that tie up the oxygen eitherthrough gases or in slag. There are various degrees ofoxidation, and the common ingots resulting from each are asfollows:

1.1.1 Rimmer Steel - The making of rimmed steels involvesthe least deoxidation. Asthe ingots solidify, a layer of nearly pure iron is formed on thewalls and bottom of the mold,and practically all the carbon, phosphorus, and sulfur segregateto the central core. Theoxygen forms carbon monoxide gas and it is trapped in the

solidifying metal as blow holesthat disappear in the hot rolling process. The chief advantage ofrimmed steel is the excellent defect-free surface that can be produced with the aide ofthe pure iron skin. Mostrimmed steels are low carbon steels containing less than .1%carbon.

1.1.2 Capped Steel - Capped steel regulates the amount ofoxygen in the molten


8/66

metal through the use of a heavy cap that is locked on top ofthe mold after the metal isallowed to reach a slight level of rimming. Capped steelscontain a more uniform corecomposition than the rimmed steels. Capped steels are,therefore, used in applications

COPYRIGHT 1999 THE ESAB GROUP, IN'


9/66

LESSON I, PART A

that require excellent surfaces, a more homogenouscomposition, and better mechanical properties than rimmedsteel.

1.1.3 Killed Steel - Unlike rimmed or capped steel, killed steel is made by

completelyremoving or tying up the oxygen before the ingot solidifies toprevent the rimming action. This removal is accomplished byadding a ferro-silicon alloy that combines with oxygen to forma slag, leaving a dense and homogenous metal.

1.1-4 Semi-killed Steel - Semi-killed steel is a compromise between rimmedand killed

steel. A small amount of deoxidizing agent, generally ferro -silicon or aluminum, is added. The amount is just sufficientto kill any rimming action, leaving some dissolved oxygen.

1.1.5 Vacuum Deoxidized Steel - The object of vacuum deoxidation is toremove

oxygen from the molten steel without adding an element thatforms nonmetallic inclusions. This is done by increasing thecarbon content of the steel and then subjecting the moltenmetal to vacuum pouring or steam degassing. The carbonreacts with the oxygen to form carbon monoxide, and as aresult, the carbon and oxygen levels fall within specifiedlimits. Because no deoxidizing elements that form solidoxides are used, the steel produced by this process is quiteclean.

H CLASSIFICATIONS OF STEEL

The three commonly used c lassifications for steel are:carbon, low alloy, and high alloy. These are referred to asthe "type"of steel.

1.2.1 Carbon Steel - Steel is basically an alloy of iron and carbon, and itattains its

strength and hardness levels primarily through the additi onof carbon. Carbon steels are classed into four groups,depending on their carbon levels.

Low Carbon Up to 0.15% carbon

Mild Carbon Steels .15% to 0.29% carbon

Medium Carbon Steels .30% to

0.59% carbon

High Carbon Steels .60% to

1.70% carbon

1.2.1.1 The largest tonnage of steel produced falls into the


10/66

low and mild carbon steel groups. They are popularbecause of their relative strength and ease with which theycan be welded.

1-2.2 Low Alloy Steel - Low alloy steel, as the name implies, contains smallamounts

of alloying elements that produce remarkable improvements in theirproperties. Alloying



11/66

LESSON I, PART A

elements are added to improve strength and toughness, todecrease or increase the response to heat treatment, and toretard rusting and corrosion. Low alloy steel is gener allydefined as having a 1.5% to 5% total alloy content. Commonalloying elements are manganese, silicon, chromium, nickel,

molybdenum, and vanadium. Low alloy steels may contain asmany as four or five of these alloys in varying amounts.

1.2.2.1 Low alloy steels have higher tensile and yield strengthsthan mild steel or carbonstructural steel. Since they have high strength -to-weight ratios,they reduce dead weight inrailroad cars, truck frames, hea vy equipment, etc.

1.2.2.2Ordinary carbon steels, that exhibit brittleness at lowtemperatures, are unreliablein critical applications. Therefore, low alloy steels with nickeladditions are often used for

low temperature situations.

1.2.2.3Steels lose much of their strength at high temperatures.To provide for this lossof strength at elevated temperatures, small amounts ofchromium or molybdenum areadded.

1.2.3 High Alloy Steel - This group of expensive and specialized steelscontain alloy

levels in excess of 10%, giving them outstanding properties.

1.2.3.1Austenitic manganese steel contains high carbon andmanganese levels, thatgive it two exceptional qualities, the ability to harden whileundergoing cold work and greattoughness. The term austenitic refers to the crystalli ne structureof these steels.

1.2.3.2Stainless steels are high alloy steels that have the abilityto resist corrosion. Thischaracteristic is mainly due to the high chromium content, i.e.,10% or greater. Nickel isalso used in substantial quantities in some stai nless steels.

1.2.3.3Tool steels are used for cutting and forming operations.They are high qualitysteels used in making tools, punches, forming dies, extrudingdies, forgings and so forth.Depending upon their properties and usage, they aresometimes referred to as waterhardening, shock resisting, oil hardening, air hardening, and hotwork tool steel.

1.2.3.4Because of the high levels of alloying elements, special


12/66

care and practices arerequired when welding high alloy steels.



13/66

LESSON I, PART A

1.3 SPECIFICATIONS

Many steel producers have developed steels that they marketunder a trade name such as Cor-Ten, HY-80, T-1, NA-XTRA,orSS-100, but usually a type of steel is referred to by its

specification. A variety of technical, gove rnmental and industrialassociations issue specifications for the purpose of classifyingmaterials by their chemical composition, properties or usage.The specification agencies most closely related to the steelindustry are the American Iron and Steel Ins titute (AISI),Society of Automotive Engineers (SAE), American Society forTesting and Materials (ASTM), and the American Society ofMechanical Engineers (ASME).

1.3.0.1 The American Iron and Steel Institute (AISI) and theSociety of Automobile Engineers (SAE) have collaborated inproviding identical numerical designations for theirspecifications. The first two digits of a four digit index number

refer to a series of steels classified by their composition or alloycombination. While the last two dig its, which can change withinthe same series, give an approximate average of the carbonrange. For example, the first two digits of a type 1010 or 1020steel indicate a "10"series that has carbon as its main alloy.The last two digits indicate an approxim ate average content of.10% or .20% carbon, respectively. Likewise, the "41"of a 4130type steel refers to a group that has chromium and molybdenumas their main alloy combination with approximately .30% carboncontent.

1.3.0.2 The AISI classifica tions for certain alloys, such asstainless steel, are somewhat different. They follow a three digitclassification with the first digit designating the main alloycomposition or series. The last two digits will change within aseries, but are of an arbitrary nature being agreed upon byindustry as a designation for certain compositions within theseries. For example, the "3"in a 300 series of stainless steelindicates chromium and nickel as the main alloys, but a 308stainless has a different overall com position than a 347 type.The "4"of a 400 series indicates the main alloy as chromium,but there are different types such as 410, 420, 430, and so forthwithin the series.

1.3.0.3 The American Society for Testing and

Materials (ASTM) is the largest organization of its kind inthe world. It has compiled some 48 volumes of standardsfor materials, specifications, testing methods andrecommended practices for a variety of materials rangingfrom textiles and plastics to concrete and metals.

1.3.0.4 Two ASTM designated steels commonly specifiedfor construction are A36 -77 and A242-79. The prefix letterindicates the class of a material. In this case, the letter "A"indicates a ferrous metal, the class of widest interest inwelding. The numbers 36 and 242


14/66



15/66

LESSON I, PART A

are index numbers. The 77 and 79 refer to the year that the standardsfor these steels were originally adopted or the date of their latestrevision.

1.3.0.5 The ASTM designation may be further subd ivided intoGrades or Classes. Since many standards for ferrous metals are writtento cover forms of steel (i.e., sheet, bar, plate, etc.) or particular productsfabricated from steel (i.e., steel rail, pipe, chain, etc.), the user mayselect from a number of different types of steel under the sameclassification. The different types are than placed under grades orclasses as a way of indicating the differences in such things aschemistries, properties, heat treatment, etc. An example of a fulldesignation is A285-78 Grade A or A485-79 Class 70.

1.3.0.6 The American Society of Mechanical Engineers (ASME)maintains a widely used ASME Boiler and Pressure Vessel Code. Thematerial specification as adopted by the ASME is identified with a prefixletter"S", while the remainder is identical with ASTM with the exception

that the date of adoption or revision by ASTM is not shown. Therefore,a common example of an ASME classification is SA 387 Grade 11,Class 1.

1.4 CRYSTALLINE STRUCTURE OF METALS

When a liquid metal is cooled, its atoms will assemble into a regularcrystal pattern and we say the liquid has solidified or crystallized. Allmetals solidify as a crystalline material. In a crystal the atoms ormolecules are held in a fixed position and are not free to move about asare the molecules of a liquid or gas. This fixed position is called acrystal lattice. As the temperature of a crystal is raised, more thermal

energy is absorbed by the atoms ormolecules and their movement increases. As thedistancebetween the atoms increases, the lattice breaks downand the crystal melts. If a lattice contains only onetype of atom, as in pure iron, the conditions are thesame at all points throughout the lattice, and thecrystal melts at a single temperature (see Figure 1).

COPYRIGHT 1999 THE ESAB GROU


16/66

LESSON I, PART A

1.4.0.1 However, if the latticecontains two or more types of atoms,as in any alloy-steel, it may start tomelt at one temperature but not becompletely molten until it has been

heated to a higher temperature (SeeFigure 2). This creates a situationwhere there is a combination ofliquids and solids within a range oftemperatures.

1.4.0.2 Each metal has a characteristiccrystal structure that forms duringsolidification and often remains the permanentform of the material as long as it remains atroom temperature. However, some metalsmay undergo an alteration in the crystallineform as the temperature is changed. This is known as phase transformation.For example,pure iron solidifies at 2795F, the delta structure transforms into a structurereferred to asgamma iron. Gamma iron is commonly known as austenite and is anonmagneticstructure. At a temperature of 1670F., the pure iron structure transformsback to the deltairon form, but at this temperature, the metal is known as alpha iron. Thesetwo phases aregiven different names to differentiate between the high temperature phase(delta) and thelow temperature phase (alpha). The capability of the atoms of a material to

transform intotwo or more crystalline structures at different temperatures is defined asallotropic. Steelsand iron are allotropic metals.

1.4.1 Grains and Grain Boundaries-As the metal is cooled to its freezingpoint, a

small group of atoms begin to assem ble into crystalline form(refer to Figure 3). These small crystals scattered throughoutthe body of the liquid are oriented in all directions and assolidification continues, more crystals are formed from thesurrounding liquid. Often, they take the form of dendrites, or atreelike structure. As crystallization continues, the crystalsbegin to touch one another, their free growth hampered, andthe remaining liquid freezes to the adjacent crystals untilsolidification is complete. The solid is now composed ofindividual crystals that usually meet at different orientations.Where these crystals meet is called a grain boundary.

1.4.1.2 A number of conditions influence the initial grainsize. It is important to know that cooling rate and temperaturehas an important influence on the newly solidified grain


17/66

structure and grain size. To illustrate differences in grainformation, let's look at the cooling phases in a weld.

COPYRIGHT 1999 THE ESAB GROUP, IN


18/66

FIGURE 3

1.4.1.3Initial crystal formation begins at the coolest spot in theweld. That spot is at thepoint where the molten metal and the unmelted base metalmeet. As the metal continuesto solidify, you will note that the grains in the center are smallerand finer in texture than thegrains at the outer boundaries of the weld deposit. This isexplained by the fact that as the

weld metal cools, the heat from the center of the weld depositwill dissipate into the basemetal through the outer grains that solidified first.Consequently, the grains that solidifiedfirst were at high temperatures for a longer time while in thesolid state and this is asituation that encourages grain growth. Grain size can have aneffect on the soundness ofthe weld. The smaller grains are stronger and more ductile thanthe larger grains. If acrack occurs, the tendency is for it to start in the area where thegrains are largest.

1.4.1.4To summarize this section, it should be understood thatall metals are composedof crystals of grains. The shape and characteristics of c rystalsare determined by thearrangement of their atoms. The atomic pattern of a singleelement can change itsarrangement at different temperatures, and that this atomicpattern or microstructuredetermines the properties of the metals.

1.5 HEAT TREATMENT

The temperature that metal is heated, the length of time it isheld at that temperature, and the rate that it is cooled, all havean effect on a metal's crystalline structure. This crystallinestructure, commonly referred to as "microstructure,"determinesthe specific properties of metals. There are various ways ofmanipulating the microstructure, either at the steel mill or in thewelding procedure. Some of the more common ways are asfollows:

1.5.1 Preheat - Most metals are rather good conductor s of heat. As a result,


19/66

the heat

in the weld area is rapidly dispersed through the wholeweldment to all surfaces where it is radiated to the atmospherecausing comparatively rapid cooling. In some metals, this rapidcooling may contribute to the formatio n of microstructures in theweld zone that are detrimental. Preheating the weldmentbefore it is welded is a method of slowing the cooling



20/66

LESSON I, PART A

rate of the metal. The preheat temperature may vary from150F to 1000F, but more commonly it is held in the 300Fto 400F range. The thicker the weld metal, the more likelywill it be necessary to preheat, because the heat will beconducted away from the weld zone more rapidly as the

mass increases.1.5.2 Stress Relieving- Metals expand when heated andcontract when cooled. Theamount of expansion is directly proportional to the amount ofheat applied. In a weldment,the metal closest to the weld is subjected to the highesttemperature, and as the distancefrom the weld zone increases, the maximum temperaturereached decreases. This nonuni -form heating causes nonuniform expansion and contraction andcan cause distortion andinternal stresses within the weldment. Depending on itscomposition and usage, the metalmay not be able to resist these stresses and cracking or earlyfailure of the part may occur.One way to minimize these stresses or to relieve them is byuniformly heating the structureafter it has been welded. The metal is heated to temperatures

just below the point where amicrostructure change would occur and then it is cooled at aslow rate.

1.5.3 Hardening - The hardness of steel may be increased byheating it to 50F to

100F above the temperature that a microstructure changeoccurs, and then placing themetal in a liquid solution that rapidly cools it. This rapid cooling,known as "quenching,"locks in place microstructures known as "martens/fe"thatcontribute to a metal's hardnesscharacteristic. The quenching solutions used in this process arerated according to thespeed that they cool the metal, i.e., Oil (fast), Water (faster),Salt Brine (fastest).

1.5.4 Tempering -After a metal is quenches, it is then usually

tempered. Tempering isa process where the metal is reheated to somewhere below1335F, held at that tempera-ture for a length of time, and then cooled to room temperature.Tempering reduces thebrittleness that is characteristic in hardened steels, therebyproducing a good balancebetween high strength and toughness. The term toughness, asit applies to metals, usuallyrefers to resistance to brittle fracture or notch toughness under


21/66

certain environmentalconditions. More information on these properties will be coveredlater in this lesson and insubsequent lessons. Steels that respond to this type oftreatment are known as "quenchedand tempered steels."

1-5.5 Annealing -A metal that is annealed is heated to a temperature 50 to 100

above where a microstructure change occurs, held at thattemperature for a sufficient time for a uniform change to takeplace, and then cooled at a very slow rate, usually in a fur nace.The principal reason for annealing is to soften steel and createa uniform fine grain structure. Welded parts are seldomannealed for the high temperatures would cause distortion.



22/66

LESSON I, PART A

1-5.6 Normalizing - The main difference between normalizing andannealing is the

method of cooling. Normalized steel is heated to a temperatureapproximately 100 above where the microstructure transforms and thencooled in still air rather than in a furnace.

1.5.7 Heat Treatment Trade-Off- It must be noted that these various waysof control-

ling the heating and cooling of metals can produce a desired property, butsometimes at the expense of another desirable property. An example ofthis trade-off is evident in the fact that certain heat treatments canincrease the strength or hardness of metal, but the same treatments willalso make the metal less ductile or more brittle, and therefore, susceptibleto welding problems.

M PROPERTIES OF METALS

The usefulness of a particular metal is determined by the climate andconditions in which it will be used. A metal that is stamped into anautomobile fender must be softer and more pliable than armor plate thatmust withstand an explosive force, or the material used for an oil rig on the

Alaska North Slope must perform in a quite different climate than a steamboiler. It becomes obvious that before a material is recommended for aspecific use, the physical and mechan ical properties of that metal and theweld metal designed to join it must be evaluated. Some of the moreimportant properties of metals and the means of evaluation are as follows:

1.6-1 Tensile Strength- Tensile strength is one of the most importantdetermining

factors in selecting a metal, especially if it is to be a structural member, partof a machine, or part of a pressure vessel.

1.6.1.1 The tensile test is performed as shown in Figure 4. The testspecimen ismachined to exactstandard dimensionsand clamped into thetesting apparatus atbothends. The specimenis then pulled to the

point of fracture andthe data recorded.

1.6.1.2 Thetensile strength testgives us 4 primary pieces ofinformation: (1)Yield Strength, (2)Ultimate Tensile Strength, (3)Elongation, and (4) Reduction in Area.


23/66

COPYRIGHT 1999 THEESAB GROUP, IN]


24/66

LESSON I, PART A

1.6.2 Yield Strength - When a metal is placed in tension, itacts somewhat like arubberband. When a load of limited magnitude is applied, themetal stretches, and whenthe load is released, the metal returns to its original shape. This

is the elastic characteristicof metal and is represented by letter A in Figure 5. As a greaterload is applied, the metalwill reach a point where it will no longer return to its originalshape but will continue tostretch. Yield strength is the point where the metal reaches thelimit of its elastic characteristic and will no longer return to its original shape.

1.6.3 Ultimate Tensile Strength - Once a metal has exceededits yield point, it willcontinue to stretch or deform, and if the load is suddenly

released, the metal will not returnto its original shape, but will remain in its elongated form. This iscalled the plastic region ofthe metal and is represented by the letter B in Figure 5. As thisplastic deformation in-

creases, the metal strainsagainst further elongation,and an increased load mustbe applied to stretch themetal. As the load isincreased, the metal willfinally reach a point where it

no longer resists and any fur-ther load applied will rapidlycause the metal to break.That point at which the metalhas

withstood or resisted the maximum applied load is its ultimatetensile strength. This information is usually recorded in poundsper square inch (psi).

1.6.4 Percentage of Elongation - Before a tensile specimen isplaced in the tensiletester, two marks at a measured distance are placed on theopposing ends of the circularshaft. After the specimen is fractured, the distance between themarks is measured andrecorded as a percentage of the original distance. See Figure 5.This is the percentage ofelongation and it gives an indication of the ductility of the metalat room temperature.

1.6.5 Reduction of Area - A tensile specimen is machined toexact dimensions. Thearea of its midpoint cross-section is a known figure. As the


25/66

specimen is loaded to the pointof fracture, the area where it breaks is reduced in size. SeeFigure 5. This reduced area iscalculated and recorded as a percentage of the original cross -sectional area. This information reflects the relative ductility or brittleness of the metal.

1.6.6 Charpy Impacts- Metal that is normally strong andductile at room temperaturemay become very brittle at much lower temperatures, and thus,is susceptible to fracture if



26/66

LESSON I, PART A

a sharp abrupt load is applied to it. An impact testermeasures the degree of susceptibility to what is called brittlefracture.

1.6.6.1 The impact specimen is machined to exact

dimensions (Figure 6) and then notched on one side. Quiteoften, the notch is in the form of a "V" and the test in thiscase is referred to as a Charpy V-Notch Impact Test. Thespecimen is cooled to a predetermined temperature and thenplaced in a stationary clamp at the ba se of the testingmachine. The specimen is in the direct path of a weightedhammer attached to a pendulum (Figure 6).

1.6.6.2 The hammer is released from a fixed height and theenergy required to fracture the specimen is recorded in ft -lbs. Aspecimen that is cooled to -60F and absorbs 40 ft -lbs ofenergy is more ductile, and therefore, more suitable for lowtemperature service than a specimen that withstands only 10 ft -lbs at the same temperature. The specimen that withstood 40ft-lbs energy is said to have better toughness or notchtoughness.

1.6.7 Fatigue Strength-A metal will withstand a load less than its ultimatetensile

strength but may break if that load is removed and thenreapplied several times. For example, if a thin wire is bentonce, but if it is bent back and forth repeatedly, it will eventuallyfracture and it is said to have exceeded its fatigue strength. Acommon test for this strength is to place a specimen in amachine that repeatedly applies the same load first in tensionand then in compression. The fatigue strength is calculatedfrom the number of cycles the metal withstands before the pointof failure is reached.


27/66



28/66

LESSON I, PART A

16 S Creep Strenqlh - If a load belowa metal's tensile strength is applied atroomlemperalure (72"F), il will cause some initial elongation, bill Ihere wrll beno further measurable elongation if the load is kept at a constant level Ifthat same load were applied to a metal heated to a high temperature, the

situation would change Although the load is held at a constant level, themetal will gradually continue to elongate This characteristics calledcreep Eventually, the material may rupture depending on thetemperature of the metal, the degree of load applied and the length oftime that it is applied All three of these factors determine a metal's abilityto resist creep, and therefore, its creep strength

16 9 Oxidation Resistance - The atoms of metal have a tendency to unite wth

oxy-

gen in the air to form oxide compounds, the most visible being rust andscale In some metals, these oxides will adhere very tightly to the skin ofthe metal and effectively seal it from further oxidation as is evident in

stainless steel These materials have high oxidat ion resistance In othermetals, the bond is very loose, creating a situation where the oxides willflake off, and the metal gradually deteriorates as the time of exposure isextended

16 10 HarctiessTest - The resistance of a metal to indentation is ameasure of its hardness and an indication of the material's strength Totest for hardness, a fixed load forces an mdenter into the test material(Figure 7) The depth of the penetration or the size of the impression ismeasured The measurement is converted into a hardness numberthrough the use of a variety of established tables The most commontables are the Brmell, Vlckers, Knoop and Rockwell The Rockwell isfurther divided into different scales, and


29/66


30/66

LESSON I, PART A

depending on the material being tested, the shape of theindenter and the load applied, the conversion tables may differ.For example, a material listed as having a hardness of Rb or Remeans its hardness has been determined from the Roc kwell "B"scale or the Rockwell "C" scale.

1.6.11 Coefficient of Expansion - All metals expand whenheated and contract whencooled. This dimensional change is related to the crystallinestructure and will vary withdifferent materials. The different expansion an d contractionrates are expressed numerically by a coefficient of thermal expansion. When two differentmetals are heated to thesame temperature and cooled at the same rate, the one withthe higher numerical coefficient will expand and contract more than the one with the lesser

coefficient.1.6.12 Thermal Conductivity - Some metals will absorb andtransmit heat more readilythan others. They are categorized as having high thermalconductivity. This characteristiccontributes to the fact that some metals will melt or undergotransformations at much lowertemperatures than others.

17 EFFECTS OF THE ALLOYING ELEMENTS

Alloying is the process of adding a metal or a nonmetal to puremetals such as copper, aluminum or iron. From the time it wasdiscovered that the properties of pure metals could be improvedby adding other elements, alloy steel has increased bypopularity. In fact, metals that are welded are rarely in theirpure state. The major properties that can be improved byadding small amounts of alloying elements are hardness,tensile strength, ductility and corrosion resistance. Commonalloying elements and their effect on the properties of metalsare as follows:

1.7.1 Carbon - Carbon is the most effective, most widely usedand lowest in cost

alloying element available for increasing the hardness andstrength of metal. An alloycontaining up to 1.7% carbon in combination with iron is knownas steel, whereas thecombination above 1.7% carbon is known as cast iron.

Although carbon is a desirablealloying element, high levels of it can cause problems; therefore,special care is requiredwhen welding high carbon steels and cast iron.

1.7.2 Sulphur- Sulphur is normally an undesirable element in


31/66

steel because it causesbrittleness. It may be deliberately added to impro ve themachinability of the steel. Thesulphur causes themachine chips to break rather than form long curls and clogthe machine. Normally, every effort is made to reduce thesulphur content to the lowest possible level because it can



32/66

LESSON I, PART A

create welding difficulties.

1.7.3 Manganese - Manganese in contents up to 1% is usuallypresent in all low alloysteels as a deoxidizer and desulphurizer. That is to say, it

readily combines with oxygenand sulphur to help negate the undesirable effect theseelements have when in their natural state. Manganese also increases the tensile strength andhardenability of steel.

1.7.4 Chromium - Chromium, in combination with carbon, is apowerful hardeningalloying element. In addition to its hardening properties,chromium increases corrosionresistance and the strength of steel at high temperatures.Chromium is the primary alloying

element in stainless steel.

1.7.5 Nickel - The greatest single property of steel that isimproved by the presence ofnickel is its ductility or notch toughness. In this respect, it is themost effective of all alloying elements in improving a steel's resistance to impact at lowtemperatures. Electrodeswith high nickel content are used to weld cast iron materials .Nickel is also used in combination with chromium to form a group known as austeniticstainless steel.

1.7.6 Molybdenum - Molybdenum strongly increases thedepth of the hardeningcharacteristic of steel. It is quite often used in combination withchromium to improve thestrength of the steel at high temperatures. This group of steelsis usually referred to aschrome-moly steels.

1.7.7 Silicon - Silicon is usually contained in steel as adeoxidizer. Silicon will addstrength to steel but excessive amounts can reduce theductility. Additional amounts of

silicon are sometimes added to welding electrodes to increasethe fluid flow of weld metal.

1.7.8 Phosphorus - Phosphorus is considered a harmfulresidual element in steelbecause it greatly reduces ductility and toughness. Efforts aremade to reduce it to its verylowest levels; however, phosphorus is added in very smallamounts to some steels toincrease strength.


33/66

1.7.9 Aluminum - Aluminum is primarily used as a deoxidizer

in steel. It may also be

used in very small amounts to control the size of the grains.

1.7.10 Copper- Copper contributes greatly to the corrosionresistance of carbon steelby retarding the rate of rusting at room temperature, but high

levels of copper can causewelding difficulties.



34/66

LESSON I, PART A

1.7.11 Columbium - Columbium is used in austenitic stainlesssteel to act as a stabilizer. Since the carbon in the stainless steel decreases thecorrosion resistance, a meansof making carbon ineffective must be found. Columbium has a

greater affinity for carbonthan chromium, leaving the chromium free for corrosionprotection.

1.7.12 Tungsten - Tungsten is used in steel to given strength athigh temperatures.Tungsten also joins with carbon to form carbides that areexceptionally hard, and thereforehave exceptional resistance to wear.

1.7.13 Vanadium - Vanadium helps keep steel in the desirablefine grain condition after

heat treatment. It also helps increase the depth of hardeningand resists softening of thesteel during tempering treatments.

1.7.14 Nitrogen - Usually, efforts are made to eliminatehydrogen, oxygen and nitrogenfrom steel because their presence can cause brittleness.Nitrogen has the ability to formaustenitic structures; therefore, it is sometimes added toaustenitic stainless steel to red ucethe amount of nickel needed, and therefore, the productioncosts of that steel.

1.7.15 Alloying Elements Summary - It should be understoodthat the addition ofelements to a pure metal may influence the crystalline form ofthe resultant alloy. If a puremetal has allotropic characteristics (the ability of a metal tochange its crystal structure) at aspecific temperature, then that characteristic will occur over arange of temperatures withthe alloyed metal. The range in which the change takes placemay be wide or narrow,depending on the alloys and the quantities in which they areadded. The alloying element

may also effect the crystalline changes by either suppressingthe appearance of certaincrystalline forms or even by creating entirely new forms. Allthese transformations inducedby alloying elements are dependent on heat input and coolingrates. These factors areclosely controlled at the steel mill, but since the weldingoperation involves a nonuniformheating and cooling of metal, special care is ofte n needed in thewelding of low and high


35/66

alloy steel.



36/66

LESSON I, PART B

1.8 ELECTRICITY FOR WELDING

1.8.1 Principles of Electricity - Arc welding is a method of joining metalsaccom-

plished by applying sufficient ele ctrical pressure to an electrode

to maintain a current path (arc) between the electrode and thework piece. In this process, electrical energy is changed intoheat energy, bringing the metals to a molten state; whereby theyare joined. The electrode (conductor) is either melted andadded to the base metal or remains in its solid state. All arcwelding utilizes the transfer of electrical energy to heat energy,and to understand this principle, a basic knowledge of electricityand welding power sources is necessary.

1.8.1.1The three basis principles of static electricity are as follows:

1. There are two kinds of electrical charges in existence - negative and

positive.

2. Unlike charges attract and like charges repel.

3. Charges can be transferred from one place to an other.

1.8.1.2Science has established that all matter is made up ofatoms and each atomcontains fundamental particles. One of these particles is theelectron, which has the abilityto move from one place to another. The electron is classified asa negative electricalcharge. Another particle, about 1800 times as heavy as the

electron, is the proton andunder normal conditions the proton will remain stationary.

1.8.1.3Material is said to be in an electrically uncharged statewhen its atoms contain anequal number of positive charges (protons) and negativecharges (electrons). This balanceis upset when pressure forces the electrons to move from atomto atom. This pressure,sometimes referred to as electromotive force, is commonlyknown as voltage. It should benoted that voltage that does not move through a conductor, butwithout voltage, there would

be no current flow. For our purposes, it is easiest to think ofvoltage as the electricalpressure that forces the electrons to move.

1.8.1.4Since we know that like charges repel and unlike chargesattract, the tendency isfor the electrons to move from a position of over -supply(negative charge) to an atom thatlacks electrons (positive charge). This tendency becomesreality when a suitable path isprovided for the movement of the electrons. The transfer of


37/66

electrons from a negative to apositive charge throughout the length of a conductor constitutesan electrical current. Therate that current flows through a conductor is measured inamperes and the word ampereis often used synonymously with the term current. To give anidea of the quantities of electrons that flow through a circuit, it

has been theoretically established that one ampere equals 6.3quintillion (6,300,000,000,000,000,000) electrons flowing past afixed point in a conductor every second.



38/66

LESSON I, PART B

1.8 ELECTRICITY FOR WELDING

1.8.1 Principles of Electricity - Arc welding is a method of joining metalsaccom-

plished by applying sufficient electrical pressure to an electro de

to maintain a current path (arc) between the electrode and thework piece. In this process, electrical energy is changed intoheat energy, bringing the metals to a molten state; whereby theyare joined. The electrode (conductor) is either melted andadded to the base metal or remains in its solid state. All arcwelding utilizes the transfer of electrical energy to heat energy,and to understand this principle, a basic knowledge of electricityand welding power sources is necessary.

1.8.1.1The three basis principles of static electricity are as follows:

1. There are two kinds of electrical charges in existence - negative and

positive.

2. Unlike charges attract and like charges repel.

3. Charges can be transferred from one place to another.

1.8.1.2Science has established that all matter is made up ofatoms and each atomcontains fundamental particles. One of these particles is theelectron, which has the abilityto move from one place to another. The electron is classified asa negative electricalcharge. Another particle, about 1800 times as heavy as the

electron, is the proton andunder normal conditions the proton will remain stationary.

1.8.1.3Material is said to be in an electrically uncharged statewhen its atoms contain anequal number of positive charges (protons) and ne gativecharges (electrons). This balanceis upset when pressure forces the electrons to move from atomto atom. This pressure,sometimes referred to as electromotive force, is commonlyknown as voltage. It should benoted that voltage that does not move t hrough a conductor, butwithout voltage, there would

be no current flow. For our purposes, it is easiest to think ofvoltage as the electricalpressure that forces the electrons to move.

1.8.1.4Since we know that like charges repel and unlike chargesattract, the tendency isfor the electrons to move from a position of over -supply(negative charge) to an atom thatlacks electrons (positive charge). This tendency becomesreality when a suitable path isprovided for the movement of the electrons. The transfer of


39/66

electrons from a negative to apositive charge throughout the length of a conductor constitutesan electrical current. Therate that current flows through a conductor is measured inamperes and the word ampereis often used synonymously with the term current . To give anidea of the quantities of electrons that flow through a circuit, it

has been theoretically established that one ampere equals 6.3quintillion (6,300,000,000,000,000,000) electrons flowing past afixed point in a conductor every second.



40/66

LESSON I, PART B

1.8.1.5Different materials vary in their ability to transferelectrons. Substances, such aswood and rubber, have what is called a tight electron bond andtheir atoms greatly resistthe free movement of electrons. S uch materials are considered

poor electrical conductors.Metals, on the other hand, have large amounts of electrons thattransfer freely. Theircomparatively low electrical resistance classifies them as goodelectrical conductors.

1.8.1.6Electrical resistance is primarily due to the reluctance ofatoms to give up theirelectron particles. It may also be thought of as the resistanceto current flow.

1.8.1.7To better understand the electrical terms discussedabove, we might compare

the closed water system with the electri cal diagram shown inFigure 8. You can see that asthe pump is running, the water will move in the direction of thearrows. It moves becausepressure has been produced and that pressure can be likenedto voltage in an electricalcircuit. The pump can be c ompared to a battery or a DCgenerator. The water flows

through the system at a certain rate. This flow rate in anelectrical circuit is a unit of measure known as the ampere.The small pipe in the fluid circuit restricts the flow rate andcan be likened to a resistor. This unit resistance is known asthe ohm. If we close the valve in the fluid circuit, we stop theflow, and this can be compared to opening a switch in an

electrical circuit.1.8.2 Ohm's Law - Resistance is basic to electrical theory and t ounderstand this

principle, we must know the Ohm's Law, which is stated asfollows: In any electrical circuit, the current flow in amperesis directly proportional to the circuit voltage applied and in -versely proportional to the circuit resistance. Dir ectlyproportional means that even though the voltage andamperage may change, the ratio of their relationship will not.


41/66

For example, if we have a circuit of one volt and three amps,we say the ratio is one to three. Now if we increase the voltsto three, our amperage will increase proportionately to nineamps. As can be seen, even though the voltage andamperage changed in numerical value, their ratio did not.The term "inversely proportional"simply means that if theresistance is



42/66

LESSON I, PART B

doubled, the current will be reduced to one -half. Ohm's Lawcan be stated mathematically with this equation:

I = E + Ror E = | xR or R = E + I

(E = Volts, I = Amperes, R = Resistance (Ohms))

1.8.2.1 The equation is easy to us e as seen in the following problems:

1) A 12 volt battery has a built -in resistance of 10 ohms. What is theamperage?

12 + 10= 1.2 amps

2) What voltage is required to pass 15 amps through a resistor of 5 ohms?

15x5 = 75volts

3) When the voltage is 80 and the circuit is limited to 250amps, what is the valueof the resistor?

80-250= .32 ohms

1.8.2.2The theory of electrical resistance is of greatimportance in the arc weldingprocess for it is this resistance in the air space between theelectrode and the base metalthat contributes to the transfer of electrical energy to heatenergy. As voltage forces theelectrons to move faster, the energy they generate ispartially used to overcome theresistance created by the arc gap. This energy becomesevident as heat. In the weldingprocess, the temperature increases to the point where itbrings metals to a molten state.

1-8.3 Electrical Power- The word "waff" is another term frequentlyencountered in

electrical terminology. When we pay our electrical bills, weare actually paying for the power to run our electricalappliances, and the watt is a unit of power. It is defined asthe amount of power required to maintain a current of oneampere at a pressure of one volt. The circuit voltage thatcomes into your home is a constant factor, but the amperagedrawn from the utility company depends on the number ofwatts required to run the electrical appliance. The watt isfigured as a product of volts times amperes and is statedmathematically with the following equation:

W =Ex | E = W+l I = W+E

(W = Watts, E = Volts, I = Amperes)


43/66

1.8.3.1 The amperage used by an electrical device canbe calculated by dividing the watts rating of the device by theprimary voltage for which it is designed.



44/66

LESSON I, PART B

1.8.3.2 For example, if an appliance is designed for the commonhousehold primaryvoltage of 115 and the wattage stamped on the appliancefaceplate is 5, then theamperage drawn by the appliance when in operation is

determined as shown:

5 * 115 = .04 amperes

1.8.3.3 Kilowatt is another term common in electrical usage.The preface "kilo"\s ametric designation that means 1,000 units of something;therefore, one kilowatt is 1,000watts of power.

1.8.4 Power Generation - Electrical energy is supplied either as directcurrent (DC) or

alternating current (AC). With direct current, the electronmovement within the conductor is in one direction only. Withalternating current, the electron flow reverses periodically. Al -though some types of electrical generators will produce currentdirectly (such as batteries, dry cells, or DC generators), mostdirect current is developed from alternating current.

1.8.4.1 Through experimentation, it was discovered thatwhen a wire is moved through a magnetic field, an electricalcurrent is induced into the wire, and the current is at its

maximum when the motion of the conductor is at right angles to the magneticlines offeree. The sketch in Figure 9 will help to illustrate this principle.

1.8.4.2 If the conductor is moved upwards inthe magnetic field between the N and S poles,the galvanometer needle will deflect plus (+).Likewise, if the conductor is moved downwardsthe needle will deflect minus ( -). With thisprinciple of converting mechanical energy intoelectrical energy understood, we can apply it tothe workings of an AC generator.


45/66

1.8.4.3Figure 10 is a simplified sketch of an ACgenerator. Starting at 0 rotation, the coil wire is movingparallel to the magnetic lines offeree and cutting none ofthem. Therefore, no current is being induced into the winding.

1.8.4.4 From 0 to 90 rotation, the coil wire cuts anincreasing number of magnetic lines offeree and reaches themaximum number at 90 rotation. The current increases to themaximum because the wire is now at right angles to the linesofferee.



46/66

LESSON I, PART B

1.8.4.5From 90 to 180 rotation, the coil wire cuts adiminishing number of lines offorce and at 180 again reaches zero.

1.8.4.6From 180 to 270, the current begins to rise again butin the opposite directionbecause now the wire is in closer proximity to the oppositepole.

1.8.4.7One cycle is completed as the coil wire moves from270 to 0 and the currentagain drops to zero.

1.8.4.8With the aid of a graph, we can vi sualize the rate atwhich the lines offeree arecut throughout the cycle. If we plot the current versusdegree of rotation, we get thefamiliar sine wave as seen in Figure 11.

1.8.4.9 With this sine wave, we can see that one complete cycle ofalternating current comprises one positive and one negative wave (negativeand positive meaning electron flow in opposing directions). The frequency ofalternating current is the number of such complete cycles per second. Formost power applications, 60 cyc les per second (60 Hertz) is the standard


47/66

frequency in North America.

COPYRIGHT 1999 THEESAB GROUP, INJ


48/66

LESSON I, PART B

1.8.4.10 Some welders use a three -phase AC supply. Three-phase issimply three sources of AC power as identical voltages brough t in bythree wires, the three voltages or

phases being separated by 120 electrical degrees. If the sine wave for thethree phases are plotted on one line, they will appear as shown in Figure 12.

1.8.4.11 This illustrates that three -phase power is smoother than single-phase because the overlapping three phases prevent the current and voltage

from falling to zero 120 times a second, thereby producing a smootherwelding arc.

1.8.4.12 Since all shops do not have three -phase power, weldingmachines for both single-phase and three-phase power are available.

1-8.5 Transformers - The function of a transformer is to increase ordecrease voltageto a safe value as the conditions demand. Common household voltage isusually 115 or 230 volts, whereas industr ial power requirements may be 208,230, 380, or 460 volts. Transmitting such relatively low voltages over longdistances would require a conductor of enormous and impractical size.Therefore, power transmitted from a power plant must be stepped up for long

distance transmission and then stepped down for final use

1.8.5.1 As can be seen in Figure 13, the voltage is generated at thepower plant at 13,800 volts. It is increased, transmitted over long distances,and then reduced in steps for the end user. If power supplied to atransformer circuit is held steady, then secondary current (amperes)decreases as the primary voltage increases, and conversely, secondarycurrent increases as primary voltage decreases. Since the current flow(amperes) determines the wire or conductor size, the high voltage line maybe of a relatively small diameter.


49/66

LESSON I, PART B

1.8.5.2The transformer in a welding machine performs much thesame as a large powerplant transformer. The primary voltage coming into the machineis too high for safewelding. Therefore, it is stepped down to a useable voltage.

This is best illustrated with anexplanation of how a single transformer works.

1.8.5.3In the preceding paragraphs, we have found than anelectrical current can beinduced into a conductor when that conductor is moved througha magnetic field toproduce alternating current. If this alternating current is passedthrough a conductor, apulsating magnetic field will surround the exterior of thatconductor, that is the magneticfield will build in intensity through the first 90 electrical degrees,or the first cycle. From thatpoint, the magnetic field will decay during the next quarter cycleuntil the voltage or currentreaches zero at 180 electrical deg rees. Immediately, the currentdirection reverses and themagnetic field will begin to build again until it reaches amaximum at 270 electrical degreesin the cycle. From that point the current and the magnetic fieldagain begin to decay untilthey reach zero at 360 electrical degrees, where the cyclebegins again.

1.8.5.4If that conductor is wound around a material with high

magnetic permeability(magnetic permeability is the ability to accept large amounts ofmagnetic lines offeree)such as steel, the magnet ic fieldpermeates that core. See Figure 14. Thisconductor is called the primary coil, andif voltage is applied to one of itsterminals and the circuit is completed,current will flow. When a second coil iswound around that same steel core, theenergy that is stored in this fluctuatingmagnetic field in the core is induced into

this secondary coil.1.8.5.5 It is the build-up and collapseof this magnetic field that excite theelectrons in the secondary coil of thetransformer. This causes an elect rical current of the samefrequency as the primary coil to flow when the secondary circuitis completed by striking the welding arc. Remember that alltransformers operate only on alternating current.

1.8.5.6 A simplified version of a welding transf ormer is


50/66

schematically shown in Figure 15. This welder would operate on230 volts input power and the primary winding has 230 turns ofwire on the core. We need 80 volts for initiating the arc in thesecondary or welding circuit, thus we have 80 turns of w ire inthe secondary winding of the core. Before the arc is struck, thevoltage between the electrode and the work piece is 80 volts.Remember that no current (amperage) flows until the welding

circuit is completed by striking the arc.



51/66

LESSON I, PART B

1.8.5.7 Since the80 volts necessaryfor initiating the arc istoo high for practicalwelding, some means

must be used tolower this voltage toa suitable level.Theoretically, avariable resistor of theproper value could beused as an output controlsince voltage is inverselyproportional toresistance as we saw when studying Ohm's Law. Ohm's Lawalso stated that the amperage is directly proportional to thevoltage. This being so, you can see that adjusting the output

control will also adjust the amperage or welding current.1.8.5.8 After the arc is initiated and current begins to flowthrough the secondary or welding circuit, the voltage in thatcircuit will be 32 volts because it is then being control led by theoutput control.

1.8.6 Power Requirements - We can make another calculation by lookingback at

Figure 15, and that is power consumption. Earlier, we explainedthat the watt was the unit of electrical power and can becalculated by the formula:

Watts = Volts x Amperes

1.8.6.1From Figure 15, we can see that the instantaneouspower in the secondarycircuit is:

Watts = 32 x 300Watts = 9600 Watts

1.8.6.2The primary side of our transformer must be capable ofsupplying 9600 wattsalso (disregarding losses due to heating, power factor, etc.), soby rearranging the formula,

we can calculate the required supply line current or amperage:Amperage = Watts *Volts A = 9600 + 230 =41.74 Amps

1.8.6.3This information establishes the approximate powerrequirements for the welder,and helps to determine the input cable and fuse size necessary.


52/66



53/66

LESSON I, PART B

1.8.7 Rectifying AC to DC -Although much welding is accomplished withAC welding

power sources, the majority of indust rial welding is donewith machines that produce a direct current arc. The

commercially produced AC

power that operates the welding machine must then be changed (rectified) todirect current for the DC arc. This is accom plished with a device called arectifier. Two types of rectifiers have been used extensively in weldingmachines, the old selenium rectifiers and the more modern silicon rectifiers,often referred to as diodes. See Figure 16.

1.8.7.1 The function of a rectifier in the circuit can best be shown by theuse of the AC sine wave. With one diode in the circuit, half-wave rectificationtakes place as shown in Figure 17.

1.8.7.2 The negative half -wave is simply cut off and apulsating DC is produced. During the positive half-cycle,current is allowed to flow through the rectifier. During thenegative half-cycle, the current is blocked. This produces a DCcomposed of 60 positive pulses per second.

1.8.7.3By using four rectifiers connected in a


54/66

certain manner, a bridge rectifier is creat ed, producingfull wave rectification. The bridge rectifier results in120 positive half-cycles per second, producing aconsiderably smoother direct current than half -waverectification. See Figure 18.

1.8.7.4Three-phase AC can be rectified toproduce an even smoother DC than single-phase

AC. Since three-phase AC power produces threetimes as many half-cycles per second as single-phase power, a relatively smooth DC voltageresults as shown in Figure 19.

COPYRIGHT 1999 THEESAB GROUP, INJ


55/66

LESSON I, PART B

1.9 CONSTANT CURRENT OR CONSTANT VOLTAGE

Welding power sources are designed in many sizes andshapes. They may supply either AC or DC, or both, and theymay have various means of controlling their voltage andamperage output. The reasons for this is that the p owersource must be capable of producing the proper arccharacteristics for the welding process being used. A powersource that produces a satisfactory arc when welding withcoated electrodes will be less than satisfactory for weldingwith solid and flux cored wires.

1.9.1 Constant Current Characteristics - Constant current power sourcesare used

primarily with coated electrodes. This type of power sourcehas a relatively small change in amperage and arc power fora corresponding relatively large change in a rc voltage or arc

length, thus the name constant current. The characteristicsof this power source are best illustrated by observing a graphthat plots the volt-

ampere curve. As can be seen in Figure 20, the curve of a constant currentmachine drops downward rather sharply and for this reason, this type ofmachine is often called a "drooper."

1.9.1.1 In welding with coated electrodes, the output current oramperage is set by the operator while the voltage is designed into the unit.The operator can vary the arc voltage somewhat by increasing or decreasingthe arc length. A slight increase in arc length will cause an increase in arcvoltage and a slight decrease in amperage. A slight decrease in arc lengthwill cause a decrease in arc voltage and a slight increase in amperage.

1.9.2 Constant Voltage Characteristics - Constant voltage power sources,also

known as constant potential, are used in welding with solid andflux cored electrodes, and as the name implies, the voltageoutput remains relatively constant. On this type of powersource, the voltage is set at the machine and amperage isdetermined by the speed that the wire is fed to the welding gun.


56/66

Increasing the wire feed speed increases the amperage.Decreasing the wire feed speed decreases the amperage.

1.9.2.1 Arc length plays an important part in welding withsolid and flux cored electrodes, just as it does in welding with acoated electrode. However, when using a constant voltagepower source and a wire feeder that delivers the wi re at aconstant speed, arc length caused by operator error, plate

irregularities, and puddle movement are automatically



57/66

LESSON I, PART B

compensated for by the characteristics of this process. Tounderstand this, keep in mind that with the proper voltagesetting, amperage setting, and arc length, the rate that thewire melts is dependent upon the amperage. If theamperage decreases, this melt -off rate decreases and if the

amperage increases, so does the melt -off rate.

1.9.2.2 In Figure 21, we see that condition #2 producesthe desired arc length, voltage, and amperage. If the arclength is increased as in #1, the voltage increases slightly;the amperage decreases considerably, and therefore, themelt-off rate of the wire decreases.

The wire is now feeding faster than it is melting off. This condition willadvance the end of the wire towards the work piece until the proper arc length

is reached where again, the melt -off rate equals the feeding rate. If the arclength is decreased as in #3, the voltage drops off slightly, the amperage isincreased considerably, and the melt-off rate of the wire increases. Since thewire is now melting off faster than it is being fed, it melts back to the properarc length where the melt-off rate equals the feeding rate. This is oftenreferred to as a self-adjusting arc. These automatic corrections take place infractions of a second, and usually without the operator being aware of them.

1.9.2.3 There are a variety of different weldingmachines, each with its own unique internal design. Ourpurpose is not to detail the function of each part of themachine, but to emphasize that their main difference is in the

way they control the voltage and amperage output.1.9.3 Types of Welding Power Sources - A great variety of welding powersourcesare being built today for electric arc welding and we shallmention some of the major types briefly. Welding powersources can be divided into two main categories: static typesand rotating types.

1.9-3.1 Static Types - Static type power sources are allof those that use commercially generated electrical power to


58/66

energize a transformer that, in turn, steps the line voltagedown to useable welding voltages. The two major categoriesof static power sources are the transformer type and therectifier type.



59/66

LESSON I, PART B

1.9.3.1.1The transformer type produce only alternating current.They are commonlycalled "Welding Transformers."All AC types utilize single -phaseprimary power and are ofthe constant current type.

1.9.3.1.2The rectifier types are commonly called "WeldingRectifiers"and produce DC or,

AC and DC welding current. They may utilize either singlephase or three phase inputpower. They contain a transformer, but recti fy the AC or DC bythe use of seleniumrectifiers, silicon diodes or silicon controlled rectifiers. Availablein either the constantcurrent or the constant voltage type, some manufacturers offerunits that are a combinationof both and can be used for coa ted electrode welding, non-consumable electrode weldingand for welding with solid or flux cored wires.

1.9.3.2 Rotating Types - Rotating type power sources

may be divided into two classifi cations:

1. Motor-Generators

2. Engine Driven

1.9.3.2.1Motor-generator types consist of an electric motorcoupled to a generator oralternator that produces the desired welding power. Thesemachines produced excellent

welds, but due to the moving parts, required considerablemaintenance. Few, if any, arebeing built today.

1.9.3.2.2Engine driven types consist of a gasoline or dieselengine coupled to a generatoror alternator that produces the desired welding power. They areused extensively on jobsbeyond commercial power lines and also as mobile repair units.Both rotating types candeliver either AC or DC welding power, or a combination ofboth. Both types are available

as constant current or constant voltage models.1.9.4 Power Source Controls - Welding power sources differ also in themethod ofcontrolling the output current or vo ltage. Output may becontrolled mechanically as in machines having a tapped reactor,a moveable shunt or diverter, or a moveable coil. Elec tricaltypes of controls, such as magnetic amplifiers or saturablereactors, are also utilized and the most modern types,containing silicon controlled rectifiers, give precise electroniccontrol.


60/66

1.9.4.1A detailed discussion of the many types of welding powersources on the markettoday is much too lengthy a subject for this course, althoughadditional information on thetype of power sources for the various welding processes will becovered in Lesson II.

1.9.4.2Excellent literature is available from power sourcemanufacturers, however, andshould be consulted for further reference.



61/66

LESSON I, GLOSSARY

APPENDIX A LESSON I -

GLOSSARY OF TERMS

AISI American Iron and Steel Institute

Allotropic A material in which the atoms are capable of transforminginto two

or more crystalline structures at different temperatures.

Alternating An electrical current which alternately travels in eitherdirection in aCurrent conductor. In 60 cycles per second (60 Hz) AC, thefrequency

used in the U.S.A., the current direction reverses 120 times every

second.

Ampere Unit of electrical rate o f flow. Amperage is commonlyreferred to as

the "current"in an electrical circuit.

ASME American Society of Mechanical Engineers

ASTM American Society for Testing and Materials

Atom The smallest particle of an element that posses all of the

characteristics of that element. It consists ofprotons, neutrons, and electrons.

Carbon Steel (Sometimes referred to as mild steel.) An alloy ofiron and carbon.

Carbon content is usually below 0.3%.

Conductor A material which has a relatively large number of looselybonded

electrons which may move freely when voltage

(electrical pressure) is applied. Metals are goodconductors.

Constant Current (As applied to welding machines.) Awelding power source which will produce arelatively small change in amperage despitechanges in voltage caused by a varying arclength. Used mostly for welding with coatedelectrodes.


62/66



63/66

LESSON I, GLOSSARY

Constant Voltage (As applied to welding machines.) Awelding power source which will produce arelatively small change in voltage when theamperage is changed substantially. Usedmostly for welding with solid or flux cored

electrodes.

Direct Current An electrical current which flows in only one dir ectionin a

conductor. Direction of current is dependentupon the electrical connections to the battery orother DC power source. Terminals on all DCdevices are usually marked (+) or ( -).Reversing the leads will reverse the direction ofcurrent flow.

Electron Negatively charged particles that revolve around the

positively

charged nucleus in an atom.

Ferrous Containing iron. Example: carbon steel, low alloy steels,stainless

steel.

Hertz Hertz (Hz) is the symbol which has replaced the term"cycles per

second." Today, rather than saying 60 cyclesper second or simply 60 cycles, we say 60Hertz or 60 Hz.

High Alloy Steels Steels containing in excess of 10%alloy content. Stainless steel is considered ahigh alloy because it contains in excess of 10%chromium.

Induced Current orInduction The phenomena of causing an electrical current to flowthrough a

conductor when that conductor is subjected to a varying

magneticfield.

Ingot Casting of steel (weighing up t o 200 tons) formed at millfrom melt

of ore, scrap limestone, coke, etc.

Insulator A material which has a tight electron bond, that is,relatively few


64/66

electrons which will move when voltage(electrical pressure) is applied. Wood, glass,ceramics and most plastics are goodinsulators.



65/66

LESSON I, GLOSSARY

Kilowatt 1,000 watts

Low Alloy Steels Steels containing small amounts ofalloying elements (usually 114% to 5% total alloycontent) which drastically improves theirproperties.

Non-Ferrous Containing no iron. Example: Aluminum, copper,copper alloys.

Ohm Unit of electrical resistance to current flow.

PhaseTransformation The changes in the crystalline

structure of metals caused by temperatureand time.

Proton Positively charged particles which are part of the nucleusof atoms.

Rectifier An electrical device used to change alternating current todirect

current.

SAE Society of Automotive Engineers

Transformer An electrical device used to raise or lower the

voltage and inversely

change the amperage.

Volt Unit of electromotive force, or electrical pressure whichcauses

current to flow in an electrical circuit.

Watt A unit of electrical power. Watts = Volts x Amperes


66/66


Date post:	14-Apr-2018
Category:	Documents
Upload:	thao-phan
View:	225 times
Download:	0 times

Giao Trinh Han-BASIC_welding

Documents