HYDROGEN EMBRITTLEMENT – High Strength Steels Achilles Heel Published on June 14, 2013 by Rob in News Scanning electron microscopy of the fracture is an essential element of the failure analysis of this Grade 8 High Strength fastener. THE PHENOMENON Sudden brittle fractures in high strength steels resulting from hydrogen embrittlement represent a dangerous threat to industry. Not only are there the usual issues of cost such as warranty claims, but in cases of personal injury or property damage, liability points clearly and directly at the manufacturer. This is because hydrogen embrittlement is usually the result of deficient procedures in the manufacturing process.
Transcript
HYDROGEN EMBRITTLEMENT – High Strength Steels Achilles Heel
Published on June 14, 2013 by Rob in News
Scanning electron microscopy of the fracture is an essential element of the failure analysis of this Grade 8 High Strength fastener.
THE PHENOMENON
Sudden brittle fractures in high strength steels resulting from hydrogen embrittlement represent a dangerous threat to industry. Not only are there the usual issues of cost such as warranty claims, but in cases of personal injury or property damage, liability points clearly and directly at the manufacturer. This is because hydrogen embrittlement is usually the result of deficient procedures in the manufacturing process.
We’ll get into the why and how of hydrogen embrittlement in the next posting in two weeks. For now though, let’s just discuss some of the characteristics of this type of failure. Perhaps you’ll recognize some of these from fractures you’ve encountered, but didn’t realize at the time that hydrogen was the cause.
Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, their resistance to fatigue fracture, their fatigue strength, is significantly reduced as well. Fracture toughness, the ability of a metal to resist fracture growth when a small crack is present, is also dramatically reduced. Brittle fracture due to hydrogen embrittlement occurs without any visible distortion or other warning signs and can happen within hours of manufacture or after years in service. Hydrogen embrittlement failures have even been observed in unassembled parts in inventory, a phenomenon known as “shelf popping”.
Generally, the higher the strength of the steel, the more at risk it is to hydrogen embrittlement and the more vulnerable it is to lower levels of hydrogen. Embrittlement at levels of 10 parts per million and less are not uncommon. Some research suggests this relationship is exponential. In other words, doubling the strength of the steel, quadruples its susceptibility to hydrogen embrittlement.
Although hydrogen embrittlement occurs in many different metal alloys, high strength steel appears to be the most sensitive, is the most widely used, and accounts for the largest number of hydrogen embrittlement failures. A professionally conducted failure analysis can definitely recognize hydrogen embrittlement when present, what caused it, and how to prevent it.
In our next post we will discuss the phenomenon of hydrogen embrittlement from a metallurgical perspective – what actually occurs, on a microscopic scale that causes hydrogen embrittlement?
Expanded grain boundaries produced during a hydrogen embrittlement failure are clearly visible in this image taken using a scanning electron microscope.
In our last post on Hydrogen Embrittlement and its effects on high strength steel we discussed the characteristics of hydrogen embrittlement failures and some of the types of material that are affected. In this post, we will describe how hydrogen embrittlement occurs, from a microscopic and metallurgical perspective.
Hydrogen atoms are the smallest of any element. So small, that they easily travel between iron atoms in steel and other ferrous alloys. Structurally, metals are composed of multi-atom crystals, or grains, that are analogous to the cells that make up biological organisms. Typically, the grains in metals are microscopic and the space between the grains, called the grain boundaries, is virtually immeasurable. But in comparison to the size of a hydrogen atom, the grain boundaries are gapping canyons.
Under unfavorable conditions individual hydrogen atoms can enter these grain boundaries and move, or diffuse, into a metal component. Once absorbed in this manner, hydrogen atoms are attracted to microscopic crystal defects, or misalignments, where there is slightly more space between grains. They are also attracted to areas under tensile stresses that result in a very slight increase in the space between grains from the opposing “pull” of the stress. A typical example of this condition is a bolt that has been tightened.
As more hydrogen atoms accumulate at these areas, they combine to form hydrogen molecules. Although composed of two hydrogen atoms, a hydrogen molecule (H2) is significantly larger than two individual hydrogen atoms. This produces pressure between grains, expanding the size of the defect or grain boundary interface, attracting more hydrogen atoms and accelerating the process.
This cycle produces a raising tensile stress inside the component which eventually results in a micro-crack. These micro-cracks grow rapidly and simultaneously at numerous locations within the part, reducing the actual intact load bearing cross section by as much as 10-20%.
In order for hydrogen embrittlement to occur, three conditions must coincide:
1. The part must have a tensile strength in excess off approximately 130,000 psi. This generally corresponds to a hardness of Rockwell C 35.
2. The part must be in contact with a source of hydrogen. This may occur during manufacture, in service, or both.
3. The part must be subjected to a tensile stress.
This last condition can be deceptive because parts do not need to be assembled or in service to be under tensile stress. Residual internal stresses from casting, forging, welding and other manufacturing processes are significant and, in fact, are probably the root cause of most hydrogen embrittlement failures. Heat treating to raise strength levels above 130,000 psi induces substantial levels of residual stress. The disturbing phenomenon of “shelf popping”, unassembled parts cracking in storage or inventory with an audible “pop”, results from hydrogen embrittlement resulting from residual stress.
Since the hydrogen is absorbed through the grain boundaries, hydrogen embrittlement cracking is primarily intergranular (fracture at the grain boundary) rather than transgranular (fracture through the grains) as in some other forms of brittle cracking.
In our next post, we will discuss potential sources of atomic hydrogen and conditions that make components susceptible to hydrogen embrittlement.
Acid cleaning of this transmission input shaft during manufacture resulted in failure by hydrogen embrittlement.
In our last post we discussed the metallurgical aspects of hydrogen embrittlement – what actually occurs that results in hydrogen absorption in metals and how it affects their material properties. In this post we will look at the potential sources of hydrogen, what materials are susceptible to hydrogen embrittlement, and why.
HYDROGEN SOURCES
One of the challenges in predicting and preventing hydrogen embrittlement is the wide range of available sources of hydrogen, in both the manufacturing and the service environments.
Thermal dissociation of hydrogen from water is a prime source of hydrogen in manufacturing processes. This can occur in the initial steel making process, as well as subsequent casting or forging operations. Hydrogen can also be absorbed during grinding, abrasive blasting or tumbling, soldering, brazing and welding. At these stages, hydrogen can be dissociated directly from high dew point atmosphere, or from water absorbed in process related media, such as welding electrode flux coatings, abrasive grinding wheels and fine absorbent “dust” in blasting and tumbling media.
Acid cleaning or pickling, and electro-plating, however, are the most common source of hydrogen in manufacturing. Hydrogen containing acids used in these operations invariably infuse susceptible parts with atomic hydrogen.
Service related sources of hydrogen include incidental contact with hydrogen containing acids or cleaning solutions, or absorption from hydrogen containing product that is being processed, such as chemicals, food, or even waste water.
The most common source of hydrogen in service by far, however, is corrosion. Corrosion can also act as a source of hydrogen in the manufacturing process as well. Rusted ingots and scrap used in casting melts, welding on parts that have corroded, and heat treating corroded parts, are potential sources of absorbed hydrogen, particularly when exposed to elevated temperatures which increase the mobility of hydrogen atoms.
SUSCEPTIBLE MATERIALS
High strength steels with tensile strengths above 130,000 psi and a hardness of Rockwell C35 or greater are the most prone to hydrogen embrittlement. Steels below these tensile and hardness levels are generally immune to hydrogen embrittlement. Why?
Increased hardness, most commonly by heat treating, is accompanied by a corresponding decrease in ductility. In simple terms, ductility is the ability of a material to deform under stress rather than crack or fracture. When hydrogen atoms combine into molecules in a steel that exceeds the tensile strength and hardness thresholds, the steel cracks under the stress increase. But if the tensile and hardness levels are below the critical threshold, the higher degree of ductility allows the steel to deform, absorbing and redistributing the stress increase, rather than cracking.
Susceptibility to hydrogen embrittlement increases in alloy steels with heat treatment to higher strength. The strength/susceptibility relationship, in fact, approaches exponential levels. In other words, doubling the strength through heat treating, quadruples the steel’s susceptibility to hydrogen embrittlement.
Identifying and sorting embrittled parts from good components before they fail is virtually impossible. The detection limit of hydrogen by chemical analysis is generally well above the 5 to 10 ppm level at which embrittlement has been shown to occur. Even if such detection capability was available, hydrogen tends to concentrate at specific locations within the part. This leaves the majority of the part with “low” or undetectable hydrogen levels. Chemical analysis of parts after failure, to determine if hydrogen embrittlement was the cause, is also not viable since the hydrogen diffuses from the part after fracture.
Since most hydrogen embrittlement results from hydrogen absorbed during the manufacturing process, parts which are “batch” processed are usually either all embrittled or all “good”. The failure of one part from a “batch”, therefore, is usually a good indication that others from the same batch will fail.
In the first three parts of this series we discussed the physical and metallurgical aspects of hydrogen embrittlement. An understanding of how the phenomenon occurs is the foundation of the ultimate question – how can hydrogen embrittlement be prevented.
PREVENTION
The two critical points at which most hydrogen embrittlement failures can be prevented are at the design stage and during the manufacturing process. Designers with limited materials engineering exposure may not realize the implications of the materials and manufacturing processes they call for in their drawing specifications. As with other failure types, hydrogen embrittlement failures can inadvertently be “designed into” a part.
On the manufacturing side, avoiding the use of reducing acids where possible removes an abundant source of hydrogen from potential exposure to a part. Pickling, etching and electroplating are common manufacturing processes in which acid exposure occurs. Because it is so widely used, electroplating requires particular attention. Minimizing plating time and maximizing current density will generally reduce the volume of absorbed hydrogen. However, consideration of electroless plating or vapor deposition as an alternative eliminates the possibility of hydrogen absorption from the coating process altogether.
Protecting components that will be heat treated or welded from corrosion, or cleaning them prior to these processes will avoid hydrogen introduction by these routes. It is also critical that welding rods are stored in a manner which prevents the absorption of moisture into their flux coating. Any process associated with elevated temperatures will also increase the mobility and absorption of hydrogen, increasing the potential for hydrogen embrittlement.
HYDROGEN MANAGEMENT
Despite the most stringent precautions, processing requirements will sometimes introduce hydrogen into parts that are at or above the tensile strength and hardness thresholds at which hydrogen embrittlement can occur. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This is an oven heating process, referred to as “baking”, that is performed within the following parameters:
1. Parts must be baked within 4 hours of hydrogen exposure. The sooner, the better.
2. Parts must be baked at a minimum of 400º F.3. Parts must be held at 400º F for a minimum of 4 hours. Longer may be required depending
on part size.
To be effective, these time and temperature parameters must be strictly followed. Short-cuts or delays will dramatically reduce the effectiveness of baking in removing absorbed hydrogen. For example, twice as much hydrogen will be baked out at 400º F versus 350ºF, and doubling the bake time doubles the amount of hydrogen that is baked out.
The sooner baking begins after exposure to hydrogen, the better. The 4 hour “window” is a maximum. Note that baking must be performed within 4 hours of each hydrogen exposure. For example, within 4 hours of pickling, AND within 4 hours of subsequent plating. No amount of baking will salvage embrittled parts if these time and temperature parameters have not been met.
A final word of caution. A 30 year analysis of hydrogen embrittlement failures in the aircraft industry found that over 70% resulted from improper baking procedures.
In the next, and final, part of the series we’ll talk briefly about the “history” of hydrogen embrittlement, and why it’s a relatively recent phenomenon.
A LITTLE HISTORY
Hydrogen embrittlement is a relatively recent phenomenon. With a few exceptions, failures by this mode did not occur prior to the middle of the last century. In a sense, the genesis of hydrogen embrittlement was the jet engine.
In the late 1940’s a revolution was underway in aviation. Jet propulsion was rapidly replacing the old piston engine driven propeller technology and aircraft performance began to exceed levels that had been considered physically impossible just ten years earlier. The dramatic increase in power provided by jet propulsion demanded airframes that could withstand the resulting higher loading. That increase in performance, and aviations never-ending quest for weight reduction, only added to the demands placed on existing materials. The result was a push for higher strength alloys from which stronger and lighter components could be made.
Low alloy steels such as 4130 had been used in aviation in the past. However, these materials were typically used in the normalized heat treated condition, at tensile strengths in the 90,000 to 120,000 psi range – well below levels susceptible to hydrogen embrittlement. In response to demands for more strength, “radical” heat treatments resulting in tensile strengths approaching 200,000 psi were applied to 4130 and other “anemic” low alloy steels. Some of the first hydrogen embrittlement failures appeared in this material, though the cause was not initially recognized.
Enhanced low alloy steels, such as 4140 and 4340 were used in response to these failures, and the cycle was repeated, with the demand for more performance from smaller components resulting in processing to ever higher tensile strength levels.
One of the unfortunate consequences of increasing the strength of low alloy steels is a corresponding reduction in corrosion resistance. To combat increased corrosion in service, a variety of electroplated coatings, such as chromium, nickel and cadmium, were applied.
With a potent source of hydrogen now available from the plating baths used to protect the new high strength alloys, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and other industries to which the new materials technology had filtered down. Once hydrogen had been identified as the Achilles heel of these materials, the prevention strategies described in part four of this series were developed.
Failure Analysis of Fatigue – Part 1 – A Railroad Disaster Led to the Discovery of Fatigue Failure
Published on April 3, 2014 by Rob in News
On May 11th, 1842 the first major railroad disaster in history set off a chain of events which led to the discovery of the phenomenon that we now know as fatigue failure.
The Paris – Versailles Express, hurtling down the tracks at the then astounding speed of 50 miles per hour, exploded in flames when the drive axle on the lead locomotive broke, digging its front end into the railbed. The second locomotive in the tandem drive set smashed into the firebox of the lead engine along with the first three cars, killing 57 passengers outright and injuring over a hundred more. It was the 1800’s equivalent of a jumbo jet crash, and the great scientific minds of the day focused their collective wisdom on perhaps the first major failure analysis in history. The result of their decade long investigation produced the beginnings of our understanding of fatigue.
Fatigue is the most common type of fracture in engineered components. Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure. The component, whether it’s the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure.
A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory. In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates. But a practical understanding of the process is extremely beneficial and has direct application to its prevention in service and the manufacturing environment.
To the non-technically inclined, the term “fatigue” suggests this type of failure is related to the age of a component; that the material is “tired”. In fact, fatigue fracture can occur within hours of a component going into service or, conversely, even highly stressed components can operate for decades with no fatigue cracking or failure.
Fatigue fractures result from repeated, or cyclic, stresses. These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation. Regardless of this variation in form, the stress on the component at the initiation point of a fatigue fracture is always tensile stress. In other words, the point of origin at which the fracture begins is being “stretched apart”, or pulled in opposite directions. To illustrate this, visualize a tube which is being repeatedly bent in one direction. The side of the tube that is concave when it is bent is being compressed. The side of the tube which is convex is being “stretched”, or subjected to a tensile stress. This is the side on which a fatigue crack will initiate.
Fatigue cracks initiate at stresses below the tensile strength of the material. Tensile strength is the stress, or load, at which a material breaks when pulled in opposite directions. Each metal alloy has a specific tensile strength, expressed as a numerical value, varying somewhat depending on heat treatment and other processing operations. These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references. The fact that fatigue cracks can initiate at stress levels below the tensile strength of a material is difficult to explain. Theories on why this occurs suggest physical and structural changes at the microscopic (0.0001” or less) area of crack initiation.
Fatigue is a progressive fracture mechanism. Once a fatigue crack initiates, it advances further into the component with each stress cycle. This crack growth process continues as long as the component is subjected to cyclic stress. Depending on the magnitude and frequency of the stresses, the crack may grow over time frames ranging from hours to years. Eventually, the crack advances to a point where the remaining intact cross section of the component cannot sustain the next cyclic stress load – “the straw that breaks the camel’s back” – and complete fracture of the component occurs.
In Failure Analysis of Fatigue – Part 2 we will discuss strategies to prevent fatigue initiation that can be implemented at the design and manufacturing stages of a components life.
Failure Analysis of Fatigue – Part 2 – Fatigue in the Real World and Crack Initaition
Published on April 8, 2014 by Rob in News
Fatigue in the “Real World”
In the “real world” fatigue usually – not always, but usually – initiates at a location that acts as a stress concentration, or “stress riser”. A component is most resistant to fracture when the stress is evenly distributed over it. A stress riser disrupts this even distribution and concentrates the stress at a geometric feature or reduction in the component’s area. Typical stress risers include holes, slots, corners and radii, rough surface finish, welds, corrosion pits, cracks and microstructural defects such as inclusions.
The exception to “usually” – the cases where fatigue fractures initiate from component surfaces that are free of stress risers – typically result from one of two causes; under-design of the component, or abusive service conditions.
Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit. Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs, even though no stress riser is present.
Fatigue failures of this type are less common than fatigue failures initiating from stress risers. Usually components are intentionally over-designed to deal with stresses several times greater than those they would be subjected to in service as a safety margin.
Fatigue Crack Initiation – The Critical Event
If the initiation stage can be prevented, fatigue fracture will not occur. It sounds obvious and simple. It’s not. Initiation is the most complex stage of fatigue fracture. A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied in thousands or millions of cycles. The cumulative effect of these cyclic loads are microscopic “shifts” in the material’s structure which ultimately produce a “dislocation” – at this scale it is too small to be called a crack – and the focal point of stress concentration is born. Vibration harmonics, dampening of the system and the service environment further complicate the issue. Collectively, these affects become difficult to predict in the design stage.
In Failure Analysis of Fatigue – Part 3 we will discuss fatigue prevention at the design stage of a component’s life, with following entries focused on the manufacturing and service environments and their relationship to fatigue failure.
Failure Analysis of Fatigue – Part 3 – Confronting Fatigue – Attack and Defense
Published on April 15, 2014 by Rob in News
Confronting Fatigue – Attack and Defense
From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment.
Design
The designer or design engineer is the first line of defense against fatigue failure. He or she can’t prevent failures originating in the manufacturing process or service environment, but the designer lays the foundation of prevention.
In an ideal world, each design would be subjected to extensive stress calculations and fatigue testing.In the real world this is rarely cost effective for non-critical components. Instead, accepted and “proven” parameters are applied. These typically include safety margins which are more than adequate. Typically, but not always. And even a common “off the shelf” fastener can take complex products out of service if it fails.
A working understanding of material strengths and properties by the designer is optimal. Unfortunately, that is a relatively rare combination of expertise. And although material strength and property data is widely available, the effective application of this information is sometimes outside the experience of the designer.
The job of the designer becomes even more challenging when the many potential variables inherent in the manufacturing process are considered. Leaving aside the production of the raw material at the mill – the bar, plate and sheet – manufacture of the designer’s envisioned component may include a host of processes that he or would ideally be familiar with through which the seeds of fatigue failure could potentially be introduced.
Computer Aided Design (CAD), Finite Element Analysis (FEA) and a variety of other computer driven design and predictive technologies can greatly enhance the fatigue resistance of a
component at the design stage. But they cannot prevent fatigue failures. That’s because the next two threats of fatigue failure are beyond the designer’s control.
In Failure Analysis of Fatigue – Part 4 we will discuss fatigue prevention at the manufacturing stage of a component’s life.
Failure Analysis of Fatigue – Part 4 – Confronting Fatigue – The Manufacturing Stage
Published on April 24, 2014 by Rob in News
Confronting Fatigue – Attack and Defense
From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss aspects of the manufacturing process to monitor in order to prevent fatigue.
The Manufacturing Process
Manufacturing processes are a rich, though unintended, source of stress concentrations from which fatigue cracks can initiate. The list is almost endless, and includes rough machined surfaces from dull tooling or excessive feeds and speeds, burrs from cutting or drilling operations, and insufficient chamfers or corner radiuses. Mechanical fasteners – bolts, screws, studs, and rivets- are highly prone to fatigue failure. Prominent among these are thread laps, folds or seems, that are formed when the threads are cut into the fastener. Threads formed by rolling are much less susceptible to laps and consequent fatigue failure. Whether threads are cut or rolled, however, insufficient tightening torque during the assembly stage of the manufacturing process is probably the number one source of fatigue failure in fasteners.
Welds, even when technically faultless, provide geometric stress concentrations. Defective welds and welding procedures may result in porosity and high hardness heat affected zones from which fatigue can initiate. Similarly, braze and solder joints, by their very nature, typically produce a geometric configuration that can potentially invite fatigue initiation. Fatigue susceptibility resulting
from these joining processes can be significantly mitigated by careful consideration in the design stage, but design quality cannot compensate for weld and joining defects such as undercut, porosity and slag inclusions.
Care in manufacturing and a good quality control program will avert many of these potential sources of fatigue initiation. However, despite the best quality control program, the manufacturer is often at the mercy of their raw material supplier. These suppliers may open the door to fatigue failure through castings which contain excessive porosity or microstructural defects, mill products which are work hardened, forgings with undetected laps or seams, or gross non-metallic inclusions in any of these products. Appropriate specifications on outsourced stock and components are vital in guaranteeing their quality, but as with so many aspects of production, they are a compromise. Loose specs solicit low cost bids, but a potentially high percentage of defective product, while tight specs limit the number of vendors capable of meeting them and drive costs higher, cutting into profits.
Failure Analysis of Fatigue – Part 5 – Confronting Fatigue – Prevention in Service
Published on May 1, 2014 by Rob in News
Confronting Fatigue – Attack and Defense
From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss a few of the many factors that can initiate fatigue failure in the service environment.
The Service Environment
Once a product leaves the factory you, the manufacturer, have lost control of the many factors that can initiate a fatigue failure once it is placed in service. Abuse and inadequate maintenance are leading preliminaries of failure by fatigue, as well as other failure modes. Failures of components or assemblies “up stream” from your product may introduce higher loads than the product or component was designed to sustain. Harsh service environments, such as road salts or ocean front installations may instigate corrosive attack, with corrosion pits providing a fatigue initiation sight. Analysis and identification of the root cause of fatigue failures in service is critical to educating your customer in the appropriate use and maintenance of your product and getting them back on track as a satisfied customer.
Identifying the root cause of service environment initiated fatigue failures can be challenging, and sometimes obscure, as the following example illustrates. Some years ago, we provided analytical support on a lawsuit which was filed after a person sustained a back injury when the metal leg of a “stacking chair” fractured. Stacking chairs are the type of institutional chairs you often see in school auditoriums and other public buildings and are designed to be stacked, one upon the other, for more compact storage when not in use. This particular chair came from a college in Ohio. Our analysis proved that low stress, high cycle fatigue was the failure mode. In other words, low magnitude stresses applied at high frequencies, in this case over a million cycles.
The chair had been in use for a relatively brief time, and even if it had seen longer service, it seemed unlikely that it could have been subjected to the number of load cycles indicated by the fracture morphology. This presented something of a mystery, as the failure mode was indisputable. Investigation of the service environment revealed that the chairs were used sporadically and when not in use, were stacked in a storeroom. The college staff was methodical in setting up the chairs in orderly rows in an adjacent auditorium, then stacking them from the same end of the same rows when they were no longer required, with the same chair ending up on the bottom of the stack before going back into storage. The stack was higher than the maximum specified by the manufacturer, providing a load in excess of the design limit. A survey of the area revealed that the storeroom was immediately above the main HVAC installation, the final and key piece of the puzzle. Vibration from the HVAC system, transmitted through the storeroom floor, and loads from the weight of chairs stacked in excess of the design limit provided the stresses required to initiate the fatigue crack. Once the crack grew to the point at which the remaining intact tubular leg could no longer sustain the load of a sitting person, final fracture occurred.
As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure. They are: 1. How did it fail? 2. Why did it fail? and 3. What will prevent future failures? If you have commissioned a failure analysis, and all three of these questions are not answered, all you have paid for is some interesting pictures and a possible lawsuit when your product fails again.
Metallurgical Associates (MAI) provides expert microstructure and heat treatment analysis. Our engineers extensive experience, combined with the latest sample preparation capabilities and techniques, identify defects or flaws, conformance to specification, and suitability of components for intended application in service.
The physical properties of metals – strength, hardness, wear resistance, etc. – and their performance in service depend to a large degree on their microstructure. The sample preparation and analysis of microstructure is referred to as “metallograpy”.
Industrial metals are composed of crystalline structures. The size, shape, orientation and chemical composition of these crystals, or grains, determine those properties. These characteristics are modified by a variety of manufacturing processes including heat treating, welding, grinding, forging, and others. Irregularities in these manufacturing processes may result in microstructural changes or defects which severely degrade a part’s performance.
Carbide defect analysis
Metallography and Microstructural Analysis applications include:
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