Embrittlement of Steels4/1/2007 by Fred HochgrafPractical advice on the embrittlement of steels and new steels that can enhance the performance of critical mechanical parts.This issue of Nuts and Bolts is a wakeup call for our clients that make parts out of hardened steels. Through-out this issue I am referring to only hardened steels. Don't read into this discussion anything about the behavior of any ductile metals.New steels are becoming available that can enhance the performance of critical parts such as bearings, gear teeth, parts subjected to high and low cycle fatigue, and large parts including turbines and paper mill rolls.The changeover to the new steels has taken place in steam turbine forgings. My crystal ball says that rolling contact bearings are the next candidates and there are murmurings that some bearings are now being produced from the new steels.These new steels take advantage of finer grain sizes than ever achieved before in mill quantities and/or lower impurity levels than we could have ever imagined. It has been found that the impurity atoms tend to migrate to the grain boundaries. The finer the grain size the greater the total grain boundary surface and the more "dilute" are the impurity atoms. It is also well established that the higher the strength and hardness, the greater the detrimental effects of the impurity atoms.Why is embrittlement important?Perhaps most crucial are its effects on damage tolerance and working stress through decreased impact strength, decreased low cycle fatigue strength, and the rather different phenomena of high cycle fatigue fracture initiation and fatigue fracture propagation. With the new steels we can operate at significantly higher levels of service stress, stress intensity and impact strength.The performance level of any hardened steel is determined by the stress levels at which cracking commences in the grain boundaries. The cracking starts as either decohesion of the grain boundaries or cracking across or around hard particles in the grain boundaries. More often than not the hard particles are carbides.How do we describe embrittlement?One way is via the Izod and Charpy tests that measure the energy to break a notched bar 1 cm square by 10 cm long at a strain rate in the ballpark of 1000 strain units per sec. Higher strain rates interest the military. Higher strain rates also occur during impacts such as railroad wheels impacting across a gap in the track at a switch or crossing. High strain rates include a cam being slammed by a cam follower. Low strain rates include the tensile tests, tests of stress rupture and hardness tests.It is very important to note that the low strain rates of a hardness test will not detect embrittlement.What are Temper Embrittlement and TemperedMartensite Embrittlement? They are the embrittlement that is seen after tempering or slowly cooling hardened steels or using them too long within a critical temperature range. The low temperature phenomenon is called "tempered embrittlement" and it is irreversible except by reaustenitizing and repeating the entire heat treating cycle. In a higher temperature band it is called "tempered martensite embrittlement". Depending on the steel chemistry there can by one embrittling band or two.ExamplesConsider the "tempered martensite embrittlement" of 4140, figure 2. Because of this embrittlement 4140 cannot be tempered or used in the range 180-500C (350-930F).Now consider the same pair of curves for 4340, figures 3 and 4. As before, "tempered martensite embrittlement" occurs in a low temperature range while this time we add "temper embrittlement" in a high temperature range. The high temperature "temper embrittlement" is reversible through appropriate retempering that either redissolves the harmful grain boundary precipitates or disperses the solute atoms that have migrated to the grain boundaries. "Tempered martensite embrittlement" is not reversible.

(Figure 1. The hardness of 4140 steel following tempering. With the low strain rates of a hardness test the curve drops smoothly with increasing tempering temperature.Ref Metals Handbook, ASM, 9th ed, vol. 1, p469)As shown in figures 1 and 3, hardness varies smoothly through the tempering range however at the higher strain rates of the impact tests there is the unacceptable embrittlement, figures 2 and 4. That's why we have to stay away from tempering in those ranges, and that's why the bands of hardness values and strengths we would expect following tempering in those ranges are, practically speaking, unavailable to the engineer.

(Figure 2. The notch toughness of 4140 steel following tempering. We see the dramatic loss in toughness accompanying tempering in the range spanning 180- 500C.Ref Metals Handbook, ASM, 9th ed, vol. 1, p469)Or suppose your bearing or hardened part is subject to soak-back heating to near 375C after normal shut down of a gas turbine or emergency shutdown of high temperature processing equipment. Well, these hardened steels can't be used. Too bad because the alternative steels tend to be expensive.The Trace ElementsThe irreversible, low temperature embrittlement is attributable to fracture in the prior austenite grain boundaries. These fractures initiate at grain boundary carbide particles. With these particles present phosphorous, manganese and silicon correlate with increased embrittlement.Sulfur and phosphorous are the principal contributors to reversible temper embrittlement. Neither can be adequately refined out of the steel so control is through pure melting stock, often made via chemical processes rather than traditional steel making. Sulfur levels of 0.002% are available.Tin and antimony contribute to embrittlement. They operate through co-segregation with nickel so that low nickel steels are sometimes preferred.Besides being one of the least expensive contributors to hardenability, manganese contributes to embrittlement. When the sulfur is extremely low there is no longer a necessity for the manganese. However we must look to other alloying elements to enhance hardenability.Carbon that preferentially diffuses to grain boundaries makes important contributions to grain boundary strength. We must be watchful lest elements such as chromium precipitate carbides in the prior austenite grain bound- aries, thereby reducing the amount of available carbon and thereby denying us the beneficial effects of carbon on grain boundary strength.Molybdenum and vanadium tend to be beneficial to grain boundary strength, apparently by not localizing the carbon atoms the way that chromium does. Additionally molybdenum is principally responsible for the secondary hardening that we exploit in the tool steels and numerous other high temperature applications for hardened steels.

(Figure 3. The hardness of 4340 varies smoothly with tempering temperature."Ref Metals Handbook, ASM, 9th ed, vol. 1, p425)Dissolved hydrogen in hardened steel migrates in response to stress, either applied or residual. Hydrogen works together with the other embrittling elements to cause grain boundary fracture. Low temperature sensitivity to hydrogen is therefore reduced when the other elements are kept at low levels.

(Figure 4. Toughness v Tempering Temperature for 4340 showing the pair of embrittlement ranges as well as the dramatic improvement that comes with 50 ppm phosphorous.Ref. J. P. Materkowski and G. Krauss, Metallurgical Tr. v10A, 1979, p1643)Looking ForwardKeep your eyes and ears open! Just as low phosphorous enhanced the 4340 in figure 4 expect that many more enhancements in the hardened steels will be coming along.Areas of embrittlement of hardened steels that I will save for another day's discussion include quench embrittlement, liquid metal embrittlement, low temperature embrittlement, weld cracking and more about hydrogen. Also available if there is a ground swell of requests would be a Nuts and Bolts discussing damage tolerance.For an excellent overview see C. J. McMahon, Jr., Brittle Fracture of Grain Boundaries, Interface Science 12, 141- 146, 2004.For a review of the metallurgy of steels see G. Kraus, Steels, ASM International, 2005

The Embrittlement Phenomena in Hardened & Tempered SteelDetailsWritten by Dan Herring Published: 05 February 2008 Created: 03 March 2008 By Daniel H. HerringWhile the end-use application of a component dictates its heat treatment, as heat treaters we know that we must achieve a delicate balance between the properties of strength and ductility. Nowhere is this fine line more evident than in the tempering process where precise control of time and temperature are critical to help produce a part with optimized microstructure and mechanical properties.Essentially, tempering is the modification of this newly formed microstructure toward equilibrium. Almost all steels that are subjected to any type of hardening process are tempered. A temper is a subcritical heat treatment that alters the microstructure and properties. In general, tempering lowers strength and hardness while improving ductility and toughness of the as-quenched martensite. However, this is not always the case. Lets learn more.What is Temper Embrittlement?In general, embrittlement is a reduction in the normal toughness of steel due to a microstructural change and chemical effects. Temper embrittlement is a phenomenon inherent in many steels, characterized by reduced impact toughness. It occurs in certain quenched and tempered steels and even in ductile irons with susceptible compositions. This form of embrittlement does not affect room-temperature tensile properties but causes significant reductions in impact toughness and fatigue performance. Although normally associated with tempered martensite, temper embrittlement can also occur if the matrix is tempered to the fully ferritic condition.

Fig. 1. Effect of temperature on impact toughness[5]

Types of Temper Embrittlement When tempering steel, several types of embrittlement must be avoided. The first type, tempered martensite embrittlement (TME), is an irreversible phenomenon that occurs in the range of approximately 250400C (480F750F) and is often referred to as blue brittleness or 350C (500F) embrittlement. The second type, temper embrittlement (TE), is a reversible phenomenon occurring when steels are heated in or slow cooled through the temperature range of 375575C (705F 1070F).Recently it has been reported that a transition from ductile to intergranular fracture in steel having greater than 0.5% C has been observed in martensitic steels tempered at low temperatures. Under tensile or bending stress, these higher-carbon steels are highly susceptible to intergranular fracture in both the as-quenched condition and after tempering at low temperatures generally considered to be safe from these embrittlement phenomena. In view of the fact that tempering is not required to render the microstructure susceptible to intergranular fracture, this type of embrittlement phenomenon is referred to as quench embrittlement. [2]Why does it happen?Tempered martensite embrittlement and temper embrittlement are examples of intergranular embrittlement. A common factor in such failures is the presence of elements that segregate to the grain boundaries. The chemical reaction rate or kinetics of segregation are such that they exhibit C curve behavior in the 350C 550C (660F 1020F) range. In other words, segregation does not occur uniformly. Both types of embrittlement are in part related to grain-boundary segregation of impurity elements (e.g. arsenic, antimony, phosphorus, and tin). Usually indicated by an upward shift in ductile-to-brittle transition temperature, both types of embrittlement develop during thermal processing after austenitizing and quenching to martensite.Tempered martensite embrittlement is thought to result from the combined effects of cementite precipitation on prior-austenite grain boundaries or interlath boundaries and the segregation of impurities at prior-austenite grain boundaries.Temper embrittlement that occurs in the range of 375575C (7051070F) is believed to be due to segregation of impurity elements (P, Sn, As, Sb) to prior austenite grain boundaries. This causes decohesion of the boundaries, resulting in the tendency for low-energy intergranular fracture under certain loading conditions.

SEM photo of intergranular temperembrittlement (Courtesy of Matco Associates Inc.)

Which steels are affected? All steels are susceptible, so the real question becomes how susceptible and what factors affect that susceptibility. For example, while plain-carbon steels may contain some of the impurity elements that will cause the embrittlement phenomenon to occur, the segregation of these elements is often enhanced by or caused by the presence of other alloying elements in substantial quantities. As a result, alloy steels generally have more susceptibility than carbon steels.It is important to understand that the degree of embrittlement is affected by the prior austenite grain size and hardness. So, if we are dealing with a fine-grained plain-carbon steel of low hardness, it may not experience embrittlement symptoms despite its phosphorous content whereas a more highly alloyed Cr-Ni steel used at higher hardness is more susceptible to impurity content.Widely used alloying elements such as chromium, nickel and manganese tend to promote temper embrittlement with the highest embrittlement effect observed in Cr-Ni and Cr-Mo steels. Small additions of molybdenum in Cr-Ni steels (0.2-0.3% in solution) can diminish temper embrittlement being caused by phosphorus. Temper embrittlement can be diminished by keeping silicon and phosphorus levels as low as possible, adding up to 0.15% molybdenum and avoiding the embrittlement heat-treating conditions.Susceptibility also depends on impurity control, and here is where the steelmaking process is critical. For example, in plain carbon and Cr-Mo steels (those with no Ni) where phosphorous is the most important embrittlement element, the percentage can be controlled by the steelmaking process. In steels that contain significant amounts of nickel, antimony and tin are more potent embrittlement elements. Phosphorous has an effect, but not as large as it has in plain carbon and Cr-Mo steels. It should be noted, however, that antimony and tin in plain-carbon steels could cause other hot-working issues.How can we correct it? Tempered martensite embrittlement (TME) is irreversible and its effects are permanent. By contrast, the effects of temper embrittlement (TE) can be reversed. This is done by re-tempering above the critical temperature of 575C (1070F), then cooling rapidly, or by re-austenitizing and cooling rapidly. Impact toughness can be restored. If necessary, this process can be repeated.A Simple Example Alloy steel, which is susceptible to temper embrittlement, will exhibit a relationship such as shown below (Fig. 1). The lower-temperature energy trough, 250400C (480F750F), is indicative of tempered martensite embrittlement while the trough at the higher temperature, 450-650C (8401200F), represents temper embrittlement.Summing Up The susceptibility of a given steel to temper embrittlement depends on a number of factors including grain size, hardness, steel grade and the impurity control in the steelmaking process itself. Not all steels and not all steelmaking processes are equal.Finally, as heat treaters we must avoid the temptation to temper to a given hardness value without understanding the consequences of our actions. Since we do not have a simple embrittlement test that can be used on the shop floor, we must understand the phenomenon and question specifications that put us into temper-embrittlement ranges.

HYDROGEN EMBRITTLEMENT High Strength Steels Achilles Heel Part 1Published on June 14, 2013 by Rob in News0

Scanning electron microscopy of the fracture is an essential element of the failure analysis of this Grade 8 High Strength fastener.THE PHENOMENONSudden 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.Well get into the why and how of hydrogen embrittlement in the next posting in two weeks. For now though, lets just discuss some of the characteristics of this type of failure. Perhaps youll recognize some of these from fractures youve encountered, but didnt 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?HYDROGEN EMBRITTLEMENT High Strength Steels Achilles Heel Part 2Published on June 20, 2013 by Rob in News0

Expanded grain boundaries produced during a hydrogen embrittlement failure are clearly visible in this image taken using a scanning electron microscope.THE METALLURGICAL PHENOMENONIn 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 ofRockwell 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.

HYDROGEN EMBRITTLEMENT High Strength Steels Achilles Heel Part 3Published on July 2, 2013 by Rob in News0

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 SOURCESOne 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 MATERIALSHigh 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 steels 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.

Hydrogen Embrittlement Part 1Posted on 29. Nov, 2010 by Rob in Failure AnalysisHydrogen Embrittlement Part 1High Strength Steels Achilles HeelThe PhenomenonSudden brittle fracture in high strength steels resulting from hydrogen embrittlement represents an extremely dangerous phenomenon to industry, particularly since it is usually the result of factors that occur during the manufacturing process.Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, fatigue strength and fracture toughness are also dramatically reduced. Brittle fracture occurs without warning and can be immediate, 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, quadruples the 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. This article offers an overview on hydrogen embrittlement as it relates to high strength steels only, though details of the phenomenon generally apply to other susceptible metals.First AppearanceIn the late 1940s 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. Weight reduction and more power propelling airframes that could withstand higher loading were critical to these improvements. This resulted in demands for higher strength alloys from which smaller, lighter and stronger 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 to tensile strengths approaching 200,000 psi were applied to 4130 and other anemic low alloy steels. Some of the first hydrogen embrittlement failures appeared, though they werent initially recognized as such.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 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 new potent source of hydrogen now available from the plating baths used in these processes, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and in other industries to which the new materials technology had filtered down.

Hydrogen Embrittlement Part 2Posted on 06. Dec, 2010 by Rob in Failure AnalysisHydrogen Embrittlement Part 2High Strength Steels Achilles Heel The Metallurgical PhenomenonHydrogen atoms are the smallest of any element. So small, that they easily travel between iron atoms. The boundaries between crystals, or grains, which are the structure of metals, are gapping canyons in relative size to hydrogen atoms. Once absorbed, 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 stress that cause a very slight increase in the space between grains from the opposing pull of the stress.As more hydrogen atoms accumulate at these areas, they combine to form relatively very large hydrogen molecules (H2) which raises internal pressure, expands the size of the defect or grain boundary interface and attracts still more hydrogen atoms, 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 associated with residual stress.Since the majority of the hydrogen is absorbed through and accumulates at the grain boundaries, hydrogen embrittlement cracking is primarily intergrannular (fracture at the grain boundary) rather than transgrannular (fracture through the grains) as in some other forms of brittle cracking.Hydrogen SourcesOne of the challenges in predicting and preventing hydrogen embrittlement is the wide range of available sources of hydrogen, both in the manufacturing and service environment.Sources from manufacturing include the original steel making process, subsequent casting or forging, grinding operations, soldering and brazing fluxes, blasting and tumbling media, welding electrodes, acid cleaning or pickling and electro-plating, etc.Service related sources of hydrogen may include incidental contact with acids or hydrogen containing cleaning solutions, or absorption from hydrogen containing product by equipment used in its processing. The most common source in service by far, however, is corrosion. Corrosion can also act as a source of hydrogen in the manufacturing process. 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.Hydrogen Embrittlement Part 3Posted on 13. Dec, 2010 by Rob in Failure AnalysisHydrogen Embrittlement Part 3High Strength Steels Achilles Heel Susceptible MaterialsWhile some stainless steel grades are susceptible, high strength steels with tensile strengths and hardness above 130,000 psi and Rockwell C35, respectively, are the most prone to hydrogen embrittlement. Steels below these tensile and hardness levels are generally immune. Why?Increasing hardness, most commonly by heat treating, is accompanied by a corresponding decrease in ductility. In simple terms, ductility is the ability 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 threshold, the steel cracks under the pressure 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 pressure 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 heat treated strength, quadruples the steels susceptibility to hydrogen embrittlement.Identifying and sorting embrittled parts from good components before they fail is virtually impossible. The detection limit of chemical analysis for hydrogen 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 isolated locations within the part. This leaves the majority of the part at low or undetectable hydrogen levels. Chemical analysis of parts after failure, to determine if hydrogen embrittlement is the cause, is also not viable since 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 over a similar time period.PreventionThe two keys to avoiding hydrogen embrittlement are at the design stage and during the manufacturing process. Designers with little metallurgical training may not realize the implications of the materials and manufacturing processes they call for in the drawing specifications. As with other failure modalities, hydrogen absorption can inadvertently be designed into a part.On the manufacturing side, avoiding reducing acids where possible removes an abundant source of hydrogen from potential exposure to the part. Where electro- plating is required, minimizing plating time and maximizing current density reduce the volume of absorbed hydrogen. Consideration of electroless plating or vapor deposition as alternatives eliminates the possibility for hydrogen absorption from the coating process altogether.Protecting furnace charges or components that will be welded from corrosion, or cleaning them prior to use will avoid hydrogen introduction by these routes. This applies equally to parts that will be heat treated. Any processing that will elevate the temperature of the part, and most do, will also raise hydrogen to a higher level of mobility, increasing the potential for absorption.Hydrogen ManagementDespite the most stringent precautions, processing requirements will sometimes introduce hydrogen to parts that exceed the hydrogen embrittlement tensile and hardness threshold. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This oven heating process, referred to as baking, is performed as follows:1. Parts must be baked within 4 hours of hydrogen exposure. Less is better. 2. Parts must be baked at 400 F. 3. Parts must be held at 400 F for a minimum of 4 hours. Longer may be required depending on part size, processing, etc. To be effective, the time and temperature parameters must be strictly followed. Short-cuts or delays on any step will dramatically reduce the effectiveness of the entire process. For example, twice as much hydrogen will be baked out at 400 F versus 350F, 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 after each hydrogen exposure if 4 hours will elapse between multiple exposures. 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. ClosingThere are competing theories on the mechanism by which hydrogen embrittlement occurs. That presented here appears to be the most widely accepted by the scientific community.The subject of this article has been limited to hydrogen embrittlement in low alloy steels. Other alloys are subject to hydrogen embrittlement, thought the mechanisms discussed are the same. However, other hydrogen driven failure mechanisms have been observed, though they appear to be less common. These include hydrogen induced blistering, internal hydrogen precipitation and hydride formation in nonferrous metals.

Microstructure Effects on Hydrogen Embrittlement in Austenitic Steels: A Multidisciplinary InvestigationS. Evers,1 T. Hickel,2 M. Koyama,3 R. Nazarov,2 M. Rohwerder,1 J. Neugebauer,2 D. Raabe,3 M. Stratmann 1 1 Department of Interface Chemistry and Surface Science 2 Department of Computational Materials Design 3 Department of Microstructure Physics and Alloy DesignHydrogen atoms, which can be absorbed into steel during production and service, often have a detrimental embrittling effect on the mechanical properties of iron and steels. It is meanwhile known that hydrogen embrittlement (HE) is also affected by the microstructure of the material. Consequently, previous indications that hydrogen atoms are trapped by vacancies, dislocations, and grain boundaries led at MPIE to investigations of superabundant vacancy formation, hydrogen-enhanced local plasticity (HELP), and hydrogen-enhanced decohesion (HEDE). Despite these efforts, any proof of a HE mechanism to be active in a given steel sample has so far been a formidable task, which cannot be achieved by a single method. A direct experimental observation of hydrogen impurities is difficult due to the low solubility and high mobility of hydrogen in steels, whereas pure theoretical investigations are challenged by the complexity and diversity of microstructural features present in steels.We therefore follow a multidisciplinary strategy to derive a deeper understanding of HE in steels.This strategy combines novel potentiometric methods based on the Kelvin probe technique to detect the local hydrogen content in materials (GO department), ab initio determination of the same quantities including the local behaviour at grain boundaries (CM department), and characterization of hydrogen induced materials failure (MA department) using orientation-optimized electron channelling contrast imaging (ECCI). Selected findings of these investigations and their relevance for austenitic steels are summarized in the following:The crucial idea for the new hydrogen detection method is the observation that hydrogen dissolved in a palladium matrix leads to the formation of a hydrogen electrode on the palladium surface, even in dry atmospheres. The origin is the presence of a nanoscopic water layer adsorbed on the surface, enabling the formation of a corresponding electrochemical double layer [1, 2]. As the electrode potential for the hydrogen electrode depends logarithmically on the activity of H in Pd, this potentiometric method is extremely sensitive especially at low activities.The idea can be employed for the investigation of steels (and various other materials) by evaporating a thin film of Pd on their surface. Since the chemical potential of H in iron-based materials is much higher than in Pd, H diffuses into the Pd film. Time dependent measurements of this accumulation can be used to perform extremely sensitive and laterally resolved measurements of H permeation through and its presence in materials. In the latter case an effective activity of H is measured, providing information about depth and density of traps sites. Main challenges of this method are the need for an exact calibration of the potential-concentration correlation for H in the evaporated Pd films, the precise calibration of the Kelvin probe tip in the dry nitrogen measurement atmosphere, as well as its long term stability.As an example the measurement of H in a H-charged austenitic steel sample, comprised of mainly austenitic and ferritic grains, is shown in Fig. 1. It can be seen that the austenite contains much more H, as the potential decreases much faster over the austenite grains. Especially active sites are located at boundaries between ferrite and austenite.

Fig.1: Detecting hydrogen on a 5050 m surface area of a H charged austenitic steel after evaporation of a 100 nm Pd film. a) A topographic image obtained by AFM indicates austenitic (due to surface preparation topographically higher) and ferritic (lower, i.e., darker) regions. b) and c) Potential maps of this area obtained after 28 h and 44 h in the Kelvin probe mode. Above the austenites the potential decreases faster than above the ferrites due to the larger amount of stored hydrogen. The dark spots in b)-c) indicate sites with especially high hydrogen concentrations (traps).An additional insight into the relevance of the different phases and their boundaries has been obtained by ab initio calculations based on density functional theory (DFT). They clearly confirm the increased solubility of H in austenite grains as compared to the ferrite grains. Mn yields further increase of the austenite solubility by straining the lattice (volume effect). One of the new insights obtained by the calculations is that small amounts of further alloying elements (like Ca, Nb, Si, Ti, and in particular Mo) considerably enhance the preference of H for austenite [3].

Fig. 2: Potential energy surface for a single H interstitial next to a 11[1-10](113) grain boundary in fcc iron. The minimum energy path for a diffusion in a (1-10) / (113) plane perpendicular / parallel to the boundary is indicated by white dots. In the upper part the corresponding energies along the same path are plotted.In order to understand the experimental results on microstructures, we have additionally used DFT to study the solubility and diffusion of hydrogen in austenite twin and grain boundaries [4]. We generally find that the solution energy of H strongly depends on the local coordination and that it is in this case only moderately correlated with the actual volume of the interstitial site. Within open structures, such as the 11[1-10](113) fcc grain boundary, various different interstitial sites are favorable for the incorporation of H atoms, providing effective trapping centres (Fig. 2). Only if these traps are filled by other H atoms, efficient diffusion channels along (113) planes might become active. We further find that the critical strain required to fracture the material is reduced by the presence of hydrogen in this grain boundary. For twin boundaries, the DFT calculations show that interstitial H atoms are actually slightly repelled. As origin for this unusual and unexpected behaviour the structural similarity between the octahedral interstial configurations in the twins and in austenitic bulk has been identified.These theoretical insights are highly relevant for experiments, which investigate the fracture mode in austenitic steels. For this purpose the recently developed orientation-optimized ECCI method has proven to be particularly useful to reveal deformation twins and complex dislocation substructures in TWIP steel. The actual measurements have been performed for a H charged Fe18Mn1.2C austenitic steel [5], for which the tensile ductility was drastically reduced by H charging during tensile testing. The central region of these samples, which have not been reached by hydrogen, showed a ductile fracture surface. In contrast, a brittle fracture surface was observed from the surface down to about 150 m. The facet size of the brittle fracture areas is about 50 m, which corresponds to the grain size, indicating that intergranular fracture was caused by H charging.

Fig. 3: ECCI micrograph for the crack propagation in Fe18Mn1.2C austenitic steel. The cracks initiate at a grain boundary, where deformation twins are intercepting. The crack propagation afterwards continues along the deformation twins.An advantage of the employed ECCI method is that in addition to the cracks primary and secondary deformation twins on (11-1) and on (1-11) planes become visible with bright contrast (Fig. 3). The measurements therefore revealed that cracks typically occur at grain boundaries with intercepting primary deformation twins. The stress concentration at these points and the reduction of the cohesive energy by hydrogen loading apparently yields crack initiation. While the primary fracture mode is intergranular, one additionally observes crack propagation following primary and secondary deformation twin boundaries (Fig. 3). Since the ab initio calculations predict that perfect twin boundaries are not sensitive to H, the stresses due to the interception of twins with grain boundaries or of primary with secondary twins need to be responsible for such a transgranular fracture along twin boundaries. Being crucially important, because deformation twinning is essentially required to achieve the superior mechanical properties of TWIP steels, further investigations of this effect are currently performed.[1] Senz, C.; Evers, S.; Stratmann, M.; Rohwerder, M., Scanning Kelvin Probe as a Highly Sensitive Tool for Detecting Hydrogen Permeation with high local Resolution, Electrochemistry Communications 13 (2011), 1542-1545, DOI: 10.1016/j.elecom.2011.10.014http://edoc.mpg.de/577499 [2] Evers, S.; Rohwerder, M., The hydrogen electrode in the dry: A Kelvin probe approach to measuring hydrogen in metals, Electrochemistry Communications 24 (2012), 85-88, DOI:10.1016/j.elecom.2012.08.019 http://edoc.mpg.de/624681 [3] Nazarov, R.; Hickel, T.; Neugebauer, J.: in preparation [4] Du, Y. J. A.; Ismer, L.; Rogal, J.; Hickel, T.; Neugebauer, J.; Drautz, R., First-principles study on the interaction of H interstitials with grain boundaries in alpha- and gamma-Fe, Phys. Rev. B 84 (2011), 144121, DOI: 10.1103/PhysRevB.84.144121http://edoc.mpg.de/581728 [5] Koyama, M.; Akiyama, E.; Sawaguchi, T.; Raabe, D.; Tsuzaki, K.: Hydrogen-induced cracking at grain and twin boundaries in an FeMnC austenitic steel, Scripta Materialia 66 (2012), 459-462, DOI: 10.1016/j.scriptamat.2011.12.015