Metallurgical Minutes Fall 2010 Materials Testing and Engineering • Failure Analysis • Manufacturing Problem Solving • Litigation Support THE PHENOMENON Sudden 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 APPEARANCE 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. 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. HYDROGEN EMBRITTLEMENT High Strength Steel’s Achilles Heel continued on next page In the mid 1950’s the F-8 Crusader became the first aircraft to exceed 1000mph. A failure in the wing structure became the first recognized hydrogen embrittlement failure. HYDROGEN EMBRITTLEMENT… FAILURE ANALYSIS Rapid Failure Analysis of suspected cases of hydrogen embrittlement is critical. If the failed component was manufactured as part of a batch or lot, it is probable that other parts in the lot have failed, or are about to. An immediate identification of embrittled parts can avoid additional costly failures. Practical cost-effective recommendations that prevent hydrogen embrittlement of your product in the future is essential to the value of a professionally performed failure analysis. Effective Failure Analysis requires experience and knowledge of hydrogen embrittlment. Analysis of tensile strength, hardness, and applied or residual stresses determine if hydrogen embrittlement continued on next page
Transcript
Metallurgical Minutes
Fall 2010
Materials Testing and Engineering • Failure Analysis • Manufacturing Problem Solving • Litigation Support
THE 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 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. 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.
In the mid 1950’s the F-8 Crusader became the first aircraft to exceed 1000mph.
A failure in the wing structure became the first recognized hydrogen
embrittlement failure.
HYDROGEN EMBRITTLEMENT…
FAILURE ANALYSISRapid Failure Analysis of suspected cases of hydrogen embrittlement is critical. If the failed component was manufactured as part of a batch or lot, it is probable that other parts in the lot have failed, or are about to. An immediate identification of embrittled parts can avoid additional costly failures. Practical cost-effective recommendations that prevent hydrogen embrittlement of your product in the future is essential to the value of a professionally performed failure analysis.
Effective Failure Analysis requires experience and knowledge of hydrogen embrittlment. Analysis of tensile strength, hardness, and applied or residual stresses determine if hydrogen embrittlement
continued on next page
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 weren’t 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.
THE METALLURGICAL PHENOMENON Hydrogen 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% or more.
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 C30.
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.
continued on page 4
thresholds were exceeded. Evaluation of the fracture by an experienced analyst using a Scanning Electron Microscope (SEM) provides critical evidence in mak-ing a hydrogen embrittlement diagnosis.
Hydrogen Embrittlement fractures are intergrannular (separation at the grain boundary or interface) but not all intergrannular fractures are caused by hydrogen embrittlement. The SEM image at the upper left of page 1 shows a quench crack in a high strength steel. The image directly below it shows classic hydrogen embrittlement features.
HYDROGEN EMBRITTLEMENT…
TESTING Like the phenomenon itself, testing for hydrogen embrittlement requires time and tensile stress, or load.
Plated high strength fasteners represent one of the most frequent hydrogen embrittlement failure component types. MAI provides static load testing of fasteners and other threaded components to determine if they have been embrittled before they are placed in service.
Complex component shapes require extensive dedicated fixturing in order to apply the required tensile load in testing. Contact MAI to discuss to discuss these requirements and test protocols.
MAI’s analysis of metallic wear particles in a processed food product identified the component in the processing equipment that they came from. The premature wear of this component was eliminated as a result of our recommended change to a different alloy. This change prevented costly unscheduled plant shut downs for its replacement and eliminated contamination of the product.
Service Life Assessment Analysis of pipes at an electric power generating utility identified their remaining safe operating life. The results of this analysis gave plant engineers the information needed to schedule replacement during a general maintenance shut down, avoiding an outage due to an unanticipated failure of the piping.
Failure analysis of welds in membrane filter housings from a pharmaceutical plant. Our evaluation and recommenda-tion for changes in the welding parameters prevented additional failures and allowed the contractor to complete construction of the plant on schedule.
MAI’s evaluation of a high hardness wear resistant overlay, developed for application to the journal of diesel engine crankshafts, confirmed that the coating met the demanding specification required for rigorous service requirements.
Our Failure Analysis of cracked track shoes from heavy earth moving equipment identified hydrogen embrittlement as the failure mechanism. Evaluation of the manufacturing process identified poor control of the heat treating process as the source of the hydrogen and the root cause of the embrittlement and subsequent failures.
MAI welder and weld procedure certification on challenging multi-pass welds quickly got a manufacturer of heavy construction equipment into production on their new product line.
Our analysis of a fractured suspension com-ponent from a semi-tractor determined that the failure resulted from operator abuse rather than from any material or design defect. As a result our client was relieved of financial liability for damages resulting from the failure and related accident.
Manufacturing Process Problem Solving by MAI identified the cause of braze join failures in testing of hydraulic fittings for a military vehicle. Following our recommended changes to the brazing process, the fittings easily passed subsequent testing.
MAI Non-Destructive Testing engineers performed on-site ultrasonic thickness testing of storage tank plates to determine if the tank could be repaired despite extensive corrosion. Our inspection revealed that the tank was not repairable and, in fact, was at risk of imminent failure, representing a major potential liability to our client.
MAI performs expert failure analysis on polymer components as well as metals. Numerous failures of glass reinforced polyphenylene water supply system components were submitted to determine the root cause of failure. Our analysis determined that improper molding procedures resulted in parallel alignment of the reinforcing glass fibers, rather than the specified random fiber orientation. This defect resulted in greatly reduced strength and cracking. Minor changes to the molding process resolved the problem.
IN THE WORKS…Thanks to our clients in a wide range of industries we have the opportunity to provide practical cost affective solutions that optimize their products, manufacturing processes and profitability. The variety of engineered products, materials, and testing challenges we address is almost unlimited. Examples of recently completed analyses include:
Using micro-chemical analysis capabilities provided by our Energy Dispersive Spectrom-eters (EDS), MAI identified the composition of contamination on field returned electronic controller modules. EDS analysis of materi-als at the service site quickly identified the source.
Dezincification is an aggressive form of cor-rosion in which zinc is preferentially attacked and depleted in brass alloys, compromising their strength and performance. MAI dezinc-ification analysis for a supplier of brass ma-rine components assures continued durabil-ity and demand for his product by customers.
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.
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 steel’s 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
Failure Analysis of heat exchanger tubes from an industrial HVAC system identified tube welding defects as the root cause of cracking. MAI engineers developed a revised weld-ing procedure that eliminated cracking and provided excellent performance in service.
Our failure analysis of a municipal water pump identified improper assembly as the root cause of the fatigue failure of the main pump shaft. Insufficient tightening torque of fasteners in the pump housing resulted in bearing movement which then led to the failure of the shaft, a chain of events that might not be immediately apparent to an inexperienced analyst.
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 400º F.
3. Parts must be held at 400º 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
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 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 pa-rameters 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.
If you have questions on hydrogen embrittlement and your product or manufacturing processes, talk with an MAI engineer at 800 798-4966 or 262 798-8098, or email [email protected].
HYDROGEN EMBRITTLEMENT CASES
With the widespread use of high strength steel, hydrogen embrittlement failures are endemic, affecting products ranging in complexity from hand tools to the space shuttle. But due to the technical nature of hydrogen embrittlement, even high profile failures are rarely reported in the media, with occasional exceptions:
In the early 1970’s General Motors responded to rapidly rising gas prices with a drastic weight reduction program to increase fuel economy. Too drastic. GM replaced carbon steel suspension bolts with smaller, lighter, high strength bolts. Hundreds of hydrogen embrittlement failures resulted and a massive recall followed.
In August, 1968 a commercial air taxi helicopter disintegrated in flight over Los Angeles when a primary rotor blade spindle failed. Accident investigation found that improper shot peening prior to plating on a repair resulted in hydrogen embrittlement of the spindle.
In 1985, solenoid valve springs failed at the Fort Calhoun Station nuclear power plant in Nebraska. Failure resulted from hydrogen embrittlement due to exposure to high temperature coolant that contained hydrogen.
Metallurgical Associates Inc. is an independent materials testing and engineering facility accredited by the American Association for Laboratory Accreditation (A2LA-ISO/IEC 17025). Our expertise includes failure analysis, process problem solving and process/material certification and selection. For a quote or discussion of your analytical requirements, contact Tom Tefelske ([email protected]), Dan Kiedrowski ([email protected]) or Rob Hutchinson ([email protected]), or call (262) 798-8098 or Toll Free (800) 798-4966. Visit MAI’s web site at www.metassoc.com.
