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Hydrogen embrittlement - when stronger isn’t always better
  • Nov 10, 2021
  • Latest Journal

by Mike Broadhurst

Man’s development through the ages has been largely governed by advances in materials science and more specifically metallurgy. Having a detailed understanding of how to win metals from ores, then alloy/improve/manipulate the mechanical and physical properties of them led to rapid advances in how man did, made or built things. Despite the development of new innovative materials such as graphene, the main stay of most engineering components and structures is still largely concentrated on iron (ferrous) based alloys. Even though research continues today to ever improve how to engineer the microstructure of steels to meet a particular demand, the majority of the detailed understanding came during a boom time for investment in ferrous metallurgy research during the 1940, 50’s 60’s and early 70’s tailing off with the demise of many steel companies’ large dedicated research groups, so there aren’t many major “unknowns”.

As with everything in life you learn by your mistakes. Investigating the mechanism and cause of failure when things go wrong has also had a major influence on alloy development and the understanding of the limitations of certain alloys in particular operating environments. The lessons learned are all well and good as long as these are widely communicated and passed on to future generations. This in itself is a problem with the way the workplace has changed over the years. The mobility of staff, few jobs for life and to some extent agism of employers means that there may not be the old experienced “sage” around to mentor younger engineers. This means that the same mistakes are often repeated down the generations.

The common drive to reduce the mass/cost of new structures and the misconception that “stronger is  better” has seen a move to high strength steels, in particular for fasteners (bolts), in many instances without adequate precautions being taken to mitigate hydrogen embrittlement (HE) failures.

The damaging effects of hydrogen on high strength steels (tensile strengths in excess of 1040MPa, 150ksi) has been a well-recognised problem by metallurgists and materials engineers for well over a 100 years. However, many mechanical/structural engineers aren’t aware of the problem and probably don’t know who to ask if there might be any issues using a particular steel in the application they are considering.

HE is a progressive cracking mechanism related to the absorption and diffusion of atomic hydrogen into the steel. Even a small quantity of hydrogen is sufficient (a few parts per million) to embrittle certain steels. Once absorbed the hydrogen collects at high energy boundaries in the microstructure, leading to the initiation of cracks. Once initiated hydrogen continues to diffuse to the high stress field ahead of the advancing crack tip causing the crack to grow until it reaches a critical size and fast overload failure of the remaining sound ligament occurs. The presence of hydrogen in high strength steels reduces the tensile ductility causing premature failure without warning under static load that depends on stress, time and temperature i.e. it is a diffusion controlled mechanism. The relevant static stress in the component is the combination of residual stresses (from machining, fabrication etc.) preload and applied stresses (external loadings and fitting). It is common in this mechanism for fasteners to be successfully installed only for the heads to be found detached at a later date, hence the mechanism is sometimes referred to as hydrogen delayed cracking.

The generally accepted consensus opinion is that steels with a tensile strength greater than 1034MPa (~320Hv) and a martensitic microstructure are susceptible to HE, with the susceptibility increasing with increasing strength/hardness. For example Grade 10.9 fasteners (tensile strength 1040MPa min) are susceptible to failure by HE with increasing susceptibility towards the top of the permitted hardness range  (320-380Hv) for this grade.

In some publications two different sources of  hydrogen, and subsequent embrittlement, are considered:
• Internal Hydrogen Embrittlement (IHE) is where the hydrogen is a residual of the steelmaking process, due to one or more of the fabrication processes, such as surface cleaning (including acid pickling/descaling), electroplating, and welding. This can normally be prevented by post fabrication treatment, such as baking, driving the hydrogen out of the metal by heating.

• Environmental Hydrogen Embrittlement (EHE) is where the hydrogen is formed by corrosion of the component in service more commonly referred to as hydrogen stress corrosion cracking (HSCC). Corrosion (i.e. the reaction of a metal/alloy with its environment) is one of the sources of hydrogen for the hydrogen embrittlement of fasteners. In acidic conditions (pH < 7), positively charged hydrogen ions are reduced to monatomic hydrogen at the metal surface. In neutral/alkaline conditions (pH > 7), water molecules are reduced, again forming monatomic hydrogen at the metal surface. In most cases a monatomic hydrogen atom at the metal surface combines with another monatomic hydrogen atom to form a hydrogen gas molecule, which is then dissolved into the water or bubbles harmlessly into the atmosphere. However, some of the monatomic hydrogen atoms can diffuse into the steel and cause HE of susceptible steels.

Irrespective of the source, the hydrogen has the same damaging effect on the steel.

HE is still a significant issue in many industries and Intertek has investigated many examples in the recent past.

• Parts of a number of high strength bolts/studs (3” diameter, 6 ft long) used in the construction of a high-rise building in a major commercial centre fell to the pavement without warning during the final completion stages. The studs were initially specified as Grade 10.9 (a grade with a strength range making it susceptible to HE) with a sherardized (zinc) coating.  These were installed via bolt boxes to secure large nodes of the building. The bolt boxes were not sealed and rainwater entry lead to corrosion at the bolt surface. Hydrogen generated by the corrosion reaction caused catastrophic fast fracture. Investigation by Intertek engineers identified the failure mechanism as environmental hydrogen embrittle (EHE) sometimes referred to as hydrogen stress corrosion cracking (HSCC). Apart from poor materials selection and a lack of adequate corrosion protection Intertek found that (despite the certification) the bolts supplied had not been zinc coated and were significantly out of specification (too hard) due to poor manufacturing control and heat treatment.

• Investigation into apparent brittle failure of high strength tower crane bolts. M45 (1¾”) Grade 10.9 alloy steel (826M40) zinc electroplated bolts suffered delayed hydrogen failure under load. The failures typically occurred between 2 and 4 weeks after the sub-assemblies or crane structures were erected. Investigations found no evidence of external corrosion but all of the fractures were consistent with hydrogen embrittlement. The bolts all had microstructures, chemical compositions and mechanical properties within the respective specifications. It was determined that in attempting to attain a consistent galvanised finish, the bolt manufacturers would sometimes acid strip and re-galvanise batches of bolts up to three times, with only one de-embrittling heat treatment at the lowest recommended temperature and for the shortest possible time.



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