Some people think they can tell if a part broke due to hydrogen embrittlement by looking at it. This is not correct. The timing of the crack is only one of the key factors that needs to be documented if hydrogen embrittlement is suspected. Four more are listed in the sidebar.
While it is possible that hydrogen embrittlement can be associated with immediate fracture on install, it would be surprising if there were no additional factors that facilitated such a rapid hydrogen-assisted damage event. Perhaps a quench crack, or a forming or casting discontinuity, concentrated the stresses beyond the design plan. Suspicion is warranted when reviewing reports that diagnose hydrogen without reporting the timing of the crack event(s).
All five of the listed parameters must be documented for a credible diagnosis of hydrogen embrittlement in hardened steel components. Note that there are other sources of hydrogen aside from pickling and plating. It is also important to realize that intergranular cracks may look very different in components that have experienced heavy cold work without additional high-temperature exposure.
Fig. 1. Scanning electron micrograph, originally captured at 150x, showing typical intergranular fracture for steel hardened by heat treating and then embrittled with hydrogen.
Fig. 2. Optical micrograph showing uniform tempered martensite. No visible separate grain-boundary phases/constituents to indicate that the material has a heat-treating problem instead of classical hydrogen embrittlement.
Historically, hydrogen embrittlement was thought to be confined to parts over Rockwell C 39. In my experience, small electroplated parts over Rockwell C 43 seem to populate the ranks of hydrogen-embrittlement-related cracks at a significantly higher rate than those between 39 and 42. This is a problematic statement since the Rockwell C test method accuracy is generally considered plus or minus 2. This is my experience, so I am reporting it.
Here is the reason that I decided it would be useful to review the issue of hydrogen embrittlement. It has come to my attention that some companies are allowing electroplaters of hardened steel to wait up to four hours, or even more, before doing a hydrogen-embrittlement prevention bake. This is a mistake. In “the old days,” hydrogen embrittlement was such a problem that the federal government was considering prison time for technicians who made mistakes when testing Grade 8 fasteners used for military applications! (My technician at the independent lab at the time made me promise to bring him clam chowder, not cream of tomato, should that have been his lot.)
The prison threat was a result of numerous hydrogen-embrittlement problems in military hardware and nuclear reactors. Adding insult to injury, SAE J429 Grade 8.8 bolts were found to have been incorrecty marked as Grade 8 bolts. Grade 8.8 do not hold up under elevated temperature conditions as well as Grade 8, which are required to be tempered at a higher temperature, and Grade 8.8 bolts may have higher residual stresses even before being placed into service. This sets the stage for hydrogen cracks to nucleate immediately after electroplating.
An additional consequence of the “counterfeit bolt scandal” was that most of the people in the industry learned that the bake had to be within one hour of the parts exiting the plating bath. Industry convinced Congress they would self-regulate. Hydrogen-embrittlement problems seemed to be few and far be-tween for some years after that. Now they seem to be more frequent. At least I hear people claiming to have hydrogen-embrittlement problems more frequently. They may not have studied the 5-step list in the sidebar.
Even though some companies have allowed the bake cycle to be delayed beyond one hour after plat-ing, it is highly preferable to bake the parts as soon as possible and as close to right after exiting the plating bath as possible. One hour is preferred as the maximum elapsed time before starting the bake cycle.
This is especially important for critical parts that have safety implications or for small parts that are going into expensive assemblies. Consider the cost to get the parts into the bake oven in a timely fashion compared to the cost to fix some indeterminate number of broken parts deep in a complex assembly. Job-shop-type manufacturers of small parts would do well to consider this when quoting small hardened and plated parts.
Minimizing the post-electroplating delay before baking is also critically important for heat-treated parts whose specified hardness is over Rockwell C 40 and for heavily cold-worked parts whatever their hardness value.
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