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Failed Fasteners

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kek78

Materials
Jun 19, 2007
6
I have several failed fasteners. After metallurgical evaluation the failure mode is classical grain boundary embrittlement (rock candy fracture surface, cracks along prior austenite grain boundaries in micro). Hydrogen embrittlement, right? However, these fasteners were not plated (cad, zinc, etc.), they only have a black oxidized coating. I would not think this type of coating process would be a source for hydrogen pick-up. Could this be temper embrittlement? The hardness on these fasteners are Rc 44 and they were in service around 4-6 months. The material appears to be clean from the micro (no massive inclusions). Any suggestions? Thanks
 
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The hardness is on the high end of SAE J1199 Class 12.9 (38-44 HRC), which is extremely sensitive to embrittlement. Such fasteners should include a post-coating treatment as kenvlach suggests.
 
To put it quite simply, there are 2 possibles, in which the details are pretty much irrelevant and probably costly to determine:
1) The loads applied to the rivets is to great (doubtfull due to your posts confirmation of the failure mechanism)
2) The rivets have failed to live up to it designated criteria.

As your left with problem (2), your solution is to change suppliers of rivets as is obvious that any failure in this way is down to a process problem rather than a loading/service problem. You could spend a lot of time/money trying to come to the actual mode of failure and the reasons behind it, or you could just envelope it within a "processing problem" and move on with better fasteners, (whilst getting money back for previous supplier)
 
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