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He may be correct, however the number of cycles that the bolts see is usually sub 100,000 so it is hard to say. - it depends on the stresses
C158 is some super hard stuff in the case but softer in the inside, if it does crack the case, the crack will stop when it gets through it.
9310 will likely be hard thru and thru giving the cracks a place to continue propogating.
This is however considering LOTS of cycles.
Its hard to say exactly which one will last longer without getting an engineer to do some serious math and testing.
Many loaded objects fail by fatigue and are way under-loaded, I've seen some examples of rotating shafts with light loads that eventually fail, they crack almost all the way through and no one notices until finally they snap.
I'd love to see some high resolution photos of bolts that have broken to see what the failure mode was.
Jon
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