I would have liked to seen that one with a magnifier. The shear line goes through the area the case hardening would not reach. The areas near the surface do not seem as fine grained as I would expect, most of the sheared surface seems coarse which to me means not so hard. First guess is a serious overload is the cause.
if I attached the photo correctly it is a tractor axle, you can see the fine grain near the surface, I do not know if the axle was case hardened or selectively hardened, but the relative hardness is indicated by the grain size.

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