Answer: The user had
attempted to save money by using rough stock whose diameter was barely
adequate to make the gauge. It had been decarburized to a normal
degree during hot rolling at the mill. The gauge was to be
hardened after rough machining, and then was to be ground to size after
tempering but before nitriding, which is OK. However, nitrides
formed at the ferrite grain boundaries, where diffusion is faster;
furthermore, diffusion is faster in the non close packed BCC ferrite
than it is in close packed FCC austenite. Therefore, a disuptive
volume increase occurred, spalling the threads. Some
Widmanstaetten precipitation of the nitrides is evident as well.
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