craig: our understanding of the definition of case hardening is essentially the same, with color being a by product of the process...

it is my understanding, that if the metal is not tempered back and left brittle like glass, small parts, like tangs and levers may crack and break...and if surface hardened only, larger parts like frames and trigger plates, may develop spider web type cracks, under the stress of firing, if they are not tempered...

it seems plausible that if a large part, like a frame or receiver breaks, it is because the part was hardened all of the way thru and not tempered enough?

keep it simple and keep it safe...