OK, lets do a wee bit of metalurgy. Anyone with differing information is most welcome to step right into this. I'm not nterested in being right, rather that we all know the truth.
...if I made two identical springs from "soft" and "hard" steel, they would behave almost identically as springs until the "soft" one was deflected beyond its yield point. The "soft" one would now be permanently bent (dimensions altered) while the hard one would return to original dimensions....
Just for conversation, if identical in every way except for hardness, I believe that what ever point you compressed the soft spring to make it yield, that will also be the same point that the hardened spring yields. The hardened spring may require more force to get to that same deflection point, but that amount of deflection to permanent deformation will be the same for both springs. Probably, identical leaf springs would be a better example because a coil may compress to solid without yielding.
I believe receivers have been shown to spring when a gun is fired, but it seems to be related to design and the nature of the iron/very mild steel and not heat treatment to spring characteristics. We may be thinking along similar lines.
