[quote=craigd 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. [/quote]

Craig
your understanding is incorrect. Steel has a Young's modulus that is unchanged by heat treat.