Hardness is resistance of material to permanent indentation,however,used extensively to characterize material and to determine if they are suitable for their intended use.degree to which a metal will resist cutting, abrasion, penetration, bending and stretching. in this way, it indicated hardness of metals will differ somewhat with the specific apparatus measuring hardness.and for springs,so that it is important to recognize that  the end users use different facilities to make a bending of spring request the hardness of material is Difference ,some need 43-45HRC,some need 44-46HRC ; some need 46-48HRC; some need 47-50HRC.for example