The results acquired during surface grinding experiments performed on hardened steel 16MnCr5(AISI-5115) workpieces were discussed. The experimental results show material modifications induced by grinding heat on different zones during grinding process depend on the contact time Δt and the maximum contact zone temperature Tmax. In particular, it is observed that rehardened zones occur within two separated ranges of temperatures and contact times.Further analyses reveal that these separated time and temperature ranges correspond to significantly different mechanical loads as well as to varying retained austenite rates.