High temperature thermal stability of nanocrystalline 316L stainless steel processed by high-pressure torsion

Differential scanning calorimetry (DSC) was used to study the thermal stability of the microstructure and the phase composition in nanocrystalline 316L stainless steel processed by high-pressure torsion (HPT) for ¼ and 10 turns. The DSC thermograms showed two characteristic peaks which were investigated by examining the dislocation densities, grain sizes and phase compositions after annealing at different temperatures. The first DSC peak was exothermic and was related to recovery of the dislocation structure without changing the phase composition and grain size. The activation energies for recovery after processing by ¼ and 10 turns were ~163 and ~106 kJ/mol., respectively, suggesting control by diffusion along grain boundaries and dislocations. The second DSC peak was endothermic and was caused by a reverse transformation of α'-martensite to γ-austenite. The hardness of annealed samples was determined primarily by the grain size and followed the Hall-Petch relationship. Nanocrystalline 316L steel processed by HPT exhibited good thermal stability with a grain size of ~200 nm after annealing at 1000 K and a very high hardness of ~4900 MPa.