Microstructure and strain hardening behavior of the transformable 316L stainless steel processed by cryogenic pre-deformation

Abstract

The dynamic formation of α′-martensite during mechanical loading is essential in achieving the desired mechanical properties of the metastable austenitic stainless steels (SS). However, the effect of α′-martensitic transformation on the mechanical behavior of 316L SS is less explored due to the over stability of austenite at room temperature (RT). Here, a thermomechanical processing method of cryogenic pre-deformation is applied to tailor the deformation-induced martensitic transformation in 316L SS and the subsequent deformation behavior during mechanical testing at RT. Detailed characterizations reveal that the α′-martensite nucleated at the intersection of shear bands by cryogenic pre-deformation can continue to grow along the shear bands at RT. The cryogenically-rolled (CryoRolled) 316L SS exhibits an excellent combination of strength and ductility in comparison with the conventional cold-rolled counterparts, due to the proper activation of α′-martensitic transformation. The CryoRolled-12% sample presents a true tensile strength of 1143 MPa and a true uniform elongation of 0.17; while similar level of true tensile strength (1135 MPa) is obtained at the expense of low uniform elongation (0.024) for the RT-rolled-50% sample. A mean-field micromechanical model is applied to analyze the influence of the dynamic formation of the strengthening α′-martensite on the strain hardening behavior.