Martensitic transformation in rolled 301 stainless steel induced by cryogenic treatment

Abstract

The martensitic transformation in rolled 301 stainless steel induced by cryogenic temperature was investigated in the present work. The methods of magnetization, thermoelectric potential combined with microscopes were adopted for revealing the microstructure evolution from the global and local scale. Hardness and tensile tests were performed at room temperature to evaluate the variation of properties. The results showed that deep cryogenic treatment improved the tensile strength of 301 steel rolled with the reduction of 10%, 20% and 30% by 93.7 MPa, 71 MPa and 125.5 MPa, and the yield strength by 131 MPa, 65.2 MPa and 208.9 MPa, respectively. The improvement in tensile strength caused by deep cryogenic treatment are equivalent to that induced by the higher rolling reduction rate, which could achieve higher strength-plasticity balance of 301 ASS at relatively low deformation. Deep cryogenic treatment promoted the formation of martensitic in the rolled microstructure, which was considered to be the main reason for the strengthening. The volume fraction of martensite in 301 ASS rolled with the reduction rate of 20% and 30% was increased by 44.5% and 28.2% after cryogenic treatment, respectively. The martensitic transformation caused by deep cryogenic treatment corresponded to the principle of conventional strain-induced martensitic transformation, which nucleated in the intersections of shear bands and formed from the transition ε-martensite. Under cryogenic temperature, the decrease of stacking fault energy (SFE) and increase of lattice internal stress reduced the critical stress of shear bands formation, as a result of increasing the amount of shear bands and nucleation sites for martensitic transformation.