Enhanced strength-plasticity synergy of 304 stainless steel by introducing gradient nanograined single austenite phase structure via USRP and induction annealing

Abstract

The drawback of low strength of 304 stainless steel could be overcome by fabricating gradient nanostructures (GNS). However, deformation-induced martensite results in magnetic generation and plasticity degradation. In this work, a single austenitic GNS 304 stainless steel is fabricated by first creating a dual-phase GNS through the ultrasonic surface rolling process (USRP), followed by rapid induction heating. The yield strength of the single austenite GNS (520 MPa) is 1.68 times higher than that of the homogeneous coarse-grained structure (310 MP) without sacrificing plasticity (elongation of 65 %). Quantitative calculations indicate that fine grain, dislocation, twinning, and back-stress strengthening contribute to the strength increment by 30 %, 17.5 %, 23.4 %, and 29.1 %, respectively. Coarse-grained regions deform mainly through FCC-HCP-BCC martensitic transformation, whereas the subsurface layer forms stacking faults and twins due to increased stacking fault energy caused by the reduction in grain size. At the topmost layer, the stress required to activate dislocations is lower than that for twinning. Under high-stress conditions, martensite forms along the nanograin boundaries via a phase transition from FCC to BCC. Consequently, the excellent plasticity of the single austenite GNS stems from the synergistic effects of high back-stress hardening, TRIP and TWIP effect.