Cryogenic work-hardening behavior for a metastable austenitic stainless steel at liquid nitrogen temperature

Abstract

An experimental study was conducted to analyze the undulated work-hardening behavior of S30408 metastable austenitic stainless steel at liquid nitrogen (LN2) temperature. The flow stress curve, work-hardening process, strength, and ductility were studied by uniaxial tensile tests at both room temperature (RT) and −196 °C. Based on the scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) observations, the effects of the cryogenic temperature and deformation degree on the dislocation evolution, phase transformation, slip behavior, and hardening behavior were analyzed. The cryogenic work-hardening mechanism regarding the initial hardening, undulated hardening, and unstable hardening throughout the whole tensile process was elaborated. The correlation between macroscopic work-hardening and microscopic martensitic transformation accompanied by a localized dislocation strengthening was illuminated. It was revealed that the high strength of the austenitic steel during the cryogenic deformation process resulted from the α′-martensite transformation accompanied by dislocation strengthening. A critical point of phase content was obtained at a strain of 0.095 at −196 °C. The undulated work-hardening at cryogenic temperature was mainly attributed to the alternating evolution of the α′-martensite and austenite phases with a non-uniform deformation mode.