Effect of solution annealing on low cycle fatigue of 304L stainless steel

Abstract

The present study investigates low cycle fatigue behaviour of the metastable austenitic 304L steel in two variants: i) as-received and ii) after additional solution annealing at 1050 °C/30 min. The applied heat treatment resulted in a decrease of δ-ferrite volume fraction and slight grain coarsening. After the solution annealing, significantly prolonged fatigue life within the whole range of studied total strain amplitudes was observed. Such improvement is attributed to the larger grain size possessing better ductility and enhanced formation of deformation-induced martensite. The phase transformation led to a significant secondary cyclic hardening stage at high strain amplitude loading. The low cycle fatigue behaviour and principal fatigue life stages were characterized by the analysis of cyclic response. A vital supplementary data were obtained by the analysis of hysteresis loop shape carried out, for the first time on a phase-transforming material, by the generalized statistical theory. The analysis was able to follow the evolution of cyclic stress sources during the fatigue life, especially the increasing contribution of deformation-induced martensite. The identified features of cyclic plasticity were supported by electron channeling contrast imaging observations. Furthermore, electron backscatter diffraction revealed a distinct preferential localization of α'-martensite into macrobands parallel to rolling direction. Energy dispersive spectroscopy mapping identified pronounced chemical segregation of Cr and Ni which correlated well with the observed macrobands. These compositional fluctuations led to local austenite metastability which is enhanced further by the applied solution annealing.