An insight into oversaturated deformation-induced sigma precipitation in Super304H austenitic stainless steel

A new mechanism of the deformation-promoted precipitation of the sigma phase during aging at 650 degrees C in nanocrystallized Super304H austenitic stainless steel was proposed for its extremely early nucleation and subsequently fast growth. Unlike the reports of sigma phase nucleation at the recrystallizing grain boundaries in other deformed steels, the sigma phase was found to nucleate before recrystallization during aging in the oversaturated shot-peened Super304H steel at stress concentrations with high distortion energy (nanograin boundaries, twinning intersections and interfaces of dispersion strengthening phases like Nb(C,N)). The high-distortion areas favored the segregation of chromium and consequently facilitated the structure transformation from face-centered cubic austenite to topologically close-packed sigma phase. Sigma phase particles were found to grow slowly in the early stage of aging due to the residual compressive stress field in the deformed austenite matrix, and began to grow abnormally quickly to be 1-2 mu m in size at the recrystallizing boundaries when recrystallization commences, due to both the release of residual compressive stress at the recrystallized region and the fast chromium diffusion at the deformed nanostructure region. To avoid the occurrence of sigma phases, the degree of surface deformation should be controlled to be lower than the deformation saturation value. (C) 2019 The Authors. Published by Elsevier Ltd.