Influence of Martensite Fraction on the Stabilization of Austenite in Austenitic-Martensitic Stainless Steels

The influence of martensite fraction (f(alpha')) on the stabilization of austenite was studied by quench interruption below M-s temperature of an Fe-13Cr-0.31C (mass pct) stainless steel. The interval between the quench interruption temperature and the secondary martensite start temperature, denoted as theta, was used to quantify the extent of austenite stabilization. In experiments with and without a reheating step subsequent to quench interruption, the variation of theta with f(alpha') showed a transition after transformation of almost half of the austenite. This trend was observed regardless of the solution annealing temperature which influenced the martensite start temperature. The transition in theta was ascribed to a change in the type of martensite nucleation sites from austenite grain and twin boundaries at low f(alpha') to the faults near austenite-martensite (A-M) boundaries at high f(alpha'). At low temperatures, the local carbon enrichment of such boundaries was responsible for the enhanced stabilization at high f(alpha'). At high temperatures, relevant to the quenching and partitioning processing, on the other hand, the pronounced stabilization at high f(alpha') was attributed to the uniform partitioning of the carbon stored at A-M boundaries into the austenite. Reduction in the fault density of austenite served as an auxiliary stabilization mechanism at high temperatures. (C) The Minerals, Metals & Materials Society and ASM International 2016