On the Nature of Retained Austenite and Its Decomposition in a Low-Alloy Pressure Vessel Steel

High-strength ferritic steels used for pressure vessels are subject to an austenitisation, quench and temper production heat treatment. Retained austenite (RA) is observed after the quenching stage, and it has been suggested that RA decomposition can form detrimental coarse carbides during tempering. Here, in-situ synchrotron X-Ray diffraction (SXRD) was used alongside dilatometry to explore the decomposition behaviour of RA in-situ , elucidating the temperature ranges over which it decomposes, and the effect of prior quench rate and the rate of heating to temper (rates of 0.1, 1, 10 and 100 °C s −1 were used in both cases). The RA was found to mostly decompose within a temperature window of 250 °C to 450 °C, but the exact bounds of the window were controlled by the initial microstructure (the quench rate), and the heating rate to temper. Indeed, heating at fast rates (≥ 10 °C s −1 ) limited the time available for RA decomposition, meaning RA could be retained post-tempering. Conversely, all RA decomposed during heating with slow rates (0.1 °C s −1 ). Importantly, the results indicate that isothermal holds at temperatures between 250 °C and 350 °C to encourage lower temperature RA decomposition are unlikely to be industrially relevant, since the RA decomposes during heating slower than 1 °C s −1 .