Séminaire: Grace Fidler, PhD student at the University of Manchester

High-strength ferritic steels are used for nuclear pressurised water reactors, with the most common grade being SA508 Grade 3 (18MND5). This steel is processed through austenitisation, quenching and tempering, and can comprise retained austenite (RA) after quenching. Previous studies have suggested that decomposition of this RA during tempering can lead to the formation of coarse carbides, which are detrimental to toughness, and have suggested isothermal holds below normal tempering temperatures to improve properties. This work explores the decomposition behaviour of RA in-situ using SXRD and dilatometry, elucidating the temperature ranges over which it decomposes, and the effect of prior quench rate. The RA was found to mostly decompose within a temperature window at 250 – 450°C, but the exact bounds of the window were controlled by the initial microstructure (the quench rate), as well as the heating rate to temper, since different quench rates led to distinct RA morphologies such as blocks and films, which decomposed following different behaviour. It was found that heating at fast rates through the decomposition window limited the time available for RA decomposition, meaning RA could be retained post-tempering. Conversely, all the RA decomposed during heating with slow rates. In-situ SEM provided further insight into the decomposition mechanism of the block type of RA, suggesting it decomposed in a bainitic mechanism. Importantly, the results indicate that isothermal holds at intermediate temperatures below normal tempering temperatures to decompose RA are unlikely to be industrially relevant, since the RA decomposes during slow heating.