Retained Austenite Decomposition, and its Effect on Microstructure and Properties in Low-Alloy Steels.

It is critically important that the low-alloy steels used in energy applications have excellent strengths and toughnesses. These properties are determined by their microstructures, which are typically controlled by a processing route referred to as 'austenitise, quench and temper'. The austenitise step is a high-temperature hold, which homogenises the microstructure across the material as a single phase - austenite. The steel is then cooled in the quench step, to form strong microconstituents such as bainite and martensite. A tempering step is then required to increase toughness, by relieving internal stresses and precipitating carbides in the martensite/bainite. Recent work has indicated that the presence of large carbides in low-alloy steel microstructures, which are detrimental for toughness, may result of the decomposition of islands of carbon-enriched retained austenite during tempering (retained austenite = austenite retained after quenching). We have already measured that significant levels of retained austenite (>10%) are likely to be present in large forgings following the quenching step, but the effect of tempering on these islands remaining less well understood. For instance, it is not clear at what stage during the tempering heat treatment these islands decompose, and whether they form different microstructures when different tempering temperatures are used. This project aims to characterise the process of retained austenite decomposition in low-alloy steels (SA540, SA508 Grade 3 and SA508 Grade 4N) in detail, and understand the conditions under which coarse carbides form. It will use techniques such as scanning and transmission electron microscopy, optical microscopy, dilatometry and synchrotron X-ray diffraction to characterise the austenite decomposition and the resulting microstructures. It will use microhardness testing and Charpy impact testing to assess the change in mechanical properties brought about by different post-temper microstructures. If time permits, a comparison will be made between the retained austenite behaviours in material that is chemical heterogeneous (i.e., standard wrought material) and material that has been homogenised to ensure a consistent chemistry throughout.