The effect of heating rate and hold time on grain refinement during the austenitisation of high strength steels with coarse prior austenite grain structures

Coarse prior austenite grain (PAG) structures can sometimes be observed in high-strength steels, e.g., in parts produced by wire-arc additive manufacturing (WAAM), and are problematic as they are often associated with poor mechanical performance. In the case of WAAM, cyclic austenitisation can refine coarse PAG structures either during or after the building process. However, in this study we show that there can be a strong effect of heating rate during austenitisation on the resulting grain refinement. The heating rate effect was systematically studied in two steels – a low-alloy steel (300M) and a maraging steel (Custom 465®) – from an as-WAAM’d state where both comprised coarse columnar PAGs. On heating, for all heating rates investigated and in both steels, the austenite ‘memory effect’ was first observed, which retained the columnar structure. This was then sometimes followed by autogenous recrystallisation (without applied external deformation), which greatly refined the PAGs. High-temperature in-situ electron backscatter diffraction was used to directly observe austenitisation in 300M, revealing that the recrystallisation shifted from a continuous to a discontinuous mechanism with decreasing heating rate, and that a further decrease in rate suppressed recrystallisation altogether, leaving only a memory effect. Conversely, in Custom 465®, recrystallisation produced significant PAG refinement across all heating rates investigated. However, the recrystallised PAGs retained some crystallographic texture inherited from the columnar PAGs and twins in the as-built material. These findings provide mechanistic insight into grain refinement strategies for steels with coarse PAGs and highlight the importance of controlling heat rates during industrial processing.