In-Situ EBSD Study of Austenitisation in a Wire-Arc Additively Manufactured High-Strength Steel

As-built high-strength steel wire-arc additive manufactured (WAAM) components often contain coarse columnar prior austenite grain (PAG) structures. These microstructures can result in poor mechanical properties, hence post-build re-austenitisation treatments are of interest to improve microstructures. Here, the potential for engineering austenite refinement was investigated using high-temperature in-situ EBSD with high temporal resolution. It was found that 300M first austenitised with a memory effect, reforming the columnar PAG structures in both morphology and crystallographic orientation. This was observed to occur by the coalescence of acicular austenite and happened earlier in the depleted dendritic regions. On further heating, austenite underwent recrystallisation without the application of external deformation via a discontinuous mechanism, involving the bulging of the prior high angle boundaries. Recrystallisation twins were also seen to form and grow alongside regular recrystallised grains. The predominance of discontinuous recrystallisation meant that a coarse grain structure was still retained after austenitisation, owing to the influence of a grain ‘swapping’ effect.