Influence of solidification cell structure on the martensitic transformation in additively manufactured steels

A key feature when using martensitic steels is the proportion of retained austenite present in the final component. Martensitic steels manufactured by laser powder-bed fusion (LPBF) have been shown to have more retained austenite than when conventionally manufactured. The LPBF microstructure is characterised by small grains containing ultrafine solidification cells (<1 µm). This study shows that the solidification cells can fully suppress thermal martensite. The retained austenite is highly metastable, and will readily transform to deformation martensite either in-build from thermal strain or post-build from deformation. This raises concerns around sample preparation methods causing incorrect phase quantification in LPBF-built martensitic steels.