Exploring the microstructural characteristics of additively manufactured 17-4PH stainless steel through HIP-integrated heat treatment

Abstract

This study investigates the effects of thermal and thermomechanical post-processing on the microstructure of laser powder bed fusion (LPBF)-fabricated 17 − 4 PH stainless steel. Three conditions were examined: as-built (AB), H900 aged, and combined hot-isostatic pressing (HIP) with H900 ageing (HIP+H900). The AB condition had exhibited columnar prior austenite grains, cellular solidification substructure, lath martensite, retained austenite, and a pronounced heterogeneous crystallographic texture. H900 ageing had promoted dislocation recovery, grain boundary stabilisation, and the formation of Cu-rich precipitates consistent with established precipitation behaviour reported in literature for this alloy. The HIP+H900 condition exhibited a predominantly martensitic microstructure containing approximately 7.5% FCC retained austenite as measured by EBSD, the remaining approximately 92.5% was indexed as BCC phase encompassing lath martensite and residual δ-ferrite. The increased retained austenite fraction in HIP+H900 relative to AB and H900 conditions was attributed to partial austenite retention on cooling from the HIP thermal cycle at 1160 °C. Among the conditions examined, HIP+H900 exhibited the most homogenous microstructure, with near-complete pore elimination, reduced crystallographic texture, and the lowest local lattice, making it suitable for applications demanding structural integrity and microstructural stability, including gun barrels, exhaust systems, and advanced automotive components.