From anisotropy to isotropy: Microstructural and oxide evolution in hot isostatic pressed additively manufactured 316L stainless steel

Powder bed fusion–laser beam (PBF-LB) additive manufacturing of 316L stainless steel offers a promising pathway for fabricating replacement reactor core internal components with tailored microstructures and mechanical properties to meet demanding service requirements. This study systematically investigates the influence of HIP temperature (1000–1250°C) on the microstructural evolution and mechanical behavior of PBF-LB-fabricated 316L stainless steel, with particular emphasis on grain structure, cellular microstructure, crystallographic characteristics, oxide evolution, and the resulting mechanical properties. The as-printed and HIP-treated materials were characterized using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, microhardness, and tensile testing, complemented by modeling and calculations of temperature-dependent oxide coarsening. The as-printed material exhibited a heterogeneous cellular microstructure, elemental segregation, high local lattice distortion, and mechanical anisotropy. HIP at 1000°C promoted microstructural recovery while largely retaining the cellular structure and associated segregation. At 1150°C, substantial dissolution of the cellular network and elemental redistribution produced the most favorable combination of strength and ductility. Increasing the temperature to 1250°C resulted in homogenization, recrystallization, grain coarsening, and nearly isotropic mechanical behavior. TEM analysis revealed progressive coarsening and redistribution of Mn–Si–O-based oxide dispersoids with increasing HIP temperature, consistent with the oxide-coarsening model and calculated temperature-dependent coarsening behavior. This study extends our previously established PBF-LB/HIP processing route by establishing relationships among HIP temperature, cellular structure evolution, oxide coarsening and redistribution, mechanical anisotropy, and tensile behavior. Among the conditions investigated, 1150°C provided the optimal balance of microstructural homogenization, strength, and ductility.