Ambient and high-temperature rotating bending fatigue of as-built and hot isostatic pressed additively manufactured 316L stainless steel

• HIP removed porosity and raised Σ3 twins in PBF-LB_SS316L from 2.4 % to 33 %. • At 450 °C, HIP improved fatigue endurance (170 MPa vs. 130 MPa as-built). • HIP increased ductility but lowered the fatigue limit to 220 MPa from 240 MPa. • As-built formed many cracks, while HIP formed a single stable crack. • EBSD showed grain coarsening at 450 °C; HIP preserved microstructural stability. This study investigates the effects of Hot Isostatic Pressing (HIP) on the rotating bending fatigue (RBF) performance of Powder Bed Fused-Laser Beam (PBF-LB) 316L stainless steel (SS316L) at both room (25 °C) and elevated temperatures (450 °C). HIP was applied at 1150 °C and 100 MPa for 4 h following stress relief to eliminate porosity and homogenize the microstructure. Advanced microstructural characterization revealed that HIP reduced porosity below detection limits and transformed the melt-pool structure into an equiaxed grain morphology with a significant increase in Σ3 twin boundary fraction (∼2.4 % to 33 %). These changes were accompanied by reduced dislocation density and internal strain. Fatigue testing showed that HIP specimens exhibited superior high-temperature fatigue endurance (170 MPa) compared to as-built samples (130 MPa), due to defect elimination and microstructural stabilization. However, at 25 °C, as-built specimens outperformed HIP ones (240 MPa vs. 220 MPa) owing to retained work-hardened microstructures. Fractographic analysis confirmed these trends: multiple crack initiation sites dominated as-built failures, while HIP fractures showed single-origin cracks with striation growth. Post-fatigue electron backscatter diffraction maps revealed temperature-dependent grain coarsening and dislocation accumulation, particularly in the as-built samples. Overall, HIP significantly enhanced the high-temperature fatigue resistance of PBF-LB_SS316L by improving microstructural integrity and delaying crack initiation, highlighting the importance of post-processing in AM components.