Contributions of sub-surface intergranular phases on fatigue crack initiation in additively manufactured austenitic stainless steel

Abstract Fatigue failures in additively manufactured 316L austenitic stainless steel produced using laser powder bed fusion processes are often attributed to the presence of process-related defects or surface roughness. To better investigate the role of surface roughness on fatigue properties, internal pores were mitigated using hot isostatic pressing, and strain-controlled fatigue testing was performed with the surface in the as-deposited condition. In the absence of internal defects, crack initiation was expected to occur at the surface, which displayed arithmetic mean surface roughness ( Sa ) values between 10 and 40 µm as the build angle decreased from a vertical (90°) to a 45° orientation. Even though a decrease in average fatigue life with this increase in surface roughness was observed, there was no evidence of fatal cracks initiating from as-deposited surface asperities. Instead, widespread brittle intergranular fracture occurred within fine-grained sub-surface regions in the contour passes along the specimen perimeter that were populated by sub-micrometer-sized Cr 2 N particles and nanometer-sized α-tridymite oxides that decorated the austenite grain boundaries. The width of these brittle fracture regions increased by 100 µm as the build angle changed from 90° to 45°. At the same time, the fraction of decorated grain boundaries (30–45%) and precipitate length (up to 10 µm) within these wider contour regions increased, driving the observed decreases in the fatigue life. Graphical abstract