Surface Condition Effects on Fatigue Behavior of Additive Manufactured 304L/316L Steel

Stainless steel is used extensively as a structural material in nuclear energy systems and for this reason its mechanical properties are constantly scrutinized. As additive manufactured (AM) parts enter nuclear systems, confidence in the AM part must be developed. In this work we focus on the fatigue behavior of both 304L and 316L stainless steel produced by laser powder bed additive manufacturing (LPBF-AM). The fatigue life of metals such as stainless steel can be affected by the surface finish of the test sample. Surface roughness measurements performed using laser confocal microscopy are compared between the as-received (AM and conventionally wrought), electropolished, mechanically polished, thermally aged, and passivated states. Initial comparisons between AM 304L and 316L in the as-received state indicate greater surface roughness in 304L, most likely as a result of powder size during fabrication. Following surface characterization, fatigue testing to failure is completed using square cross section bar fatigue tests in bending with R values of 0.1 in each condition. Lastly, fracture surface analysis using a Hitachi SU8200 cold field emission gun (CFEG) scanning electron microscope (SEM) in each condition is compared. Relations between surface post processing, the resulting finish, and fatigue performance will be drawn to shed light on practical considerations for the processing-structure-properties relationships which apply to AM austenitic steels in fatigue limited service.