Fatigue behavior of vacuum-sintered binder jetted fine 316L stainless steel powder

Abstract

The study examined the impact of surface roughness on the high-cycle fatigue (HCF) properties of vacuum-sintered binder jetted gas atomized fine 316L stainless steel (SS) powder. The microstructure and density of the as-sintered specimens were analyzed using micro-computed tomography, indicating a relative density of ∼99.8 ± 0.1% with equiaxed grains surrounded by delta-ferrite phase at the grain boundaries (volume fraction of ∼2%). The as-sintered specimens had a rough surface with an arithmetical mean roughness of Ra = 6.56 ± 0.58 μm and root-mean-square roughness of Rq = 8.29 ± 0.68 μm. After mechanical grinding, the surface roughness was reduced to Ra = 0.21 ± 0.03 μm and Rq = 0.25 ± 0.04 μm. Microhardness analysis revealed an increase of nearly 70% up to 125 μm beneath the surface of mechanically ground specimens. The enhanced surface hardness was found to be related to an in-plane compressive residual stress on the mechanically ground sample. The fatigue endurance limit was evaluated using the staircase test methodology, with an average value of ∼170 and 225 MPa for the as-sintered and mechanically ground parts, respectively. High-cycle fatigue experiments were conducted at a stress ratio of Rσ = −1, and the results showed that surface grinding improved fatigue life at higher stress levels due to in-plane compressive stress and the reduction in surface roughness. Fractography revealed that the fracture mechanisms in the crack propagation and final fracture zones were striation marks and dimple features. Finally, the electron backscatter diffraction technique was used to study the deformation history at different sites close to the fracture surface.