Influence of porosity on deformation and fatigue behavior of P/M austenitic stainless steel

The monotonic and cyclic deformation response of powder metallurgy austenitic stainless steels resembling the commercial grade 3161 has been evaluated. Two porous steels at densities 6.9 and 7.2g/cm(3) respectively were studied and compared to a fully dense HIPed steel. Methods used were tensile and low cycle fatigue testing. Additionally, a study of short cracks that developed during low cycle fatigue was made with a replication technique and by optical microscopy of longitudinal sections. Stress and strain concentrations caused by the pores and the variation in load-bearing area along the specimen originating from the pore structure, result in microplasticity (early deviation from elastic behavior) during monotonic tensile straining. Tensile behavior is dependent strongly on specimen dimensions. The porous steels showed pronounced hardening during cyclic straining caused by the creation and expansion of plastic zones near pores. Surface fatigue cracks nucleate early (similar to 10% of the lifetime) and grow continuously until failure. Internal cracks were seldom found. Crack linking was observed in the low density porous steel.