Characterization of high-strength high-nitrogen austenitic stainless steel synthesized from nitrided powders by spark plasma sintering

A high-N austenitic stainless steel (HNASS) was fabricated using a novel approach, in which duplex stainless steel powders were firstly gas nitrided in the temperature range of 700-900 degrees C and then consolidated using spark plasma sintering, resulting in an FCC matrix with average grain sizes of 1.6-8.2 mu m. The Cr2N precipitates transformed from cellular precipitates (lamellar within the grains) to intergranular precipitation (equiaxed along the grain boundaries) as the nitriding temperature rose from 700 degrees C to 750 degrees C, and the N content rose from 0.68 wt% to 1.71 wt%. The 700 degrees C-treated alloy exhibits a yield strength of 783 MPa, a high tensile strength of 1091 MPa, and an elongation of 41% while the 900 degrees C-treated alloy exhibits an ultra-high compressive strength of 3017 MPa, and hardness of 520 HV. These values dramatically exceed the published values of similar HNASSs. Quantitative calculations showed that these superior mechanical properties could be attributed to increased solid solution and grain boundary strengthening. Furthermore, it can be inferred that while the equiaxed Cr2N phase increased the yield strength, it greatly degraded the fracture toughness.