High-Performance Porous High-Nitrogen Nickel-Free Austenitic Stainless Steel via High-Vacuum Reduction Sintering Combined with Solution Nitriding

Porous high-nitrogen nickel-free austenitic stainless steel (PHNASS) combines low elastic modulus with high strength and corrosion resistance, but thermodynamically stable oxides on powder particle surfaces weaken sintering-neck bonding and hinder nitrogen ingress. Here, PHNASS was prepared from Cr–Mn stainless steel powder with 40 vol.% NH4HCO3 as the space holder via high-vacuum reduction sintering (1200–1300 °C, ≤1 × 10−3 Pa) followed by solution nitriding at 1150 °C. Compared with ordinary-vacuum sintering, high vacuum removes the Cr/Mn surface oxides (including spinel MnCr2O4) through carbothermal reduction, yielding clean, well-developed sintering necks and promoting nitrogen dissolution (relative XPS peak-area fraction of solid-solution nitrogen rising from 42% to 61%). The nitrided matrix is fully austenitic, with dispersed fine, equiaxed Cr2N precipitates (200–400 nm); selective Mn volatilization produces a surface-to-interior Mn gradient and, most probably, Kirkendall-type micropores, whereas nitrogen remains uniform. At 42.5% porosity, the 1300 °C sample exhibits a compressive yield strength of 378 MPa and a peak compressive stress (after substantial densification) of 2160 MPa at 42.6% fracture strain (5.4 times the ordinary-vacuum counterpart); its corrosion current density in 3.5 wt.% NaCl is 1.78 × 10−6 A·cm−2, 4.5 times lower. High-vacuum oxide reduction thus activates sintering-neck formation and nitrogen alloying simultaneously, providing a basis for PHNASS process optimization.