Nitride-Hardening of an Austenitic Stainless Steel Containing Titanium

Abstract

The internal nitriding of Fe : 20 wt.-% Cr, 25 wt.-% Ni, 1·5 wt.-% Ti has been investigated at 900–1150° C using nitrogen pressures of 0·1–1·0 atm. The conditions under which Cr2N and CrN are formed and then subsequently reduced in hydrogen are described. The precipitated phase previously reported as TiN is identified as (Ti, Cr)N in which the chromium content decreases with (1) increasing nitriding temperature, (2) decreasing nitrogen pressure, and (3) hydrogen reduction. The particle size of the precipitated (Ti,Cr)N increases linearly with distance from the surface and the precipitate density is inversely proportional to the cube of this distance. The particle size also increases slowly with decreasing nitrogen potential and markedly with increasing temperature. For the experimental conditions used, the nitriding rates are in accordance with the kinetics deduced from internal nitriding or oxidation theory. The nitrided alloy shows significant improvements in high-temperature tensile properties compared with conventional dispersion-strengthened stainless steels.