High Nitrogen Steels. Effect of Nitride (Cr2N) Precipitation on the Mechanical, Corrosion, and Wear Properties of Austenitic Stainless Steel.

Abstract

High-nitrogen austenitic stainless steels are thermally unstable and susceptible to nitride precipitation (primarily Cr2N) during elevated temperature exposure. This paper describes the effect of Cr2N precipitation on the mechanical, corrosion, and abrasive wear properties of a high-nitrogen austenitic stainless steel, nominally Fe-19Cr-5Mn-5Ni-3Mo-0.02C-0.7N. In the annealed state, Cr-rich nitrides (Cr2N) precipitate sequentially as intergranular, cellular, and finally, intragranular precipitates. Cold working, prior to aging, increases grain boundary and intragranular precipitation kinetics, but retards cellular phase formation. Nitride precipitation has only a minor influence on the yield and ultimate tensile strength of annealed materials, but causes embrittlement which is enhanced by high strain rate conditions such as impact testing and by the accelerated pricipitation kinetics associated with prior deformation. Nitride precipitation results in sensitization which is also accelerated by prior deformation. The degree of sensitization, as measured by electrochemical potentiokinetic reactivation (EPR) testing, correlates to intergranular, cellular, and intragranular Cr-depletion. Although the degree of sensitization is defined by both the Cr-minimum and width of Cr-depleted region, a Cr level below approximately 14 wt% is required for sensitization. Precipitation also results in a greater corrosion susceptibility as determined by potentiodynamic polarization scans. The wear behavior of the 0.7 wt% N alloy is little affected by the precipitation of Cr2N since the volume fraction and size (compared to the abrasive) of the Cr2N precipitates is insufficient to alter the wear resistance of the alloy.