Low-Temperature Ion Nitriding of Austenitic Stainless Steels

Abstract

The surface nitrided layer of austenitic stainless steels ion-nitrided at a relatively low temperature of about 623 K was investigated in order to elucidate the nature of an f.c.t. nitride Y_N formed in this layer with high corrosion resistance and high hardness of about Hv 1500. In this layer, only Y_N nitride was produced and X-ray diffraction peaks of chromium nitride were not detected. The surface nitrogen concentration was found to increase with the nitriding time from about 6 to about 9.5 mass%. Furthermore, the concentration profile of nitrogen in this layer showed a monotonous decrease from the maximum value at the surface to the minimum value of about 6 mass% at the nitriding front. The tetragonal crystal structure of Y_N phase was recognized to be derived from the f.c.c. cell of γ′-Fe₄N by an expansion in the a direction and a contraction in the c direction, according to the location of the nitrogen atoms on the centers of the four c axes.
Y_N nitride was formed only in the low-temperature ion-nitrided layer of austenitic stainless steels of Fe–Cr–Ni, but not formed in the austenitic steels of Fe-high Ni or Fe-high Mn and in the ferritic stainless steels of Fe–Cr.