1999 Volume 40 Issue 1 Pages 13-19
Three types of austenitic stainless steels, SUS304, SUS316 and SUS310 were preheated in air in a temperature range of 573 to 973 K for 1 h, followed by nitriding in NH3 gas in a temperature range of 773 to 873 K. The influences of nitriding temperature and time on nitriding behavior were examined in connection with the kinetics of the nitriding reaction.
The nitriding reaction occurs locally at an early stage of the nitriding treatment. The whole surface becomes covered with a nitrided layer by coalescence of nitrided regions. Thereafter, the nitrided layer with a uniform thickness is formed by the growth in the depth direction.
Diffusion of nitrogen through the nitrided layer is the rate-determining process in the growth of the nitrided layer. Atmospheric preheating has no influence on the growth of the nitrided layer. On the other hand the nucleation of the nitriding reaction is greatly accelerated by atmospheric preheating. This is attributed to both considerable dissociation of NH3 gas and the rapid diffusion of nitrogen through the surface film due to the change in the composition of the surface film with atmospheric preheating. The hardness of the nitrided layer decreases with increasing nitriding temperature. This tendency is especially significant for SUS304 and SUS316 steels.
