KINETICS OF NITRIDING IN TITANIUM BEARING AUSTENITIC STAINLESS STEEL

The kinetics of nitriding in titanium bearing austenitic stainless steels in cylindrical coordination has been investigated. Nitriding at nitrogen partial pressure of 0.5 atm and temperature from 980 degrees C up to 1160 degrees C resulted in the formation of titanium and chromium nitride and above the Cr2N solvus temperature (1100 degrees C), no chromium nitride was formed. The nitrided case may consist of up to three zones: nitrogen in solid solution, precipitation of TiN and finally mixed TiN+Cr2N precipitation. These are in consistent with the affinity of titanium and chromium towards nitrogen which has been thermodynamically justified. To assess the kinetics of nitriding, a mathematical model for nitrogen diffusion in cylindrical system has been developed via solving Fick's equation for cylindrical coordinate by numerical method. For this purpose the use has been made of the austenite stabilizing effect of nitrogen to correlate the movement of the boundary of the nitrogen stabilized austenite and plastic deformation induced martensite. Since the crystal structure of the material used in this study is of fcc type and it is nitrided in a fully annealed and unstrained state, therefore the effect of excess nitrogen on the kinetics of nitride fronts growth has been assumed as negligible. The results are in good agreement with the previous investigations. Unlike previous works, the present model takes into account the change of nitrogen diffusion coefficient due to nitrogen concentration profile and formation of titanium nitride.