INTERNALY NITRIDING BEHAVIOUR OF RAPIDLY SOLIDIFIED TITANIUM MODIFIED AUSTENITE STAINLESS STEEL

according to Fick's diffusion laws. The ribbons were observed by optical microscopy, transmission electron microscopy, STEM and microprobe as well as measurement of microhardness.
The results show that the microstructure of internally intrided, rapidly solidified Ti-modified 316L stainless steel consists of fine-scale grains (<3μm) and finely dispersed nitride particles. The size of nitride particle was about 0.2μm (at grain boundaries) and about 0.02μm (at cell boundaries).The number densities of particles are estimated to be of the order of 10^17-19 particles/m³ and 10^20-21 particles/m³ respectively. The nitriding condition of 900℃/7 min, is optimal with respect to the size and number density of particles in this study. The calculation of the standard free-energy and STEM analyses suggest that there is a possibility of simultaneous formation of both chromium nitride and titanium nitride and preferential formation of chromium nitride. After heat treatment for 1h at 1150℃, most of chromiun in the particles dessoives into the matrix. In this case the precipitates transform to titanium nitrides. Heat treatment of nitrided ribbon at 1150℃ for 1h causes very little change in grain size and in morphology, distribution and size of nitrideparticles.lt means that titanium nitride particles are very stable and suppress grain growth during recrystallization# High microhardness values (218-254) are obtained by nitriding the rapidly solidified Ti-modified stainless steel, which confirms that rapid solidification process has a promotive effect on precipitation hardening.