Fluidized-bed nitridation of stainless steel powder

An attempt was made to produce high-nitrogen stainless steel powder, that can be consolidated into a solid compact with uniform, high-nitrogen concentration. The nitridation of 316L stainless steel powder of average particle size of 70 mu m with a minimum fluidization velocity of 4.5 cm s(-1), was nitrided in a 55 mm I.D. fluidized-bed reactor at temperatures ranging from 550 degrees C to 750 degrees C using NH3H/(2) gas mixtures. NH3 concentration and temperature had pronounced effects on the progress of nitridation while the effect of H-2 was minor. Nitrogen contents reached as high as about 10% by mass, completely converting Cr and Fe into CrN and Fe4N, at temperatures below 600 degrees C. Only Cr was nitrided to form CrN at 750 degrees C, reaching a saturated level of 6% nitrogen by mass. A nitrided shell was always developed on the particle, resulting from the precipitation of CrN, that progressed inward.