Production of ultra high nitrogen stainless steel by ammonia solution nitriding

Abstract

Nitrogen addition to stainless steel is very effective for improving not only mechanical properties but also corrosion resistance. Moreover, if steel contains enough nitrogen to maintain austenitic structure, it is possible to make a nickel-free austenitic stainless steel. However, nitrogen-alloying offers unique challenges since the solubility of nitrogen in liquid Fe and Fe-based alloy is very low at atmospheric pressure. To overcome this problem, ammonia solution nitriding is investigated as a promising process to produce high nitrogen stainless steel.In the present work, N2 and NH3 solid solution nitriding are compared with each other. As a result, when we applying NH3 nitriding instead of N2, enhanced equilibrium N contents and nitriding rate are achieved. Moreover, equilibrium N contents and rate can be controlled by NH3 and H2 ratio. Therefore, Ni-free austenitic steels are effectively acquired by ammonia solid solution. Because of high nitridation potential of ammonia, however, chromium nitride formation is observed. And it is confirmed that the nitrides are homogenized and removed by austenitizing at 1573K. In addition, the grain size of ultra high nitrogen stainless steel is reduced from 500㎛ to 20㎛ through the reversion of eutectoid structure.