Fabrication of Ultra High Nitrogen Austenitic Stainless Steel by NH3 Solution Nitriding

Abstract

Nitrogen addition to stainless steels is very effective for improving mechanical properties and corrosion resistance. N-2 solution nitriding is one of the possible methods to produce high nitrogen stainless steel. However, the nitriding rate by N-2 gas is significantly small and also the attainable nitrogen contents in Fe-Cr alloys is less than 1 mass% in general. NH3 nitriding is an attractive alternative since NH3-H-2 gas mixtures have high corresponding nitrogen potential than that of pure N-2 gas. In the present study, aiming for the enhancement of the nitriding rate and the increase of nitrogen contents more than 1 mass%, NH3 and N-2 solution nitriding of Fe-Cr alloys under various conditions has been carried out and their behaviors of nitrogen absorption in Fe-Cr alloys are investigated. It is found that the suppression of the pre-decomposition of NH3 before it reaches the sample is critical for the effective NH3 nitriding. Compared with the N contents after 1 h nitriding, NH3 nitriding is found to be about 5 times faster than that of N-2 (0.1 MPa) nitriding under the present experimental conditions. In addition the N contents more than 1 mass% were easily achieved by NH3 nitriding. By using devised NH3 gas injection system to minimize the pre-decomposition of NH3 gas, austenitic Fe-23.5Cr alloys with more than 3 mass% of nitrogen (ultra high nitrogen stainless steel) that had never been achieved by solution nitriding was successfully produced.