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. N2 solution nitriding is one of the possible methods to produce high nitrogen stainless steel. However, the nitriding rate by N2 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–H2 gas mixtures have high corresponding nitrogen potential than that of pure N2 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 N2 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 N2 (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.