A Study on Nitrogen Permeation and Tempering Heat Treatment of AISI Type 409L Ferritic Stainless Steel

Abstract

Generally, solution nitriding (nitrogen permeation) is not applied to ferritic stainless steel, which has low nitrogen solubility in the ferrite phase. This study has investigated phase changes, nitride precipitations and hardness variations of Fe-11Cr-0.1Ti (409L) ferritic stainless steel following nitrogen permeation and tempering heat treatments. The strong affinity between nitrogen and Ti enabled the permeation of nitrogen to 409L ferritic stainless steel. The nitrogen-permeated surface changed to a martensitic phase with a hardness range of between 520 and 585Hv, depending on the nitrogen permeation temperature and time, while the surface nitrogen content was about 0.04%~0.05%. When tempering the NPSA (solution annealing after nitrogen permeation) treated specimen at 450 °C, a maximum hardness of 550Hv was obtained, probably due to the precipitation of very fine rod and square type titanium nitrides, while the minimum hardness of 365Hv was obtained at a tempering temperature of 650°C, owing to the precipitation of coarse TiN.