Structural Control of Stainless Steel by Nitrogen Absorption in Solid State

Abstract

Nitrogen addition to stainless steels is very effective for improving mechanical properties and corrosion resistance. Powder metallurgy process, which is the sintering of stainless powder in N₂ gas atmosphere, has some beneficial points to the production of high nitrogen stainless steels, because a large amount of nitrogen can be absorbed into powder particles in a very short time.
In this paper, the behavior of nitrogen absorption into an Fe-23mass%Cr ferritic alloy powder was investigated in association with the structural changes. The results obtained are as follows: (1) Since the amount of nitrogen absorbed is dependent on the surface equilibrium between N₂ gas and nitrogen content of steels, the saturation nitrogen content increases with lowed sintering temperature. (2) At 1473 K in 1 atm-N₂ gas, the saturation nitrogen content of an Fe-23mass%Cr alloy is about 1 mass%N and the nitrogen absorption causes a structural change of the matrix from ferrite to austenite. The nitrogen content is also enough to give a stable austenitic structure at room temperature for the alloy. (3) The Fe-23mass%Cr-1mass%N alloy developed by the above sintering treatment has a high yield strength and a moderate elongation in spite of containing about 12 vol% of retained pores. Such good mechanical properties are attributed to the solid-solution of a great deal of nitrogen into the steel.