Influence of the Chemical Composition on the Behavior of Austenitic Stainless Steel in Fluid Layer Nitriding Process

Abstract

The main factors that influence the stainless steel behavior to the nitriding process are: chemical composition, nitriding temperature, treatment time. Researches have revealed that, with low temperature nitriding, the chemical composition expressed directly by the alloying elements or indirectly by the austenite or ratio E_Cr/E_Ni stability index decisively influences the characteristics of the nitride layer. The high alloyed steels having over 50% alloying element and high stability index S = 30, produce a diffusion layer of a low nitrogen concentration, below the saturation limit, and hardness less than 500 HV₀₀₅. The usual stainless steels CrNi, CrNiMo of low stability index S = 17.5-24 at 500 ^oC, within the sensitive temperature range form high hardness combinations (1200-1400 HV₀₀₅) and at temperatures of 450 ^oC under the sensitive range, diffusion layers of high nitrogen concentrations close to the saturation limit, featuring high hardness 1100-1200 HV₀₀₅ and slightly lower depths, tens of microns.