Investigation of Low-Temperature Liquid Nitriding Conditions for 316 Stainless Steel for Improved Mechanical and Corrosion Response

Abstract

Combined influence of liquid nitriding time and liquid nitriding temperature on the properties of nitrided region developed on 316 stainless steel was investigated. Studies were restricted to low temperatures of 425 °C and 475 °C while the nitriding time was varied from 4 to 16 h. Developed nitrided layers were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction. Mechanical and electrochemical properties of the nitrided layers were characterized, respectively, by Vickers micro-hardness measurements and cyclic polarization studies in 3.5 wt% NaCl solution. After 4-h nitriding at 425 °C or at 475 °C, the developed expanded austenite started to contract with simultaneous decrease in nitrided layer thickness, hardness and corrosion resistance. These prolonged nitriding time effects have been attributed to the decreased chemical potential of nitrogen in salt bath leading to the outward diffusion of N (from sample to salt bath). However, the 4-h nitriding at 475 °C resulted in larger expansion of austenite as compared to 4-h nitriding at 425 °C. Due to this, better mechanical and electrochemical property was realized after 4-h nitriding at 475 °C suggesting that the increased amount of N solubility in sample led to simultaneous increase in hardness and corrosion resistance of stainless steels.