MEASUREMENT AND CALCULATION OF CARBON CONCENTRATION AND DIFFUSION-INDUCED STRESS IN STAINLESS STEEL AFTER LOW TEMPERATURE GAS CARBURIZING

Abstract

Low temperature gas carburizing has been established as a surface hardening process to improve the wear and fatigue resistance of austenitic stainless steel.In the gas carburizing treatment of stainless steel at low temperature,carbon diffuses into the steel to a depth up to dozens of or even hundreds of micrometers.In industrial practice,it would be useful to establish a model that can predict the carburizing depth from carburizing condition.However,to date a satisfactory model does not exist,and the classic Fick's law has been proved to be inaccurate to describe the carbon diffusion during carburation.It has been observed that the insertion of carbon can lead to the evolution of very high compressive stresses in the surface layer of stainless steel.Since it has well established that the stresses induced by diffusion of atoms can in turn affect the diffusion behavior,the high compressive stresses due to carbon diffusion during carburation are supposed to play a role in the further diffusion of carbon.In this work,316L stainless steel block specimens were gas carburized at low temperature,and the carbon concentration,diffusion-induced stress along the depth direction of specimens were measured.Based on the coupled stress-diffusion theory,a model was built to calculate the carbon concentration and diffusion-induced stress in the specimens after carburizing.Then the calculated carbon concentration and stress distribution were compared to that obtained by above measurement.The results show as follow:(1) After carburizing,a diffusion layer containing high amounts of carbon and compressive stress was formed near the surface of specimens.The concentration and compressive stress were the maximum at the surface,and decreased with increasing depth.There was a linear relationship between the stress and carbon concentration.With increasing carburizing time,the depth and concentration level of the carbon diffusion layer increased.(2) To describe the carbon diffusion during carburation of stainless steel at low temperature,the model based on coupled stress-diffusion theory is more appropriate than the classical Fick's second law.The diffusion-induced compressive stress played an important role in diffusion of carbon.(3) The compressive stress highly increased the apparent diffusion rate of carbon.This explained the phenomenon that measured diffusion depth of carbon is much higher than that excepted from Fick's law.The interaction between diffusion and diffusion-induced stress should be considered when studying diffusion mechanism of carbon or nitride in carburizing or nitriding similar to the gas carburizing at low temperature.