Experimental research and thermodynamic simulation of low temperature colossal carburization of austenitic stainless steel

Because of excellent corrosion resistance, good toughness and machinability, austenitic stainless steels are widely used in many industries. In order to improve the corrosion resistance, the carbon content of austenitic stainless steel is ultra-low, resulting in low surface hardness, poor wear and fatigue resistance properties which limit its application. Low temperature colossal carburization (LTCC) is a kind of novel surface strengthening technology for significantly increasing the surface hardness of austenitic stainless steels, while keeping their original excellent corrosion resistance because of no formation of carbides. The wear, fatigue and corrosion resistance of austenitic stainless steel of low temperature carburized layer have been investigated in recent years. However, the researches on key technical parameters, especially the carburizing atmosphere and the alloying element, have been rarely reported due to intellectual property protection limits. In this work, OM, EPMA, XRD and IXRD are used to investigate the effects of CO concentration on the microstructure, carbon concentration distribution, phase constitution and residual stress of the carburized layer on 316L austenitic stainless steel surface. Based on thermodynamic theory, the model of carbon transfer and diffusion was also built by software DICTRA to calculate the distribution of carbon concentration and activity of low temperature carburized layer. The results reveal that S phase is detected on 316L austenitic stainless steel surface treated by LTCC, and the compressive residual stress is formed at the same time. The increment of CO concentration can significantly increase the carbon concentration of carburized layer, which improve the hardness and compressive residual stress. The simulated carbon concentration and activity distributions are in accordance with the experimental results when the carbon concentration is lower, but when the carbon concentration is higher, the simulated carbon concentration is lower than experimental results due to the decrease of trapping sites and high compressive residual stress.