Analysis of surface hardening characteristics of gas carburized chromium alloy steel for automotive constant velocity joint

Abstract

The purpose of this study is to investigate the factors affecting the surface hardening characteristics of gas carburization for internal race and cage which are actually used as parts for constant velocity joints of chromium alloy steel. The specimens were made to the actual size used for the automobile parts. The carrier gas was made by adding RX gas mixed with propane and air. Propane gas and butane gas were mixed to control the carburizing gas atmosphere, and heat treatment was performed in two steps for carburization and diffusion. Hardness distributions for the carburized depths of the heat treated automotive parts were measured by micro Vickers Hardness550Hv and Rockwell hardness tester for surface and internal hardness. The test results showed that the lower carbon content, the deeper carburized layer from the surface to the inside and low surface and internal hardness values. These results show that the lower the carbon content, the shorter the carburization time. It is related diffusion time required to reach the surface and internal hardness. This may be related to the diffusion gradient of carbon on the surface and inside of the material. Acicular structure was martensite and decrease gradually from the surface to the inside. The shape of the structure changed according to the degree of carburizing. For 0.2%C chromium alloy steel, the Vickers hardness was maintained at 700Hv up to 0.7mm and the effective hardening depth hardness was 550Hv at carburizing depth of 1.4mm. For 0.15%C chromium alloy steel, the Vickers hardness value was found to be 714Hv at a carburizing depth of 0.1 mm and 550Hv at a carburizing depth of 0.7 mm.