Study On Carburization Dynamics And Kinetics Of Bainite Transformation Of Steel With Al And Si Elements

Gear play a very important role in mechanical industry and has been widely used in automotive, machine tools, lifting machinery and other areas. In this paper, Our reserch team developed a heat treatment process to manufacture gear: First, carburizing with low-carbon steel. There exist a great difference of carbon concentration between surface and substrate after carburizing, So there must be a difference in martensitic transformation point of these areas in tested steel. Then, the specimens were isothermally treated at the temperature above martensitic transformation point of the sample surface. Microstructure of the surface is hard bainite, the heart area is a low-carbon martensite, and the transition layer is a mixture of bainitic and low-carbon martensite.Transformation temperature and CCT curve of low-carbon steel with addtion of Al/Si were investigated by Gleeble-3500 thermal simulator. Carburizing test was performed by a RQ2-25-9 furnace at temperarue of 900°C, 930°C and 950°C. The kinetics of carburizing was studied. The bainite transformation kinetics and its influencing factors of test steel was also studied.The results show that:Low-carbon steel with Al/Si elements has good characteristics of carburizing. Al and Si have less effect on the carburizing kinetics of low carbon steel. The carburizing kinetic equation is as follows: 900°C,ξ= e?0.60τ0.5; 930°C,ξ= e?0.42τ0.5; 950°C,ξ= e?0.34τ0.5.The diffusion activation energy of carbon in the austenite is 121 kJ/mol.Both the transformation beginning temperature and the transformation speed of bainite increases with the increasing of Al content. Accordingly, CCT curve Move to the left. Microstructure of the surface is hard bainite, the heart area is a low-carbon martensite, and the transition layer is a mixture of bainitic and low-carbon martensite can be gained by isothermally treatment.