Study On Re Carburizing Of Steel 20CrMnTi And First Principles Calculation Of Effect Of Re On The Diffusion Of Carbon

This paper deals with the kinetics of layer growth and the effect of rare earth (RE) on the structure and properties of carburized layer during carburizing of steel 20CrMnTi with rare earth added in craburizing gas. Microstructure evolution, microhardness profiles, the kinetics laws of layer growth and the main positions of rare earth existing for RE carburizing had been revealed in this paper as well.Microstructure observation and microhardness profiles measurement results show that the addition of RE could refine microstructure in surface layer, accelerate the carburizing process, decrease the carburizing temperature and increase the hardness of carburized layer. Obviously, it can be seen that a large amount of dispersed granular carbides precipitate in surface layer of steel 20CrMnTi RE carburized at 880 and 900?C for 5h under carbon potential 1.3%. The microstructure of surface layer after directly quenching consists mainly of acicular martensite, fine granular carbides and a little residual austenite. EDS analysis demonostrates that rare earths exist mainly in carburized layer and carbides.Based on pseudopotential plane wave method of the first principles, the crystal cells of austenite and carbide had been constructed and optimized, as the crystal cell energy, the lattice parameters, the electronic structure, the diffusion activation energy of carbon atom had been calculated as well. The calculated results of energy and lattice parameters of austenite and carbide indicated that the addition of RE enhances the energy and the volume, and reduce the stability of crystal cells, which is beneficial for carbon to diffuse into the matrix and accelerate the carburizing process. Electronic structure of austenite and carbide showed that adding RE could make the total density of states of crystal cell and carbon atoms aggregate around feimi energy which reduce the stability of crystal cell and carbon atoms, the stability of partial iron atoms also decrease, and all those is in favor of carbon diffusion. Total charge density of austenite crystal cell increases, the ionic bond effect between carbon and lanthanum atoms is also very strong, which enhances the surface hardness of carburized layer. Carbon atoms tend to diffuse on crystal plane which the bonds are weaker and adding RE could reduce the diffusion activation energy, which could prove resulting from the calculated results of carbon diffusion activation energy in austenite with concentration gradient and in carbide. The presence of rare earths in austenite crystal cell could decrease the carbon diffusion activation energy. The carbon diffusion activation energy increases with decreasing the concentrations of carbon and RE in austenite crystal cell as well as the effect of RE on the carbon diffusion activation energy decreases gradually.