Solid carburizing in ferritic phase region of DIEVAR steel: Microstructure evolution and formation mechanism of carburizing layer

A solid carburizing technology of ferritic phase region is developed to enhance the surface properties of DIEVAR steel. By optimizing the composition and ratio of carburizing agent to energizing reagent, a uniform carburized layer with a depth of more than 1400 mu m is successfully introduced on the DIEVAR steel. The carbon concentration is gradually increased from 0.35 wt% at the matrix to 3.14 wt% at the carburized layer surface. Meanwhile, the hardness of the surface achieves as high as 517 HV0.9807N, which is increased by 272 % compared to the matrix (190 HV0.9807N). The results show that the carburized layer has a good diffuse-type interface characteristic and a good combination with the matrix. The change of microstructure indicates carbon atoms have been diffused into the DIEVAR steel even in the ferritic phase region, which shows an extremely low solid solubility of carbon (0.0218 wt%). The efficient diffusion of carbon in the ferritic phase region is ascribed to the existence of large amount alloying elements in the matrix. According to theoretical calculations based on lattice constant, the presence of alloying elements can increase the diameter of octahedral interstice in ferrite, which can accommodate more atoms and thus increase the carbon solubility in ferrite at room temperature to 0.1431 wt %. With the help of lattice expansion at the solid-carburizing temperature, enormous diffusion paths will be formed in ferrite, which can remarkably facilitate the inward diffusion of carbon atoms. With the increase of carbon content, carbides and tertiary cementite will be formed and precipitated due to the reaction with Cr, Mo and V carbide forming elements. These results suggest that ferritic phase region solid carburizing is a promising process to modify the surface properties of alloy steels.