Failure mechanism and improvement process of preparing carbide coating on high speed steel by thermal diffusion reaction

Thermal diffusion process (TD) is an important surface modification technique to prepare carbide coating on steel, which can effectively improve the hardness and wear resistance, as well as prolong the service life of the substrate. However, cracking problems are often encountered in the TD processing of high-speed steel. In this work, TD process was used to prepare vanadium carbide coating on M35 high speed steel. The results indicated that the coarse crack after TD treatment was quenching crack. The mechanism of such failure were attributed to the diffusion of elements (mainly C and V) and the volume expansion caused by martensitic transformation during the TD process. Bainite isothermal quenching and three times tempering process were selected as the way of solving this cracking failure. The microstructure and properties of the coating prepared by the improvement process were studied. The coating was composed of fine equiaxial crystals containing V6C5 and VC. The hardness of the coated sample could achieve 2644 HV, which increased greatly compared with uncoated M35 matrix (750 HV), and the friction/wear performance of the samples treated by TD process increase by 82.8%.