Microstructure And Properties Of C-B Double Cementation Of 18CrNiMo7-6 Steel

In this paper, the layer microstructure and properties of C-B double cementation of 18CrNiMo7-6 steel were studied.18CrNiMo7-6 steel was carburized in UBE-600 multipurpose heat treatment furnace with gas carburizing method. The thicknesses of carburized layers are 0.5mm,1.0mm and 1.5mm. Then they were put into box type resistance furnace to pack boronizing at 850℃,880℃,910℃, and 940℃ for 3,5,7, and 9h together with non carburized samples. Using the self-made granular boronizing agent. After boriding, one sample was air cooled, and another sample was quenched and low temperature tempered.The microstructure of the layers was observed by optical microscope and SEM. The phase composition of layers was analyzed by XRD. The micro hardness distribution from the surface to core of the layers was measured by micro hardness tester. Analyzed the effect of process parameters on the microstructure, hardness, thickness and phases boriding layer. Analyzed the impact of different carburizing layer thickness on the thickness, the hardness gradient and transition layer thickness of boronizing layer by surface properties testing. The wear resistance of the sample surface is tested by the HT-1000 type high temperature friction and wear tester. The corrosion resistance of the sample surface is tested by the CHI660E type electrochemical workstation. Meanwhile, the thickness of boride layers was measured to carry out the kinetic study of the steels and the contour diagrams were drawn to predict the relationships of boride layers’ thickness and processing parameters.The experiment results show that:with the self-made boronizing agent, the higher boronizing temperature and the longer holding time, the thicker boronizing layer is. boronizing layer mainly as Fe2B phase, in the higher temperature and longer time of heat preservation, boronizing layer FeB+Fe2B dual phase microstructure; boronizing layer is serrated and substrate binding and boron carbon composite permeability in the transition zone of whisker boride along the grain boundary carbide growth, increase the hardness of the transition zone, enhances the supporting role of high hardness boride layer, reduce the boronizing workpieces surface cracking and spalling tendency in the service. The main phase of boronizing layer is Fe2B. At too high temperature and too long time of heat preservation, boronizing layer appear FeB+Fe2B dual phase microstructure. The boronized layer serrated combined with the matrix, C-B double cementation has a wide and hard transition zone, and enhance the supporting role of higher hardness boride layer, reducing the surface cracking and spalling tendency of boronizing surface during use.The reciprocating wear experiments and high temperature friction and wear test showed that the friction coefficient of surface after C-B double cementation is smaller than the friction coefficient of the surface after carburizing, the compound layer wear loss quality was 60% of the carburized steel at room temperature. At 400 ℃ wear loss quality was 36% of the carburized steel, proving that the wear resistance of 18CrNiMo7-6 steel was greatly improved by C-B double cementation. The hardness gradient is less because of the spread of boronizing layer growth.Carburizing can increase the carbon concentration of the steel surface. On the one hand, this is beneficial to the nucleationon of boride, on the other hand, On the other hand it also hinders the boride preferential growth, weakens the sharpness of boronizing layer zigzag, promote lateral growth. Thinner carburized layer promotes the growth of boronizing layer,can promote boronizing layer thickness growth rate, reduces the average activation energy of of boron atoms; thicker carburized layer hinder growth rate boronizing layer and increases the average diffusion activation energy of boron atoms.