Microstructure and Tribological Properties of Mo-coating Prepared by Pack-cementation on 40Cr Steel

Thermal-diffusion molybdenizing is one of the methods uesd for modifying the chemical composition of steel surface. Mo can improve hardenability of steel, interact with carbon to form carbides with high melting point, and improve the wear resistance of the steel surface. In order to explore the thermal-diffusion molybdenizing process, Mo-coatings are prepared on 40Cr steel by pack-cementation treatment via box-furnace heating and induction heating at 1 000-1 300 degrees C. Microstructure, element distributions, phases and tribological properties of the treated samples are characterized and studied by the field emission gun scanning electron microscope ( FEG-SEM) , X-ray diffraction (XRD) and friction and wear testing machine. The evolution mechanism of the microstructure of molybdenizing by induction heating is described. The results showed that no obvious Mo-containing layer was observed when heated by the box- furnace at 1 100 degrees C, but a 30-70 mu m thick Mo-containing layer is formed by induction heating at 1 000-1 300 degrees C. The cross-sectional structure of the samples after induction heating pack-cementation is composed of Mo-coating, transition layer, affected-layer and substrate. The Mo-coating is mainly composed of Fe-Mo solid solution and Mo containing carbides phase, the transition layer consisted of alloy pearlite, and the affected layer was a carbon-poor zone. Studies show that the highest hardness of the Mo-coating is 560 HV0.2, which is about twice that of the untread sample. The friction coefficient of the IHM-1200 sample is 0. 73, which is 0. 12 lower than the untreated sample. The mass loss is slightly lower than the untreated sample. Friction properties was significantly improved after the formation of the Mo-coating on the 40Cr steel.