Effects of carburizing process on the wear resistance of plow tips of high speed hydraulic reversible plough

Abstract: Wear resistance of plow tips has been one of the most important issues in a high-speed hydraulic reversible plough. In this study, the Carburizing-Quenching-Tempering (CQT) process was employed to improve the wear resistance of plow tips fabricated by the 33MnCrB5 steel with the hard alloy coatings. A systematic investigation was also made to elucidate the influence mechanism of the process on the microstructure and wear resistance of carburized plow tips. The results show that the maximum content of near-surface carbon and the depth of the carburized layer were 0.70 % and 2.5 mm, respectively, for the 33MnCrB5 plow tips after CQT process. The surface carbon content of the plow tip was significantly higher than that treated by Quenching-Tempering (QT) process. In addition, the surface microstructure of the CQT plow tip was composed of plate martensite (high hardness), retained austenite, and a small amount of carbide. Specifically, the core microstructure was mainly lath martensite (high strength and toughness), which was different from the microstructure of lath martensite and pro-eutectoid ferrite in the QT plow tip. The microhardness and tensile strength of CQT 33MnCrB5 plow tips were greatly enhanced, due to the evolution of the microstructure. The yield strength and tensile strength of the carburized plow tip in the core region reached up to 1 584 and 1 898 MPa, respectively, while, the surface microhardness reached 699 HV1. Among them, the yield strength, tensile strength, and surface microhardness increased by 198 and 314 MPa, and 252 HV1, respectively, compared with the QT plow tip. The friction tests show that the CQT plow tip (0.29) presented a lower friction coefficient, but a higher abrasive resistance, compared with the QT plow tip (0.37). The improvement was attributed to the fact that the carburized layer was produced into the abrasive chips with the higher hardness carbides during wear, thus increasing the contact area between the carburized layer and the friction substrate. Furthermore, the field tests indicated that the total wear of the CQT plow tip after 120 hm2 of operation (91 g) was significantly lower than that of the QT plow tip (144 g), in terms of the profile dimensions in the length, width and thickness directions. Nevertheless, the carbide layer on both CQT and QT plow tips was completely fractured, indicating that the overlaying carbide layer failed to protect the tips for a long time. The combined strengthening was achieved to integrate the high hardness plate martensite and strain-induced martensitic transformation in the carburized layer. The high hardness of plate martensite and diffusely distributed carbide organization in the carburized layer effectively hindered the abrasive particles from pressing into the substrate, and then weakened the cutting of abrasive particles on the plow tip material, particularly for the higher local resistance of the material to destructive deformation. At the same time, the residual austenite in the near-surface layer of the carburized layer produced significant strengthening, due to the dislocation strengthening and strain-induced martensitic phase transformation during the frictional wear of the plow tip. The strengthening layer was used to transfer the external load into a greater depth, thus triggering the dislocation movement at stresses above the yield point. As a result, the strain-induced martensite appeared within the slip band. As such, the internal microhardness gradient of the sample inward development resulted in significant work hardening, especially for the higher wear resistance of the CQT plow tip. Moreover, the surface area of the CQT plow tip after wear was significantly larger than that of the QT one. Correspondingly, there was a decrease in the load and friction heat release per unit area of the CQT plow tip, in order to reduce the creep softening tendency of the material to be worn. The higher wear resistance of the CQT plow tip was finally achieved in this case. The finding can shed light on the wear resistance of key soil-engaging components in the agricultural machinery and manufacturing industry.