In order to overcome the coordination between surface wear resistance and impact toughness of transmission parts (such as gears and shafts), surface chemical heat treatment is needed for surface chemical heat treatment. It is a special composite heat treatment technology, through vacuum carburizing to form a carburizing layer with high hardness and wear resistance, followed by critical annealing treatment can improve the properties of the modified layer without changing the matrix material, such as wear resistance. In this work, the effect of critical annealing heat treatment on the microstructure, micro-hardness and wear resistance of vacuum carburizing layer was investigated. Critical annealing post-treatment was used to ensure the surface wear resistance of 17CrNiMo6 carburized steel while reasonably regulating the surface hardness. Vacuum carburizing was carried out at 930 ℃ in a vacuum carburizing furnace (ECM, ICBP-200-TG, including boost and diffusion), followed high temperature tempering (500-650 ℃), quenching (850-880 ℃), cryogenic treatment (‒80 ℃) and low temperature tempering (150-180 ℃), then by critical annealing heat treatment at 790 ℃×10 min and 840 ℃×20 min for the preparation process. The microstructure and phase composition of the modified layer were observed by optical microscope (OM, OLYMPUS-311U), scanning electron microscope (SEM, SU5000), energy dispersive spectrometry (EDS, ULTIMATELY MAX40) and X-ray diffraction (XRD, PANalytical B.V XPert Pro PW3040/60). The hardness and tribological properties of the modified layer were studied by Vickers hardness tester (HV-1000) and high-temperature friction and wear tester (HT-1000), etc. The 3D morphology of wear scars and wear rate were observed by laser confocal microscope. A detailed analysis of the microstructure evolution, element distribution, hardness and tribological behavior of the modified layer after critical annealing heat treatment was carried out. As the number of critical anneals heat treatment increased, the surface martensite content decreased, the residual austenite increased, while the tempered martensite in the core remained essentially unchanged, and the surface scan EDS maps showed a more uniform elemental distribution. The corresponding surface hardness showed a decreasing trend, while the core hardness was basically unchanged, and the surface hardness of the 1 cycle heat treatment was the highest (730HV0.5). In the multiple critical annealing heat treatment, the wear resistance was in the order of 1 cycle heat treatment > 2 cycles heat treatment > 3 cycles heat treatment. Among the cyclic heat treatment samples, compared with the 3-cycles heat treatment, the friction coefficient was reduced by 42.8% after 1 cycle heat treatment, and the wear rates between the modified layers were 2.07×10‒13 m3/(N·m) (930 ℃ carburized treatment specimens), 0.71×10‒13 m3/(N·m) (1-cycle heat treatment specimens), 5.23×10‒13 m3/(N·m) (2-cycles heat treatment specimens) and 4.21×10‒13 m3/(N·m) (3-cycles heat treatment speci-mens). The wear mechanisms of the modified layer in the room temperature environment were abrasive wear, oxidation wear and adhesive wear. After critical annealing heat treatment of 17CrNiMo6 carburized steel, the modified carburized layer with both high hardness and wear resistance was obtained. To a certain extent, reasonably regulation of the number of critical annealing cycles heat treatment will improve the wear resistance and core toughness of the carburized modified layer. However, vacuum carburizing composite critical annealing heat treatment provides the theoretical basis for the later heat treatment modification technology. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.
