| Mold play an important role in modern industrial production. With the continuous development of industrial technology in China, various performance requirements of mold increases. Often due to die in high temperature and high pressure, such as poor working environment lead to the surface of the mold wear and thermal fatigue problems. It causes mold scrap, and the problem of the mold scrap accounts for above 80% of the total number, serious impacting on productivity. High velocity oxygen fuel (HVOF) technology as a new surface modification technology, spray powder is WC-Co. Compared with other thermal spraying technique, HVOF has obvious advantage:fast firing rate of spraying powder, coating low spraying temperature reducing decomposition rate of WC and cost of production. The hardness, density, wear resistance, and corrosion resistance of coating are higher. But still there are some defects in the coating, such as porosity, that reduces the quality of the coating. In order to further improve the microstructure, to enhance the mechanical properties such as hardness, wear resistance of the coating, the cryogenic treatment technology of coating processing is adopted. The purpose is to improve the material’s microstructure, and at the same time also can improve the performance of materials, including the density, hardness, strength, dimensional stability, the cutting performance and corrosion resistance, etc. Coolant, time of heat preservation, heat preservation temperature and the order of process have more influence on treatment effect in cryogenic treatment.This article selects the HVOF process of 40 Cr steel spraying WC-10Co-3Cr coating, and adopt different cryogenic process on the coated parts processing. By single factor variable method, this paper discusses the thermal insulation of cryogenic treatment temperature and holding time on microstructure and mechanical properties, corrosion resistance of the coating. Using scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively, on the coating surface to analyze morphology, and crystal structure; Using the digital micro Vickers hardness tester to test the coated with a coating of micro hardness; examines the cryogenic treatment process conditions on the coating, coating microstructure porosity and micro hardness. The results showed that: (1) The lower the heat preservation temperature, and the longer the holding time, the lower void fraction of the coating. Void fraction from 4% without cryogenic treatment when reduced to under -196C° heat preservation after 24 h of 2.8%.(2) The heat preservation time of 2 h, WC grain to contract at low temperature, the lower the heat preservation temperature, WC grain size is smaller;-196C° for heat preservation temperature, WC solubility in the Co is reduced, the WC carbide, preferential attachment on the original WC grain growth. The longer the time of heat preservation, precipitation is, the greater the WC grain size. Micro hardness of the coating is closely relative to the WC grain size, the greater the WC grain size, the greater the coating micro hardness, and vice versa.(3) Cryogenic processing make α-Co (fcc) the ε-Co (hcp) martensitic phase transformation, the longer the holding time shift more fully. The other η has refined and increase trend. These factors are beneficial to the improvement of the micro hardness of 40Cr steel WC-10Co-3Cr coating for WC grain size increases, the coating of micro hardness increase;(4) Cryogenic treatment reduce coating porosity and make the Co martensite-transformation, reducing the gap in the stress concentration and plastic of the coating. Thereby reducing the friction coefficient of the coating, improve wear resistance.(5) Cryogenic treatment on the coating resistance corrosion influence is complex, hand to reduce void ratio and makes the corrosion channels decreases and the corrosion resistance of the coating is improved, on the other hand, the shrinking of Co phase and martensitic transformation decrease the coating resistance corrosion. The synthesis of cryogenic treatment on the corrosion resistance of coating was not significantly improved. |
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