| Since the 1980's, China's mold industry entered a period of rapid development and has become an important part of industrial production. Molds are the most consumed parts in industrial production and ways to maximize its service life has been a research focus at home and abroad. China's steel production molds' variety, specifications and quality is far behind Sweden, Japan, the United States and other developed countries, the service life of molds is only 1/2-1/3 that of imports. Molds rely mainly on their surface to work, therefore, more than 80% of failures is due from surface damage, such as wear, fatigue, and corrosion. Therefore, the development and application of surface modification technology is the key to improving mold lifetime. Not only can the use of surface technology repair a failed mold, it can also improve the surface hardness of the mold and change the surface chemical composition and, in turn, can improve wear resistance, anti-adhesion properties, fatigue resistance and corrosion resistance.This paper introduces several common hot die surface treatment methods and their advantages and disadvantages. These include thermo-chemical treatment, surface hardening, arc welding, thermal spraying, brush plating, laser cladding technology and spark machining. Afterwards, H13 hot die steel is widely used as an object of study, so that it is adequate for a tough service environment, it is sprayed with a WC system ceramic powder and self-fluxing alloy powder Ni60A. As WC-12Co powder has a high melting point, and is susceptible to oxidation at high temperature, this study used the HVOF process. The properties of the coating structure and performance can be detected using:scanning electron microscopy, X-ray diffraction, micro hardness tests, combined with strength testing, high temperature coefficient of friction and high temperature wear resistance tests.The results show that:(1) By observing the powder particles deposited on the substrate of the state, the spray particles melt in a way to form a deformation and coating mechanism. The results showed that spraying with appropriate parameters, the composition of the coating particles are in a completely molten and semi-molten state, then there is a higher deposition efficiency and the result is relatively strong, obtaining an excellent quality coating.(2) Both the domestic and imported WC-12Co powders result in homogeneous and dense coatings, low porosity of 1.5%, a relatively high deposition rate; using Ni60A powder spray, they can result in a relatively dense coating with the porosity of 2.7%, while its deposition rate is relatively low. We can extend the spraying time to obtain a coating with the same thickness as WC-12Co samples, but microcracks were easily caused due to the thermo-stressconcentration.(3) The domestic and imported WC-12Co powders sprayed under parameter:kerosene flow 20L/h (pressure:0.72Mpa), oxygen flow rate 30m3/h (pressure:1.3Mpa), feeding gas flow rate 700ml/min (pressure:0.4Mpa), spray distance of 350mm obtained bonding strength of more than 70Mpa; while domestic WC-12Co coating with 62.7Mpa and imported one with more than 70Mpa when the kerosene flow decreased to 16L/h (pressure:0.64Mpa). The Ni60A coating's tensile fracture surface within the coating, has a bonding strength of 51.8Mpa.(4) The imported WC-12Co coating has the highest hardness, an average of 1370HV0.2 the domestic WC-12Co coating, reached 1304HV0.2, the Ni60A coating had a relatively low hardness of 820HV0.2 These three coatings at room temperature, had a friction coefficient close to 0.32 or so, but at 500℃, the friction coefficients were 0.54,0.52 and 0.59. For high temperature wear and tear, the WC coating system has excellent red hardness and wearing resistance than Ni60A coating, with the weight lose is only 1/4-1/3 compared to the latter.(5) Domestic WC-12Co powder can be used as an economical and effective coating material for the modification and repair on the H13 hot die steel surface, the appropriate spray parameters are:kerosene flow 20L/h (pressure:0.72Mpa), oxygen flow rate 30m3/h (pressure:1.3Mpa), feeding gas flow rate 700ml/min (pressure:0.4Mpa), spray distance of 350mm. |
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