Preparation of Tungsten Carbide Base Coatings by Supersonic Flame Spraying

       High velocity oxyfuel spraying (HVOF) is a new type of thermal spraying technology developed on the basis of ordinary flame spraying in the early 1980s. It uses hydrogen, acetylene, propylene, kerosene, etc. as fuel, and oxygen as combustion accelerant. It burns in a combustion chamber or special nozzle to generate a supersonic flame with a speed of more than 2100m/s, and at the same time, the powder is fed into the flame. Melting or semi-melting particles are produced, which are deposited on the surface of the substrate by high-speed impact to form a coating. Supersonic flame spraying has the advantages of high flame speed, low temperature, high coating bonding strength, good compactness, high hardness, low porosity and low oxide content. The process has now developed into a large number of applications in aviation, metallurgy, textile machinery, automobiles, railways and other fields, representing the development direction of modern thermal spraying technology. Supersonic flame spraying technology not only brings significant social benefits, but also has significant economic benefits. According to market estimates, in 2015 the HVOF process will have a 25% market share in the thermal spray industry. This technology has relatively low flame temperature (about 2600~3000℃), high flame velocity, can effectively inhibit and reduce the decomposition of tungsten carbide during the spraying process, and the obtained coating can retain the wear resistance of the tungsten carbide phase. The process is particularly suitable for spraying tungsten carbide materials with good wear resistance, and the obtained tungsten carbide coating materials can provide very good wear resistance and corrosion resistance.

The work of preparing tungsten carbide-based coating on the surface of 45# steel by supersonic flame spraying method has achieved good results. They used WC, Co, Cr, etc. as raw materials, and prepared raw materials according to the composition of WC10Co4Cr. The raw materials were subjected to stirring ball milling, spray granulation and vacuum sintering to obtain powder blocks, and then through crushing, sieving, grading and batch control. The particle size of the sprayed powder is 15 ~45μm. After testing, the obtained spray powder has high sphericity, good fluidity (~13 s/50g), similar powder bulk density (4.8~5.0 g/cm3), and a single powder has a loose structure; the powder phases are both WC and Co. Mutually. The substrate material of the coating sample is 45# steel, and the surface of the substrate is sandblasted with 50-70 mesh white corundum before spraying, and then the surface of the sample is blown with compressed air. The tungsten carbide-based coatings were prepared using a Praxair JP8000 supersonic flame spray gun fixed by a robot.

The test of the obtained coating shows that the thickness of the coating reaches 300 microns, the deposition efficiency reaches 55.15%, and the porosity is 0.32%; the coating phase is mainly composed of WC phase, and also contains a small amount of W2C phase and amorphous or nanocrystalline phase. There is no W or η phase (Co6W6C, Co3W3C), indicating that the particles stay in the flame for a very short time, the degree of decarburization of the powder is small, and the WC hard phase in the powder is preserved in the coating, which is conducive to the wear resistance of the coating. Sexual enhancement. In the experiment, the flow rate of oxygen and kerosene was set unchanged, that is, the temperature of the flame was controlled to be the same during spraying; in addition, a new spray gun and a high-efficiency cooling system were used to reduce the degree of decarburization. The main reasons for the small degree of decarburization of the coating are: (1) there is no η phase in the powder; (2) the design of the oxygen flow and kerosene flow in this experiment is beneficial to obtain a relatively low flame temperature, (3) the speed of the spray gun is fast and the spraying The cooling of the process compressed air keeps the coating less heated.

The tests of hardness, abrasive wear and resistance to salt spray corrosion show that the microhardness of the coating reaches nearly 1300HV, and the volume wear is quite small; the coating still maintains a smooth surface and metallic luster in a neutral salt spray corrosion environment for 360 hours , no corrosion occurred, indicating that the coating has strong corrosion resistance. Since it is easy to form a thin Cr2O3 ceramic layer on the coating surface after adding Cr, the corrosion resistance of the coating is improved.