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Tungsten-carbide coating was prepared by supersonic flame spraying
Oct 26, 2018

Supersonic flame spraying, high velocity oxyfuel spraying, HVOF) is on the basis of the ordinary flame spraying in the early 1980 s developed a kind of new thermal spraying technology.It USES hydrogen, acetylene, propylene, kerosene and other fuels, and USES oxygen as the fuel booster. It burns in the combustion chamber or special nozzle, generating supersonic combustion flame with a speed of over 2100m/s. At the same time, powder is sent into the flame to produce melted or semi-melted particles.Supersonic flame spray has the advantages of high flame speed, low temperature, high bonding strength, high density, high hardness, low porosity and low oxide content.The technology has been widely used in aviation, metallurgy, spinning machine, automobile, railway and other fields, representing the development direction of modern thermal spraying technology.Supersonic flame spray technology brings remarkable social benefits as well as economic benefits.According to market estimates, supersonic flame spraying technology will have 25% of the market in the thermal spraying industry in 2015.This technology has the relatively low flame temperature (2600 ~ 3000 ℃), high flame flow speed and the spraying process can effectively restrain and reduce the decomposition of tungsten carbide, coating obtained preserves the wear-resisting properties of tungsten carbide phase, so the process particularly suitable for spraying of tungsten carbide materials wear-resisting performance is good, the tungsten carbide coating materials obtained can provide very good wear resistance and corrosion resistance.

The preparation of tungsten-carbide coating on 45# steel surface by supersonic flame spraying has been reported recently by central south university.According to the composition of WC10Co4Cr, they prepared raw materials with WC, Co, Cr and other raw materials. The raw materials were prepared by mixing ball grinding, spray granulation and vacuum sintering, and then the powder particle size was controlled by crushing screen, classification and batch.After testing, the obtained powder has high globular degree, good fluidity (~ 13s /50g), similar density (4.8~ 5.0g /cm3), and a single powder shows loose structure.The powder phase is both WC and Co phase.The substrate material of the coating sample is 45# steel. The surface of the substrate is treated with 50~70 eyes white corundum sandblasting before spraying, and then the surface of the sample is cleaned with compressed air.The tungsten carbide coating was prepared with Praxair JP8000 supersonic flame gun fixed by mechanical arm.

The test of the coating shows that the thickness of the coating is 300 microns, the deposition efficiency is 55.15% and the porosity is 0.32%.The coating phase is mainly composed of WC phase, which also contains a small amount of W2C phase, amorphous phase or nanocrystalline phase, and no W or WC phase (Co6W6C and Co3W3C), which indicates that the particle stays in the flame stream for a very short time, and the powder decarburizes to a small extent.In the experiment, the oxygen and kerosene flow were set unchanged, that is, the flame temperature was controlled consistent when spraying.Combined with a new type of spray gun and an efficient cooling system, the degree of decarbonization is reduced.The main reasons for the low decarburization degree of the coating are :(1) there is no significant phase in the powder;(2) the design of oxygen flow and kerosene flow is conducive to obtaining relatively low flame temperature.

The test of hardness, abrasive wear and salt spray corrosion resistance showed that the microhardness of the coating reached nearly 1300HV and the volume wear amount was relatively small.Under the corrosive environment of neutral salt fog for 360 hours, the coating still remains smooth and has metallic luster, and no corrosion occurs, indicating that the coating has strong corrosion resistance.As Cr can easily form a very thin Cr2O3 ceramic layer on the coating surface, the corrosion resistance of the coating is improved.

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