Corrugated Roll Tungsten Carbide Cobalt Coating Reinforcement

Thermal spraying technology is a key technology in the field of surface modification. As a wear-resistant and corrosion-resistant coating, tungsten carbide/cobalt coating is widely used in aerospace, metallurgy, machinery and other fields because of its high hardness and good toughness. Compared with hard chromium plating, thermal sprayed WC-Co coating has obvious advantages in wear resistance, corrosion resistance, fatigue resistance and so on, and has the advantages of rapid preparation and less environmental pollution.

However, when using traditional WC-12Co thermal spray powder, the higher the hardness of the coating, the lower the toughness. According to the actual data, when the hardness is increased by 2.4%, the fracture toughness index of the corrugated roll can be reduced by 8.6%; The hardness decreases obviously; the contradiction between hardness and toughness makes it impossible to balance the hardness requirements and toughness requirements during spraying, and it is impossible to ensure that the quality of the corrugated roll coating is optimal.

To this end, the researchers changed the formula of WC-12Co coating, developed a new WC-12Co thermal spraying powder, composed of tungsten carbide, cobalt and trace titanium carbide metal, tungsten carbide mass percent, metal cobalt mass percent is 88%, metal cobalt mass percent is 11.4%, inhibitor titanium carbide mass percent is 0.6%. The process is as follows:

1. Mixture
Tungsten carbide consists of Nanocrystalline Tungsten Carbide and medium-sized tungsten carbide in proportion. The mass percentage of Nanocrystalline Tungsten carbide is 22-66% and that of medium-sized tungsten carbide is 22-66%. According to the technological requirements, Nanocrystalline Tungsten Carbide of 44 kg (22%), tungsten carbide of 132 kg (66%) and cobalt powder of 22.8 kg (11.4%) were added to the ball mill, and then deionized water of 25% of the raw material, 50 kg of the deionized water at 5-8 ℃ and 4.0 kg (2.0%) of polyvinyl alcohol were added to the ball mill. The equipment could be rolled in tilting mode. The ball mill or vertical stirred ball mill has a ball milling time of 22 hours.

2. Granulation
Due to the existence of Nanocrystalline Tungsten carbide, the flow properties of the materials are poor. Therefore, the open centrifugal spray drying tower is used to dry granulation, and the spherical mixture is obtained to make the ultrafine powder with poor flow achieve good flow performance after being aggregated into large particles, and the rotating speed of the centrifugal atomizer is 8000 revolutions / min.

3.Sintering
The mixture was loaded into a three-layer graphite boat and sintered continuously in a molybdenum wire furnace in a hydrogen-protected atmosphere at a speed of 12 minutes per boat and a sintering temperature of 950 C (zone 1), 1150 ℃ (zone 2), grain growth inhibitors and Nanocrystalline Tungsten Carbide selectively heat-treated below the grain growth temperature. The grain growth inhibitors are more effectively dispersed and ultrafine materials are formed, in which the grain growth inhibitors are uniformly distributed in the grain boundaries.

4. Fragmentation classification
After sintering, the product was broken by mechanical method, and the final thermal sprayed powder was obtained by sieving or air flow classification according to the particle size distribution. The particle size of the powder was 15-45 micron 65kg, 20-53 micron 130kg. The loss of metal was about 5kg (2.5%) during the production process, and the bulk specific gravity was 4.9-5.1g/cm³.

The new WC-12Co thermal spraying powder has good fluidity, excellent heating performance, excellent hardness and toughness. According to the actual data, compared with conventional thermal spraying powder, the toughness of the new thermal spraying powder is 4% higher than that of conventional powder when the same hardness is obtained, and the toughness index of the new thermal spraying powder is only reduced by 2.11% when the hardness is increased by 3.06% by adjusting parameters. When the hardness of lattice powder increased by 2.4%, the fracture toughness index decreased by 8.6%. Therefore, the powder is suitable for coating with high wear resistance and toughness.

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