How to apply tungsten carbide spray coating?

How to apply tungsten carbide spray coating?

The construction of tungsten carbide spraying needs to be combined with specific processes (such as plasma spraying, supersonic flame spraying, etc.). The following are the construction processes and key points of the two mainstream processes of supersonic flame spraying (HVOF) and plasma spraying:

1、 Construction steps of supersonic flame spraying (HVOF)

Principle: Using fuel (such as propane, hydrogen) to burn with oxygen to generate a high-temperature and high-speed flame flow, tungsten carbide powder is heated to a molten or semi molten state, and sprayed and deposited onto the surface of the workpiece to form a coating.

1. Surface pretreatment

Oil and rust removal: Clean the surface grease of the workpiece with acetone or ethanol, and then remove the oxide scale by sandblasting (alumina or quartz sand, particle size 80-120 mesh), with a roughness of Ra 12.5-25 μ m, to enhance the adhesion of the coating.

Bottom treatment (optional): For low surface energy materials such as stainless steel, a nickel chromium (NiCr) transition layer (thickness 50-100 μ m) can be sprayed first to improve the bonding strength.

2. Equipment debugging and parameter setting

Fuel to oxygen ratio: propane: oxygen ≈ 1:5 (adjusted according to equipment model), flame velocity>1000m/s, ensuring sufficient powder melting and high particle flight speed to form a dense coating.

Spray distance: Control within 200-300mm. If the distance is too close, it may cause oxidation of the coating, while if it is too far away, the powder will cool too quickly and the adhesion will decrease.

Powder conveying rate: The conveying rate of tungsten carbide powder (particle size 15~45 μ m) is about 20~40g/min, which needs to be matched with the flame temperature to avoid powder melting or over melting.

3. Spraying operation

Uniformly moving spray gun: The angle between the spray gun and the workpiece surface is ≥ 60 °, and it moves uniformly along a straight or circular trajectory (speed 50-100mm/s). The thickness of a single spray is ≤ 0.2mm, and multiple layers are stacked to the design thickness (such as 0.5-1mm).

Control interlayer temperature: After each layer of spraying, wait for the workpiece to cool to * * ≤ 100 ℃ * * before proceeding to the next layer to avoid stress cracking of the coating caused by high temperature.

4. Post processing (optional)

Sealing treatment: For occasions with high corrosion resistance requirements, resin or ceramic sealant is used to fill the coating pores after spraying to improve density.

Mechanical processing: If high-precision surface is required, the coating roughness can be reduced to Ra 0.8~3.2 μ m by grinding or polishing.

2、 Construction steps of plasma spraying

Principle: Using high-frequency arc to generate plasma (temperature up to 16000 ℃), tungsten carbide powder is heated to a molten state and sprayed at high speed onto the surface of the workpiece to form a coating.

1. Surface pretreatment

Similar to HVOF process, strict degreasing and sandblasting are required to roughen the surface, with slightly higher roughness requirements (Ra 25-50 μ m).

2. Equipment debugging and parameter setting

Plasma gas: argon gas is used as the main gas (flow rate 30-50L/min), and hydrogen gas is used as the auxiliary gas (flow rate 5-10L/min) to increase the flame temperature and velocity.

Spray power: usually 30-50kW, adjusted according to the melting point of the powder (tungsten carbide melting point is about 2600 ℃) to ensure sufficient melting.

Spray distance: Control within 100-150mm. If the distance is too close, the coating may be burned, while if it is too far away, the molten particles will cool quickly and the bonding force will be poor.

3. Spraying operation

Stable arc current: After starting the arc, maintain the stability of the arc, and evenly feed the powder into the center of the flame flow from the powder feeding port. The spray gun moves at a speed of 30-80mm/s, and the single coating thickness is ≤ 0.3mm.

Cooling control: Large workpieces can be cooled synchronously with compressed air to avoid local overheating and deformation.

4. Post processing

Remelting treatment: Laser remelting or induction remelting is performed on the coating to eliminate pores and improve the microstructure, enhancing hardness and wear resistance (hardness can reach 1200-1500HV).

3、 Key precautions

Powder purity: Use composite powders such as WC-12Co (cobalt bonding phase), with a cobalt content of ≥ 10%, to ensure coating toughness and bonding strength, and to avoid brittle fracture of pure tungsten carbide.

Safety protection: During construction, it is necessary to wear dust masks and goggles, and operate in a closed sandblasting room or ventilated environment to prevent inhalation of tungsten carbide dust (long-term inhalation may cause "hard metal lung disease").

Process selection:

HVOF process: suitable for coatings with extremely high wear resistance requirements and thin thickness (such as valve sealing surfaces), coating porosity<1%, bonding strength>70MPa.

Plasma spraying: suitable for complex shaped workpieces or thick coatings (such as shaft repairs), but with high porosity (3%~5%), it needs to be combined with sealing treatment.