Anti-wear Technology & Spraying Process for Water Wall of CFB Circulating Fluidized Bed Boiler

There are numerous thermal spraying manufacturers in the current market, among which supersonic electric arc thermal spraying is widely adopted for anti-corrosion and wear-resistant thermal spraying treatment on metal surfaces.

Apart from anti-wear beams, blue mud, surfacing welding and flow guide plate anti-wear technology developed by us, thermal spraying was once a highly popular anti-wear process for water walls of CFB circulating fluidized bed boilers.

Widely applied thermal spraying methods include flame spraying, electric arc spraying, plasma spraying, detonation spraying and supersonic spraying. Their technical characteristics and application scopes are elaborated as follows:

It is implemented via flame spray guns. High-temperature flame generated by oxy-acetylene combustion melts spraying materials, and surrounding compressed air sprays molten materials or particles to make them adhere to the substrate surface. Featuring simple operation and uncomplicated equipment, this technology is widely used in industry, including wire flame spraying and powder flame spraying.

Two consumable metal wires serving as spraying materials are electrified to form short circuit and generate continuous electric arc, which melts the wire ends. High-speed cold air jet atomizes the molten metal and sprays it onto the substrate surface. Coating wires are automatically fed by wire feeding wheels. When heavy current passes between the two wires, electric arc forms to melt the wires rapidly, and compressed air breaks the molten metal into tiny droplets to form coatings on the base material surface.

Direct current arc discharge partially ionizes high-temperature argon, nitrogen, helium and other gases into ion beams. Low-temperature gas flows around the arc zone to form thermal shrinkage effect, narrowing the arc section and raising energy density and current density, with the maximum temperature reaching 20,000℃. It is applicable to powdery coating materials. The plasma spray gun converts electric energy into thermal energy to produce high-temperature and high-speed plasma jet with temperature up to 50,000℃, capable of melting all spraying materials.

Thanks to ultra-high temperature and controllable atmosphere, it can spray various high-melting-point metals, oxides and other materials. Vacuum plasma spraying equipment developed in the past decade has expanded coating categories, improved coating quality, and realized new material synthesis and surface modification.

It utilizes the energy released by the detonation of mixed flammable gas and oxygen. Combustion and detonation generate heat energy and shock waves; heat melts spraying powder, while shock waves eject molten powder onto workpieces at a speed of 700-800 meters per second to form coatings.

Its core advantage lies in ultra-high particle flying speed and strong kinetic energy, suitable for spraying metals, cermets and ceramic materials. However, it is not widely used domestically and overseas due to high equipment cost, loud noise and oxidizing working atmosphere.

Short for High Velocity Oxygen Fuel spraying. Gaseous or liquid fuels such as hydrogen, propane and propylene are mixed with high-pressure oxygen and combusted in specific combustion chambers or nozzles to form high-temperature and high-speed flame flow, which melts and accelerates powder materials to form coatings on workpiece surfaces.

Adopting oxy-propane or oxy-propylene as fuel, its spraying speed doubles the speed of sound, and the velocity of molten powder particles can hit 400 meters per second, 4 times that of flame spraying and 2 times that of plasma spraying. The formed coatings are denser with higher bonding strength, ideal for carbide coating spraying. The ultra-high particle impact speed greatly improves the bonding strength, hardness, compactness and wear resistance of coatings.

1. Pretreat the material surface: cleaning, sand blasting and drying
2. Spray multi-element composite alloy powder on the surface by HVOF equipment (gun-workpiece distance: 17-23 cm, gun moving speed: 27-35 m/min, methane flow: 35-45 L/min, oxygen flow: 35-45 L/min, nitrogen flow: 24-34 L/min, alloy powder flow: 35-45 g/min)
3. Conduct laser irradiation

This process can refine and homogenize the microstructure of laser cladding layers, eliminate cracks, pores and other defects, and greatly enhance the overall quality of cladding layers.

Electric arc melts continuously fed wire materials at wire ends. Supersonic airflow accelerated by Laval nozzle atomizes molten wires into fine and evenly distributed particles to form coatings on workpieces. Molten particles combine with the base material through mechanical, physical and metallurgical bonding with bonding strength over 60MPa.

Compared with ordinary electric arc spraying and flame spraying, it features faster particle flying speed, higher bonding strength, lower porosity, uniform and dense coatings. The surface temperature of workpieces is kept below 100℃ during construction without workpiece deformation, so as to prepare high-quality coatings. It is a continuous process integrating melting, atomization and deposition.

Pores inevitably exist in thermal sprayed coatings, even HVOF wear-resistant coatings have a porosity of 0.1%~0.9%. In humid environments, the base material is prone to corrosion, resulting in coating peeling and equipment failure. The special sealing agent for thermal sprayed wear-resistant coatings can effectively improve the corrosion resistance and lubricity of wear-resistant coatings.