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Plasma surfacing technology is one of the effective technical methods to improve the wear resistance, corrosion resistance and impact resistance of metal surfaces. Plasma surfacing technology is a surface metallurgical method that uses the nozzle of the water cooling system to constrain the arc, uses the transferred arc as the heat source to obtain a high energy density plasma arc, and cooperates with alloy powder materials. The two parts of the plasma torch, the cathode electrode and the nozzle (anode), are connected to the negative and positive electrodes respectively. The working gas passes through the plasma torch and is ignited using high-frequency sparks. The gas is heated to a very high temperature (approximately) to produce a compression effect. Under the combined action of magnetic compression, thermal compression and mechanical compression, a plasma jet with extremely high temperature and extremely high speed is ejected from the nozzle. After the alloy powder enters the plasma beam, it is instantly heated and accelerated to form particle rays. The particles are in a molten or semi-molten state and are sprayed onto the surface of the workpiece to be processed. Physical and chemical changes such as melting, mixing, and solidification occur on the surface of the part. Finally, they are mixed with the surface of the part. Metallurgical bonding.
Characteristics of plasma surfacing due to the combined influence of thermal compression effect, magnetic compression effect and mechanical compression effect, the formation of non-transfer plasma arc can obtain or higher temperature, heat concentration, it can melt high melting point, high Hardness of powder material. Therefore, a coating with dense structure, low porosity, high bonding strength to the substrate and easy control of the coating thickness can be obtained. During the plasma surfacing process, the heat-affected zone is relatively small, and the deformation of the parts is small. During the surfacing process, the microstructure and properties are not changed, and the properties are not changed. Especially suitable for high-strength steel, thin-walled parts, and slender parts.
Plasma surfacing has the following advantages?
The heat affected zone is small and the structure does not change. The temperature of the workpiece can be controlled to less than ℃.
The coating contains less oxides and impurities, and is thicker, harder, and more anti-corrosive than electroplating, carburizing, and nitriding coatings.
The surfacing layer is metallurgically bonded to the workpiece, resistant to impact and will not fall off.
The coating is smooth and flat, and the thickness can be precisely controlled.
The cladding speed is fast and the dilution rate of plasma arc cladding can be controlled at one or lower.
The cladding layer has a dense structure and beautiful shape. The cladding process can be easily mechanized and automated.
Good controllability. The arc parameters such as the atmosphere temperature of the plasma arc can be adjusted by changing the power, changing the gas type, flow rate and the structural size of the nozzle, thereby achieving efficient automated production and improving labor productivity.
Wide range of materials used. As a deposited material, cladding alloy powder is not restricted by casting, rolling, wire drawing and other processing techniques. Alloy powders of different compositions can be configured according to different performance requirements. It is especially suitable for hard wear-resistant alloys that are difficult to wire but easy to powder. , to obtain the surfacing layer with the required performance.
With the development of modern technology, more and more valves work in high temperature, high pressure, high corrosion, high wear resistance and other environments. In order to improve the wear resistance, high temperature resistance and corrosion resistance of valves, it is now common in factories to Surfacing technology is used to increase the hardness of the valve shell or internal parts to improve the wear resistance and service life of the valve. The so-called overlay welding is to weld a layer of special wear-resistant, anti-corrosion and anti-oxidation materials on the surface of the workpiece. Make the workpiece have sufficient hardness, wear resistance and corrosion resistance to meet the use requirements of the valve, and be easy to operate. However, if you want to obtain a valve cladding surface that meets the design and usage requirements, you must strictly follow the work instructions and operating requirements. At the same time, you must select appropriate welding materials based on the base material (workpiece material) and welding method.
At present, commonly used cladding alloys include iron-based nickel-based cobalt-based and other cladding materials. Among them, cobalt-based alloys have good high-temperature properties, excellent thermal strength, corrosion resistance and thermal fatigue resistance, and have better wear and corrosion resistance than iron-based and nickel-based alloys. Therefore, in the development and production of valves, cobalt-based cemented carbide is widely used in the surfacing of various valve shells and sealing surfaces due to its irreplaceable and excellent comprehensive properties.
The process of plasma arc welding cladding valve requires mechanical cleaning of the workpiece before welding. The surface of the workpiece must have a metallic luster and remove rust, oil, and oxides. Some workpieces need to be preheated before surfacing. For example, welding of carbon steel with cobalt-based powder whose nominal size is larger than surfacing requires preheating. For surfacing iron-based powder, when the selected powder type and base material have preheating requirements, Preheating is required, and for pearlitic martensitic steels, even smaller parts must be preheated to prevent the occurrence of cracks in the cladding layer. The preheating temperature depends on the base material. Parts to be cladded in batches should be preheated in the furnace. The preheating temperature and time depend on the size and shape of the workpiece. The workpiece should be heat treated immediately after surfacing. For carbon steel workpieces using cobalt-based powder surfacing, the nominal size is smaller than that. Iron-based powder surfacing can be used without stress relief heat treatment. When the selected powder type and base material have heat treatment requirements, then Corresponding heat treatment is required.
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