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            Spray welding is to reheat the preheated self-soluble alloy powder coating to 1000-1300 C, melt the particles, float the slag onto the surface of the coating, and disperse the boride and silicide in the coating, so as to achieve good bonding between the particles and the surface of the substrate.
            Spray welding process is suitable for the following situations:
            1. Surface loads of various kinds of carbon and low alloy steels are large, especially those subjected to impact loads. For workpieces with bonding strength of 350-450N/mm2, spray welding hardness HRC150 < 65, coating thickness from 0.3 to several millimetres, and surface roughness of spray welding layer after grinding can reach Ra0.4-0.1 micrometres.
            2. When used in corrosive media, the coating is required to be compact and porous.
            3. Hardening, carburizing, nitriding and hard chromium plating are used in the original design of workpiece surface, which requires high hardness.
            4. Workpiece working environment is bad, such as strong abrasive wear, erosion wear, cavitation and so on.
            5. Oxygen-acetylene flame alloy powder spray welding process is suitable for surface strengthening or repairing of various carbon steel and low alloy steel parts, but some characteristics of parts material should be noticed. When the linear expansion coefficient of matrix material differs greatly from that of alloy spray welding layer, it should be used cautiously when the linear expansion coefficient of matrix material is less than 12 *10-6/~C and more than 12 *10-6/~C, so as to avoid manufacturing. Cracks can also be difficult to spray welding if the content of elements with high affinity to oxygen in matrix metals is more than 3%, the total content of elements such as tungsten and molybdenum is more than 0.5%, or the sulfur content in steel is more than 0.5%. This is because these materials are easy to form dense and stable oxide film by interaction with oxygen to prevent melting. The wetting effect of the alloy on the matrix will cause the liquid alloy to roll like a "sweat bead" during remelting. Therefore, the adaptability of the spray welding process to the sprayed matrix material should be paid attention to.
            6. Metal materials that can be sprayed without special treatment:
            7. Metal materials that need to be preheated at 250-375 C after spray welding need to be slowly cooled.
            8. Metal materials requiring isothermal annealing after spray welding