Preheating method and preheating points before thermal spraying

       Preheat method



There are generally two preheating methods, namely furnace preheating and torch flame preheating. However, when using the flame preheating of the spray gun, it should be noted that the flame flow should not be too close to the surface of the workpiece, so as to avoid sudden heating on the surface of the workpiece, and non-uniform heating should not occur. This phenomenon will not only not improve the bonding strength of the coating, but also reduce the bonding strength between the coating and the substrate.



Precautions for Preheating



Since improper preheating may cause oxidation and deformation of the workpiece, special attention should be paid to controlling the preheating temperature and preheating method during implementation. For different base materials, different preheating temperatures and preheating methods should be selected. For ordinary steel, the preheating temperature is generally controlled at 80~120℃. For aluminum substrates, it is best to use indirect preheating (that is, preheating from the back or side, not directly on the spray surface, and oven preheating is not recommended), and the preheating temperature is generally controlled at 65~ 95 ℃ , If indirect preheating is impossible or inconvenient, preheating may not be performed, and during spraying, the substrate temperature should be kept below 200 °C. For magnesium and its alloy substrates, due to the rapid surface oxidation, it should not be performed. Preheating treatment, because preheating will produce oxide film on the sprayed surface, the existence of this oxide film will seriously affect the bonding strength of the coating and the substrate.



Different spraying parts, preheating temperature and preheating method should also be different. When spraying the inner hole, the preheating temperature should be higher than the preheating temperature when spraying the outer circle.



During spray welding, the surface preheating temperature is different from that of spraying. Careful selection should be made according to the thermal expansion characteristics and oxidation resistance of the base material, as well as the size and shape of the workpiece. When the preheating temperature is too low, not only the powder deposition rate is low, but also it is easy to fall off during melting. When the preheating temperature is too high, the base Severe expansion and serious oxidation will not only affect the wetting of the matrix and the micro-diffusion of atoms during remelting, but also lead to obvious slag inclusions in the spray-welded layer. Therefore, for ordinary carbon steel, the preheating temperature is usually 250~350℃, and the steel with good oxidation resistance, such as stainless steel, can be selected at 350~450℃. For smaller parts, the preheating temperature should be appropriately lowered; for larger parts, it can be appropriately increased; for parts with complex shapes, attention should be paid to the preheating range and location to reduce the tendency of deformation and cracking; for locally sprayed parts , local preheating should be used.