The key points of spray welding process for iron castings

       

The key points of spray welding process for iron castings are as follows

① There are quite a lot of parts made of cast iron in vehicles and mechanical equipment. It is inevitable that there are various problems in the process of manufacture or use. Adopting oxygen acetylene flame spray welding process is not only an effective method to strengthen cast iron, but also an ideal means to repair various defects and damages (such as sand holes, air holes or wear and other damages in use) of castings;

② Spray welding is mostly used to repair local defects of iron castings, and the defects are different in size and depth, so it is suitable for one-step spray welding. According to the workpiece and repair parts, small power spray gun should be used as far as possible, so as to reduce the heat input to the matrix. Generally, QH-1 / h, QH-2 / h, qh-4 / h, etc. are selected;

③ In spray welding, because of the poor weldability of cast iron parts, nickel based alloy powder (ni-b-si Series) should be preferred. The melting point of nickel based alloy powder is generally 950 ~ 1050 ℃, and the matrix will not be melted during remelting. At the same time, the spray welding layer of nickel based powder with low carbon content has low hardness and good plasticity, which can relax the spray welding stress and prevent cracks. At this time, it is very important to operate unskilled;

④ When selecting the spray welding specification, the material of cast iron, the size of defects and the requirements of working conditions should be considered. On the premise of ensuring the necessary flame energy, the heat input to the matrix should be reduced as far as possible. The lower limit of oxygen and acetylene gas pressure should be taken. During powder spraying and remelting, the spray and melting distance should be adjusted appropriately to control the heat input;

⑤ Small local defects, such as air holes and sand holes, may not be preheated before spray welding, so as to minimize the heating area around the spray welding layer and minimize the high temperature area.

⑥ Spray welding is suitable for continuous spray welding for local small and deep defects. This method has high powder deposition rate, rapid thickness growth and high efficiency, but requires skilled operation, so as to coordinate powder feeding rate with spray and fusion speed, and achieve uniform spray and penetration;

⑦ For the defects with large area and deep depth, in order to prevent the matrix from being heated too much and causing the thermal stress to increase, the intermittent spray welding method can be used. If necessary, the composite process of electric welding and spray welding can also be used. The welding rod can be used to fill the bottom and spray welding the upper part. If the wall thickness of the casting is large, the wire can be planted before spraying, which can not only increase the bonding strength, prevent the peeling of the spray welding layer from the base metal, but also eliminate part of the thermal stress of spray welding;

⑧ The thermal stress of spray welding large complex castings is large, so the measures should be taken to reduce the stress accumulation, such as heating stress reduction method and sectional symmetry method; Preheating before welding and slow cooling after welding can achieve good results;

⑨ If the remelting temperature is too high, not only the alloy elements will be burned, the matrix will be overheated, or even the matrix will be melted, resulting in the carbon in cast iron entering the spray welding layer, which will increase the carbon content in the spray welding layer, improve the hardness, and reduce the plasticity. Because the carbon content in the matrix is reduced, it is easy to appear white mouth. Moreover, if the temperature is too high, it will cause greater thermal stress, leading to cracks. The remelting temperature should not be too low, Otherwise, it is easy to form ash inclusion or incomplete fusion, which will affect the bonding strength. The control of the remelting temperature mainly depends on the operator's attention to observe the change of the coating surface state during remelting. With the help of the "mirror reflection" during remelting, the "mirror reflection" indicates that the powder has melted and the slag has floated up. At this time, the remelting gun should be removed immediately, and the remelted part should avoid repeated heating.

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