Welding types are mainly divided into: 1. Shielded metal arc welding; 2. Submerged arc welding (automatic welding); 3. Carbon dioxide gas shielded welding (automatic or semi-automatic welding); 4. MIG/MAG welding (Gas Metal Arc Welding); 5. TIG welding (Gas Tungsten Arc Welding); 6. Plasma arc welding. Each method has different principles and applications. Since this is not the focus of this article, a general understanding will suffice. Below, we will focus on the welding technology of ball valves:

　　Welding ball valves is different from ordinary welding techniques because the valve cavity contains non-metallic sealing materials such as rubber and polytetrafluoroethylene plastic, which cannot undergo post-weld heat treatment. The multi-layer thick-wall welding process of ball valves is a process of repeated heating and cooling of metal materials. Typically, its weld joints are designed as narrow-gap thick-wall submerged arc welds, and some use fully welded valve bodies, mostly those with extra-large thickness cylindrical weld joints.

　　Therefore, stress concentration at the root of the valve body weld joint, residual stress, and deterioration of the microstructure become weak links in the valve body structure. The two current treatments are: 1. Heat treatment; 2. Joint treatment. These two treatments will cause stress concentration several times higher than the normal working stress, making it difficult for engineers to handle.

　　Therefore, how to solve the stress concentration, residual stress, and microstructure deterioration at the root of the weld joint of the ball valve body has become the biggest problem in valve welding. Here, we summarize the key points of the valve body welding technology, hoping to help friends who want to understand ball valve welding technology!

　　The welding process of this valve body is introduced as follows.

　　1. Technical Requirements of the Valve Body

　　(1) After welding, the valve body is free of deformation, including the upper and lower shaft flange (45 steel).

　　(2) The heated area of the valve body cannot be too wide, and the temperature cannot be too high, otherwise it will damage the sealing device inside the valve body.

　　(3) No weld bead is allowed.

　　(4) Ensure that the pressure resistance is above 10 MPa.

　　　2. Weld Structure

　　Based on the above technical requirements, the weld is determined to be a non-penetrating weld. Therefore, the joint form of the weld must ensure both assembly and non-penetration. After testing, the final weld form is determined.

　　3. Valve Body Material

　　Because this valve body is ZG15Ⅱ, the chemical composition of the valve body was tested before processing. The test results are consistent with the results of the casting, meeting the design requirements.

　　4. Assembly

　　Using a modified oil press to position the inverted tire, and fastening the two halves with bolts, adjustment, and performing a sealing test, the external ball and the valve body meet the design requirements.

　　5. Welding Process

　　Considering the technical requirements of the valve body, MAG CO2 welding machine was finally selected.

　　The welding material is ER50-6 (Ø1.0) welding wire.

　　According to the principle of equal strength of welding materials, the selected ER50-6 (Ø1.0) welding wire meets the requirements of ZG15Ⅱ.

　　The shielding gas uses a mixture of 82% Ar + 18% CO2.

　　 5.1 Positioning Weld

　　Due to the serious hardening tendency of cast steel and 45 steel, preheating is required before positioning welding. Use an oxy-acetylene flame to uniformly heat the area on both sides of the weld seam to 2 times the plate thickness to 75~100℃ (leaning towards the 45 steel side), ensuring that the sealing device is not affected, measured using a far-infrared thermometer.

　　Before welding, remove the scale and rust within a 20mm range on both sides of the weld seam using an angle grinder to achieve a metallic luster.

　　Place 6 positioning points 50mm away from the edge on both sides of the weld seam and measure the distance between the two points with a caliper to measure the shrinkage.

　　 5.2 Bottom Weld

　　Considering the characteristics of high rigidity and poor plasticity of ZG15Ⅱ material, small welding parameters, i.e., small welding heat input, should be adopted in the selection of welding specifications. Under the premise of ensuring weld quality, try to increase the welding speed, reduce the fusion ratio, and adopt a multi-layer, multi-pass, 6-segment symmetrical welding method for deformation control; pay attention to measuring the positioning points with a caliper for each layer and controlling the shrinkage amount until the welding is completed. Except for the first and last layers, stress relief is performed by hammering between other layers.

　　6. Final Step

　　After welding is completed, the weld is leak-free after pressure testing (airtightness), and the sealing surface meets the technical requirements. After X-ray inspection, the weld itself has no porosity, slag inclusion, lack of fusion, cracks, and other welding defects, and the valve core rotation meets the technical requirements.