Technical requirements for the welding process of ball valve bodies

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Technical requirements for the welding process of ball valve bodies: The structure of a certain ball valve’s body has been changed from a split-type to an integrated type, and the technical challenge lies in the welding process. The welding process for this valve body is described as follows. 　　1. Technical requirements for the valve body 　　(1) After seam welding, the valve body shall show no deformation, including the flanges at the upper and lower shaft ends (made of 45 steel). 　　(2) The heating surface area of the valve body should not be too large, nor should the temperature be too high, otherwise it will damage the sealing mechanism inside the valve body. 　　(3) Weld beads must not appear. 　　(4) Ensure the pressure is above 10 MPa. 　　2. Weld structure 　　Based on the above technical requirements, it is determined that this weld shall be a underfilled weld. Therefore, the butt joint configuration of the weld must ensure both proper assembly and prevention of underwelding. After testing, the finally determined weld form is shown in Figure 1. 　　3. Material of the valve body: Since the valve body is made of ZG15Ⅱ, its chemical composition was analyzed before processing. The analysis results were consistent with those obtained during casting, meeting the design requirements. The chemical element contents and mechanical properties of ZG15Ⅱ are shown in Table 1 and Table 2. 　　4. Assembly: Use a hydraulic press to modify the flip tire and position it as shown in Figure 1; then fasten the two halves around it with bolts. Conduct a sealing test to ensure that the gap between the outer surface of the sphere and the valve body meets the design requirements. 　　5. Welding process 　　Considering the technical requirements of the valve body, a MAG CO2 welder was ultimately selected. 　　The welding material is ER50-6 (Ø1.0) wire. The chemical composition and mechanical properties of the welding wire are shown in Tables 3, 4, and 5. 　　Based on the equal strength principle for welding materials, the selected ER50-6(Ø1.0) wire meets the requirements of ZG15Ⅱ. 　　The shielding gas is a mixture of 82% Ar + 18% CO2. 　　5.1 Provisional welding Due to the high hardening tendency of cast steel and 45 steel, preheating is necessary before provisional welding. An oxy-acetylene flame is used to uniformly heat the area on each side of the weld by a distance of 2 times the plate thickness, to a temperature of 75–100°C (with a bias toward the 45 steel side). The measurement is carried out using an far-infrared thermometer, ensuring that the sealing mechanism is not affected. 　　Before welding, the scale and rust within a 20 mm range on both sides of the weld surface should be removed using an angle grinder to achieve a metallic finish. 　　Mark 6 positioning points 50 mm from the edge on each side of the weld, and use a caliper to measure the distance between these two points in order to determine the amount of contraction. 　　5.2 Backside welding: Given the high stiffness and poor plasticity of ZG15Ⅱ material, it is necessary to use low welding parameters when selecting the welding specifications, that is, a low amount of welding heat input. While ensuring the quality of the weld, efforts should be made to increase the welding speed and reduce the fusion ratio. To minimize deformation, a method of welding in multiple layers and passes, across 6 symmetric sections, should be employed; the welding sequence is shown in Figures 2 and 3. It is important to use calipers to measure the positioning points after each layer is welded, in order to control the amount of contraction until the welding is complete. Except for the first and last layers, stress relief is carried out using hammering between all other layers. 　　6. Conclusion After welding is completed, a pressure test is conducted to ensure that there are no leaks at the weld seam; once the sealing surface meets the technical requirements, X-ray inspection is carried out to verify that the weld itself is free from welding defects such as pores, slag inclusions, lack of fusion, or cracks. The rotation of the valve core also meets the technical specifications.