Do you know the technical requirements for welding process of ball valve body?
The technical challenges of the ball valve body manufacturing lie in the welding process. In this short article we will introduce the welding process of the ball valve body.
1. Technical Requirements of the Valve Body
(1) The valve body has no deformation after seam welding, including the upper and lower shaft flanges (45 steel).
(2) The heated area of the valve body cannot be too wide and the temperature cannot be too high, otherwise the sealing device in the valve body will be damaged.
(3) There shall be no weld bead.
(4) Ensure that the pressure is above 10MPa.
2. Weld Structure
According to the above technical requirements, the weld is determined to be a non-penetration weld. Therefore, the butt joint form of the weld is required to ensure both assembly and non-penetration.
3. Assembly
The sealing test is carried out after positioning and fastening with bolts around the two parts. The design requirements are met between the outside of the ball and the valve body.
4. Welding Process
Considering the technical requirements of the valve body, MAG CO2 welding machine was selected.
ER50-6 is selected as welding material（ Ø 1.0) welding wire.
According to the principle of equal strength of the welding material, ER50-6 is selected（ Ø 0) the welding wire meets the requirements of zg15 Ⅱ.
The shielding gas is a mixture of 82%Ar+18%CO2.
4.1 Tack Welding
Due to the serious hardening tendency of cast steel and 45 steel, it is necessary to preheat them before the positioning welding. The oxygen acetylene flame is used to evenly heat both sides of the weld within the range of 2 times of the plate thickness to 75 ~ 100 ℃ (inclined to 45 steel side). And the far-infrared thermometer is used to measure, subject to not affecting the sealing device.
Before welding, the oxide scale and rust within 20 mm on both sides of the weld surface shall be polished to metallic luster by angle grinder.
Mark 6 positioning points 50mm from the edge on both sides of the weld and measure the distance between the two points with a caliper to measure the shrinkage.
4.2 Back Cover Welding
Considering the characteristics of zg15 Ⅱ material such as high rigidity and poor plasticity, small welding parameters, i.e. small welding heat input, should be adopted in the selection of welding specifications. On the premise of ensuring the weld quality, the welding speed should be increased as much as possible, the fusion ratio should be reduced, and the multi-layer, multi pass and six section symmetrical welding method should be adopted in the aspect of deformation; The welding sequence is shown in Fig. 2 and Fig. 3. Pay attention to measure the positioning point with caliper for each welding layer, and control the shrinkage until the welding is finished. In addition to the first and last pass, the other passes are treated by hammering.
5. Conclusion
After the welding is completed, there is no leakage through the pressure test (air tightness) of the weld joint. The X-ray inspection should show the weld having no welding defects such as porosity, slag inclusion, lack of fusion and cracks, and the valve core rotation should meet the technical requirements.
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