How to prevent cracks when welding the disc of Gate valves
1. Strictly control the content of harmful impurities in the base metal and welding materials. 2. For materials that are prone to welding hot cracks due to large linear expansion coefficients, use small specifications and no swing welding during welding to reduce heat input.
3. Use low-hydrogen, ultra-low-hydrogen welding materials or technological measures to reduce the amount of hydrogen in the weld, and perform hydrogen elimination treatment immediately after necessary welding.
4. Carefully treat the strip welding area and both sides before welding, and take good protective measures to the molten pool during welding.
5. For easily hardenable steels and rigid components, preheat before welding, keep warm during welding, warm and cool after welding, and if necessary, take corresponding treatments to eliminate welding residual stress after welding.
Specific operation: lay the gate flat and fix it with DC reverse connection. In order to avoid cracks, use small welding heat input, fast welding with small current, heat the welding plate with oxyacetylene, and heat it to 200℃ for welding , First perform four-section welding from the outer ring of one side of the sealing surface, after 1/4 circle of welding, hammer the surface of the weld bead with a small hammer to eliminate stress or avoid cracks, then weld 1/4 circle from a symmetrical position, use a small hammer Strike the surface of the weld bead to eliminate stress. Use the same method to weld the remaining 2 1/4 turns. After the first round is welded, clean the slag, and weld the second round of bead. You must melt 1/3-1/2 of the width of the first bead to prevent Defects such as slag inclusion or incomplete penetration. Then weld a weld bead until the entire sealing surface is full. After that, the welding slag is cleaned up, and the size of the surfacing ram is measured. If the size does not meet the technical requirements, a second layer of surfacing should be performed on the ram. The second layer of surfacing should be perpendicular to the direction of the first layer of weld bead, and the surface of the weld bead must be hammered every time a section of the bead is welded to eliminate stress and avoid cracks. After the second layer is welded, heat preservation is required. The ram is placed in a heat preservation furnace to cool slowly, and then taken out after 24 hours for turning, then grinding and assembling. If there are defects on both sides of the gate and need to be welded, one side should be welded first, heat preservation and cooling, after turning, use the same method, then weld the other side, and finally grind and assemble the valve for sealing test.