Welding is mainly used for surfacing of valve sealing surface, repair welding of casting defects and welding required by product structure.
The selection of welding materials is related to its process method.
The materials used in shielded metal arc welding, plasma arc welding, submerged arc welding and carbon dioxide gas shielded welding are different.
The most common and commonly used welding methods and various materials used in shielded metal arc welding.
1. Requirements for valve welders
The valve belongs to the pressure pipeline element.
The skill level and welding process of the welder directly affect the product quality and safety production, so it is urgent to strictly require the welder.
Welding is a special process in the valve production enterprise, and special processes need special means, including the management and control of personnel, equipment, process and materials.
Welders shall pass the basic knowledge and actual control examination of Boiler and Pressure Vessel Welder Examination, hold the certificate (certificate), and can only engage in welding operations within the validity period.
2. Storage requirements for valve welding rods
1) Pay attention to the ambient humidity to prevent the welding rod from being affected with damp.
The relative humidity in the air is required to be less than 60% and a certain distance from the ground or wall.
2) Distinguish the type and specification of welding rod.
3) Pay attention not to damage the coating during transportation and stacking, especially for stainless steel electrode, surfacing electrode, cast iron electrode, etc.
3. Repair welding of valve castings
1) In case of sand inclusion, crack, air hole, sand hole, looseness and other defects in the valve casting, welding repair is allowed.
However, before welding repair, oil stain, rust, moisture and defects must be removed.
After removing the defects, sand paper shall be used to polish the metal luster.
The shape shall be smooth, with a certain slope, and no sharp edges are allowed.
If necessary, powder or liquid penetration shall be used for non-destructive control, and welding repair can be carried out only when there is no defect.
2) Repair welding is not allowed if there are serious penetrating cracks, cold shuts, honeycomb pores, large areas of looseness on the pressure bearing steel castings, and there are no defects to be removed or the parts cannot be repaired and polished after welding.
3) The repeated welding repair times of pressure bearing steel casting shell after leakage test shall not exceed two times.
4) Castings must be polished flat and smooth after welding repair, and no obvious welding repair marks are allowed.
5) NDT requirements for castings after welding repair shall be implemented according to relevant standards.
4. Stress relief treatment after valve welding
1) For important weldments, such as insulation jacket welds, valve seat inset welds on the valve body, surfacing sealing surfaces requiring post welding treatment, and pressure-bearing castings whose repair welding exceeds the specified range, the welding stress shall be eliminated after welding.
For those that cannot be put into the furnace, the method of local stress relief can also be used.
The process of eliminating welding stress can refer to the electrode manual.
2) The welding stress shall be eliminated after welding if the repair depth exceeds 20% of the wall thickness or 25 mm or the area is greater than 65c ㎡ and the shell test leakage.
5. Valve welding procedure qualification
The correct selection of welding rods is only an important link in this special process of welding, and it is only the correct selection of welding rods.
Without the guarantee of the previous items, it is impossible to obtain good welding quality.
Since the welding quality of the shielded metal arc welding is different from the quality of the electrode itself, the diameter of the electrode, the base metal, the thickness of the base metal, the weld position, the preheating temperature, and the important parameters specified by the current used, attention should be paid to the changes of these important parameters.
In valve products, welding procedure qualification is required for the surfacing of the sealing surface, inlay welding of the valve seat and valve body and welding repair of pressure parts.