cast steel valves are widely utilized due to their cost-effectiveness and design flexibility. However, various casting defects such as sand holes, porosity, cracks, shrinkage, and inclusions can occur in castings due to constraints in casting processes related to dimensions, wall thickness, climate, raw materials, and construction operations, especially in alloy steel castings produced by sand casting. The presence of more alloying elements in steel decreases the fluidity of the molten steel, making casting defects more likely to occur. Therefore, defect identification and the development of rational, economical, practical, and reliable welding processes to ensure that the welded valves
Defect Judgment
In practical production, some casting defects are not suitable for welding repair, such as through cracks, penetration defects (penetration to the bottom), honeycomb porosity, sand and slag inclusions that cannot be removed, and shrinkage exceeding 65cm², as well as other significant defects agreed upon in the contract between both parties. The type of defect should be determined before welding.
In factories, carbon arc air gouging can be used to remove casting defects, followed by grinding with a handheld angle grinder until a metallic luster is exposed. However, in practical production, casting defects are often directly removed with carbon steel welding rods at high current, and the defect area is ground with an angle grinder to achieve a metallic luster. Generally, for defect removal in castings,
Preheating of Defect
Area For carbon steel and austenitic stainless steel castings, if the area of the repair weld is
When welding austenitic stainless steel castings, it should be done in a well-ventilated area to facilitate rapid cooling. For pearlitic low-alloy steel castings and carbon steel castings with large welding areas, welding should be done with a wind shield or shielded from wind to avoid cracking caused by rapid cooling. After welding a bead, the slag should be immediately removed, and the welded area should be uniformly hammered outward from the center of the defect to reduce welding stress. If welding is done in multiple layers (generally 3~4mm per layer), the slag should be promptly removed and the welded area hammered after each layer. In winter welding, for pearlitic steel castings like ZG15Cr1Mo1V, the welded area should be repeatedly heated with oxygen-acetylene flame after each layer of welding and then quickly welded again to prevent welding cracks.
Handling of Welding Rods
The welding repair height of the casting is generally about 2mm higher than the casting plane to facilitate machining. If the welding repair layer is too low, welding scars may be exposed after machining. If the welding repair layer is too high, it will be time-consuming, labor-intensive, and wasteful of materials.
