In the pressure bearing valves of industrial pipelines, cast steel valves are widely used because of their cost economy and design flexibility. However, due to the constraints of casting size, wall thickness, climate, raw materials and construction operation, castings will have various casting defects such as sand holes, pores, cracks, shrinkage porosity, shrinkage cavities and inclusions, especially alloy steel castings cast in sand mold. Because the more alloying elements in the steel, the worse the fluidity of the liquid steel, the casting defects are more likely to occur. Therefore, defect identification and the formulation of reasonable, economic, practical and reliable repair welding process to ensure that the repaired valve meets the quality requirements have become the common concern of valve cold and hot processing. This paper introduces the repair welding methods and experience of several common steel casting defects (the welding rod is represented by the old brand).

2. Defect treatment

2.1 defect judgment

In production practice, repair welding is not allowed for some casting defects, such as penetrating cracks, penetrating defects (bottom penetration), honeycomb pores, sand and slag inclusion that cannot be removed, shrinkage porosity with an area of more than 65 square centimeters, and other major defects that cannot be repaired as agreed in the contract between both parties. The type of defect shall be judged before repair welding.

2.2 defect elimination

In the factory, carbon arc air gouging can be used to blow away casting defects, and then hand-held angle grinder can be used to grind the defective parts to expose metallic luster. However, in production practice, it is more to directly use carbon steel electrode to remove defects with high current and grind metal luster with angle grinder. For the removal of general casting defects, use < 4mm-j422 welding rod with 160 ~ 180A current to remove the defects, and grind the defect mouth into U shape with angle grinder to reduce welding stress. The defects are completely removed and the repair welding quality is good.

2.3 preheating of defective parts

For carbon steel and austenitic stainless steel castings, preheating is generally not required where the area of the repair welding part is less than 65cm2 and the depth is less than 20% or 25mm of the casting thickness. However, pearlitic steel castings such as ZG15Cr1Mo1V and ZGCr5Mo shall be preheated at a temperature of 200 ~ 400 ℃ (repair welding with stainless steel electrode, the temperature shall be taken as the minimum), and the holding time shall not be less than 60min. If the casting cannot be preheated as a whole, it can be heated to 300-350 ℃ with oxygen acetylene after the defect part is expanded by 20mm (slightly dark red through visual observation in the dark back). The large cutting torch neutral flame gun first swings rapidly around the defect part and its periphery for a few minutes, and then moves slowly for 10min (depending on the defect thickness), so that the defect part can be preheated fully and then repaired quickly.