Among the pressure-bearing valves of industrial pipelines, cast steel valves are widely used due to their cost economy and design flexibility. However, because the casting process is restricted by the size, wall thickness, climate, raw materials and construction operations of the casting, various casting defects such as blisters, pores, cracks, shrinkage porosity, shrinkage cavities and inclusions will appear in the castings, especially sand casting alloys. Steel castings for more. Because the more alloying elements in the steel, the poorer the fluidity of the molten steel, the more likely to produce casting defects. Therefore, the identification of defects and the formulation of a reasonable, economical, practical and reliable repair welding process to ensure that the valve after repair welding meets the quality requirements has become a common concern in hot and cold processing of valves. This article introduces the repair welding method and experience of several common steel casting defects (the welding rod is represented by the old brand).
1. Defect judgment
In production practice, some casting defects are not allowed to repair welding, such as penetrating cracks, penetrating defects (penetrating bottom), honeycomb pores, sand inclusions that cannot be removed, and shrinkage porosity with an area exceeding 65 square centimeters, etc., and Other major defects that cannot be repaired as agreed in the contract between the two parties. The type of defect should be judged before repair welding.
2. Defect removal
In the factory, carbon arc air gouging is generally used to blow out casting defects, and then a portable angle grinder is used to polish the defective parts to reveal the metallic luster. But in production practice, it is more to use carbon steel electrode with high current to remove defects, and use angle grinder to grind out metallic luster. Generally, casting defects can be eliminated by using a <4mm-J422 electrode and a current of 160-180A to remove the defects. An angle grinder grinds the defect into a U shape to reduce welding stress. The defects are completely removed, and the repair welding quality is good.
3. Preheating of defective parts
For carbon steel and austenitic stainless steel castings, where the area of the repair welding part is less than 65cm2 and the depth is less than 20% or 25mm of the thickness of the casting, preheating is generally not required. However, for pearlitic steel castings such as ZG15Cr1Mo1V and ZGCr5Mo, due to the high hardening tendency of steel and easy cracking in cold welding, preheating should be performed. The holding time should be at least 60min. If the casting cannot be preheated as a whole, it can be heated to 300-350°C with oxygen-acetylene at the defect site and expanded by 20mm (observation of the dark red in the dark place), and a large torch neutral flame gun is used first at the defect and surrounding areas. Swing the circle quickly for a few minutes, then move slowly for 10 minutes (depending on the thickness of the defect), so that the defect is fully preheated and then repaired quickly.