In the realm of pressurized valves in industrial pipelines, 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 meet quality requirements have become a common concern in valve cold and hot processing. This article introduces several common methods and experiences for welding cast steel defects.
Defect Treatment:
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.
Defect Removal
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, <4mm-J422 welding rods and a current of 160~180A are used to clean the defects, and the defect openings are ground into a U-shape with an angle grinder to reduce welding stress. Thorough defect removal results in good welding quality.
Preheating of Defect
Area For carbon steel and austenitic stainless steel castings, if the area of the repair weld is <65cm² and the depth is <20% of the thickness of the casting or 25mm, preheating is generally not required. However, for pearlitic steel castings such as ZG15Cr1Mo1V and ZGCr5Mo, due to the tendency for steel to quench harden and the susceptibility to cold cracking, preheating treatment should be carried out. The preheating temperature is between 200 and 400°C (taking the lower value when using stainless steel welding rods), and the holding time should be no less than 60 minutes. If the casting cannot be preheated as a whole, oxygen-acetylene flame can be used to heat the defect area and extend 20mm beyond it to a temperature of 300-350°C (visually observed as a faint red color in the shaded area). With a large cutting torch neutral flame gun, rapid circular oscillation should be performed at the defect and its vicinity for a few minutes, followed by slow movement for 10 minutes (depending on the thickness of the defect), ensuring that the defect area is adequately preheated before quickly welding.
Welding Method:
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
Before welding, check whether the welding rods are preheated. Generally, welding rods should be dried at 150~250°C for 1 hour. Preheated welding rods should be kept in a heat-insulated box for immediate use. Welding rods should be preheated three times. If there is peeling, cracking, or rusting on the surface of the welding rod, it should not be used.
Number of Welding Repairs
For pressure-bearing castings, such as valve housings that leak during hydraulic testing, generally only one welding repair is allowed at the same location. Multiple welding repairs are not permitted because repeated welding can cause coarsening of the grains in the steel, affecting the pressure-bearing performance of the casting, unless the casting can undergo post-weld heat treatment. For non-pressure-bearing castings, welding repairs at the same location are generally limited to three times. For carbon steel castings with more than two welding repairs at the same location, stress relief treatment should be performed after welding.
Welding Repair Layer Height
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.