Common Cast Steel Valve Welding Repair Methods

In the pressure valves of industrial pipelines, cast steel valves are widely used because of their economical cost and flexibility of design. However, due to the casting process is restricted by the casting size, wall thickness, climate, raw materials and construction operations, there will be various casting defects such as sand holes, pores, cracks, shrinkage, shrinkage holes and inclusions, especially in the alloy steel castings of sand casting. Because the more alloying elements in the steel, the worse the fluidity of the liquid steel, casting defects are more likely to occur. Therefore, defect identification and formulation of reasonable, economic, practical and reliable repair welding process to ensure that the valve meets the quality requirements after repair welding has become a common concern of valve hot and cold processing. This paper introduces the methods and experience of several common defects in steel casting.

Defect Treatment

Defect Judgment

In production practice, some casting defects are not allowed to repair welding, such as penetrating cracks, penetrating defects (through the bottom), honeycomb pores, sand and slag inclusion that cannot be removed and shrinkage loosening that exceeds 65cm, as well as other major defects that cannot be repaired as agreed in the contract between the two parties. The type of defect should be determined before repairing welding.

Defect Removal

In the factory, it is generally possible to use carbon arc gouging to blow away casting defects, and then polish the defects with a hand-held Angle grinder until the metallic luster is exposed. But in the production practice, more is to directly use carbon steel welding rod high current to remove defects, and grind out metal luster with Angle grinder. General casting defects can be removed with <4mm-J422 current 160~180A to remove the defects, and the Angle grinder will grind the defect mouth into U shape to reduce welding stress. Defect removal thoroughly, repair welding quality is good.

Defect Parts Preheat

Carbon steel and austenitic stainless steel castings, where the area of the welding part is <65cm, the depth is < 20% of the thickness of the casting or 25mm, generally no need to preheat. However, ZG15Cr1MO1V, ZGCr5Mo and other pearlite steel castings, due to the hardening tendency of steel, cold welding is easy to crack, should be preheated treatment, preheating temperature of 200 ~400℃(with stainless steel welding rod, the temperature is taken as a small value), the holding time should not be less than 60min. If the casting cannot be preheated as a whole, oxygen can be used Acetylene is heated to 300-350℃ after expanding 20mm in the defect part (slightly dark red for visual observation in the back), the large torch neutral flame gun first moves in a circle at the defect and surrounding area for a few minutes, and then changes to a slow movement to maintain 10min(depending on the thickness of the defect), so that the defect part is fully preheated and quickly repaired welding.

Repair Welding Method

Requirements

When repairing welding of austenitic stainless steel castings, it is necessary to cool them quickly in a ventilated place. For pearlitic low-alloy steel castings and carbon steel castings with large welding area, the lee should be selected or shielded by a windshield to avoid cracks caused by fast cooling. If a pile layer is repaired, the slag should be removed immediately after the repair welding, and the hammer evenly outward along the defect center to reduce the welding stress. If the repair welding is divided into several layers (generally 3~4mm is a repair welding layer), the residue and hammer the repair welding area should be removed in time after each layer of repair welding. If welding in winter,ZG15Cr1MO1V class pearlite alloy steel castings, each layer of repair welding should also be repeatedly heated by oxygen-acetylene, and then quickly repair welding to avoid welding cracks.

Welding Rod Treatment

Before repairing welding, check whether the electrode is preheated first, and generally the electrode should be dried for 1 hour at 150~250℃. After preheating, the electrode should be placed in an incubator, so that it can be used with it. The electrode is preheated repeatedly for 3 times. If the surface skin of the electrode falls off, cracks and rusts, it should not be used.

Repair Welding Times

Pressure castings, such as valve shell after pressure test seepage, the same part is generally only allowed to repair welding once, can not be repeated repair welding, because multiple repair welding will make the steel grain coarse, affect the pressure performance of the casting, unless the casting can be re-heat treatment after welding. The repair welding of other non-pressure parts is generally not more than 3 times. For carbon steel castings with more than two times of repair welding in the same part, stress relief treatment should be performed after welding.

Repair the Height of the Welding Layer

The welding height of the casting is generally about 2mm higher than the casting plane to facilitate machining. The repair welding layer is too low, and the welding scar is easily exposed after machining. Welding layer is too high, time-consuming and laborious materials.

Ending

The defect treatment of cast steel valves is a complex and meticulous process, which requires us not only to have accurate defect identification ability, but also to master scientific and reasonable repair welding technology. Through careful defect removal, proper preheating treatment, precise repair welding operations, and strict electrode management, we can significantly improve the quality of repair welding, extend the service life of valves, and ensure their reliability and safety in a variety of operating conditions.

It is worth noting that repair welding is not a one-size-fits-all solution. For some major defects that are not allowed to repair welding, we must strictly abide by the relevant norms and contract agreements to avoid the potential risks caused by blind repair welding. At the same time, for the castings after repair welding, appropriate post-treatment, such as stress relief and possible heat treatment, is also a key step to ensure the effect of repair welding.