Repair Methods for Defects in Aluminum Bronze Valve Castings

Abstract

This paper reviews standard approaches for addressing defects in aluminum bronze valve castings and explores the use of impregnation, repair welding, and surface treatment to repair these defects.

1. Overview

Cast aluminum bronze valves are widely used in corrosion-resistant environments, particularly in applications exposed to seawater, sulfates, and chlorides. These valves typically operate at temperatures below 300°C and pressures between 1.6 and 4.0 MPa. The intricate geometry of aluminum bronze valve castings often leads to defects such as shrinkage cavities, porosity, and oxidation inclusions. Secondary oxidation inclusions formed during pouring, along with numerous pinhole defects, can significantly weaken the material’s mechanical strength and sealing performance. Repairing cast aluminum bronze valves is complex due to the material’s susceptibility to cracking under stress, which often results in the casting being scrapped due to leakage. Common aluminum bronze alloys used in large and medium-sized valve bodies are ZCuAl9Fe4Ni4Mn2 (Chinese standard) and C95800, C95500, and C63200 (American standards, mainly for export). This paper proposes process improvements to reduce leakage in cast aluminum bronze valve bodies, thus improving overall casting yield and operational efficiency.

2. Related Requirements

(1) Foreign Standards

MSS SP-80-2003, the standard published by the Manufacturers Standardization Society (MSS) of the Valve and Fittings Industry, mandates that all bronze castings be clean, structurally sound, and free from defects that could affect performance. Plugging, repair welding, and impregnation are prohibited under this standard. ASTM B62-2002, the American standard, prohibits repair, plugging, or remelting of bronze or high-copper brass castings.

(2) Domestic Standards

The Chinese national standard GB 12225-2005 prohibits the removal of defects in copper alloy castings for general valves by hammering, plugging, or impregnation. However, the standard allows repair by welding or other approved methods for defects considered repairable. Welding is prohibited on the sealing surface, threaded components, and areas exposed to high temperatures or severe corrosion.

3. Current Situation Analysis

Industry standards for cast aluminum bronze valves prohibit the use of impregnation for leakage in valve castings but permit repair welding under specific conditions. Research and practical applications of repair welding and impregnation for pressure-bearing aluminum bronze castings have been conducted both in China and internationally. Advances in non-destructive testing technology now enable detection of casting defects regardless of the treatment method used. Thus, it is necessary to reassess the previous restrictions on aluminum bronze valve castings.

4. Repair Welding Process

4.1 Impregnation Method

(1) Impregnation Process

The process begins with rough machining of the valve body or valve bonnet casting, ensuring no holes on the flange surface, followed by high-pressure water jet cleaning of the inner and outer surfaces. The casting is then placed in a vacuum furnace at -0.09 MPa to remove impurities from the pores. Next, the casting is immersed in an impregnating solution, which is forced into the pores at a pressure of 0.4–3 MPa. After impregnation, the casting is cleaned, and the impregnant is cured at 90–100°C using an air furnace for inorganic impregnants or hot water for organic impregnants. Finally, the casting undergoes surface finishing and is subjected to a hydrostatic pressure test.

(2) Impregnants

Bronze valve casting impregnants are classified as either inorganic or organic. Inorganic impregnants, primarily silicate-based solutions such as water glass, are effective for sealing larger capillaries (less than 0.5 mm). Organic impregnants, typically acrylic resin-based, are suited for finer pores (less than 0.3 mm). Aluminum bronze castings, which generally have larger pores than stainless steel, are typically sealed more effectively with inorganic impregnants. Inorganic impregnants are used at temperatures below 230°C and withstand pressures of 2–3 MPa. Organic impregnants tolerate similar temperature ranges but offer greater resistance to pressure. Both types of impregnants are suitable for standard operational conditions of bronze valves.

(3) Scope of Repair

Impregnation is effective for sealing pinhole leaks caused by defects such as porosity and oxidation inclusions. These defects typically involve small pore diameters, irregular channels, and large affected areas, making impregnation the most effective method for sealing internal pores. Secondary impregnation may be required for additional sealing. Since leakage often occurs during hydrostatic testing, the test duration for cast aluminum bronze valves should be extended, with close monitoring.

4.2 Repair Welding Method

Defects in aluminum bronze valve castings, such as shrinkage cavities, cracks, sand holes, and holes, can be repaired through welding, provided that the process is performed correctly and the defects are thoroughly removed.

(1) Repair Welding Process

• Defect Identification: Inspect the casting for defects.
• Defect Removal: Use mechanical tools, such as a grinding machine, metal reamer, or drill, to eliminate defects. For cracks, drill stop holes at both ends and grind the defect area into a smooth bevel with at least a 30° angle. Fusion welding techniques should not be used for defect removal.
• Surface Preparation: Ensure the defect area exhibits a metallic luster. Perform a penetrant test (PT) to confirm that no defects remain.
• Preheating: Preheat an area 50 mm around the defect using an oxygen-acetylene torch. Maintain a temperature of 500–600°C, with a minimum preheating time of 5 minutes, depending on the casting thickness.
• Welding: Perform argon arc welding at a controlled, consistent speed.
• Interpass Cleaning: Use a stainless steel brush to remove contaminants between welding passes.
• Stress Reduction: Uniformly strike the repaired area with a flat hammer to relieve stress.
• Stress Relief Tempering: Heat treat and temper the 50 mm area surrounding the repair using an oxygen-acetylene torch at approximately 600°C for at least 5 minutes.
• Slow Cooling: Cover the repaired area with asbestos cloth to facilitate slow cooling.
• Final Finishing & Inspection: Grind the repaired area to a smooth finish, matching the surrounding surface. Perform a PT inspection. If the repair passes, perform a pressure retest and a final leakage inspection.

(2) Key Considerations for Repair

• Defects must be completely removed using mechanical methods to reveal a smooth, clean metal surface.
• The defect area should not exceed 40 mm × 40 mm and must be ground to a smooth bevel to minimize repair-induced stress.
• For cracks, perform a PT inspection before welding; preheat the workpiece, and apply tempering after welding.
• Strict temperature control is essential throughout the process.
• Welding should be performed in a wind-free environment to prevent defects.
• After welding, cover the repaired area with asbestos cloth to ensure slow cooling.

4.3 Surface Treatment

The surface of aluminum bronze valve castings may exhibit defects such as sand detachment and gas porosity with a diameter smaller than 42 mm. Although these defects do not lead to leakage during a water pressure test, they negatively impact the aesthetic quality of the casting. In such cases, a copper-based repair agent can be applied to the defective area. After application, the repair agent should either be heat-cured or allowed to cure naturally for 12 hours. Once cured, the surface should be ground and subjected to stainless steel shot peening to achieve a uniform surface finish that matches the surrounding material. The repair agent has a compressive strength of 92 MPa and an operating temperature limit of 175°C, complying with industry performance standards for bronze valves.

5. Conclusion

After repairing aluminum bronze valve castings through impregnation, repair welding, and surface restoration, the pressure retention time should be extended by two to three times during re-pressure testing. Additionally, the original leakage points and areas prone to defects should be closely monitored. When performed correctly, the majority of defective valves can still meet operational requirements.