Spray Welding Process for Coating Valve Sealing Surfaces

Spray welding is a critical surface treatment technology that involves spraying an alloy coating onto the sealing surface of valves. This process significantly improves the valve's wear resistance, corrosion resistance, and overall service life. It is widely used in the repair and enhancement of industrial valves, particularly in industries like oil, chemicals, and power generation. This article will outline the process flow of alloy coating on valve sealing surfaces, covering the preparations before spray welding, surface pretreatment, spray welding techniques, cooling after spraying, and protection measures.

Preparation Before Spray Welding

Spray welding is a technical process that requires careful preparation to ensure a smooth operation and optimal coating performance.

1. Material Selection for Spray Welding

Choosing the right materials for spray welding is crucial. The selection should be based on the valve's material, operating environment, and technical requirements. Generally, spray welding coatings consist of a bonding layer and a working layer. The bonding layer ensures strong adhesion to the base material, while the working layer focuses on enhancing wear resistance and corrosion resistance. For valves requiring high wear resistance, a high-hardness alloy powder is used, while valves with high corrosion resistance demands are coated with anti-corrosion alloy powders.

2. Determining Process Parameters

Several process parameters need to be precisely controlled during spray welding. These include spray pressure, powder particle size, and the relative movement speed between the spray gun and the workpiece. Incorrect pressure or inappropriate powder size can affect the quality and adhesion of the coating.

3. Presetting Coating Thickness

Since the valve usually requires further machining after spray welding, the coating thickness must allow for adequate machining allowance. The preset thickness should also account for thermal expansion and contraction, ensuring the final quality is not compromised.

Surface Pretreatment of Valve Workpieces

Surface pretreatment is essential to ensure a strong bond between the spray coating and the base material. Proper surface preparation significantly enhances the adhesion and durability of the coating.

1. Surface Cleaning

Before spray welding, the workpiece surface must be thoroughly cleaned of oils, rust, paint, and other impurities. Oil can be removed using solvent cleaners, while rust can be eliminated through acid etching, mechanical grinding, or sandblasting. For oil contamination that has penetrated the base material, flame heating is used to remove it. Using efficient cleaning agents and appropriate mechanical methods during this stage ensures a strong bond between the coating and the base material.

2. Surface Roughening

To enhance the adhesion between the coating and the workpiece, the surface must be roughened. Common methods include sandblasting, grooving, threading, or knurling. Sandblasting is the most common method, using materials such as quartz sand, aluminum oxide, or chilled iron sand. Factors such as sand grain shape (sharp and hard), particle size, air pressure, sandblasting angle, and duration all impact the roughening effect. For shaft and sleeve components, grooving or threading is often used to achieve the required surface roughness.

3. Special Surface Treatment

For harder workpieces, electrical discharge machining (EDM) is often used to roughen the surface. This method involves using an electric arc to melt nickel or aluminum wires that fuse with the base material, creating a rough surface. EDM is suitable for some special materials but should be used cautiously for thin coatings.

Spray Welding Process

The spray welding process involves applying both the bonding layer and the working layer. The spraying of each layer needs to be strictly controlled to ensure coating quality, adhesion, and performance.

1. Spraying the Bonding Layer

Before spray welding, the valve workpiece should be preheated, typically to a temperature between 100°C and 200°C. This helps reduce thermal stress caused by temperature differences, preventing thermal cracks. The bonding layer is generally sprayed to a thickness of 10 to 20 microns. During the process, a neutral or carburizing flame is used. Flame color can indicate whether the spraying process is optimal: a white, bright flame suggests overheating, while a dark red flame indicates inadequate melting, resulting in poor coating quality. The spray gun should be perpendicular to the workpiece surface, with a spraying distance of 180mm to 200mm.

2. Spraying the Working Layer

After spraying the bonding layer, the surface should be cleaned with a steel wire brush to remove residual ash and oxide films. The appropriate flame type is selected based on the type of powder (such as iron, copper, or nickel powder). The spray gun should maintain the same distance of 180mm to 200mm from the workpiece, and the spraying speed should be controlled between 70 and 150mm/s. To ensure the process temperature does not exceed 250°C, the workpiece temperature should be monitored regularly with a thermometer.

Cooling After Spray Welding

After spray welding, the workpiece needs to be cooled properly to prevent cracks in the coating or deformation of the workpiece. Special protective measures should be taken, especially for complex or large-sized components. Cooling methods may vary depending on the workpiece's shape. For example, long shaft components can be rotated while cooling on a machine tool, or they can be suspended vertically for cooling.

Post-Spray Welding Protection and Cleaning

The surface of the workpiece after spray welding requires cleaning and protection to ensure the coating's integrity and extend the valve's service life.

1. Protecting Non-Spray Welded Areas

Non-spray-welded areas near the coating, such as keyways or oil holes, should be sealed using graphite blocks or chalk to prevent contamination during the spray welding process. It's essential to choose blocking materials that are compatible with the workpiece's material and temperature requirements to avoid using low-melting-point alloys that could contaminate the coating.

2. Surface Protection

Areas that need to be protected after spray welding can be covered with high-temperature resistant glass cloth or asbestos. If needed, custom fixtures can be designed to protect the workpiece. The fixture material should be strong enough to withstand the required conditions and avoid contamination from low-melting-point materials.

Conclusion

The alloy coating process for valve sealing surfaces is a highly precise technology that requires strict control of process parameters and operational steps. Proper selection of spray welding materials, careful control of temperature, speed, and pressure during spraying, and thorough surface pretreatment and protection are all key factors in ensuring the valve's performance and service life. By meticulously managing each step, high-quality coatings can be achieved, significantly enhancing the valve's wear and corrosion resistance, as well as its reliability under harsh operating conditions. This process not only meets industrial production demands but also provides effective solutions for valve maintenance and repair.