Alloy Spraying of Valve Sealing Surfaces

Alloy Spraying of Valve Sealing Surfaces

The process flow of alloy spraying of valve sealing surfaces mainly includes four steps: preparation before spray welding of valves, surface pretreatment, spray welding of valves, and post-processing of spray welding of valves.
Preparation before spray welding of valves
Determine spray welding materials, spray welding tools, spray equipment and processes. Before formulating the process, you should first understand the actual condition and technical requirements of the spray welding workpiece and conduct an analysis.

(1) Determination of the thickness of the coating 
Generally speaking, the valve must be mechanically processed after spray welding, so machining allowance must be reserved for the thickness of the coating, and the thermal expansion and contraction of the valve during spray welding must also be taken into consideration.

(2) Determination of coating materials
The selection basis is that the coating material should meet the requirement for the material, coordination, technique and working conditions of the workpiece to be spray welded, and the bonding layer and working layer materials should be selected respectively.

(3) Determination of the parameters
Determine pressure, powder particle sizes, and relative movement speeds of the spray gun and the workpiece.
Pretreatment of workpiece surfaces of valves
Surface preparation is an important process to ensure the bonding strength between the coating and the substrate.

(1) Concave cutting treatment
When there is a fatigue layer and partially severely strained groove marks on the surface, under the premise of strength permitting, auger cutting can be performed to provide room for thermal spray welding.

(2) Surface cleaning
Remove oil stains, rust and paint layers to make the surface of the workpiece clean. Oil stains and paint can be removed with solvent cleaners. If the oil stains have penetrated the base material, they can be removed by heating with a flame. The rust layer can be acid-leached, mechanically ground or sandblasted.

(3) Surface roughening
The purpose of surface roughening is to enhance the bonding force between the coating and the substrate and eliminate stress effects. Commonly used methods include sandblasting, slotting threading, and roughening.
A: Sandblasting is the most commonly used. The sand materials can be quartz sand, alumina sand, chilled iron sand, etc. The sand materials should be sharp and hard, clean, and dry, and have sharp edges and corners. Its size, amount of air pressure, blasting angles, distance, and time should be carried out according to the specific situation.
B: Grooving, threading, and rolling: For surface roughening of shafts and sleeve parts, one groove and threading can be used. The surface roughness of the groove and threads is preferably RA6.3-12.5. No coolant or moisturizer is added during the processing. For workpieces with threads, knurls can be knurled on the surface, but sharp corners should be avoided.

C: Workpieces with good hardness can be roughly treated by EDM roughening. However, thin-coated workpieces should be used with caution. The EDM roughening uses thin nickel or aluminum wires as electrodes. Under the action of the arc, the electrode material and the substrate surface are partially fused to produce a rough surface. Contamination should be prevented for fresh surfaces after surface roughening. Touching is strictly prohibited and kept the workpiece in a clean, dry environment. After roughening, the valve should be spray welded as soon as possible. Generally, the valve spray welding time should not exceed two hours.
(4) Protection of non-spray welding parts
The non-spray welding near the spray welding surface of the ball valve needs to be protected. It can be shielded with heat-resistant glass cloth or asbestos. If necessary, make a corresponding clamp according to the part. However, it should be noted that the clamp material must have a certain strength and alloys with low melting points should not be used to avoid contaminating the coating. For parts such as keyways and oil holes on the surface of the substrate that are not allowed to be spray welded, graphite blocks or chalk can be used to make them flat or slightly above the surface. When cleaning after spraying, be careful not to damage the coating and blunt the edges and corners.
Spray welding of valve sealing surfaces
Preheat the valve to 100 to 200 degrees before spray welding to reduce the temperature difference. Small pieces can be preheated in the baking oven. The base of the bonding layer should be controlled between ten to twenty wires. When spraying powder, use a neutral or northern carbonized flame. After sending, a concentrated bright red flame will appear, with blue and white smoke. If the end is bright white, it means there is excessive damage and the flame should be adjusted or the amount of powder fed should be reduced. If it is the terminus, it means it is not well-fused. Increase the flame and control the amount and flow rate of powder. If adjusting the flame and powder amount is ineffective, you can change the powder particle size and nickel content, and use coarse powder or powder with more nickel content. When spraying, it should be perpendicular to the spray welding surface of the valve, and the distance should generally be controlled at 180 to 200MM.
Valve spray welding working layers: After the bonding layer is sprayed, use a wire brush to remove the ash powder and oxide film. When using iron powder, use a northern carbonizing flame. When using copper powder, a neutral flame should be adopted.
When using nickel powder, it is between the two, depending on its composition. The distance is controlled between 180 to 200MM. If the spraying distance is too great, the melt temperature will decrease, the speed will slow down, and the energy will be insufficient. The bonding strength is low; the structure is loose; the spraying distance is too small, and the powder particles will not melt well; the impact force will be great to cause rebound. The deposition efficiency is low and the bonding strength is also low. The relative movement speed is best between 70 to 150MM/S. It should be measured frequently with a thermometer. If the temperature exceeds 250 degrees, valve spray welding should be suspended.
Cool the workpiece after spray welding: During cooling, the main purpose is to prevent the cracking of coatings and deformation of workpieces. Precautions should be taken especially for some special shapes, naturally cooling the long axis while rotating on the machine tool, or hanging them vertically for cooling.