The surface treatment of ball part in ball valve
The ball valve has been widely used in industrial applications due to its small flow resistance, a wide range of pressure and temperature use range, good sealing performance, short opening and closing time, easy installation and other advantages. The ball is an important part that plays a key role in the opening and closing function for the ball valve. In order to enhance the sealing performance and hardness of the ball, it is necessary to pre-treat the surface of the ball. So what do you know about common surface treatments for the ball body?
- Nickel or chromium plating
Carbon steel body soft seated ball valve has poor corrosion resistance, the surface of the ball can avoid corrosion by electroplating a layer of alloy metal. Electroplating is the process of plating a thin layer of other metals or alloys on the metal surface by using the principle of electrolysis, so as to improve the corrosion resistance, abrasion resistance and surface aesthetics of the metal. When the ball is Austenitic stainless steel and the sealing ring is PEEK, it is suggested that the surface of the ball be plated with Nickel (ENP) or Chromium (HCr) to improve the hardness of the ball and seal. The coating thickness is generally 0.03mm ~ 0.05mm and above if there are special requirements that can be appropriately thickened, through this the plated ball hardness can be up to 600HV ~ 800HV.
2. Cold sprayed tungsten carbide
Cold spraying is a process in which compressed air accelerates metal particles to a critical velocity (supersonic) and physical deformation occurs after the metal particles impact directly on the surface of the substrate. The metal particles are firmly attached to the substrate surface, and the metal particles are not melted during the whole process. The advantage of cold spray is that the ball does not need to be heated, deformation and internal stress will not be generated in the spraying process, the thickness is well controlled, but the surface adhesion is not as good as that of spray welding.
Tungsten carbide is characterized by high hardness and good wear resistance, but the melting point is much higher than the general metal material point, about 2870℃, so only cold spraying tungsten carbide (WC) process can be used. The 0.15mm~0.18mm thickness of spraying tungsten carbide can achieve the ideal surface hardness, if there are special requirements can be up to 0.5mm ~ 0.7mm, the thicker the thickness of cold spray, the lower the surface adhesion, not recommended to use a thick cold spray thickness. The hardness of cold sprayed on the surface is generally 1050HV~1450HV (about 70HRC).
- Spray welding or cold spraying of Nickel-base alloy/Cobalt-based alloy
Ball valves usually use spray welding or cold spraying of nickel-based alloy inclnel600 on the ball. The process of spray welding is basically the same as that of thermal spraying, but the remelting process is added in the process of powder spraying. The commonly used cobalt-based alloy on the ball valve ball is STL20, STL6 and STL1, which is usually used for spray welding. The general thickness of spray welding cobalt-based alloy is 0.5mm ~ 0.7mm, and the actual maximum thickness can be up to 2.5mm~3mm. The hardness after spray welding is generally STL20:50~52HCR; STL6:38 ~ 40 HCR; STL1:48 ~ 50 HCR4，
- Nitriding treatment
Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms penetrate into the surface layer of the workpiece at a certain temperature and medium. Nitriding treatment can improve the wear resistance, fatigue resistance, corrosion resistance and high-temperature resistance of metal. The essence of nitriding treatment is to infiltrate nitrogen atoms into the surface layer of the ball. During the friction process between the seat and the ball, the nitride layer is easy to be worn or thinned for the hard seated ball valve, resulting in the ball is easy to be scratched by impurities in the medium, affecting the seal and even making the torque increase.
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