The Hardening Process of Metal-seated Ball Valves

Metal seated ball valves are ideal for handling high-viscosity fluids, mixtures with dust and solid particles, highly corrosive fluids, and other challenging media in various industries like thermal power plants, petrochemical systems, and coal chemical industry. The hardening process of the valve spheres and seats is crucial for their performance and durability.

Hardening Methods for Valve Spheres and Seats

Currently, there are several common hardening processes for the surface of metal seated ball valves:

(1) Surfacing or spray welding the sphere's surface with cemented carbide achieves a hardness of over 40HRC. However, this process is complex, inefficient, and may cause deformation, making it less commonly used.

(2) Hard chromium plating on the sphere's surface provides a hardness of 60 to 65HRC with good wear and corrosion resistance. While simple and cost-effective, the coating's hardness diminishes at high temperatures and may peel off.

(3) Plasma nitriding yields a surface hardness of 60 to 65HRC with a nitrided layer thickness of 0.20 to 0.40mm but lacks corrosion resistance for certain industries.

(4) Supersonic spraying (HVOF) on the sphere's surface achieves a hardness of 70 to 75HRC with high strength and low porosity, making it ideal for demanding applications.

Supersonic spraying utilizes oxyfuel combustion to accelerate powder particles onto the workpiece surface, creating a dense coating with high bonding strength and low oxide content. This advanced technology ensures minimal heat-affected zones and maintains the workpiece's composition and structure.

Hardness Matching of Ball and Valve Seat

Proper hardness matching between the ball and valve seat is essential to prevent blockages and ensure longevity. Typically, a 5 to 10HRC difference in hardness is maintained to optimize the valve's service life. The ball is usually harder than the valve seat to withstand wear and damage.

Two widely used hardness combinations for the contact surface are:

Ball: 55HRC, Valve Seat: 45HRC
Ball: 68HRC, Valve Seat: 58HRC

Considerations for Hardening Processes

Selecting appropriate hardening processes is crucial for the durability of metal seated ball valves. Materials like duplex stainless steel require special attention to avoid damaging their metallographic structure during hardening. Careful consideration of material selection and hardening processes is necessary to ensure optimal performance and longevity.

Conclusion

The hardening process of metal seated ball valves significantly impacts their performance and lifespan. Proper hardening methods not only enhance durability but also reduce manufacturing costs. Continued advancements in technology will lead to more effective hardening processes. Designers and manufacturers must prioritize material selection, hardening processes, and hardness matching to ensure the reliability and longevity of metal seated valves.