The hardening methods for the ball body and valve seat of metal hard-sealed ball valves are diverse. Here are some of the main hardening methods and their characteristics: 

1. Hard alloy overlay welding: 

Features: The hardness can reach above 40HRC, and it has excellent wear resistance. 

Disadvantages: The process is complex, the production efficiency is low, and large-scale welding can easily cause deformation of the parts. 

Application: Currently, the process for hardening the surface of spherical objects is not widely used. 

2. Hard Chrome Plating: 

Features: Hardness can reach 60-65 HRC, thickness is 0.07-0.10mm. The chrome coating has high hardness, good wear resistance, corrosion resistance and can maintain a bright surface for a long time. The process is relatively simple and the cost is low. 

Disadvantages: The hardness of the hard chromium coating will rapidly decrease as the temperature rises due to the release of internal stress. The working temperature cannot exceed 427℃. Additionally, the adhesion of the chromium coating is poor and the coating is prone to peeling off. 

Application: Suitable for situations where temperature requirements are not strict. 

3. Plasma Nitriding: 

Features: The surface hardness can reach 60-65 HRC, and the nitriding layer thickness is 0.20-0.40 mm. 

Disadvantage: Poor corrosion resistance, unable to be used in fields with strong chemical corrosion. 

Application: Suitable for situations where the requirement for corrosion resistance is not high. 

4. Supersonic Flame Spraying (HVOF): 

Features: The hardness can reach up to 70-75 HRC. The bonding strength is high, with a thickness of 0.3-0.4mm. The coating is mechanically bonded to the workpiece, with a high bonding strength of 77 MPa and a low porosity of (<1%). The heating temperature for the workpiece in this process is low (<93℃), and the workpiece does not deform. It can be subjected to cold spraying. During spraying, the particle speed of the powder is high (1370 m/s), there is no heat affected zone, and the composition and structure of the workpiece remain unchanged. The coating has high hardness and can be machined. 

Application: It is the main process method for hardening the surface of a sphere. It is widely used in thermal power plants, petrochemical systems, and coal chemical industries for high-viscosity fluids, mixed fluids containing dust and solid particles, and corrosive fluid media. 

5. Induction Hardening: 

Features: Fast heating speed, high efficiency, excellent quenching quality, and easy to achieve automation. 

Disadvantage: The equipment investment is relatively high. 

Application: Suitable for mass production of metal hard-sealed ball valve components. 

6. Carburizing: 

Features: Place the low-carbon sphere and valve seat in a carbon-rich medium. Under high temperature, carbon atoms penetrate the surface of the workpiece, forming a high-carbon infiltration layer. Then, through quenching and low-temperature tempering, the surface acquires high hardness and wear resistance, while the core remains with good toughness. Carburizing treatment can enhance the fatigue strength and wear resistance of the parts. 

Disadvantages: The processing period is long, and after carburizing, complex subsequent treatments are required. 

Application: Suitable for ball valve components that are subjected to impact loads and wear. 

7. Nitriding: 

Features: At a certain temperature, nitrogen atoms penetrate into the surface of the sphere and valve seat, forming a nitrided layer. This nitrided layer has high hardness, good wear resistance, and certain corrosion resistance. After treatment, the deformation of the parts is minimal. 

Disadvantage: The nitriding layer is relatively thin and has a higher brittleness. 

Application: Suitable for ball valve components that require strict deformation tolerance and operate under conditions of no impact or light impact. 

8. Carburizing and Nitriding: 

Features: It simultaneously infiltrates carbon atoms and nitrogen atoms into both the spherical surface and the valve seat surface, combining the advantages of carburizing and nitriding, such as low processing temperature, short processing time, small deformation, high surface hardness and good wear resistance. 

Application: Widely applied, it can effectively enhance the overall performance of ball valve components.