Hardening Process of Metal-seated Ball Valves

1. Overview
Metal seated ball valves should be elected for high-viscosity fluids, mixed fluids with dust and solid particles, highly corrosive fluids and other media in thermal power plants, petrochemical systems the coal chemical industry, and the hardening process of ball valve spheres and valve seats are very important.

2. Hardening methods of metal seated ball valve spheres and valve seats
At present, the commonly used hardening processes on the surface of metal seated ball valves mainly include the following:

(1) The surface of the sphere is surfaced, or spray welded with cemented carbide, and the hardness can reach more than 40HRC. The process of surfacing the cemented carbide on the surface of the sphere is complex, and the production efficiency is low. Large-area surfacing can easily cause deformation of parts. Currently, the surface hardening process of the sphere is used less frequently.

(2) The surface of the sphere is plated with hard chromium; the hardness can reach 60 to 65HRC, and the thickness is 0.07 to 0.10mm. The chromium plating layer has good hardness, wear resistance, and corrosion resistance and can keep the surface bright for a long time. The process is relatively simple, and the cost is low. However, the hardness of hard chromium coating will decrease rapidly due to the release of internal stress when the temperature rises, and its operating temperature cannot be higher than 427°C. In addition, the binding force of the chromium plating layer is low, and the plating layer is prone to falling.

(3) The surface of the sphere is plasma nitrided, with a surface hardness of up to 60 to 65HRC and a nitrided layer thickness of 0.20 to 0.40mm. The plasma nitriding hardening process cannot be used in fields such as chemical industries with strong corrosion due to poor corrosion resistance.

(4) The supersonic spraying (HVOF) process on the surface of the sphere has a hardness of up to 70 to 75HRC, high collective strength, and a thickness of 0.3 to 0.4mm. Supersonic spraying is the main process for surface hardening of the sphere. This hardening process is mostly used in highly viscous fluids, mixed fluids with dust and solid particles, and highly corrosive fluid media in thermal power plants, petrochemical systems, and coal chemical industries.

The supersonic spraying process is a process in which oxyfuel combustion generates high-speed airflow to accelerate powder particles to impact the surface of the workpiece to form a dense surface coating. In the impact process, high bonding strength, low void ratio, and low oxide content can be obtained after impacting the workpiece surface due to the fast particle speed (500 to 750m/s) and low particle temperature (-3000°C). The characteristic of HVOF is that the alloy powder particle speed exceeds the speed of sound, even 2 to 3 times that of sound, and the airflow speed is 4 times that of sound. 

HVOF is a new processing technology. The spraying thickness is 0.3 to 0.4mm. The coating and the workpiece are mechanically bonded. The bonding strength is high (77MPa) and the porosity of the coating is low (less than 1%). This process heats the workpiece to a low temperature (less than 93°C); the workpiece does not deform and can be cold sprayed. When spraying, the powder particle speed is high (1370m/s); there is no heat-affected zone, and the composition and structure of the workpiece do not change; the coating hardness is high, and it can be machined.

Spray welding is a thermal spray treatment process on the surface of metal materials. It uses a heat source to heat the powder (metal powder, alloy powder and ceramic powder) until it melts or reaches a high plasticity state, then sprays it by airflow and deposits it on the pre-treated workpiece surface to form a firmly bonded coating layer with the workpiece surface. In the spray welding and surfacing hardening process, both the cemented carbide and the matrix have a melting process. There is a hot melt zone where the cemented carbide and the matrix meet. To fully achieve the performance of the spray welding or surfacing cemented carbide layer, we can avoid welding heat melting after processing. The area is the metal contact surface. It is recommended that the thickness of spray welding or surfacing cemented carbide should be greater than 3mm.

3. Hardness matching of the contact surface between the ball and the valve seat of the metal seated ball valve
The metal sliding contact surface needs to have a certain hardness difference, otherwise blocking will easily occur. In actual application, the hardness difference between the valve ball and the valve seat is generally 5 to 10HRC. This hardness difference enables the ball valve to have a better service life. Since the processing of the ball is complex and the processing cost is high, to protect the ball from damage and wear, the hardness of the ball is generally selected to be higher than that of the valve seat surface.

There are two widely used hardness combinations for the contact surface hardness of the valve ball and valve seat, which are as follows: 
(1)The surface hardness of the valve ball is 55HRC, and the surface hardness of the valve seat is 45HRC. Supersonic sprayed Stellite20 alloy can be selected for the surface of the valve ball, and Stellite12 Alloy can be used for the surface of the valve seat. This hardness combination is the most widely used hardness combination for metal seated ball valves and can meet the regular wear requirements of metal seated ball valves.

(2)The surface hardness of the valve ball is 68HRC; the surface hardness of the valve seat is 58HRC, and that of the valve ball can be sprayed with tungsten carbide at supersonic speed. The surface of the valve seat can be supersonic sprayed with Stellite20 alloy. This hardness is widely used in the coal chemical industry and has good wear resistance and service life. 

In foreign countries, there are valves with the same surface hardness as the valve ball. The surfaces of the valve ball and the valve seat are sprayed with a supersonic tungsten carbide process, and the surface hardness is greater than 72HRC. Even in the case of ultra-high hardness, the ball and the valve seat are not in contact with each other. The surface is not prone to blocking. However, there is currently no mature grinding process for domestic valve balls and valve seats with surface hardness greater than 72HRC. It is difficult to ensure the coordination of the valve ball and valve seat, so they are rarely used.

4. Matters needing attention when hardening metal seated ball valves and valve seats
The ball and seat materials of metal seated ball valves are generally made from stainless steel or corrosion-resistant materials. Otherwise, the bonding layer between the carbide and the valve seat (or ball) is easily corroded by the medium, causing the carbide layer to fall, which affects the life of the ball valve. In addition, appropriate hardening processes should be selected for different valve seat (or valve ball) materials. Duplex stainless steel is widely used in the coal chemical industry. Duplex stainless steel has good resistance to corrosion fatigue and wear and corrosion.

Duplex stainless steel is a type of steel with both ferrite and austenite structures. The ferrite and austenite structures each account for about 50%, and the two-phase structure exists independently. It has the characteristics of both austenitic stainless steel and ferritic stainless steel. Among the characteristics of ferritic stainless steel, when the temperature is between 400 to 500°C, strong embrittlement will occur after long-term heat preservation. This phenomenon is generally called embrittlement at a temperature of 475°C; when the temperature exceeds 400 to 500°C, duplex stainless steel performance will be ruined. 

If duplex stainless steel adopts the spray welding or surfacing cemented carbide process, the cemented carbide and the matrix will undergo a melting process. The temperature is generally greater than 900°C, which will destroy the metallographic structure of duplex stainless steel. Duplex stainless steel is not suitable for the hardening process of spray welding (or surfacing) cemented carbide. The surface hardening process of duplex stainless steel is suitable for the supersonic spraying process. The hardening process must ensure that it does not damage the metallographic structure of the matrix of the duplex stainless steel.

5. Conclusion
The ball and valve seat of the metal seated ball valve adopt a reasonable hardening process, which can directly determine the service life and performance of the metal seated valve. The reasonable hardening process can reduce manufacturing costs. With the continuous emergence of new technologies, there will be more hardening processes. The hardening process and hardness matching of metal seated valves is a very complex issue. Therefore, hard seated valve design, material selection, hardening process, and hardness matching should be well-paid attention to.