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Introduction to metal sealing ball valve hardening process introduction
1 Overview
In thermal power plants, petrochemical systems, high-viscosity fluids in coal chemical industry, mixed fluids with dust and solid particles, and highly corrosive fluids, ball valves require metal hard-sealed ball valves, so use a suitable metal hard seal. The hardening process of the ball valve ball and seat is very important.
2 Metal hard seal ball valve ball and seat hardening
At present, the hardening processes commonly used on the surface of metal hard seal ball valve spheres are as follows:
(1) Hardfacing of the surface of the sphere (or spray welding), the hardness can reach above 40HRC, the process of surfacing the hard alloy on the surface of the sphere is complicated, the production efficiency is low, and the large-area surfacing is easy to deform the part. The surface hardening process is less used.
(2) The surface of the sphere is hard chrome plated, the hardness can reach 60~65HRC, the thickness is 0.07~0.10mm, the chrome plating layer has high hardness, wear resistance, 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 the hard chrome plating layer is rapidly lowered due to the release of internal stress when the temperature is raised, and the working temperature cannot be higher than 427 ° C. In addition, the chrome plating layer has low bonding force, and the plating layer is liable to fall off.
(3) The surface of the sphere is plasma nitrided, the surface hardness can reach 60-65HRC, and the thickness of the nitride layer is 0.20~0.40mm. The plasma nitriding hardening process is not suitable for chemical corrosion and other fields due to poor corrosion resistance.
(4) The supersonic spraying (HVOF) process on the surface of the sphere, the hardness is up to 70-75HRC, the collection intensity is high, the thickness is 0.3-0.4mm, and the supersonic spraying is the main process method of the surface hardening of the sphere. Highly viscous fluids in thermal power plants, petrochemical systems, coal chemical fields; mixed fluids with dust and solid particulates, and highly corrosive fluid media.
The supersonic spraying process is a process in which oxyfuel combustion produces high-speed gas flow to accelerate the impact of powder particles on the surface of the workpiece to form a dense surface coating. During the impact process, due to the faster particle speed (500-750 m/s) and lower particle temperature (-3000 °C), high bonding strength, low void ratio and low oxide content can be obtained after impacting the surface of the workpiece. coating. HVOF is characterized by the speed of the alloy powder particles exceeding the speed of sound, even 2 to 3 times the speed of sound, and the air velocity is 4 times the speed of sound.
HVOF is a new processing technology with a thickness of 0.3 to 0.4 mm. The coating is mechanically bonded to the workpiece. The bonding strength is high (77 MPa) and the coating porosity is low (<1%). The process has a low heating temperature of the workpiece (<93 ° C), the workpiece is not deformed, and cold spray can be performed. When spraying, the powder particles have a high speed (1370m/s), no heat affected zone, no change in the composition and structure of the workpiece, and high hardness of the coating, which can be machined.
Spray welding is a surface thermal spraying process for metal materials. It heats the powder (metal powder, alloy powder, ceramic powder) to a high-plastic state by a heat source, sprays it by airflow, and deposits it on the surface of the pre-treated workpiece to form a layer with the surface of the workpiece. (Substrate) bonded to a strong coating (welding) layer.
In the spray welding and surfacing hardening process, both the cemented carbide and the matrix have a melting process, and the cemented carbide and the matrix are provided with a hot melt zone, which completely achieves the performance of the spray-welded or surfacing hard alloy layer, and avoids welding and melting after processing. The area is a metal contact surface. It is recommended that the thickness of the spray-welded or hard-welded cemented carbide be more than 3 mm.
3 Hard seal ball valve ball and seat contact surface hardness matching
The sliding contact surface of the metal needs to have a certain hardness difference, otherwise it is prone to seizure. In practical use, the hardness difference between the general valve ball and the valve seat is 5-10 HRC, and the hardness difference enables the ball valve to have a good service life. Due to the complicated processing of the sphere and the high processing cost, in order to protect the sphere from damage and wear, the hardness of the sphere is generally higher than the hardness of the surface of the valve seat.
The hardness of the contact surface between the valve ball and the valve seat is widely used. There are two hardness combinations: 1 ball surface hardness 55HRC, seat surface 45HRC, supersonic spray Stellite20 alloy on the ball surface, and surfacing Stellite12 on the valve seat surface. Alloy, the hardness is the most widely used hardness of metal sealed ball valve, which can meet the normal wear requirements of metal hard seal ball valve; 2 the surface hardness of the ball is 68HRC, the surface of the valve seat is 58HRC, and the surface of the ball can be supersonic sprayed with tungsten carbide. The seat surface can be supersonic sprayed with Stellite20 alloy. This hardness is widely used in the field of coal chemical industry, with high wear resistance and service life.
In foreign countries, the valve ball and the seat surface have the same hardness. The ball and seat surface are supersonic-coated with tungsten carbide. The surface hardness is greater than 72HRC. Even in the case of ultra-high hardness, the ball and the valve seat are in contact. The surface is also less prone to seizure. However, at present, the valve ball and valve seat with a surface hardness of more than 72HRC in the domestic valve ball have no mature grinding process, and it is difficult to ensure the cooperation of the valve ball and the valve seat, and the use is less.
4 hard seal ball valve ball and seat hardening attention
Metal hard seal ball valve ball and seat material are generally selected from stainless steel or corrosion-resistant materials, otherwise the bond layer between cemented carbide and valve seat (or valve ball) is easily corroded by the medium, and the hard alloy layer falls off, which affects the life of the ball valve.
In addition, suitable hardening processes should be selected for different valve seat (or ball) materials. Duplex stainless steel materials are widely used in the coal chemical industry. Duplex stainless steel materials have good corrosion fatigue and wear corrosion resistance.
Duplex stainless steel is a kind of steel with both ferrite and austenitic structure. The ferrite and austenitic structure each account for about 50%, and the two-phase structure is independent. Its performance characteristics It combines the characteristics of austenitic stainless steel and ferritic stainless steel. In the characteristics of ferritic stainless steel, when the temperature is in the range of 400-500 °C, strong embrittlement will occur during long-term heat preservation. This phenomenon is generally called 475 °C embrittlement; when the temperature exceeds 400-500 ° C, duplex stainless steel Performance will be destroyed.
If the duplex stainless steel material adopts the spray welding or surfacing hard alloy process, the process of the cemented carbide and the matrix melts (the temperature is generally greater than 900 ° C), which will destroy the metallographic structure of the duplex stainless steel, so double Phase stainless steel is not suitable for the hardening process of spray-welded (or surfacing) cemented carbide. The surface hardening process of duplex stainless steel is suitable for the supersonic spraying process, and the hardening process must ensure that the metallographic structure of the duplex stainless steel matrix cannot be destroyed.
5 Conclusion
The ball and seat of the metal hard seal ball valve adopt a reasonable hardening process, which can directly improve the service life and service performance of the metal hard seal valve, and the reasonable hardening process can reduce the manufacturing cost. With the continuous emergence of new technologies, there must be more hardening treatment processes. The hardening process and hardness matching of metal hard-sealed valves is a very complicated problem, so in the design of hard-sealed valves, material selection, hardening process and hardness matching Should be given full attention.
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