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Views: 11     Author:     Publish Time: 2021-05-26      Origin:

Overview of the Hardening Process of Metal Sealed Ball Valves

· Seal valve widely used in thermal power generation, oil extraction and transport , coal mine highly viscous fluids, the mixed fluid and solid particles of dust, fluid, etc. strong corrosion fluid , the need to use the ball valve metal seal valve, so the selection of suitable The hardening process of the ball and seat of the metal hard seal ball valve is very important.

· The hardening method of the ball and seat of the metal hard seal ball valve

Currently metal seal valve sphere surface often see the hardening process:

(1) The surface of the sphere is welded (or sprayed) with hard alloy, and the hardness can reach more than 40HRC. The surfacing process of hard alloy on the surface of the sphere is complicated, the production efficiency is low, and the large-area surfacing is easy to cause deformation of the parts. The surface hardening process is used less.

(2) The surface of the sphere is plated with hard chrome , the hardness can reach 60-65HRC, and the thickness is 0.07-0.10mm. The chrome-plated 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 chromium coating will decrease rapidly due to the release of its internal stress when the temperature rises, and its working temperature cannot be higher than 427°C. In addition, the bonding force of the chromium plating layer is low, and the plating layer is easy to fall off.

(3) Plasma nitriding is used on the surface of the sphere, the surface hardness can reach 60-65HRC, and the thickness of the nitrided layer is 0.20-0.40mm. The plasma nitriding hardening process cannot be used in fields such as chemical strong corrosion due to its poor corrosion resistance. (4) Supersonic spraying (HVOF) process on the surface of the sphere, with a hardness of up to 70-75HRC, high aggregate strength, and a thickness of 0.3-0.4mm. The supersonic spraying is the main process method for surface hardening of the sphere. The hardening process is mostly used in high- viscosity fluids in thermal power plants, petrochemical systems, and coal chemical industries ; mixed fluids with dust and solid particles, and highly corrosive fluid media. The supersonic spraying process is a process in which oxy-fuel combustion generates high-speed airflow to accelerate powder particles to hit the surface of the workpiece to form a dense surface coating. During the impact process, due to the faster particle speed (500~750m/s) and the lower particle temperature (-3000℃), after impacting the surface of the workpiece, high bonding strength, low porosity, and low oxide content can be obtained . coating layer. The characteristic of HVOF is that the speed of alloy powder particles exceeds the speed of sound, even 2 to 3 times the speed of sound, and the airflow speed is 4 times the speed of sound.

HVOF is a new processing technology. The spraying thickness is 0.3-0.4mm. The coating and the workpiece are mechanically bonded. The bonding strength is high (77MPa) and the porosity of the coating is low (<1%). This process has low heating temperature for the workpiece (<93℃), the workpiece is not deformed, and can be cold sprayed. When spraying, the powder particle speed is high (1370m/s), there is no heat-affected zone, the composition and structure of the workpiece are unchanged, 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, ceramic powder) to a molten or high plastic state, and then spray it by airflow and deposit it on the surface of the pre-treated workpiece to form a layer with the surface of the workpiece. (Substrate) Bonding firmly to the coating (welding) layer.
In the spray welding and surfacing hardening process, both the cemented carbide and the substrate have a melting process, and there is a hot melting zone at the assembly of the cemented carbide and the substrate, in order to fully achieve the performance of the spray welding or surfacing hard alloy layer, and avoid the welding hot melt after processing The zone is the metal contact surface, and it is recommended that the thickness of the spray welding or surfacing hard alloy should be greater than 3mm.

· Seated ball valve hardness of the ball and seat contact surface matching

The metal sliding contact surface must have a certain hardness difference, otherwise it is prone to seizure. In actual application, the hardness difference between the valve ball and the valve seat is generally 5-10HRC. This difference in hardness enables the ball valve to have a better service life. Due to the complex 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 selected to be higher than the hardness of the valve seat surface.

The contact surface hardness of the valve ball and the valve seat is widely used. There are two types of hardness coordination: ①The surface hardness of the valve ball is 55HRC, the surface of the valve seat is 45HRC, the surface of the valve ball can be supersonic sprayed Stellite20 alloy, and the surface of the valve seat can be surfacing Stellite12. alloy, the hardness of the complex is the most widely used metal sealing ball range of hardness fit to meet a conventional metal seal valve wear requirements; ② ball surface hardness of 68HRC, 58HRC seat surface, the valve ball surface can be supersonic spray tungsten carbide, The surface of the valve seat can be supersonic sprayed Stellite20 alloy. This hardness is widely used in the coal chemical industry and has high wear resistance and service life.

In foreign countries use the valve ball and the same surface hardness of the mating seat, the valve ball and the seat surface are used supersonic spray carbide tungsten process, the surface hardness of greater than 72HRC, even in the case of high hardness, ball and valve seat of the surface seizure hardly occur, but the domestic ball making the surface hardness of greater than 72HRC of the valve ball, valve seat no mature grinding process, it is difficult to ensure that the valve ball and with the valve seat, less use.

· Ball Valve valve ball and seat hardening Notes

Metal hard seal ball valve ball and valve seat materials generally choose stainless steel or corrosion-resistant materials, otherwise the cemented carbide and valve seat ( or valve ball ) bonding layer will be easily corroded by the medium, and the cemented carbide layer will fall off, which will affect the life of the ball valve. In addition to different valve bodies ( or ball ) should select the appropriate material hardening process, in coalification engineering art duplex stainless steel materials are widely used, the duplex stainless steel material having good wear resistance and corrosion fatigue resistance.

Duplex stainless steel material is a kind of steel with both ferrite and austenite structure. Ferrite and austenite structure each account for about 50%, and the two- phase structure is independent, and 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℃, long-term heat preservation will produce strong embrittlement. This phenomenon is generally called 475℃ embrittlement; when the temperature exceeds 400~500℃, duplex stainless steel Performance will be destroyed. If the duplex stainless steel material adopts the spray welding or surfacing cemented carbide process, the process of melting the cemented carbide and the matrix in this process (the temperature is generally greater than 900 ℃) will destroy the metallographic structure of the duplex stainless steel material, so the double Phase stainless steel materials are not suitable for spray welding (or surfacing) hard alloy hardening process. The surface hardening process of duplex stainless steel material is suitable for the supersonic spraying process, and the hardening process must ensure that it does not damage the metallographic structure of the duplex stainless steel material matrix.

·Tips :

The ball and seat of the metal hard seal ball valve adopts a reasonable hardening process, which can improve the service life and performance of the metal hard seal valve, and a reasonable hardening process can reduce the manufacturing cost. With the continuous emergence of new technologies, there will be more hardening treatment processes. The hardening process and hardness coordination of metal hard-seal valves is a very complicated problem. Therefore, the design, material selection, hardening process and hardness coordination of hard-seal valves , Manufacturers and users should be fully cautious .



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