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Metal sealing ball valve hardening process

Ⅰ. Overview

In thermal power plants, petrochemical systems, high-viscosity fluids in the coal chemical industry, mixed fluids with dust and solid particles, and highly corrosive fluids, ball valves need to use metal hard-sealed ball valves, so choose appropriate metal hard-sealed ball valves. The hardening process of the ball and seat of the ball valve is very important.

Ⅱ. Hardening method of ball and seat of metal hard-sealed ball valve

At present, the commonly used hardening processes for the surface of metal hard sealing ball valve balls mainly include the following:

(1) Hard alloy surfacing (or spray welding) on the surface of the sphere, the hardness can reach more than 40HRC, the surfacing process of hard alloy on the sphere surface is complicated, the production efficiency is low, and the large-area surfacing welding is easy to deform the parts. The process of case hardening is used less frequently.

(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 hard chrome plating will decrease rapidly due to the release of internal stress when the temperature increases, and its working temperature cannot be higher than 427 °C. In addition, the bonding force of the chrome plating layer is low, and the plating layer is prone to fall off.

(3) The surface of the sphere adopts plasma nitriding, the surface hardness can reach 60~65HRC, and the thickness of the nitride layer is 0.20~0.40mm. Due to the poor corrosion resistance of the plasma nitriding treatment hardening process, it cannot be used in the fields of chemical strong corrosion.

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

The supersonic spraying process is a process method in which the combustion of oxygen fuel produces high-speed airflow to accelerate the powder particles to hit the surface of the component to form a dense surface coating. During the impact process, due to the fast particle velocity (500-750m/s) and the low particle temperature (-3000°C), high bonding strength, low porosity and low oxide content can be obtained after hitting the surface of the part. coating. 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 air velocity is 4 times that of the speed of sound.

HVOF is a new processing technology, the spray thickness is 0.3-0.4mm, the coating and the component are mechanically bonded, the bonding strength is high (77MPa), and the coating porosity is low (<1%). This process has a low heating temperature for the parts (<93°C), the parts are not deformed, and can be cold sprayed. When spraying, the powder particle velocity is high (1370m/s), there is no heat-affected zone, the composition and structure of the parts 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 heats the powder (metal powder, alloy powder, ceramic powder) to a molten or high plastic state through a heat source, and then sprays it by air flow and deposits it on the surface of the pre-treated part to form a layer with the surface of the part. (Substrate) combined with a strong 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 melt zone where the cemented carbide and the substrate meet. The area is the metal contact surface. It is recommended that the thickness of the cemented carbide should be more than 3mm by spray welding or surfacing.

Ⅲ. Hardness of the contact surface between the ball and the seat of the hard-sealed ball valve

The metal sliding contact surface needs to have a certain hardness difference, otherwise it is easy to cause seizure. In practical application, the hardness difference between the valve ball and the valve seat is generally 5-10HRC, which 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 higher than the hardness of the valve seat surface.

There are two kinds of hardness combinations that are widely used in the contact surface hardness of the valve ball and the valve seat: ①The surface hardness of the valve ball is 55HRC, and the surface of the valve seat is 45HRC. Alloy, this hardness match is the most widely used hardness match for metal-sealed ball valves, which can meet the conventional wear requirements of metal-sealed ball valves; ②The surface hardness of the valve ball is 68HRC, the surface of the valve seat is 58HRC, and the surface of the valve ball can be sprayed with supersonic tungsten carbide. The surface of the valve seat can be made of Stellite20 alloy by supersonic spraying. This hardness is widely used in the coal chemical industry and has high wear resistance and service life.

Ⅳ. Epilogue

The valve ball and valve seat of the metal hard-sealed ball valve adopt a reasonable hardening process, which can directly determine the service life and performance of the metal hard-sealing valve, and a reasonable hardening process can reduce the manufacturing cost.

Post time: Oct-26-2022