Overview Of Hardening Process Of Metal Sealed Ball Valve

1 Overview

In the medium of high viscous fluid, mixed fluid with dust and solid particles, and strongly corrosive fluid in the field of thermal power plant, petrochemical system and coal chemical industry, the ball valve needs to select the metal hard seal ball valve, so it is very important to select the appropriate hardening process of metal hard seal ball valve ball and valve seat.

2 hardening method of metal hard seal ball valve ball and valve seat

At present, the hardening processes commonly used for the surface of metal hard seal ball valves mainly include the following:

(1) Surfacing (or spray welding) cemented carbide on the surface of the sphere has a hardness of more than 40HRC. The surfacing cemented carbide process on the surface of the sphere is complex, the production efficiency is low, and large-area surfacing is easy to deform the parts. At present, the process of surface hardening of the sphere is rarely used.

(2) The surface of the sphere is plated with hard chromium with a hardness of 60 ~ 65hrc and a thickness of 0.07 ~ 0.10mm. The chromium 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 hard chromium coating will decrease rapidly due to the release of internal stress when the temperature increases, and its working temperature cannot be higher than 427 ℃. In addition, the bonding force of chromium plating layer is low, and the coating is easy to fall off.

(3) Plasma nitriding is adopted on the surface of the sphere, with a surface hardness of 60 ~ 65hrc and a nitriding layer thickness of 0.20 ~ 0.40mm. Due to poor corrosion resistance, the plasma nitriding hardening process can not be used in the fields of strong chemical corrosion.

(4) The high-speed spraying (HVOF) process on the surface of the sphere has the highest hardness of 70 ~ 75hrc, high aggregate strength and thickness of 0.3 ~ 0.4mm. Supersonic spraying is the main process means for the surface hardening of the sphere. High viscous fluid in thermal power plant, petrochemical system and coal chemical industry; The hardening process is mostly used in mixed fluids with dust and solid particles and strongly corrosive fluid media.

Supersonic spraying process is a process method in which oxygen fuel combustion produces high-speed air flow to accelerate powder particles to impact the workpiece surface and form a dense surface coating. In the process of impact, because the particle velocity is fast (500 ~ 750m / s) and the particle temperature is low (- 3000 ℃), the coating with high bonding strength, low porosity and low oxide content can be obtained after impacting the workpiece surface. The velocity of particles is 4 times the speed of sound, even more than the speed of sound of particles.

HVOF is a new processing technology, with spraying thickness of 0.3 ~ 0.4mm, mechanical bonding between coating and workpiece, high bonding strength (77MPa) and low porosity of coating (< 1%). The heating temperature of the workpiece is low (< 93 ℃), the workpiece is not deformed, and cold spraying can be carried out. When spraying, the powder particle velocity is high (1370m / s), there is no heat affected zone, the composition and structure of the workpiece have no change, and the coating hardness is high, so it can be machined.

Spray welding is a kind of thermal spraying process for the surface of metal materials. After the powder (metal powder, alloy powder and ceramic powder) is heated to melt or reach a high plastic state through a heat source, it is sprayed by air flow and deposited on the pre treated workpiece surface to form a coating (welding) layer firmly combined with the workpiece surface (substrate).

In the spray welding and surfacing hardening process, the cemented carbide and the matrix have a melting process, and there is a hot melt zone at the collection of cemented carbide and the matrix. In order to fully achieve the performance of spray welding or surfacing cemented carbide layer and avoid the welding hot melt zone as a metal contact surface after processing, it is recommended that the thickness of spray welding or surfacing cemented carbide should be more than 3mm.

3 hardness matching of contact surface between hard seal ball valve and valve seat

The metal sliding contact surface needs to have a certain hardness difference, otherwise it is easy to bite. 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 and high processing cost of the ball, in order to protect the ball from damage and wear, the hardness of the ball is generally higher than that of the valve seat surface.

There are two kinds of hardness matching widely used for the hardness matching of the contact surface between the valve ball and the valve seat: ① the surface hardness of the valve ball is 55HRC and the surface of the valve seat is 45hrc. The surface of the valve ball can be sprayed with Stellite 20 alloy at supersonic speed, and the surface of the valve seat can be overlaid with Stellite 12 alloy. This hardness matching is the most widely used hardness matching for metal sealed ball valves, which can meet the conventional wear requirements of metal hard sealed ball valves; ② The surface hardness of the valve ball is 68hrc, the surface of the valve seat is 58hrc, the surface of the valve ball can be sprayed with tungsten carbide at supersonic speed, and the surface of the valve seat can be sprayed with stellite20 alloy at supersonic speed. This hardness is widely used in the field of coal chemical industry, with high wear resistance and service life.

In foreign countries, the matching with the same surface hardness of the valve ball and valve seat is used. The surface hardness of the valve ball and valve seat is greater than 72hrc by supersonic spraying tungsten carbide process. Even under the condition of ultra-high hardness, the contact surface of the valve ball and valve seat is not easy to bite. However, at present, there is no mature grinding process for the valve ball and valve seat with surface hardness greater than 72hrc in China, so it is difficult to ensure the matching between the valve ball and valve seat, so it is rarely used.

4 precautions for hardening of hard seal ball valve ball and valve seat

The ball and valve seat of metal hard sealed ball valve are generally made of stainless steel or corrosion-resistant materials. Otherwise, the bonding layer between cemented carbide and valve seat (or valve ball) is easy to be corroded by the medium, and the cemented carbide layer falls off, affecting the service life of the ball valve.

In addition, appropriate hardening process should be selected for different valve seat (or valve ball) materials. Duplex stainless steel materials are widely used in the field of coal chemical industry. Duplex stainless steel materials have good corrosion fatigue resistance and wear corrosion resistance.

Duplex stainless steel is a kind of steel with both ferrite and austenite structure. Ferrite and austenite structure account for about 50% respectively, and the two-phase structure exists independently. Its performance is characterized by the characteristics of both austenitic stainless steel and ferritic stainless steel. In the characteristics of ferritic stainless steel, when the temperature is in the range of 400 ~ 500 ℃, strong embrittlement will occur after long-time insulation. This phenomenon is generally called 475 ℃ embrittlement; When the temperature exceeds 400 ~ 500 ℃, the properties of duplex stainless steel will be destroyed.

If spray welding or surfacing cemented carbide process is adopted for dual phase stainless steel, the melting process of cemented carbide and matrix (temperature is generally greater than 900 ℃) will damage the metallographic structure of dual phase stainless steel, so dual phase stainless steel material is not suitable for spray welding (or heap welding) cemented carbide hardening process. The surface hardening process of duplex stainless steel is suitable for supersonic spraying process. The hardening process must ensure that it cannot damage the metallographic structure of the matrix of duplex stainless steel.