At present, the commonly used hardening processes on the surface of metal hard seal ball valve balls are as follows:
(1) Ball surface surfacing (or spray welding) cemented carbide, the hardness can reach more than 40HRC, the spherical surface surfacing cemented carbide process is complex, the production efficiency is low, and the large-area surfacing is easy to deform the parts, and the process of spherical surface hardening is used less.
(2) Hard chrome plating on the surface of the sphere, 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 will decrease rapidly due to the release of internal stress when the temperature increases, and its operating temperature cannot be higher than 427°C. In addition, the adhesion of the chrome plating layer is low, and the plating layer is easy to fall off.
(3) The surface of the sphere is made of plasma nitriding, the surface hardness can reach 60~65HRC, the thickness of the nitriding layer is 0.20~0.40mm, and the plasma nitriding treatment hardening process cannot be used in the field of chemical strong corrosion due to poor corrosion resistance.
(4) Supersonic spraying (HVOF) process on the surface of the sphere, the hardness can reach up to 70~75HRC, the collective strength is high, the thickness is 0.3~0.4mm, and the supersonic spraying is the main process means of surface hardening of the sphere. Highly viscous fluids in thermal power plants, petrochemical systems, coal chemical industries; This hardening process is used in most of the highly corrosive fluid media mixed fluids with dust and solid particles.
The supersonic spraying process is a process method in which oxygen fuel combustion generates a high-speed air flow to accelerate the impact of powder particles on the surface of the workpiece to form a dense surface coating. In the impact process, due to the high speed of the particles (500~750m/s) and the low particle temperature (-3000°C), the coating with high bonding strength, low porosity and low oxide content can be obtained after hitting the surface of the workpiece. HVOF is characterized by the fact that the velocity of alloy powder particles exceeds the speed of sound, even 2~3 times the speed of sound, and the speed of the air flow 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 combined, the bonding strength is high (77MPa), and the coating porosity is low (<1%). This process has a low heating temperature (<93°C) for the workpiece, and 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, there is no change in the composition and structure of the workpiece, and the coating hardness is high, and it can be machined.
Spray welding is a thermal spraying process on the surface of metal materials. It is to heat the powder (metal powder, alloy powder, ceramic powder) to a molten or high plastic state through a heat source, and then spray it by air flow and deposit it on the surface of the pre-treated workpiece to form a coating (welding) layer that is firmly combined with the surface (substrate) of the workpiece.
In the process of spray welding and surfacing hardening, the cemented carbide and the substrate have a melting process, and there is a hot melting zone at the collection of the cemented carbide and the matrix, in order to fully achieve the performance of the cemented carbide layer of spray welding or surfacing welding, and avoid the metal contact surface of the hot melt area after processing, it is recommended that the thickness of the cemented carbide needs to be greater than 3mm.
