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

(1) Hard alloy surfacing (or spray welding) on the surface of the sphere has a hardness of more than 40HRC. Hard alloy surfacing on the surface of the sphere has a complex process, low production efficiency, and large-area surfacing is easy to deform parts. At present, the process of Case-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 0.07-0.10ram. The chromium plating layer has high hardness, wear resistance, corrosion resistance, and can maintain surface brightness for a long time. The process is relatively simple and the cost is relatively low. However, the hardness of the hard chromium coating will rapidly decrease as the temperature increases due to the release of internal stress, and its working temperature cannot be higher than 427 ℃. In addition, the bonding force of the chromium plating layer is low, and the coating is prone to detachment.

(3) The surface of the sphere adopts plasma nitriding, 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 cannot be used in fields such as strong chemical corrosion.

(4) The sphere surface is sprayed with HVOF technology, with the highest hardness of 70~75HRC, high collective strength and thickness of 0.3-0.4mm. The ultrasonic spraying is the main process method for Case-hardening of the sphere. High viscosity fluid in Fossil fuel power station, petrochemical system and coal chemical industry; This hardening process is mostly used in mixed fluids with dust and solid particles, as well as highly corrosive fluid media. The supersonic spraying process is a process method that produces Jet stream generated by oxygen fuel combustion to accelerate the powder particles to hit the workpiece surface and form a dense surface coating. In the impact process, due to the high particle velocity (500-750m/s) and low particle temperature (-3000 ℃), the coating with high bonding strength, low void fraction and Suboxide content can be obtained after impacting the workpiece surface. The characteristic of HVOF is that the velocity of alloy powder particles exceeds the speed of sound, even 2-3 times that of sound, and the airflow velocity is 4 times that of sound. HVOF is a new processing technology, with a spraying thickness of 0.3-0.4ram. The coating is mechanically bonded to the workpiece, with high bonding strength (77MPa) and low coating porosity (< 1%). This process has a low heating temperature for the workpiece (< 93 ℃), and the workpiece does not deform, allowing for cold spraying. When spraying, the powder particle velocity is high (1370m/s), there is no heat affected zone, the composition and structure of the workpiece do not change, and the coating hardness is high, which can be machined. Spray welding is a process of Thermal spraying on the surface of metal materials. It heats the powder (metal powder, alloy powder, ceramic powder, etc.) to a molten or highly plastic state through a heat source, and then sprays it onto the pre treated surface of the workpiece using a gas flow, forming a coating (welding) layer that is firmly bonded to the surface of the workpiece (substrate). In both spray welding and surfacing hardening processes, the hard alloy and the substrate undergo a melting process, and there is a hot melt zone at the junction of the hard alloy and the substrate. In order to fully achieve the performance of the spray welding or surfacing hard alloy layer and avoid welding the hot melt zone as a metal contact surface after processing, it is recommended that the thickness of the spray welding or surfacing hard alloy should be greater than 3mm.