What is the use of supersonic spraying?

       1 Overview

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

 

2. Hardening method of ball and valve seat of metal hard sealing ball valve

      At present, the commonly used hardening processes on the surface of metal hard-sealed ball valves are:

    (1) The surface of the sphere adopts hard alloy surfacing (or spray welding), and the hardness can reach above 40HRC. The surfacing process of the spherical surface is complicated, the production efficiency is low, and the large-area surfacing is easy to cause deformation of the parts. Use less case hardening process.

    (2) The surface of the sphere is plated with hard chromium, the hardness can reach 60-65HRC, and the thickness is 0.07-0.10mm. The chrome layer has high hardness, wear resistance, corrosion resistance, and can keep the surface bright for a long time. The process is relatively simple and inexpensive. However, as the temperature increases, the hardness of the hard chromium coating decreases rapidly due to the release of its internal stress, and its operating temperature cannot be higher than 427°C. In addition, the bonding force of the chrome plating layer is low, and the plating layer is easily peeled off.

   (3) Plasma nitriding is performed on the surface of the sphere, the surface hardness can reach 60-65HRC, and the thickness of the nitriding layer is 0.20-0.40mm. The plasma nitride hardening process cannot be used in fields such as chemically aggressive corrosion due to its poor corrosion resistance.

  (4) The supersonic spraying process is carried out on the surface of the sphere, the hardness is as high as 70-75HRC, the aggregation strength is high, and the thickness is 0.3-0.4mm. Supersonic spraying is the main process method for surface hardening of balls. The hardening process is mainly 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 HVOF process is a process in which oxy-fuel combustion creates a high-velocity gas stream to accelerate powder particles against the workpiece surface to form a dense surface coating. During impact, high bond strength, low porosity and low oxide content can be achieved after impacting the workpiece surface due to faster particle velocity (500 ~ 750m/s) and lower particle temperature (-3000°C) get. 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, while the air velocity is 4 times the speed of sound.

    HVOF is a new processing technology. The spray thickness is 0.3-0.4mm. The coating and the workpiece are mechanically bonded. The bond strength is high (77MPa) and the porosity of the coating is low (<1%). The heating temperature of the workpiece is low (<93℃), the workpiece is not deformed, and it can be cold sprayed. When spraying, the particle velocity is high (1370m/s), there is no heat affected zone, the composition and structure of the workpiece remain unchanged, the coating hardness is high, and it can be machined.

    Spraying is a thermal spraying process for the surface of metal materials. It uses a heat source to heat the powder (metal powder, alloy powder, ceramic powder) to a molten or highly plastic state, and then sprays it through an air stream and deposits it on the pretreated workpiece surface to form a layer of the workpiece surface. (substrate) is firmly bonded to the coating (welding).

In the process of spray welding and surfacing hardening, both the cemented carbide and the base have a melting process, and there is a hot melt zone at the assembly of the cemented carbide and the base to fully realize the performance of spray welding. Or surfacing the cemented carbide layer, and avoid welding hot melt after processing. This area is the metal contact surface, and it is recommended that the thickness of the cemented carbide for spray welding or surfacing welding should be greater than 3mm.

3. The hardness of the contact surface between the ball and the seat of the hard-sealed ball valve is matched

     The metal sliding contact surface must have a certain hardness difference, otherwise it is easy to get stuck. In practical applications, the hardness difference between the valve ball and the valve seat is generally 5-10HRC. This difference in hardness gives the ball valve a better service life. Due to the complex machining and high machining cost of the ball, in order to protect the ball from damage and wear, the hardness of the ball is usually selected to be higher than that of the valve seat surface.

     The contact surface hardness of the ball and 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 sprayed with Stellite20 alloy by supersonic speed, and the surface of the valve seat can be surfacing with Stellite 12. 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, and the surface of the valve seat is 58HRC. The surface of the valve ball can be ultrasonically sprayed with tungsten carbide. The seat surface can be made of Stellite 20 alloy which is supersonic sprayed. This hardness is widely used in the coal chemical industry and has high wear resistance and service life.

     In foreign countries, the ball and seat surfaces have the same hardness. The surface of the valve ball and valve seat adopts the supersonic spraying tungsten carbide process, and the surface hardness is greater than 72HRC. The ball and seat are in contact even with ultra-high hardness. The surface is also not easy to get stuck. However, in China, there is no mature grinding process to produce valve balls and valve seats with surface hardness greater than 72HRC. Difficult to ensure ball and seat fit and are rarely used.