1. An overview of the

Metal hard seal ball valve in the petroleum, chemical, metallurgical, and has been widely used in the construction of city.By choosing different material, metal hard seal ball valve can be applied to water, steam, oil, nitric acid, acetic acid and urea, and other media.Through the structural optimal design, metal hard seal ball valve can be realized without friction and low torque opening and closing.In use, the main failure form of metal hard seal ball valve is erosion wear.Though, through methods such as the surface heat treatment or welding carbide can improve the service life of metal hard seal ball valve and abrasion resistance, but still can't meet conveying pulverized coal ash, steel, silicon powder and alumina powder special medium such as request.If application of supersonic flame spraying technology processing ball, ball can form a carbide metal ceramic coating, the coating and substrate bonding strength of up to 70 mpa, porosity < 2%, uniform and dense structure, relatively complete keep the characteristics of the hard phase WC, hardness > 1000 hv, wear-resisting corrosion resistant performance is good, have good mechanical performance, can significantly improve the use of the ball valve performance.

Working principle of the 2.

Figure 1  HVO/AF supersonic spraying device

Supersonic spraying device (figure 1) using kerosene as fuel, oxygen and compressed air as the accelerant, control device of kerosene and propellants at a certain pressure and flow rate to the spray gun, by high performance liquid atomizing nozzle mix into fog after injection gun chamber, the liquid fog by the spark plug of high temperature and high pressure gas is formed after combustion, laval spray gun to accelerate it to Seattle.Powder feeding system from laval spray powder spray gun nozzle of a low pressure area into a supersonic jet, the jet from the gun after heating speed, high speed spray to the workpiece surface coating sedimentary formation.Because of multifunction supersonic spraying technology can be used two propellants, oxygen and compressed air at the same time have the function that HVOF and HVAF, flame flow speed and temperature over a range of continuous adjustable.

3. The spraying process

Before spraying, must pass a clean surface of workpiece and coarsening of sandblasting processing, general coating material for the WC - 12 Co, the quality of the WC and Co % Wt were 88% and 12% respectively, sintering method of manufacture.Tungsten carbide hardness at room temperature is higher, especially the hot hardness is very good, to 1000 ° C, its hardness drop rarely (figure 2).

Figure 2  temperature hardness curve  

Through sample to WC - 12 co coating and substrate in 30 ∀ and 90 ∀ erosion wear test contrast, of 4.5 times, when 30 ∀ ∀ in 90 of 5.2 times.Because of its strong adhesive force, high hardness, has a full set of wear-resisting coating microstructure, and thus the erosion wear resistance is higher.Tests show that WC - 12 co erosion is part of the coating and plastic characteristics of erosion.Co binding phase due to plastic deformation, can absorb part of the plastic erosion energy of particles, and hard phase WC has against the erosion of the particles, the two complement each other, so the coating has good resistance to erosion and wear performance.

4. The application

In petroleum, chemical industry and coking plant, due to the high temperature in the pipeline, the medium easily within the valve cavity and seal surface adhesion or coking, ordinary hard seal ball valve (figure 3) frequently because of sphere surface coking and untight seal card even plug.To this end, the valve seat sealing surface structure to the blade with a sharp blade shape (figure 4), can be in the process of the valve opening and closing skiving off adhesion or coking layer on the surface of the sphere, so as to ensure the reliable seal of sealing pair.

Figure 4  scraper type high temperature metal hard seal ball valve

In the process of manufacture, using F304 forgings sphere, surface nitriding treatment in order to improve the hardness.Seat is made of F304, surfacing too hard alloy.Use found in normal temperature condition, medium under the condition of clean, can guarantee the reliable seal, when the temperature of the medium to 400!Above, because the company too set alloy hot hardness is not high, the sphere surface nitriding treatment, wear resistance is poor, the scraping of coking at the same time, it is easy to damage the valve seat sealing surface and sphere surface, so it is difficult to meet the conditions required.Through the analysis, instead of using supersonic spraying technology in the sphere surface spraying WC - 12 co, formation of about 0.3 mm thick erosion abrasion resistant coatings, 60 ~ 62 HRC hardness.Shape of the valve seat sealing surface substrate is processed into scraper, in its surface spray coating on 0.3 mm thickness.By changing the spraying process, make the coating hardness is 55 ~ 58 HRC, the valve seat sealing surface hardness and sphere surface form a certain hardness difference.Spraying is completed, the sphere by the diamond abrasive grinding, surface roughness Ra0.8 to microns, the valve seat sealing surface and sphere running-in by hand.Running-in natural diamond powder materials with adding oleic acid modulation (size 1200 mesh), grinding surface roughness of sealing surface to Ra0.2 mu m after assembly test, reliable sealing at room temperature.With a special heating equipment, heating to design the valve after the limit temperature of 540 ° c, insulation 8 h, operation test, the valve opening and closing flexible, the sealing test is normal.After repeatedly testing, spherical has no obvious change.As a result of the supersonic spraying WC - 12 co coating red hardness and resistance to erosion wear performance is good, even if the temperature in the process of use to design the ceiling above 540 ° c, can still maintain good erosion resistance and abrasion resistance, and the scraper with spherical surface hardness has about 5 HRC hardness difference between.When the seat edge skiving coking things will not damage the sphere surface on the surface of the sphere and valve seat sealing surface, can ensure the sealing performance and service life.

5  epilogue

Supersonic spraying technology applied in hard seal ball valve, greatly improving the scope of application and service life of the valve.If choose department too set alloy powder and other materials can produce metal coating that could satisfy the requirement of different working conditions.According to user's feedback information, the use of supersonic spray coating processing technology of hard seal ball valve in the medium such as water, steam and oil service life by about 20 times, in the coal powder, silicon powder line erosion tough conditions such as life expectancy can also be increased by more than five times.