Ball valves are widely used due to their small flow resistance, wide range of pressure and temperature, good sealing performance, short opening and closing time, and non-directional installation requirements (except for one-way ball valves with special requirements). As the opening and closing part of the ball valve, the ball plays the most important role. So what are the common spherical surface treatment methods? Before we talk about the topic, we first understand several concepts: 1, electroplating; 2, cold spray; 3, thermal spray, 4, spray welding, 5, nitriding treatment.
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Plating: A process in which a thin layer of other metal or alloy is plated on certain metal surfaces using electrolysis. Thereby improving corrosion resistance, wear resistance and improving aesthetics.
Cold spray: Compressed air accelerates the metal particles to a critical speed (supersonic), and the metal particles physically deform after hitting the surface of the substrate. The metal particles collide on the surface of the substrate and adhere firmly. The metal particles are not melted during the whole process, but if the metal particles do not reach the supersonic speed, they cannot adhere. The shortcoming of cold spray is that the thickness of the sprayed layer is not high, and the surface adhesion is not well sprayed. The advantage is that the ball does not need to be heated. No deformation or internal stress is generated during the spraying process, and the thickness of the spray is well controlled. As shown in Figure 1, the cold spray schematic.
Thermal spraying: a method of heating a sprayed material to a dissolved or semi-melted state by using a heat source, and spraying the deposited material onto the surface of the pretreated substrate to form a coating at a certain speed
Spray welding: The process of spray welding is basically the same as that of thermal spraying, and the difference is added to the remelting process in the dusting process. Spray welding has a one-step spray welding method and a two-step spray welding method. One-step spray welding method is to melt one section after spraying, and spray and melt alternately, using the same spray gun. After the workpiece is preheated, the protective layer of 0. 2mm is sprayed first, and the surface is sealed to prevent oxidation. The spray is sprayed from one end, and the spray distance is 10 to 30 mm. The protective layer is locally heated to melt and begins to wet. When flowing, the powder is sprayed again, and the melting is repeated until the predetermined thickness is reached, the surface is mirror-surface reflective, and then spreads forward to reach the surface completely covering the spray-welded layer. If the thickness is insufficient at one time, the thickening can be repeated. The one-step method is suitable for small parts or small area spray welding. The two-step spray welding method is to complete the spray layer and then remelt it. The remelting is the key process of the two-step process and is carried out immediately after spraying. High-power soft flame with neutral flame or weak carbonized flame, the spray distance is about 20 ~ 30mm, the angle between the flame and the surface is 60 ° ~ 75 °, starting from about 30mm from the coating, properly grasp the remelting speed, will be coated The layer is heated until the coating has a mirror reflection and then the next part is remelted. Because the spray welding process needs remelting, when the melting point (melting temperature) of the base material (such as sphere) is lower than that of the alloy, it is not suitable for the spray welding process. Because the alloy material is remelted, if the melting point of the matrix material is lower, when the alloy reaches remelting Temperature
Before, the matrix material has already melted or partially melted.
Nitriding treatment: refers to a chemical heat treatment process in which a nitrogen atom penetrates into the surface layer of a workpiece in a certain medium at a certain temperature. The nitrided product can improve the wear resistance, fatigue resistance, corrosion resistance and high temperature resistance. The common treatment methods for the surface of the sphere are electroplating, cold spraying, spray welding and nitriding. The following are the surface treatment methods of the sphere that I often encounter in my work. It is not comprehensive, but it is commonly used to ensure that:
1, the surface of the sphere is plated with nickel (ENP) or chrome (HCr):
When the spherical material is carbon steel and the valve is soft sealed, such as A105, 25, LF2, etc., the corrosion resistance of these materials is very poor, so the surface of the sphere must be treated with nickel plating (ENP) or chrome plating (HCr) to avoid spherical corrosion. , affecting the seal. When the sphere is austenitic stainless steel or other similar soft material, when the sealing ring of the ball valve is a relatively hard material such as PEEK, it is recommended that the surface of the sphere be plated with nickel (ENP) or chrome (HCr) to improve the spherical hardness to avoid the spherical surface being Scratches such as PEEK affect the seal.
Nickel plating (ENP) or chrome plating (HCr) plating thickness is generally 0.03mm ~ 0.05mm to meet the basic requirements, if special requirements, can be properly thickened; nickel plating (ENP) or chrome plating (HCr) after plating hardness It is generally 600 HV to 800 HV.
2, cold spray tungsten carbide WC on the surface of the sphere:
Tungsten carbide is characterized by a particularly high hardness and good wear resistance. However, the disadvantage is that the melting point of tungsten carbide is particularly high, about 2870 ° C. The melting point of general metal materials is much lower than the melting point of tungsten carbide, so only the process of cold spraying tungsten carbide WC can be used. The thickness of the sprayed tungsten carbide WC is generally 0.15mm~0.18mm, which can achieve the ideal surface hardness. If special requirements, the thickness can reach 0.5mm~0.7mm, the thicker the cold spray thickness, the lower the surface adhesion. Thicker cold spray thicknesses are not recommended. The surface cold-coated tungsten carbide WC hardness is generally 1050HV~1450HV, which is about 70HRC.
3, spray welding or cold spray nickel-based alloy, the nickel-based alloy commonly used on the ball valve sphere is NI60, NI55. NI60 and NI55 have a melting point of 1027 ° C and can be spray welded or cold sprayed.
The thickness of spray-welded nickel-based alloy is generally 0.5mm~0.7mm, which can meet the basic requirements. The actual maximum thickness can reach 2.5mm~3mm. For cold spray, the spray thickness is 0.15mm~0.18mm.
The hardness after spray welding is generally NI55:50~55 HRC, NI60:55~60HRC
4, spray-weld cobalt-based alloy, cobalt-based alloy is generally spray welded. Commonly used cobalt-based alloys on ball valve spheres are STL20, STL6, STL1
The thickness of spray-welded cobalt-based alloy is generally 0.5mm~0.7mm, which can meet the basic requirements. The actual maximum thickness can reach 2.5mm~3mm.
The hardness after spray welding is generally STL20:50~52HCR; STL6:38~40HCR; TL1:48~50HCR
5, nitriding treatment,
The essence of nitriding treatment is to infiltrate nitrogen atoms into the surface of the workpiece. In fact, the depth of penetration is not well controlled, and the surface hardness is also unstable. The most critical part of the process of hard-sealing ball valves is the grinding between the seat ring and the ball. During the grinding process, the nitride layer is easily worn away. Or it is so thin that when the valve is actually used on the pipeline, the spherical surface is easily scratched by impurities in the medium, affecting the seal and even making the torque larger. Therefore, if you are not trying to ask for trouble, or if your customer will quickly replace the new valve, it is strongly recommended that the nitriding process should not be used on hard-sealed ball valves. Export products have never had a hard ball valve that is nitrided. Of course, our company often has such requests from customers, mainly because of the low price, but the problems are endless. This is all done by the customers themselves. Who will let him not listen?
Careful friends must have found that the above hardness values are in HV and some are HCR. the reason
Yes: HCR Rockwell hardness is a hardened steel ball with a apex angle of 120 degrees diamond cone or a diameter of 1.58 mm as the indenter, and is pressed into the surface of the sample with a specified test force. In the test, first add the initial test force, then add the main test force. After pressing the surface of the sample, the main test force is removed, and the Rockwell hardness value of the metal material to be tested is determined according to the depth of the surface indentation of the sample while retaining the initial test force. According to such a test method, if the surface to be tested is a plating layer or a cold-sprayed layer, the thickness is too low, and it is easily pressed by the indenter, so that the true hardness value cannot be measured. Therefore, it is replaced by another hardness value HV Weis hardness, which is measured by measuring the indentation area. HCR and HV have no conversion relationship, only correspondence.