1. Overview of cast iron valves
Due to the low pressure of the water system, the main materials of the valves and pipe fittings used are cast iron, but cast iron is not corrosion resistant, and a single material cannot meet the requirements, so it is necessary to carry out appropriate coating treatment for it. This requires research and analysis on the coating of this type of products, and Shanghai valve manufacturers choose appropriate materials and processes. Cast iron valve materials include gray iron, malleable iron and ductile iron. As ductile iron valves are used more and more widely in water system, especially gate valves, especially Z45X resilient seated gate valves, this paper takes ductile iron valves as an example to discuss.
2. Current situation of cast iron valves
According to the investigation and analysis, the damage and replacement of the valve are mainly due to the corrosion of the main body. The inner chamber is seriously corroded due to the contact with the medium. After the corrosion, the valve diameter becomes smaller, the flow resistance increases, and the medium transmission is affected. Moreover, most valves are installed on the ground or underground, and the surface is in contact with the air. The air is humid, and the surface is prone to rust. This requires that the main material of the valve be protected to extend its service life. On the other hand, due to the direct contact between cast iron and water, the existence of rust constitutes a secondary pollution to the water quality, which must be controlled. Therefore, the requirements for coating are mainly functional protection, supplemented by decoration. The surface treatment methods of valves mainly include paint coating, zinc plating and powder coating. The protection period of painting is short, and it cannot be used under working conditions for a long time. Galvanizing process is mainly used for pipelines. Hot dip galvanizing and electro galvanizing are both used. The process is complex. The pretreatment adopts pickling and phosphating processes. There will be acid and alkali residues on the surface of the workpiece, leaving a hidden danger of corrosion, making the zinc coating easy to fall off. The corrosion resistance of the zinc coating is 3 to 5 years. Powder coating has the characteristics of coating thickness, corrosion resistance, erosion resistance, etc., which can meet the requirements of water system service conditions on valves.
3. Cast iron valve material
The powder coating is solvent-free, which reduces environmental pollution and organic solvent poisoning. It is not flammable, and it is safe, convenient, simple and economical to store and transport. The powder coating can be recycled, and the utilization rate is about 95%. The coating process is easy to realize automation, and 4O-5001xm thick coating can be obtained for one painting. The coating has no primer, which improves the working efficiency. The coverage of corners is excellent, and the mechanical properties, insulation properties and corrosion resistance of the coating film are excellent. However, the color change procedure of powder coating is complex. It is easy to agglomerate due to the influence of pressure, temperature and humidity during storage. The manufacturing equipment and coating process of powder coatings are complex. There are many kinds of powder coatings, including nylon 1l, nylon 12, epoxy resin and epoxy polyester. Considering its performance, cost and process complexity, epoxy resin powder is suitable for valve coating. If the cost factor is not considered, nylon series powder is a better solution with high comprehensive performance index. There are successful cases of using nylon powder as valve coating abroad.
4. Cast iron valve process
The cast iron parts to be sprayed in the valve include valve body, bonnet, gland, handwheel, gate, etc. Spraying process is to remove dust for workpiece pretreatment_+ Preheating, spraying, curing and cooling.
(1) The surface to be coated for pretreatment must be dry and clean, and have a certain roughness. The porous structure of cast iron makes the structure of parts not compact. During acid treatment, it is easy to have acid infiltration, which leads to hydrogen storage in materials and hydrogen embrittlement. Therefore, pickling and phosphating process cannot be adopted for cast iron before coating, and shot blasting or sand blasting process shall be adopted to reach Sa2.5 level. The workpiece shall be preheated within 6h after treatment, and the parts shall not be exposed to rain to prevent rusting again. Oil contamination must be prevented during handling, and canvas gloves must be worn. When designing the spraying line, if conditions permit, Shanghai Mingjing Anti corrosion Valve Manufacturing Co., Ltd. can consider online shot blasting (sand blasting) to avoid multiple manual contact with the workpiece and prevent secondary pollution of the workpiece. Consumables for shot peening can be mixed with high-quality stainless steel shot (S 1.5) and stainless steel cutting head (1.2) in a proportion of 1: l, and replenished regularly and proportionally according to consumption.
(2) The preheating must ensure that the solid temperature of the casting is 190-210 ℃. The air temperature of the drying chamber cannot be used as the basis for determining the workpiece temperature, and the air temperature can only be used as a reference. As the valve casting is thick and loose, and there is thermal inertia, there must be enough preheating time to ensure the temperature of the workpiece, and at the same time, the gas in the workpiece should be dried out to prevent the overflow of the powder after spraying and the formation of bubble defects. The specific time is set according to the specification and size of the workpiece.
(3) Spraying and spraying mainly include electrostatic spraying and fluidized bed process. Electrostatic spraying can be divided into powder electrostatic spraying production line and powder electrostatic spraying unit according to the size of the workpiece. The two processes are the same, and the main difference is the turnover mode of the workpiece. The spraying production line is automatically transmitted by the transmission chain, while the spraying unit is manually hoisted. The thickness of the coating shall be controlled within 250-3001a, m. If the thickness is less than 1501xm, the protective property will be reduced; if the thickness is greater than 500p, m, the coating adhesion will be reduced, the impact resistance will be reduced, and the powder consumption will be increased. The fluidized bed process is that the powder is suspended in the fluidized bed. When the powder meets the hot workpiece, it will gel on the surface and form a coating of no less than 300p, m. The film thickness of the workpiece constructed by the fluidized bed process is uniform, and it can also be well coated for the inner cavity and other areas, with good coating quality and high efficiency. The electrostatic spraying operation process is simple, and the one-time input is small. The fluidized-bed process has high coating quality and can be invested in large amount at one time. The spraying shall be completed in the shortest possible time, preferably in the powder gel time. For threads, planes, holes and other places that do not need spraying, silicone plugs, high temperature tapes and other tools can be used to protect them, and they can be removed after spraying.
(4) The curing temperature is 180 ℃, and the curing time is 10-15 min. The curing process is the process of finishing the coating, which has a great impact on the performance of the final coating, and its temperature and time should be reasonably controlled. Low temperature, short time, incomplete curing. High temperature or long time will cause discoloration of the coating. The time from the completion of spraying to entering the curing oven shall be as short as possible, preferably within 2min, otherwise the curing effect will be affected. The cured workpiece shall be cooled to room temperature, and natural cooling method is recommended.
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