Effect of solid solution treatment on the quality of the stainless steel pipe fittings
The production cost of stainless steel pipe fittings should be considered, but the factors of preventing corrosion and improving product quality should also be taken into account. Therefore, it is necessary to adopt solid solution process in the production process of stainless steel pipe fittings to enhance the safety of pipe fittings. Heat treatment process plays a vital role in the quality and safety of pipe fittings. It is known that in order to save cost in the production of pipe fittings, a few enterprises did not take solid solution treatment, only used polishing and sandblasting methods to deal with the defects and shortcomings of pipe fittings, which will bring long-term hidden dangers to the use of pipe fittings.
|The Role Of Heat Treatment Of Stainless Steel Pipe Fittings||Solid Solution Characteristics Of Stainless Steel Tube Fittings|
|Three Elements Of Solid Solution Of Stainless Steel Pipe Fittings||Solid Solution Technology Operation And Use Of Pipe Fittings|
For this reason, this paper shares the importance, methods and rules of the solid solution process for stainless steel pipe fittings, so that more people can understand the knowledge and necessity of the solid solution process for stainless steel pipe fittings, and ensure the quality and safety of stainless steel pipe fittings in use.
Austenitic stainless steel is softened by heat treatment. The stainless steel is heated to 950-1150 ℃ for a certain period of time, so that carbides and various alloy elements are evenly dissolved in austenite, which is called solid solution treatment.
I. The Role of Heat Treatment of Stainless Steel Pipe Fittings
As we all know, the molecule structure, magnetic properties and physical properties of stainless steel pipes have changed after forming and welding. The corrosion resistance of the stainless steel can be restored by the solution process with atmosphere protection. At the same time, the required hardness of the stainless steel can be obtained to ensure the better service performance of the stainless steel. The stainless steel tube fittings after solution treatment have good improvement effect.
- Eliminate the modification of stainless steel pipes in the process of processing, reduce the hardness of stainless steel to below 220HV, improve the plasticity and toughness of stainless steel, and make the pipes more convenient and safe in installation.
- Restore the stress and intergranular change in the production process of stainless steel pipes, reduce the intergranular corrosion and stress corrosion of stainless steel, and enhance the anti-corrosion ability.
- Remove the magnetism of stainless steel due to processing and stabilize the austenite structure.
- Natural brightness of reduced stainless steel (different from polished brightness).
Stainless steel is a special kind of steel. Because of the existence of alloying elements such as nickel and chromium, its heat treatment has different characteristics from that of ordinary steel.
- The thermal conductivity of stainless steel is low, which is only 27% of that of carbon steel at room temperature. With the increase of heating temperature, the thermal conductivity of stainless steel decreases gradually. Therefore, when stainless steel is heated at low temperature, the heating process should be carried out slowly.
- Austenitic stainless steel can inhibit carbide formation by solution heating to about 1100 ℃, then rapidly cool to room temperature, so that carbon reaches supersaturation state, which can greatly improve the corrosion resistance of stainless steel.
- Gas protection (hydrogen) is used in the solid solution process to avoid the formation of viscous iron oxide on the surface of stainless steel, reduce the brightness of the surface of stainless steel, and improve the appearance of the reputation.
The solid solution process of stainless steel pipe fittings has a great influence on the corrosion resistance and appearance brightness of stainless steel, and plays a decisive role in the processing performance of stainless steel. Therefore, the heat treatment process of stainless steel is very important in the production of stainless steel pipe fittings.
- Solution temperature. According to the characteristics of chemical composition of stainless steel, the solution temperature of stainless steel should be 950 – 1150 ℃ to achieve softening effect, so that the hardness of stainless steel can be reduced to less than 220HV, to meet the quality requirements of pipe installation and clamping. If the temperature control is unreasonable, it is easy to produce various quality defects.
- Solution time. During the heating process of austenitic stainless steel, the residual ferrite content in the steel decreases with the prolongation of heating time. Therefore, the solution treatment of stainless steel pipes should be controlled at about 1050 C to saturate carbon and improve corrosion resistance. Then it is cooled rapidly in the air to achieve solid solution effect.
- Solution speed and heat preservation. The thermal conductivity of stainless steel is low, which is only 27% of that of carbon steel at room temperature. Therefore, the heating process of stainless steel at low temperature should be carried out slowly. If the heating speed is too fast, it is easy to produce deformation. In solid solution, the heating speed of stainless steel should be controlled, and the holding time should be paid attention to. If 316L steel is kept at about 1100 C for a long time, the content of residual ferrite will decrease continuously.
In solid solution production, the solid solution of stainless steel pipe fittings is heated, insulated and cooled, and the whole process takes about 40 minutes.
In the process of production, Yaang Pipe Industry Co., Limited attaches great importance to solid solution treatment in order to enhance the anti-corrosion ability of stainless steel pipes and improve the safety factor, and attaches importance to the improvement of production process and equipment to ensure the quality of products.
- All stainless steel pipe fittings produced by the company are treated by solid solution process. Pipe fittings are annealed in a solution furnace and pipe fittings are dissolved in an on-line solution equipment. In order to improve product quality and meet the requirements of national standards.
- Solution cost. The use of solid solution process will bring about a considerable increase in the cost of pipe fittings. The main costs are: electricity costs, gas costs, labor costs.
- Selection of protective gas (hydrogen). Hydrogen is used in the production of solid solution gas of pipe fittings, and hydrogen (mixed gas) after liquid ammonia pyrolysis is used as protective gas. There is no difference between the two gases as solid solution protective gas.
The principle is that 2.64 Nm3 (Nm3 = standard cubic meter) mixed gas can be produced by pyrolysis of liquid ammonia per kilogram. One standard cubic meter contains 75% hydrogen and 25% nitrogen. Secondly, the impurity content of the mixed gas obtained by pyrolysis is less (about 2g/cubic meter of water vapor, 1000PPM of residual ammonia); thirdly, the residual ammonia can be reduced to 3 ppm through molecular sieve (UOP) adsorption purifier. (3 ppm = 3 ppm) below, nitrogen content is very small, nitrogen in general conditions is inert gas, solid solution will not be affected.
- Solution can increase corrosion resistance.
- Reduce the hardness of stainless steel to less than 220HV, and improve the quality requirement of pipeline installation.
- In solid solution, we should master the temperature and time to prevent quality defects caused by unreasonable control of the temperature and time of solid solution.
- Using solid solution process, the cost per ton will increase by about 2600-3000 yuan.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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