Effect of solid solution on stainless steel pipe fitting and stainless steel flange
Stainless steel pipe fittings and flange manufacturers should consider the production cost of pipe fittings and flange, but also consider the factors of corrosion prevention and product quality improvement. Therefore, it is necessary to adopt solution process in the production process of stainless steel pipe fittings and flange to enhance the safety of pipe fittings and flange. Heat treatment process plays a crucial role in the quality and safety of pipe fittings and flange. It is understood that in order to save costs in the production of pipe fittings and flanges, a few enterprises did not take solution treatment, only used polishing and sand blasting, which covered up the defects and deficiencies of pipe fittings and flanges, which will bring permanent hidden dangers to the use of pipe fittings and flanges.
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 uniformly dissolved in austenite, which is called solution treatment.
The purpose of solution treatment is to dissolve the carbides and γ ‘in the matrix so as to obtain uniform supersaturated solid solution, which is convenient for the precipitation of fine particles, evenly distributed carbides and γ’ and other strengthening phases during aging. At the same time, it can eliminate the stress caused by cold and hot processing and make the alloy recrystallize. Secondly, the solution treatment is to obtain the appropriate grain size to ensure the high temperature creep resistance of the alloy. The temperature range of solution treatment is about 980 ~ 1250 ℃, which is mainly selected according to the precipitation and dissolution rules of each alloy and the use requirements, so as to ensure the necessary precipitation conditions and a certain grain size of the main strengthening phase. For the alloy used for a long time at high temperature, it is required to have better high temperature durability and creep performance, and higher solution temperature should be selected to obtain larger grain size; for the alloy used at medium temperature and requiring better room temperature hardness, yield strength, tensile strength, impact toughness and fatigue strength, lower solution temperature can be used to ensure smaller grain size. During the solution treatment at high temperature, all kinds of precipitates are gradually dissolved, and the grains grow up; during the solution treatment at low temperature, there are not only the dissolution of the main strengthening phase, but also the precipitation of some phases. For the alloy with low supersaturation, the faster cooling speed is usually selected; for the alloy with high supersaturation, the cooling is usually in air.
The solubility and temperature of carbon in austenitic stainless steel have great influence. In the temperature range of 400 ℃ ~ 850 ℃, high chromium carbide will precipitate in austenitic stainless steel. When the chromium content falls below the limit of corrosion resistance, there is poor chromium on the grain boundary, intergranular corrosion will occur, and in serious cases, it will become powder. Therefore, austenitic stainless steel with intergranular corrosion tendency should be treated by solution heat treatment or stabilization.
Solution heat treatment: the austenitic stainless steel is heated to about 1100 ℃, so that the carbide phase is completely or basically dissolved, and the carbon is solid dissolved in the austenite, and then rapidly cooled to room temperature, so that the carbon reaches the supersaturated state. This heat treatment method is solution heat treatment.
The rapid cooling in solution heat treatment seems like the quenching of ordinary steel, but the “quenching” at this time is different from that of ordinary steel, the former is softening treatment, the latter is hardening. In order to obtain different hardness, the heating temperature of the latter is different, but less than 1100 ℃.
The function of heat treatment of stainless steel pipe fitting and flange
After forming and welding of stainless steel pipe fitting and flange, the molecular structure, magnetic properties and physical properties of metal have changed. The corrosion resistance of stainless steel can be restored by the solution process under atmosphere protection, and the hardness required by stainless steel can be obtained to ensure the best performance of stainless steel. After solution treatment, the stainless steel pipe fittings and flanges have good improvement effect:
- 1. It is mainly to improve the plasticity and toughness of steel and alloy and prepare for precipitation hardening treatment.
- 2. The results show that the structure and composition of stainless steel are uniform: at high temperature, the atomic activity is intensified, σ phase dissolves, and the chemical composition tends to be uniform.
- 3. It can fully dissolve all kinds of phases in the alloy, strengthen solid solution, improve toughness and corrosion resistance, eliminate stress and soften, so as to continue processing or forming.
- 4. Eliminate work hardening to facilitate continuous cold work. After solution treatment, the distorted lattice is restored, the elongated and broken grains are recrystallized, and the internal stress is eliminated.
- 5. Restore the corrosion resistance of stainless steel. Due to the carbide precipitation and lattice defect caused by cold working, the corrosion resistance of stainless steel decreased. After solution treatment, the corrosion resistance of stainless steel recovered to the best state.
Features of solid solution of stainless steel pipe fittings and flanges
Stainless steel is a special kind of steel. Due to the existence of alloying elements such as nickel and chromium, its heat treatment is different from that of ordinary steel
- 1. The thermal conductivity of stainless steel is relatively 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.
- 2. After the austenitic stainless steel is heated to 1100 ℃, the carbide can be restrained, and then it can be rapidly cooled to room temperature to make the carbon supersaturated, which can greatly improve the corrosion resistance of stainless steel.
- 3. The pipe fittings and flanges are protected by (hydrogen) gas in the process of solid solution to avoid the formation of viscous iron oxide on the stainless steel surface, reduce the brightness of the stainless steel surface and improve the reputation of the appearance.
Three elements of solid solution for stainless steel pipe fittings and stainless steel flanges
The solid solution process of stainless steel pipe fitting and stainless steel flange 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 and flanges.
According to the characteristics of chemical composition of stainless steel, the solution temperature of stainless steel should be 950-1150 ℃, so as to achieve the softening effect, reduce the hardness of stainless steel to within 220hv, and meet the quality requirements of pipe installation and clamping. If the temperature control is not reasonable, it is easy to produce various quality defects. The solution temperature is mainly determined by chemical composition. Generally speaking, the solid solution temperature of alloy with many kinds and high content should be increased accordingly. Especially for the steel with high content of manganese, molybdenum, nickel and silicon, the softening effect can be achieved only by raising the solution temperature and making it fully dissolved. However, for the stabilized steel, such as 1Cr18Ni9Ti, the carbide of the stabilized element is fully dissolved in austenite when the solution temperature is high, and precipitates at the grain boundary in the form of Cr23C6 in the subsequent cooling, resulting in intergranular corrosion. In order to keep the carbide (TIC and NBC) of stable elements from decomposition and solid solution, the lower limit of solid solution temperature is generally adopted.
During the heating process of austenitic stainless steel, the content of residual ferrite decreases with the increase of heating time. Therefore, the solution treatment of stainless steel pipe fittings and flanges should be controlled at about 1050 ℃, so that the carbon can reach saturation state and improve the corrosion resistance. Then it is cooled rapidly in the air to achieve the solid solution effect.
Solution speed and heat preservation
The thermal conductivity of stainless steel is relatively low, which is only 27% of that of carbon steel at room temperature. Therefore, the temperature rise process of stainless steel should be carried out slowly at low temperature. If the heating speed is too fast, it is easy to produce deformation, the temperature rise speed of stainless steel should be controlled in the solid solution, and attention should be paid to the holding time. For example, when 316L steel is kept at 1100 ℃ for a long time, the content of residual ferrite will continue to decrease.
In solution production, the solid solution of stainless steel pipe fittings and flanges should go through the process of heating, heat preservation and time cooling. The whole process takes about 40 minutes.
Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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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