Why do stainless steel pipe fittings need to be solution treated?

What is the solution treatment of stainless steel pipe fitting?
The austenitic stainless steel pipe fitting is softened by heat treatment, and the stainless steel pipe fitting is heated to about 950 to 1150 ° C for a certain period of time, so that the carbide and various alloying elements are uniformly dissolved in the austenite, which is called solid solution of the stainless steel pipe fitting.
First, the role of heat treatment of stainless steel pipe fittings.
Stainless steel pipe fittings manufacturers must take into account the production cost of pipe fittings, but must also consider the factors that prevent corrosion and improve product quality. Therefore, it is necessary to adopt a solid solution process in the production process of stainless steel pipe fittings to enhance the safety of pipe fittings. Quality and safety play a vital role. It is understood that a small number of enterprises in the production of pipe fittings to save costs, did not take solution treatment, only polished blasting and other methods to cover the defects and deficiencies of the pipe fittings, which will bring permanent hidden dangers to the use of pipe fittings.
Everyone knows that after the stainless steel pipe fitting is processed by molding and welding, the molecular structure, magnetic properties and physical properties of the metal have changed. The atmosphere-protected solution treatment can restore the corrosion resistance that is affected after processing, and at the same time obtain the hardness required for stainless steel to ensure the best performance of stainless steel. The stainless steel pipe fitting after solution treatment has a good improvement effect:
1. Eliminate the modification phenomenon of stainless steel pipe fittings during processing, reduce the hardness of stainless steel to below 220HV, improve the plasticity and toughness of stainless steel, and make the pipe fittings more convenient and safe in installation.
2. Restoring stress and intergranular changes in the production process of stainless steel pipe fittings, reducing intergranular corrosion and stress corrosion of stainless steel, and enhancing corrosion resistance.
3. Remove stainless steel due to the magnetic properties generated during processing, and stabilize the austenite structure.
4. Restore the natural light on the surface of the stainless steel material (natural light is different from polished light).
Second, the characteristics of solid solution of stainless steel pipe fittings.
Stainless steel is a special type of steel. Due to the presence of alloying elements such as nickel and chromium, the heat treatment has different characteristics from ordinary steel heat treatment:
1. The thermal conductivity of stainless steel is low. At room temperature, the thermal conductivity is only 27% of carbon steel. With the increase of heating temperature, the thermal conductivity of stainless steel is gradually reduced. Therefore, when the stainless steel is heated at a low temperature, the temperature rise process should be carried out slowly.
2. After austenitic stainless steel is heated to about 1100 °C by solid solution, it can inhibit the generation of carbides, and then rapidly cool to room temperature to make the carbon supersaturated, which can greatly improve the corrosion resistance of stainless steel.
3. The pipe fittings are protected by (hydrogen) gas during the solid solution process to avoid the formation of viscous iron oxide on the stainless steel surface, to restore the brightness of the stainless steel surface, and to improve the reputation of the appearance.
Third, the three elements of stainless steel pipe fitting solid solution.
The advantages and disadvantages of the solid solution process of stainless steel pipe fittings have a great influence on the corrosion resistance and appearance brightness of stainless steel, and play a decisive role in the processing performance of stainless steel. Therefore, the heat treatment process of stainless steel is in a very important position in the production of stainless steel pipe fittings.
1. Solution temperature. According to the characteristics of the chemical composition of stainless steel, the solid solution temperature of stainless steel should be 950-1150 degrees Celsius to achieve the softening effect, and the hardness of stainless steel can be reduced to within 220 HV, which meets the requirements of pipe installation and compression quality. If the temperature control is unreasonable, various quality defects are likely to occur.
2. Solid solution time. During the heating process of austenitic stainless steel, the residual ferrite content in the steel decreases with the heating time. Therefore, the solid solution treatment of stainless steel pipe fittings should be controlled at around 1050 °C to achieve carbon saturation and improve corrosion resistance. It is then rapidly cooled in air to achieve a solid solution effect.
3, solid solution speed and insulation. Stainless steel has a low thermal conductivity, and its thermal conductivity is only 27% of that of carbon steel at normal temperature. Therefore, the temperature rise of stainless steel should be slow when heated at low temperatures. If the heating rate is too fast, it is easy to deform. In the solid solution, the heating rate of the stainless steel should be controlled, and the holding time should be paid attention to. For example, 316L steel is around 1100 °C, the holding time is long, and the residual ferrite content is continuously reduced. In solid solution production, the solid solution of stainless steel pipe fitting is subjected to heating-heating-cooling process, which takes about 40 minutes.
Fourth, the pipe solid solution process operation and use.
In the production process, Yaang Pipe Industry Co., Ltd. attaches great importance to solution treatment to enhance the corrosion resistance of stainless steel pipe fittings and improve the safety factor. It attaches importance to the improvement of production process and equipment to ensure the production quality of products.
All the pipe parts produced by the company are processed by the solid solution process. The pipe fittings are annealed by a solid solution furnace; the pipes are solid-dissolved by a tube-based online solid solution device. In order to improve product quality and meet the requirements of Chinese standards.
1, solid solution can increase the ability to resist corrosion. It is recommended to adopt a solid solution process in the production of pipe fittings.
2. Stress and intergranular changes will occur during the production process of the pipe fittings. The solid solution process can eliminate these changes, so it should be solid solution after the pipe fittings are formed, in order to truly protect.
2. Restore the hardness of stainless steel to within 220 HV to ensure the quality of pipeline installation.
3. When solid solution, it is necessary to master the temperature and time to prevent quality defects due to unreasonable temperature control and time control.
4, using solid solution process, each ton will increase the cost between 2600-2800 yuan.
The austenitic stainless steel pipe fittings are not subjected to the solid-melting heat treatment after the factory is not carried out or strictly in accordance with the solid-melting process, and the appearance cannot be checked at the time of site acceptance. The article briefly introduces the principle of solid solution, and expounds the method of rapid hardness testing of austenitic non-induced steel pipe parts by using the Leeb hardness tester. The criterion for judging whether the stainless steel pipe fittings have been qualified for solid solution heat treatment is given. The engineering application of the method shows that the effect is good.
According to China’s standard specifications, stainless steel pipe fittings must be subjected to solid-melting heat treatment before leaving the factory to refine the grain structure and increase the corrosion resistance and low temperature resistance of the pipe fittings. However, some manufacturers are pursuing profits and reducing costs before the stainless steel pipe fittings are shipped. It is directly shipped without solid-solution or solid-melt treatment according to the process requirements. After the unqualified stainless steel pipe fittings are applied in petrochemical projects, they will usually crack at low temperature, causing oil production accidents such as oil leakage and oil running, resulting in the shutdown of oil and gas processing equipment, treatment plants or gathering stations, resulting in serious safety. Hidden dangers and huge economic losses. We can produce pipe fittings according to the standards of different countries.

How to check if the stainless steel pipe fittings are solidified?
Since the normally produced pipe fittings need to be pickled and passivated after the solution treatment, the stainless steel pipe fittings subjected to the solid solution treatment and the unsolidified treatment are substantially identical in appearance. The traditional stainless steel pipe inspection method is metallographic inspection, but at the construction site, the metallographic inspection has high cost and long time, which is not conducive to full implementation on the site. We have studied the metallographic organization and manufacturing process of stainless steel pipe fittings, and finally chose The method of hardness detection is used to identify the quality, and the detection cost is effectively reduced.
1, parameter determination
According to the national standards for Chinese pipe fittings: SH/T 3408-2012 and GB/T 12459-2005, the hardness requirements for the austenitic non-induced steel pipe parts are: Brinell hardness HB ≤ 190 is acceptable, but due to Brinell hardness The instrument is large in size, complicated in operation, and difficult to apply in the field. Under normal circumstances, the hardness test on the site adopts the Leeb hardness tester (corresponding to the Leeb hardness HL). We applied the HLN-200 Leeb hardness tester on site. Refer to Appendix B of GB/T17394-1998, Brinell hardness 190 is equivalent to the Leeb hardness H LC523 when using C-type impact, when using vertical downward inspection, HL = HLC, so the test hardness value HL ≤ 523 is qualified.
1.1 Material inspection
In the actual application process, due to the variety of austenitic stainless steels, the hardness values are different, and the actual detection value of stainless steel pipe fittings is easy to meet the requirements. Taking 0Cr18Ni9 as an example, the pipe fittings with unqualified solution heat treatment were spot-checked. The results are shown in Table 1.
Table 1 Richardson hardness test of unqualified pipe fittings
20180728112852 67988 - Why do stainless steel pipe fittings need to be solution treated?

Data from liningge (SINOPEC Zhongyuan Oilfield Company)
Note: According to the discrete requirements of the standard measurement data, the difference between the measured value and the average value cannot be greater than 15. Since the tube is not solution treated, the metallographic structure inside the material is not uniform, so the discrete value is too large. The underlined data in the table is the unqualified data required by the standard.
If the hardness test value is accepted according to the standard, the hardness of the above stainless steel pipe fittings is acceptable. In fact, during the field application, the stainless steel pipe fittings are generally cracked at a low temperature, and the chemical composition is qualified by chemical composition analysis. Therefore, only the hardness required by the standard is tested, and the acceptance cannot fully meet the requirements for use. Subsequently, a batch of qualified stainless steel pipe fittings were selected for hardness testing. The results are shown in Table 2:
Table 2 Leeb hardness test of qualified pipe fittings
20180728112936 37584 - Why do stainless steel pipe fittings need to be solution treated?
Comparing Table 2 with Table 1 shows that the measurement dispersion of qualified pipe fitting is much smaller than that of unqualified pipe fittings, indicating that the uniformity is better. At the same time, in order to better compare, we have carried out spot checks on some stainless steel pipe fittings that have been qualified and used by different manufacturers. The specific values are shown in Table 3.
Table 3: Hardness test of stainless steel pipe fittings
20180728113013 29493 - Why do stainless steel pipe fittings need to be solution treated?
As can be seen from Table 3, the discreteness of individual data is too large. Analysis of the reason, because the length of the pipe is large, each pipe is considered according to the uniform distribution of the structure and alloy elements during the spot check, so the individual data dispersion is too large, but the hardness value is still close to the solid solution treated pipe, far less than Unsolidified fittings.
1.2 Test data modeling
After the material inspection is completed, the statistical data is compiled, and the centralized or discrete state of the data distribution is judged, and the model is modeled according to the normal distribution.
1.2.1 Analysis of unqualified pipe fittings data
(1) The average value of the Leeb hardness test was calculated to be 411.9.
(2) After calculation, the variance of the unqualified pipe inspection data is 18.65.
1.2.2 Analysis of qualified pipe fittings data
(1) The average value of the Leeb hardness test was calculated to be 359.9.
(2) After calculation, the variance of the qualified pipe inspection data is 6.57.
1.3 Test Analysis and Conclusion
According to the principle of normal distribution, in the case of the same sample capacity, the larger the variance, the greater the fluctuation of the data and the more unstable. Through the above calculations combined with the mapping analysis, it is concluded that the data variance of the qualified pipe fittings is small, and the case of the unqualified pipe fittings is opposite.
In the case of 95% confidence, the hardness of the unacceptable pipe is in the range of 370-450, and the hardness of the qualified pipe is in the range of 340-380. The two sides overlap slightly, but considering that the normal distribution curve can only approximate the actual situation on the spot, the extreme situation will not occur, and considering the quality to be ensured, the critical value of the Leeb hardness of the pass and fail is finally determined. 370, that is, the average value of the Leeb hardness average of more than 370 is unqualified pipe fittings, and further testing can be carried out if there is any objection.
2, the application effect
After determining the acceptable range of the hardness of the Richter scale, the stainless steel pipe fittings used in the engineering are checked and accepted according to the Leeb hardness, and the solidification test is carried out by using the Leeb hardness test method. After the project was put into production, there was no further serious accident caused by the cracking of the stainless steel pipe fittings at low temperature.
3, the conclusion
With the development of the market economy, counterfeit and shoddy products are gradually increasing. The means of material testing and inspection must keep pace with the times, and constantly explore innovation and innovation in production practice. On the one hand, strengthen the inspection and acceptance process, and spare no effort to prevent unqualified materials from flowing into the construction site; on the other hand, establish a self-restraint mechanism, establish a blacklist of manufacturers of unqualified materials, and strictly prohibit re-feeding of materials, so that unqualified materials are not sold in the market, fundamentally Guarantee the quality of the project.

Source: China Pipe Fittings 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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

Please notice that you might be interested in the other technical articles we’ve published:

  • Dimensions of Pipe Fittings

  • Specifications of Steel Pipe Fittings

  • Development trend of high pressure pipe fittings

  • How to test pipe fitting

  • How to Calculate a Pipe Bend

  • Stainless steel pipe bend

  • How to get high quality pipe fittings

References:

  1. https://www.yaang.com

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