Cold drawing, cold rolling, cold rolling, cold bending, cold expansion, and cold twisting are all common processing methods for manufacturing stainless steel pipes or heat-resistant steel pipes for heat exchangers, etc. using stainless steel seamless/welded steel pipes. The excellent plasticity of stainless steel, especially austenitic stainless steel, makes the above cold working (CW) generally easy to achieve; however, all these cold workings, like welding, will inevitably damage the performance of stainless steel pipes, especially corrosion resistance or heat resistance. Eliminating, reducing, or controlling such damage has always been the focus of attention in the manufacture and subsequent processing of stainless steel pipes. The final solution or annealing heat treatment before delivery is the most effective way to eliminate the harmful effects of the above cold working. However, this method, on the one hand, requires high-temperature heating and pickling treatment, which will significantly increase the manufacturing cost and production cycle. In addition, there are also problems such as the discharge, treatment, and evaluation of waste gas and wastewater such as acid mist. Therefore, a few manufacturers omit this process to reduce costs or meet deadlines. Some users purchase such products to save money, which is extremely unwise and unprofitable. On the other hand, it may be difficult to implement this process for certain products or application conditions. Therefore, controlling the degree of cold working (cold working deformation) and performing local low-temperature stress relief annealing have become two other practical ways to reduce or control its harmful effects, but their application conditions, including the difference in steel types, are still controversial.
1. Damage and elimination of cold working on the performance of stainless steel pipes
1.1 Damage to the performance of stainless steel pipes by cold working achieved by plastic deformation at room temperature will cause cold work hardening, that is, the hardness and strength of the material increase, and the original plasticity of the material is partially or completely lost, and it will inevitably damage the corrosion resistance or heat resistance of the material.
1.2 Methods to eliminate damage
The final solution heat treatment of austenitic and duplex stainless steel pipes before delivery, and the final annealing heat treatment of ferritic stainless steel pipes before delivery, are all aimed at effectively eliminating the performance damage caused by the above-mentioned cold working and welding and other hot working. This is why the stainless steel pipe standards of most countries, especially the European unified stainless steel pipe standards, stipulate that all stainless steel seamless pipes must be supplied in a solid solution or annealed state. Domestic users have reported that 316L austenitic stainless steel seamless pipes are found to have pitting corrosion as soon as they are soaked in seawater (316L is not an ideal material for seawater corrosion or soaking, but it is not normal to find pitting corrosion after one soaking). The final solution treatment or inadequate treatment is likely a “cheap product”. The final solution or annealing heat treatment is a very important and indispensable process in the manufacture of stainless steel seamless pipes.
2. Cold bending and stress relief heat treatment of stainless steel pipes
Cold bending is a common cold processing method for stainless steel pipes, which can be completed by steel pipe manufacturers, users, or professional pipe fitting manufacturers. Whether or not to perform stress relief heat treatment after cold bending and how to perform heat treatment are often a point of contention between manufacturers or users in orders. This issue has been stipulated in foreign pipeline standards, but there are indeed some points worth discussing.
2.1 Cold bending and stress relief heat treatment of stainless steel pipes
Cold bending is a common cold processing method for stainless steel pipes, which can be completed by steel pipe manufacturers, users, or professional pipe fitting manufacturers. Whether or not to perform stress relief heat treatment after cold bending and how to perform heat treatment are often a point of contention between manufacturers or users in orders. This issue has been stipulated in foreign pipeline standards, but there are indeed some points worth discussing.
2.2 For applications that withstand cyclic loads or stress corrosion environments
Regarding applications that withstand cyclic loads or stress corrosion environments, the regulations of European and American standards are slightly different. The American standard stipulates that for materials with impact test requirements, stress relief or solid solution treatment should be performed when the maximum calculated fiber elongation is 5% after bending or when there are other requirements. U-shaped tubes used in heat exchangers such as feedwater heaters and condensers in power plants working in high-temperature and high-pressure water/steam media are sensitive to stress corrosion cracking due to the chloride ions and oxygen content in the medium. Therefore, the two (the only in the world) American ASTMA688/A688M and A803/A803M standards for stainless steel seamless and welded steel pipes for feedwater heaters, and Japan’s JISG3463 standard for stainless steel pipes for boilers and heat exchangers stipulate that users can require that U-shaped tubes must be subjected to local stress relief heat treatment after bending. The French nuclear reactor manufacturing standard RCC-M3319 stipulates that U-shaped tubes must pass the MgCl2 stress corrosion test after bending to determine whether stress relief heat treatment is required after bending.
3. Twisted tubes and twisted heat exchangers
A heat exchanger composed of twisted tubes (twisted cold-worked tubes, TwistedTube) is used abroad. Its characteristics are that a single steel tube is twisted at 60° per pitch, and 7 steel tubes form a twisted heat exchanger unit. It is said that its advantages are compact structure, high thermal efficiency, and can reduce the dead zone of outer flow stagnation. It is an ideal structure for heat exchangers in narrow spaces. Calculation and analysis show that the plastic deformation formed by twisting cold working is only 4%~14%, and its working temperature is not more than 540℃. According to the provisions of the “ASME Boiler and Pressure Vessel Code”, stress relief annealing is not required. However, after stress corrosion testing according to ASTMG36 standard, it is proved that 316 and 321 austenitic stainless steel twisted tubes must be subjected to stress relief annealing or solution annealing to obtain good stress corrosion resistance, and the performance of 321 stainless steel pipes is much better than that of 316 stainless steel pipes.
4. Duplex stainless steel twisted tubes and U-tubes
The test results abroad have shown that it is not appropriate to perform stress relief annealing on duplex stainless steel twisted tubes or U-tubes. The current test results require R≥5.33d0 for 2205 duplex stainless steel pipes, but R≥1.5d0 for 2507 super duplex stainless steel pipes. The reasons are: ① Duplex stainless steel has excellent resistance to pitting and stress corrosion, and the higher the pitting equivalent PRE value, the better the material’s stress corrosion resistance. ② Local low-temperature stress relief annealing will affect the phase equilibrium and intermetallic compounds of the matrix, that is, the precipitation of brittle phases will cause greater damage to corrosion resistance. These research results show that duplex stainless steel pipes may be a more suitable material for heat exchangers, and it is also the basis for the American Welding Society and ASMEB31.3 standards to be very cautious about their heat treatment regulations.
5. Annealing method for stress relief of U-shaped tubes
Both domestic and foreign methods use resistance heating or local heating in the furnace to perform local stress relief annealing on U-shaped tubes, but which method is more effective or reasonable is often the focus of controversy. The latest research results in the United States show that resistance heating is a more reasonable and effective method. The reasons are as follows: ① The power frequency alternating current can be directly introduced from the U-shaped tube cutting point 250mm away through the clamping electrode, and the bent tube section can be heated to 1010~1065℃ in a short time (about 10s), with very low energy consumption; ② The optical pyrometer can be used to automatically control the temperature of the heating zone; ③ The inner wall is filled with Ar gas to effectively prevent oxidation; ④ After heating, forced air cooling can be used to quickly cool to below 425℃ within 2~3min, showing a yellow or light blue, thin and dense oxide film, which can meet high standard use requirements without pickling.
6. Conclusion
(1) Cold processing such as cold drawing and cold rolling will cause cold hardening of stainless steel, especially austenitic stainless steel, and induce lattice dislocation, martensitic phase transformation, carbide precipitation, increased magnetism, and residual stress, thereby reducing its corrosion resistance. Annealing or solution heat treatment after cold working can effectively eliminate these adverse effects; therefore, austenitic stainless steel seamless pipes and deep cold processed stainless steel welded pipes must be supplied in a solution or annealed state to effectively ensure their corrosion resistance.
(2) In addition to stress corrosion cracking and environmental conditions where there is a risk of corrosion fatigue due to alternating stress, controlling the degree of cold working is another way to avoid its adverse effects. This is especially important for local cold work such as cold bending and cold expansion, which are difficult to anneal. As long as the cold bending radius of austenitic stainless steel pipes is controlled to be no less than 1.5d0, and the cold bending radius of ferritic and duplex stainless steel pipes is greater than 2.5d0, it is generally not necessary to perform stress relief annealing after cold bending.
(3) For bends that require resistance to stress corrosion cracking, such as U-shaped austenitic stainless steel bends for heat exchangers working under high temperature and high-pressure water or steam conditions, effective stress relief annealing must be performed after cold bending regardless of the size of the cold bending radius.
(4) 06Cr19Ni11Ti (321) stainless steel has better resistance to stress corrosion cracking than 316L and is a more suitable austenitic steel pipe material for U-tube heat exchangers. Duplex stainless steel U-tubes are not suitable or should not be subjected to local stress relief annealing after cold bending or twisting.
(5) Cold twisted stainless steel pipes can form a new type of heat exchanger with a compact structure and higher heat exchange efficiency, which deserves the attention exploration and development of relevant design and application departments.
(6) Duplex stainless steel U-tubes are not suitable or should not be subjected to local stress relief annealing after cold bending or twisting.
(7) Resistance heating is a local stress relief heat treatment method that is more energy-saving and time-saving than indirect heating in the furnace is easy to realize automatic control, and should be promoted as a priority.
(8) European and American pipeline standards (ASMEB31.1-2012, ASMEB31.3-2012, BSEN13480-4:2012) have some updated and detailed regulations on the heat treatment of stainless steel pipes after cold/hot forming, which deserve high attention.
Post time: Nov-06-2024