Hey, take a look – the various aspects of stainless steel expansion jointing

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You understand all of these: • Austenitic stainless steel performs fairly well in halogen environments when it is not subjected to cold deformation, and this is something that not many people know. •Ordinary austenitic stainless steels also contain about 5% ferrite, a fact that few people are aware of. •It is well known that excessive cold deformation of ordinary austenitic stainless steels leads to martensitic transformation, but people forget this in practice. •It is little known that excessive cold deformation of ordinary austenitic stainless steel can impart magnetism. •No one pays any attention to the fact that grain sliding occurs during expansion bonding, which can even lead to the rupture of slip zones (in terms of microstructure)… • Some people who work on standards are not those engaged in technical research; the issue raised here concerns the adverse effects that expansion bonding of materials has on their microstructural properties. Qiqiu: •It is incorrect and imprudent to require, as per GB151, that all materials be expanded joined. The corporate standards of German company BASF state that no form of expansion joining should be used in any environment where stress corrosion may occur! ! Many stainless steels, when used in the same environment, become defective after a short period of time due to expansion bonding, whereas those that are not expanded bonded can last for a very long time. Cold working stress is very harmful; welding stress is less significant, provided that the welding quality is excellent. •The North Steel Research Institute once conducted experiments using stainless steel pipes joined by mechanical expansion; the stress generated at the expansion joints exceeded 100 MPa, which is a prerequisite for stress corrosion. •In duplex stainless steel, martensitic transformation occurs when there is significant cold working deformation, and this is the key factor behind chloride corrosion, as demonstrated by numerous cases. •There are no standards specifying the solution treatment for stainless steel heads and U-tubes, the design requirements are vague, and carrying out such treatment is complicated; as a result, many companies omit it, which in fact creates potential corrosion risks. Eight-tongued: •1. Regarding stainless steel heat exchange tubes, the view that they cannot all be joined by expansion fitting can be viewed with skepticism. The key expansion degree must be appropriate, with as little residual stress as possible. 2. After expansion joining at the tube ends, stress-relief heat treatment is generally not applied to the tube ends of the heat exchange tubes, mainly because it is difficult to handle the sensitization temperature range. 3. The expansion of the heat exchange tubes must be measured; let the data speak for itself. Usually, the factory documentation for heat exchangers does not include such expansion measurement data. . . 4. It is recommended to use the pre-expansion–welding–expansion method for heat exchange tube ends (depending on the technical requirements specified in the drawings, either light expansion or forceful expansion should be used). 5. Expander devices have precision requirements; the longer their service life and the more times they can be used, the greater the variation in their precision, until they become unusable and must be discarded. Are there any mandatory requirements in this regard, or is it simply a matter of saving money? . . 6. It is recommended to use flexible expansion methods such as fluid bag expansion or rubber expansion. 7. Pay attention to the matching of the tube sheet hardness with that of the heat exchange tubes; carry out the necessary heat treatment and annealing – don’t cut corners. . . 8. The drilling accuracy of the tube sheet must be strictly controlled, as defects such as burrs and roughness are likely to occur. . . The domestic industry remains extensive; it’s unable to engage in meticulous work, and money is spent without proper planning. . . The deadline is tight. . . Seven Mouths: • In an environment with chloride ions, 1. Austenitic stainless steel heat exchange tubes are not suitable for use with strength expansion. The expansion rate (deformation rate) for strength expansion is 12%-18%; it is possible to exceed the 15% limit specified in GB150 for cold forming, in which case solution treatment is required ； 2, 304, and 304L are suitable only for micro expansion or expansion joining, with an expansion rate (deformation ratio) of 3%-8% ； Studies have shown that in austenitic stainless steels with a Ni content of over 9% (such as 316L, which typically has a Ni content of 10%), martensite formation is minimal during cold working, and cracks are rare. In contrast, in steels with a Ni content of less than 9% (such as 304L, which usually has a Ni content of 8%), martensite formation occurs more easily during cold working, leading to the appearance of cracks. Bashé: • The hazard of stress corrosion in stainless steel actually lies in cold-work stress. There will be a crimping step at the crimping area, and the additional stress resulting from crimping is greater than 100 MPa. This leads to the following problems: 1. Martensitic transformation caused by cold working stress ； 2. At the expansion joint step, the fibers are elongated, and grain sliding occurs, leading to the fracture of the ferrite fibers. As long as it is ordinary austenitic stainless steel (with nickel content greater than 10% and nitrogen content greater than 0.1%, or a nickel content of 12% or more, which constitutes a stable austenite state), there will be a small amount of ferrite remaining. As long as there is residual ferrite, it is metastable austenite. These are all the reasons for the fibrous fracture of martensite and ferrite.