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Material selection for expansion joint of bellows compensator

The bellows compensator expansion joint is widely used in many industries. In addition to considering good compensation capacity, reliability is the key to the bellows compensator expansion joint. However, reliability is ensured through multiple links such as design and manufacturing. Any negligence of any link will lead to the reduction or even failure of the life of the compensator.
Most bellows compensator expansion joints manufacturers have analyzed the failure reasons of the bellows compensator expansion joints. It is found that the failures during operation are mainly manifested by corrosion leakage and instability deformation, among which corrosion failure is mostly. Anatomical analysis of corrosion-infected bellows found that corrosion failure is usually characterized by point corrosion perforation and stress corrosion cracking, in which chloride stress corrosion cracking accounts for about 95% of the total corrosion failure. Therefore, the correct selection of corrugated pipe fabrication materials and structures, reasonable design of waveform parameters and fatigue life, and assurance of installation quality can greatly improve the safety and reliability of the bellows compensator expansion joint.
In design, the stability of the bellows should be considered to prevent the instability of the bellows. The data show that the compensation amount of the bellows depends on its fatigue life, and the higher the fatigue life, the smaller the single wave compensation amount of the bellows. In order to reduce the cost and increase the single-wave compensation amount, the lower the allowable fatigue life, the greater the bending stress of the corrugated pipe caused by the displacement, and the higher the comprehensive stress, the stability of the bellows is greatly reduced.
When the permissible life of the bellows design is low, not only the meridional comprehensive stress is high, but also the hoop stress is relatively high, so that the bellows partially enters the plastic deformation quickly, resulting in failure of the bellows instability. In addition to design, the choice of material for the bellows is also critical. For the selection of corrugated pipes, in addition to the working medium, working temperature and external environment, the possibility of stress corrosion, the effect of water treatment agent and pipe cleaning agent on the material should be considered, and the corrugation should be combined on this basis. The welding material of the pipe material, the molding and the cost performance of the material are preferably made of a corrugated pipe material which satisfies working conditions and is practical.

Experts suggest that the material selected for the bellows should meet the following conditions:

  • (1) Good plasticity, facilitating the forming of the bellows, and obtaining sufficient hardness and strength by a subsequent treatment process (cold work hardening, heat treatment, etc.).
  • (2) High elastic limit, tensile strength and fatigue strength to ensure the normal operation of the bellows.
  • (3) Good welding performance, meeting the welding process requirements of the bellows in the manufacturing process.
  • (4) Good corrosion resistance, meeting the requirements of bellows working in different environments.

At present, most manufacturers use austenitic stainless steel (such as 304, 304L, 316, 316), 254SMO, 904L, AL6XN and other super austenitic stainless steels have become important materials. For the heat pipe network laid in the trench, when the pipeline at the expansion section of the bellows compensator is low, the rainwater or accidental sewage will be immersed, and materials with stronger corrosion resistance, such as high-nickel alloy and iron-nickel alloy, should be considered. Etc., such as INCOLOY800, INCOLOY825, INCONEL625 and Hastelloy C-276.

The special materials for the bellows compensator expansion joints and expansion joints are as follows:

  • 1. Super austenitic special stainless steel: 254SMO, 904L, AL6XN
  • 2, duplex stainless steel: 2205 duplex steel (S31803)
  • 3. Nickel-based alloy steel: INCOLOY800/800H/800HT/840/825, INCONEL600/601/690/625/X-750 Hastelloy C-276/C-22/X, MONEL400/K500 4. Pure nickel: N4 N6 (Ni200, Ni201) 5. Titanium plate for press: GR1, GR2, TA1, TA2 6, austenitic stainless steel: SUS304, SUS304L, SUS316, SUS316L, SUS310S, SUS321, SUS316Ti.

Source: China Stainless Steel Bellows Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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