| Annealing bellows is more common in Japan than in U.S. and Europe. In
radial outward forming a small thinning occurs but this is compensated
by an increase in yield strength due to cold work. When the bellows are
subsequently annealed, the increase in yield is eliminated.
As-formed yield strength is usually double (or more) the
specification minimum annealed strength. Increased yield results in
increased convolution deformation pressure and in-plane squirm pressure
increase. Also, plastic strain due to deflection is reduced and this
results in increased fatigue life.
The argument for annealing usually revolves about stress corrosion
cracking. However, tests have shown that the threshold of stress
required for stress corrosion is very low and the normal pressure stress
in an annealed bellows exceeds this level.
"The work hardening of austenitic stainless steel induced during
the forming of convolutions generally improves the fatigue life of an
expansion joint, often to a marked degree; thus it is not normally
considered beneficial to either stress relieve or anneal after
forming." (1985 EJMA Standards, for example para C-5.15)
Appendix B also contains a good article on stress corrosion
cracking.
In forming high yield strength materials such as Inconel 625, an
interstage anneal may be required. However, annealing is not recommended
after forming except in special cases. |
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