High Temperature Bellows Applications
Application
A bellows is usually protected from direct exposure to the
flow stream and the actual bellows material temperature
(ABMT) is rarely the same as the temperature of the media
flowing through the expansion joint. However, it is still possible
that the ABMT may be in the high temperature range.
The high temperature range is nominally considered to be
that range of temperature where the time-dependent properties
of creep rate and stress rupture become signifi cant
(see table below).
Heat Treatment of High Temperature Bellows
Creep-rupture is a time dependent property that is affected
by cold work. In the high temperature range, cold work can
reduce the stress for rupture to below that for annealed
material. More importantly, rupture ductility is severely
affected. Rupture ductility represents the elongation that
occurs during a stress rupture test. For T316 stainless steel
at 1000°F, rupture ductility values range from 9-18% for annealed
material compared with only 0.18-0.6% for 10-15%
cold worked material. Other high alloy and nickel base
alloy bellows materials such as Alloy 800 show a similar
reduction in rupture ductility. Consequently, cold work in
the material can limit the amount of inelastic strain that can
be accumulated over the expected lifetime of the bellows.
According to the American Society for Metals, “low ductility
in a rupture test almost always indicates high resistance to
the relaxation of stress due to creep, and possible sensitivity
to stress concentrations. There is also ample evidence
that rupture ductility has a major infl uence on creep-fatigue
interaction.”
Based upon the above’ annealing after forming to eliminate the
effect of cold work should be considered for bellows that have
an ABMT in the high temperature range (Note: if the bellows
are annealed, consideration must be given in the calculations
to account for the reduced material strength).
The photos below show a bellows that failed after exposure in
the high temperature range. This bellows was installed in the
cold worked condition without heat treatment and externally
insulated such that the ABMT was in the high temperature
range. A metallurgical analysis indicated that the characteristics
of the cracking in the bellows were consistent with low
creep ductility. The report recommended that the replacement
bellows receive heat treatment after forming.
Alloy 625 Bellows*
A commonly specified bellows material for high temperature
expansion joints is Alloy 625. Unfortunately, even in the annealed
condition, this material is subject to severe loss of
room temperature ductility after exposure to temperatures in
excess of 1000°F. Therefore, it is important to design expansion
joints containing Alloy 625 bellows properly so that the
ABMT will not exceed 1000°F during operation.
* Manufacturer’s data indicates that Alloy 625LCF is capable
of long term exposure at 1100 F without significant loss of
room temperature ductility