Page 513 - Mechanical Engineers' Handbook (Volume 4)

502 Cryogenic Systems

Figure 35 Coefficient of linear thermal expansion of several metals as a function of temperature.
(Courtesy American Institute of Chemical Engineers.)

tivity. One measure of the suitability of a material for cryogenic service is the ratio of tensile
strength to thermal conductivity. On this basis stainless steel looks very attractive and copper
much less so.
The most common materials used in cryogenic service have been the austenitic stainless
steels, aluminum alloys, copper alloys, and aluminum-alloyed steels. Fine grained carbon-
manganese steel and aluminum-killed steel and the 2.5% Ni steels can be used to tempera-
tures as low as 50 C. A 3.5% Ni steel may be used roughly to 100 C; 5% Ni steels have
been developed especially for applications in liquified natural gas processing, that is, for
temperatures down to about 170 C. Austenitic stainless steels with about 9% Ni such as
the common 304 and 316 types are usable well into the liquid H range ( 252 C). Aluminum
2
and copper alloys have been used throughout the cryogenic temperature range. However, in
selecting a particular alloy for a given application the engineer should consider carefully all
of the properties of the material as they apply to that application.
Stainless steel may be joined by welding. However, the welding rod chosen and the
joint design must both be selected for the material being welded and the expected service.
For example, 9% nickel steel can be welded using nickel-based electrodes and a 60–80
single V joint design. Inert gas welding using Inconel-type electrodes is also acceptable.
Where stress levels will not be high types 309 and 310 austenitic-stainless-steel electrodes
can be used despite large differences in thermal expansion between the weld and the base
metal.
Dissimilar metals can be joined for cryogenic service by soft soldering, silver brazing,
or welding. For copper-to-copper joints a 50% tin/50% lead solder can be used. However,
these joints have little ductility and so cannot stand high stress levels. Soft solder should not
be used with aluminum, silicon-bronze, or stainless steel. Silver soldering is preferred for
aluminum and silicon bronze and may also be used with copper and stainless steel.

Polymers
Polymers are frequently used as structural materials in research apparatus, as windows into
cryogenic spaces, and for gaskets, O-rings, and other seals. Their suitability for the intended