Page 509 - Mechanical Engineers' Handbook (Volume 4)

498 Cryogenic Systems

Figure 29 Effect of residual gas pressure on the effective thermal conductivity of a powder insulation—
perlite, 30–80 mesh, 300 to 78 K.

crystal lattice forms or close-packed-hexagonal lattices are usually subject to a brittle trans-
formation as the temperature is lowered. Such materials include the low-carbon steels and
certain titanium and magensium alloys. Figure 30 shows these crystal forms and gives ex-
amples of notch toughness at room temperature and at liquid N temperature for several
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example metals. In general carbon acts to raise the brittle transition temperature, and nickel
lowers it. Additional lowering can be obtained by fully killing steels by deoxidation with
silicon and aluminum and by effecting a fine grain structure through normalizing by addition
of selected elements.
In selecting a material for cryogenic service, several significant properties should be
considered. The toughness or ductibility is of prime importance. Actually, these are distinc-
tively different properties. A material that is ductile, as measured by elongation, may have
poor toughness as measured by a notch impact test, particularly at cryogenic temperatures.
Thus both these properties should be examined. Figures 31 and 32 show the effect of nickel
content and heat treatment on Charpy impact values for steels. Figure 33 shows the tensile
elongation before rupture of several materials used in cryogenic service.
Tensile and yield strength generally increase as temperature decreases. However, this is
not always true, and the behavior of the particular material of interest should be examined.
Obviously if the material becomes brittle, it is unusable regardless of tensile strength. Figure
34 shows the tensile and yield strength for several stainless steels.
Fatigue strength is especially important where temperature cycles from ambient to cry-
ogenic are frequent, especially if stresses also vary. In cryogenic vessels maximum stress