Journal of the Royal Naval Scientific Service. Volume 27, Number 2 ...

Materials Development: Conde and Godfrey 117
TABLE 3.
Some Sub Zero Temperatures of Technological
Importance.
Chemical Substance
or Process
Liquifying Temp.
Boiling Point or
Operating Temp.
°C
GasjLiquid
Volume Ratio
Ammonia 334 —
Propane 42 3 316
Propylene 477 —
Carbonyl Sulphide - 50 2 —
Chlorine 55 —
SO., dewaxing in
refining
- 60 —
Hydrogen sulphide 59-7 —
Carbon dioxide - 785 —
Acetylene 84 —
Ethane - 8S7 —
Purification of
nitrous oxide
Butyl rubber
production
- 90 —
100 —
Ethylene — 104 485
Krypton —1532 —
Methane —162 —
Natural Gas 162 600
Oxygen -183 843
Argon - 186 —
Fluorine -188 —
Nitrogen 196 683
Neon 246 —
Dueterium 2496 —
Hydrogen -253 850
Helium —269 753
packed hexagonal structure (e.g. magnesium,
zinc) become brittle at low temperatures and
face centred cubic metals (e.g. aluminium,
copper, nickel) remain ductile. There are exceptions
however and to demonstrate the
suitability of a material for low temperature
service the properties must be evaluated. The
growth of cryogenic engineering in the past
thirty years has led to extensive literature on
properties. Important properties at low temperatures
include conventional mechanical
strength, elastic modules and ductility, fatigue
strength, impact behaviour, notch ductility
and resistance to crack propagation, thermal
conductivity and expansion. In addition
fabricability by brazing or welding and the
properties of the resulting joints are important
as well as any adverse effects which may arise
from structural changes in heat-affected zones
of the parent material. The effect of lung term
exposures at low temperatures is also significant
as well as that of thermal cycling from
ambient to sub-zero levels since some materials,
such as certain types of stainless steel, may
be unstable and embrittle due to transformation
effects. Economic factors are important
in commercial applications such as liquid
gas storage or transport but in specialised
aerospace applications specific strength
(strength/density ratio) may be of greater
significance. Corrosion rates reduce with reduction
in temperature and corrosion is not
normally a serious factor but in certain instances
may have to be taken into account
since the corrosion rate in the cryogenic substance
may be small but significant or corrosion
may occur from the environment in contact
with the outside of the cryogenic containment.
The mechanical properties at low temperatures
of a selection of metals and alloys are
given in Table 4, and the chemical analyses of
these materials are presented in Table 5. In
general fatigue properties improve with deceasing
temperature in parallel with the increase
in tensile and yield properties. Conventional
mechanical properties may indicate
useful ductility at low temperatures but other
data are required to assess behaviour under
shock loads or triaxial stresses. Charpy Vee
notch impact data together with notched
tensile strength (see Table 4) provide some
indications of performance. The cast nickelaluminium-bronze
in Table 4 shows evidence
of serious degradation of impact value at low
temperatures and hence must be considered
suspect for cryogenic applications. To examine
the notch ductility behaviour and resistance
to crack propagation under severe service conditions
of low temperature and complex
stresses more sophisticated tests such as the
Tipper notched tensile test and the U.S. Navy
tear test have been evolved. In such tests a