CRYOGENICS—MATERIALS OF CONSTRUCTION
MATERIALS OF CONSTRUCTION AT CRYOGENIC TEMPERATURES
Ductility is measured from the energy required to break the specimen. Some data in this respect is given in table 3.
TABLE 3: Change of ductility with low temperature
|Metal||Crystal lattice||Energy to break (FT lb.) at 700F||Energy to break (FT lb.) at 3000 F|
FCC structures are most suitable at cryogenic temperatures as these do not lose their ductility at low temperatures. Thus, copper and its alloys, aluminum and its alloys are most suitable. From the data, it can be noted that the ductility of FCC structure metals improves at low temperatures while that of iron falls to a very low value. However, iron can be alloyed in such a way that the ductility improves. Most metals show super-conductivity between temperatures of 4 to 15 K. Fig. shows the phenomenon of super-conductivity.
Fig. Super conductivity( FCC Lattice materials)
(a) Copper and its alloys
Alloys of copper and Copper are easily formed and soldered. Storage and process vessels required at cryogenic temperatures are made from copper and its alloys. Addition of 3.5 % of silicon greatly improves the properties. Tensile strength improves from 200 to 400 N/mm 2.
(b) Aluminum and its alloys
It has been found that aluminum and its alloys are less costly than copper. These are light. These have excellent ductility and conductivity at low temperatures. These are easy to work with. Thus, storage vessels for liquid oxygen, Nitrogen, Hydrogen Argon and Helium and nitrogen chilling heat exchangers are made from aluminum and its alloys.
(c )Nickel steel alloys
These can be used up to -200 0 C with 8.5% of nickel
(d) Austenitic stainless steel
It is FCC structure. It is non magnetic. It has 3 to 19 % of Ni and 16 to 22 % Cr. It can be used for cryogenic applications.
(e) Body centered lattice materials
(f) Iron and steel become brittle. The alloying elements must be added to make these suitable at low temperatures.
NON METALS AT LOW TEMPERATURES
These must be used under compression load applications. These are
( i) Plastics
PROPERTIES REQUIRED AT LOW TEMPERATURES
Such materials should have a higher values of the followings:
- Yield strength
- Ultimate strength
- Creep strength
- Weld strength
It is a compromise for various requirements namely
- Manufacturing cost
- Stress level required
- Corrosion resistance
- Welding characteristics
Keeping safety first principle cost becomes the most predominant factor in the final selection.
- Mechanical properties of structural materials at low temperatures-NBS monograph 13, McClintock R.M. & gibbons, H.P., Washington
- Cryogenic materials data hand book- Durban, T.F., McClintock, R.M., Reed and R.P.
3. Scurlock, Ralph G., ed. (1993). History and Origins of Cryogenics, Oxford, Clarendon Press.
- Weisend, John G. II, ed. (1998). Handbook of Cryogenic Engineering, Philadelphia, Taylor and Francis
- A brief overview of cryogenics in China, S.M. Li, Cryogenic Laboratory,Zhejiang University, Hangzhou, 310027, China, 2000
- Flynn, T.M., Cryogenic Engineering, Dekker, New York, 2005