MATERIALS OF CONSTRUCTION AT CRYOGENIC TEMPERATURES

 

MATERIALS OF CONSTRUCTION

AT CRYOGENIC TEMPERATURES

 

All materials do not work satisfactorily at

cryogenic temperatures. Properties undergo

a tremendous change from room temperature

down to cryogenic temperatures. Mechanical,

electrical, thermal, and magnetic properties are

most affected. At cryogenic temperatures,

properties of concern include permeability,

thermal conductivity, specific heat, linear thermal

expansion, thermal expansion coefficient, and

Young’s modulus. All these properties are not

available for all materials. Ductile materials suit

cryogenic temperatures. Measure ductility

from the energy required to break

the specimen. The table gives the available

data in this respect.

Thermal, mechanical, & electrical properties of a material change on cooling. The electrical resistance of a conductor decreases at a fast rate as the temperature decreases. Metals contract at lower temperatures. There is fast cooling as specific heat decreases at low temperatures.

TABLE: Change of ductility of materials at low temperature

Metal

Crystal lattice energy

Energy to break (FT lb.) at 700F

Energy to break  (FT lb.) at 3000 F

Al
Face Centered
Cubic
19
27
Cu
FCC
43
50
Ni
Face Centered
Cubic (FCC)
89
99
Iron
BCC
78
1.5
Titanium
Hexagonal Close
Packed (HCP)
14.5
6.6
Mg
HCP
4
30-105
FCC structures are most suitable at cryogenic temperatures as these do not lose their ductility at low temperatures.  Copper and its alloys, aluminum and its alloys are most suitable.  The ductility of metals with FCC structure improves at low temperatures. The ductility of iron falls to a very low value. Add alloys to iron to improve ductility. Most metals show super-conductivity between temperatures of 4 to 15 K.  Fig. shows the phenomenon of superconductivity.

Fig. Superconductivity( FCC Lattice materials) at cryogenic temperatures

(a) Copper and its alloys
 Copper & its alloys are easily formed and soldered. Storage and process vessels of copper & its alloys are used at cryogenic temperatures. The 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
 These are cheaper. These are light. These have excellent ductility and conductivity at low temperatures. These are easy to work with.  Vessels of aluminum & its alloys store liquid gases.
(c )Nickel steel alloys
 Used up to -200 C with 8.5% of nickel
(d) Austenitic stainless steel
Structure is FCC & non-magnetic. It has 3 to 19 % of Ni and 16 to 22 % Cr. Used for cryogenic applications.
(e) Body-centered lattice materials
(f) Iron and steel become brittle. Add alloying elements to make it suitable at low temperatures.

NON METALS AT LOW TEMPERATURES

Use these under compressive load applications. These are
( i) Plastics
(ii) Neoprene
(iii) Thiokol
(iv) Polyacrylate
(v) Asbestos
(vi) Teflon
(vii) Plexiglas
PROPERTIES REQUIRED AT LOW TEMPERATURES
Such materials should have higher values of the followings:
  1.  Yield strength
  2.  Ultimate strength
  3.  Toughness
  4.  Creep strength
  5. Weld strength
  6. Permeability

FINAL SELECTION

It is a compromise for various requirements namely
  1. Cost
  2. Manufacturing cost
  3. Stress level required
  4. Corrosion resistance
  5. Welding characteristics
Keeping safety first principle cost becomes the most predominant factor in the final selection.

Types of welding suitable on materials

(i) Submerged arc welding

(ii) Shield metal arc welding

(iii) Gas metal arc welding

(iv) Tungsten inert gas  welding

Practical applications of materials at cryogenic temperatures

1.  Austenitic stainless steel used for vessels storing liquid nitrogen (-195.80C).

2. Use aluminum alloy in the storage of liquid hydrogen (-253.00C).

3. Use materials having a magnetocaloric effect in magnetic cooling.

These materials are FeRh,  La0.8Ca0.2MnO3, Gd5Si2Ge2 & paramagnetic salt Gd2(So4)3.8H2o.

Gadolinium (Gd) metal used in Magnetic cooling at 0.001 K.

 5. Use shrink fitting for making aluminum vessels for low temperature service.

6. niobium-titanium & niobium-tin (Nb3Sn) are used in superconducting magnets

7. Plastic and glass rigid containers are suitable for frozen food.
8. The standard plastic blood bags store blood plasma up to −260°C.
9.  Thin plastic sheet bags of black and white colors are in use for storing dead bodies

10. PVC Remains Material used for Life-Saving Devices.

11. There are two types of superconducting materials. These are high-temperature (HTS) and low-temperature super (LTS)conductors. These are Type I and Type II. Type I are pure elemental superconductors. Type II are alloys and compounds superconductors. High-temperature superconducting materials find use in superconducting transformers and motors. The most promising HTS materials for high current applications are mercury, lead, indium, and tin.

12. Low-temperature superconducting materials are Type II. Type II includes Nb3Sn (18.1 K), Nb3Si (19 K), and Nb3Ge (23.2 K). The most common is niobium-tin.

13. Plastic straws are in use in handling bull insemination.

14. Materials used for cryopumps pistons, cylinder, sleeve are high nickel/copper alloys (such as Monel or Inconel) & stainless steel. PTFE is in use for piston rings. Cryo-pumps produce a very high vacuum of value 10-12 mm of mercury.

15. Low carbon steel with 3.5 %Ni, and aluminum-magnesium alloys are in use in satellites parts having low temperatures.

16. High alloy steels containing 18-21% chromium and 9-14% nickel suit for the storage of liquid hydrogen and helium.

REFERENCES

  1. Mechanical properties of structural materials at low temperatures-NBS monograph 13, McClintock R.M. & gibbons, H.P., Washington
  1. Cryogenic materials data handbook- Durban, T.F., McClintock, R.M., Reed, and R.P.
       3. Scurlock, Ralph G., ed. (1993). History and Origins of Cryogenics,                        Oxford, Clarendon Press.
  1. Weisend, John G. II, ed. (1998). Handbook of Cryogenic Engineering, Philadelphia, Taylor and Francis
  2. A brief overview of cryogenics in China, S.M. Li, Cryogenic Laboratory, Zhejiang University, Hangzhou, 310027, China, 2000
  1. Flynn, T.M., Cryogenic Engineering, Dekker, New York, 2005