Invar 36

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Invar, also known as Nickel Alloy 36, is a nickel-iron alloy noted for its uniquely low coefficient of thermal expansion. The name Invar comes from the word invariable, due to its relative lack of expansion or contraction with temperature changes. Invar was invented in 1896 by Swiss scientist Charles Édouard Guillaume, who received the Nobel Prize in Physics in 1920 for the discovery, which enabled improvements in scientific instruments.

Invar is an austenitic alloy. It is strong, tough, ductile and possesses a useful degree of corrosion resistance. It is magnetic at temperatures below its Curie point and non-magnetic at temperatures above. Invar is therefore always magnetic in the temperature range in which it exhibits the low expansion characteristics. Invar is the standard alloy for low expansivity up to 400 oF. For applications at higher temperatures, the higher nickel alloys are recommended.

Typical Analysis in Percentage:

  Ni              35.0 - 38.0                   Cr             0.25

  Mn             0.50                            Si              0.25

  Al              0.10                            Mg             0.10

  Zr              0.10                            Ti              0.10

   P              0.02                             S              0.02

  Fe             Balance

Properties:

Typical Tensile Strength (1000 psi):

Annealed:          85 Maximum

¼ Hard :            90 to 115

½ Hard :            105 to 125

Hard :                120 Minimum

Typical Hardness, Rockwell B:

Annealed :         70 Maximum

¼ Hard :            78 to 83

½ Hard :            84 to 88

Typical Coefficient of Thermal Expansion (Annealed):

Temp. Range         in/in/ oF x 10-6

-200 - 0 oF                      1.1

0 - 200 oF                       0.7

200 - 400 oF                    1.5

400 - 600 oF                    6.4

600 - 800 oF                    8.6

800 - 1000 oF                  9.5

Physical Properties (Annealed):

Specific Gravity : 8.08

Density : 0.292 lb/in

Electrical Resistivity : 494 ohms/mil/ft

Temperature Coefficient of Electrical Resistivity : 0.67 x 10-3/oF

Melting Point : 2600 oF

Curie Temperature : 530 oF

Inflection Temperature : 375 oF  

Modulus of Elasticity : 21.0 psi x 106

Heat Treatment:

The effect of heat treatment upon the expansion of the alloy is dependent upon the method of cooling. Rapid cooling (quenching) decreases the rate of expansion while the reverse is true when slow cooling is employed. Cold working is even more effective than quenching in lowering the expansivity. Subsequent annealing will remove the lowering of the coefficients induced by cold work in proportion to the temperatures employed, the alloy assuming the values corresponding to the annealed condition when a temperature of about 1100 oF is reached. Invar which has been subjected to cold working or machining may require a stress-relieving heat treatment for stabilization if the material is to be used for high precision work. This material is never used above its thermal inflection point.

Anneal : The alloy softens progressively when heated in the range of 1000 to 2300 oF. Pronounced grain growth does no occur until 1900 oF has been passed. It can be air cooled or water quenched from the annealing temperature.

Stress Relieve : Heat to 600 - 700 oF for about one hour, air cool, reheat to a temperature somewhat above the top operating temperature, cool slowly to somewhat below the lower operating temperature, again heat slowly to above the operating temperature, cool slowly to room temperature (cooling very slowly through the Curie temperature is also considered to improve stability).

Harden : Cannot be hardened by any thermal treatment.

Stabilize : Water quench from 1500 oF, then age for one hour at 600 oF, air cool.

Machinability:

Being tough and ductile, Invar is somewhat difficult to machine. High speed steel or sintered carbide should be used and the cutting edges kept sharp. The machinability characteristics of Invar are quite similar to austenitic stainless steels. Because of its high ductibility, the chips formed during machining tend to be stringy and tough, thus imposing rapid wear on cutting tool edges. In general, slow speeds and light feeds should be used to avoid excessive heat and minimize the possibility of the generated heat affecting the expansion characteristics. The use of soluble oil cutting compound is recommended for all machining operations.