Invar Alloy

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FeNi(INVAR36) resistance alloy

1.Description

Invar is an iron-nickel austenitic alloy of lowest thermal expansivity. 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.

INVAR36

Chemical Content(%)

Physical Properties(Annealed)

Temperature Coefficient of Thermal Expansion

Typical Hardness,Rockwell B

Typical Tensile Strength(1000 psi)

2.Usage

It is  widely used in molds for composite aircraft parts,molds for composite boat parts,precision condenser plates, resonant cavities for radars,precision measurements constructions,special joints and washers, thermostatic bimetals,laser part applications,clock pendulums,balance wheels,watch parts,tooling

3.Machinability

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.

4.Workability

Forging: The principal precaution to observe in forging is to heat quickly and avoid soaking in the furnace. Long soaking may result in a checked surface due to absorption of sulfur from the furnace atmosphere and/or oxide penetration.

Blanking and Forming: Alloy 36 presents no unusual problems in blanking and forming. For cleanest blanking

properties, a Rockwell hardness of B90 is suggested. This hardness will allow mild bending and forming operations. Where deep drawing operations are involved a finish annealed strip of a Rockwell harness of about B75 is usually desirable.

Grinding: A silicon carbide wheel is desirable, preferably a soft wheel which will wear without loading.

Welding: Alloy 36 can be welded by the conventional methods. Caution must be taken so as not to overheat the

molten metal. This will avoid spattering of the molten metal and pits in the welded area. When welding with

rod, it is essential that the rod be of the same composition if similar properties in the weld are desired.The rods should be heavily coated. In oxyacetylene welding, the flame is maintained slightly on the reducing side.

Brazing: Silver and zinc-free alloys have been used for brazing Invar 36. This alloy should be annealed prior to

brazing. Joints should be designed to avoid placing material in tension during brazing. If silver soldering is done manually, the parts should be preheated to 800 oF and slowly cooled.

Plating: Alloy 36 can be chromium, cadmium and nickel plated or zinc coated by the usual methods used for ferrous alloys.

Pickling: Pickling is best done in hydrochloric acid solutions. A 25% solution at 160 oF is particularly effective.

Distortion:Invar has poor heat conductivity, a low coefficient of expansion at temperatures below 300 oC,

normal coefficient of expansion at higher temperatures, and an elastic modulus 79% of steel. These conditions

promote internal strain and distortion. Note carefully the section on stress relief. We suggest a rapid air cool

instead of a water quench whenever possible.