Low Expansion Alloy
Invar (36% NI-Balance Iron) Alloy has been the metal of choice for low expansion applications for years. "Super-Invar" (31% NI-5% Co-Balance Iron) has found some favor because it has a near zero coefficient of thermal expansion over a limited temperature range. The useful range of "Super Invar" is limited between -32° to + 275°C because the material begins to transform from Austenite to Martinsite at temperature below-32°F.
To develop the lowest coefficient of thermal expansion the following heat treatment is recommended
The C.T.E. crosses over the zero threshold frequently. Each lot of heat behaves a little differently, but these results are typical for material between falling between 0°F and 200°F.
Formability: Super Invar is easily formed, deep drawn, and fabricated.
Weldability: Super Invar is welded using a special weld wire and a variety of other high nickel rods and wires.
Machinability: Super Invar is tough and gummy, not hard or abrasive. Tools tend to plow instead of cut, resulting in long stringy "chips." Tools must be sharp. Feed and speed should be low to avoid heat and distortion. The use of a coolant is recommended for all machining operations. Machinability has been reported to be similar to that of Kovar, Stainless 300 series, and Monel Alloys. Ni-Fe Alloys generally have a tendency to develop a surface scale during hot working that penetrates the surface. Machining allowances must be increased to eliminate the deep surface oxide. The initial cut is frequently the most difficult.
Typical Linear Coefficient of Thermal Expansion ( cm per cm. c x 10 -6 )