Invar is an iron-nickel austenitic alloy with low thermal expansion characteristics. It is strong, tough, and ductile, and possesses a useful degree of corrosion resistance. Invar is always magnetic in the temperature range in which it exhibits the low expansion characteristics. It is the standard alloy for low expansion up to 204 oC. For applications at higher temperatures, higher nickel alloys are recommended.
Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, valves in motors, and antimagnetic watches. In land surveying, when first-order (high-precision) elevation leveling is to be performed, the leveling rods used are made of Invar, instead of wood, fiberglass, or other metals. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.
Apex has a long history of working with Invar. Parts produced with Invar are typically used for precision instrumentation. Parts are usually small because the price of Invar can range between $20 and $30 per pound. Machining of Invar is reasonable with the use of carbide tooling. It machines at about half the rate of carbon steel. The main challenge with machining Invar is removing burrs generated by the cutting process. Every part machined requires its own deburring method. The deburring will often be done under magnification due to the small size of parts.
Because of their small size, parts tend to have much tighter tolerance callouts. The tightest tolerance achieved at Apex was .002mm (.00008”) in a small hole, using Wire EDM and measuring with an OPG Vision System. Choosing the right tolerance on a part is crucial. One decimal place can change the process and price dramatically. The parts also need to be checked with the proper measurement technology. Feel free to email Apex with any of your Invar fabrication requirements.