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Knowing more about Invar, a popular high-tech alloy material, can help shops boost production.
Today's metalworking shops frequently machine parts and components from Invar alloy (UNS K93600), a 36% nickel-iron alloy known for its unique low-expansion properties. Although the material has been fairly popular for years, shops can still benefit from a better understanding of its machining parameters
Produced by Carpenter Technology Corp., Reading, Pa., the alloy has a rate of thermal expansion approximately one tenth that of carbon steel at temperatures up to 400 deg F (204 deg C).
The alloy is available in two variations. One is the conventional Invar alloy, used generally for its optimum lowexpansion properties. The second is a variation of the basic alloy known as Free-Cut Invar "36" alloy (UNS K93050 and ASTM F1684). This alloy has shown improved machinability for applications where high productivity is important. It is also a 36% nickel-iron alloy, but with a small addition of selenium to enhance machinability.
Free-machining variation
Free-Cut Invar "36" alloy, the first free-machining Invar alloy, is used by machine shops producing high volumes of parts like controls for hot water heaters, filters for microwave instruments, and precision parts for optical mounting in lenses.
Compared with the conventional Invar grade, the downside for Free-Cut Invar "36" alloy is negligible. Its coefficient of thermal expansion is slightly higher than that of the basic alloy, which is not enough to make a difference in part performance.
With the freemachining alloy, there is a minimal loss in both transverse toughness and corrosion resistance. It may be necessary to clean and passivate the free-cut alloy to remove selenides from the surface.
However, a good case can be made for the free-cut alloy because it machines without a hassle and often boosts production by as much as 250%.
Both Invar 36 alloys are soft like Type 304 and Type 316 austenitic stainless steels and have the same high workhardening rate, requiring care in machining. The free-cut variation, in particular, machines similar to the two stainless grades.
The standard Invar alloy produces stringy, gummy chips which "birdnest" around the tools and interfere with coolant flow. Chips have to be broken up using chipbreakers. Chipbreakers are also used with the free-cut alloy, but they do not...