Engineers and product designers prize Invar 36 because of its ability to maintain its size and shape over a wide range of temperatures. This makes it a very useful and common material in a variety of engineered products ranging from watch parts to satellites.
However, Invar is a tricky material to work with. It can be very difficult to machine and requires special considerations to avoid problems. In this article, we’ll discuss Invar 36 machinability challenges and some methods for overcoming them.
What Is Invar 36?
Invar 36 is an iron-nickel alloy containing 36% nickel. This composition is optimized to have an exceptionally low coefficient of thermal expansion—one-tenth that of carbon steel at temperatures up to 400°F. This means it doesn’t expand or shrink at extreme temperatures nearly as much as other metals do. Invar 36 is also a low-expansion alloy.
Because it’s so resistant to thermal expansion, Invar 36 is commonly used for applications that demand high dimensional stability under temperature variation. For example, radio and electronic devices, aircraft controls, optical systems, surveying tape, and clockwork components are resistant to thermal expansion.
Invar 36 Machinability Challenges
Although it looks and feels similar to carbon steel, Invar 36 alloy is much more difficult to machine. It is soft and very ductile, especially in the annealed condition. This can make Invar 36 machinability a challenge.
Cutting tools tend to plow into the material instead of cutting it cleanly. Because of this, Invar 36 produces stringy, gummy chips that tend to “bird nest” around the cutting tool, interfering with cutting and coolant flow.
Like other nickel alloys, Invar is susceptible to work hardening. Work hardening occurs when the surface of the metal ahead of the cutting tool plastically deforms, forming a hardened layer that is very difficult to penetrate in subsequent passes or operations. This can lead to poor surface finish, part deformation, and broken tools.
Tips for Machining Invar 36
Though it’s a challenging material to cut, there are ways to improve Invar 36 machinability, including the following best practices.
Getting good results when milling Invar requires a careful balance of tool diameter, number of teeth, cutting speed, feed, and chip space. Machine power, condition, and setup are also very important to reduce chatter and prevent work hardening.
The gummy chips Invar produces can interfere with the cutting tool if too much accumulates. Use a chip breaker and ensure your tooling is sharp and rigidly supported when machining Invar. High-speed steel and carbide tools both work well on Invar, though with carbide tools, it is important to use a powerful machine and a rigid setup to minimize vibration. Helical tools resist chatter and will generally provide the best surface finish. Descaling the surface before machining can make machining significantly easier.
Prevent work hardening by keeping the tool moving and minimizing heat. Maintain a positive feed rate and avoid dwelling so heat doesn’t build up in one area. Machining Invar requires higher feeds and lower speeds than other metals. Ensure you have adequate lubrication volume and pressure to keep the cutting surface cool. When turning Invar 36, use the largest tool possible to provide a large heat sink.
Finally, machining Invar introduces stresses that can change the thermal expansion behavior of the finished product. It may need to be annealed after machining to relieve these stresses and ensure dimensional stability.