Properties and Machinability of Invar

Invar is a low-expansion alloy containing 36% nickel in iron. It exhibits virtually no dimensional change over a wide temperature range of -250℃ to +200℃. This extremely low thermal expansion is achieved because the expansion resulting from temperature rise is offset by the contraction resulting from the transformation from ferromagnetic to paramagnetic. Compared to iron's coefficient of thermal expansion of 11.8, Invar is extremely low at approximately 2.0 (one-sixth of iron and one-tenth of nickel).

Due to these characteristics, Invar is used in applications where minimizing dimensional fluctuations caused by temperature changes is essential. Examples of its use include precision and measuring instruments, astronomical telescopes, and LNG-related equipment. Its stable dimensional retention also makes it in high demand for scientific experiments and high-precision machined parts. However, Invar's special composition that achieves low expansion also makes it a difficult material to machine.

The three main issues in cutting Invar material are listed below:

(1) Chip entanglement and tool wear

Because invar is highly viscous, chips generated during cutting tend to get entangled in the tool. This accelerates tool wear and adversely affects the machined surface finish.

(2) Local overheating and increased tool temperature due to low thermal conductivity

Due to Invar’s low thermal conductivity, heat generated during machining tends to accumulate in the material, causing the tool temperature to rise. This can result in reduced tool sharpness. Additionally, dimensional accuracy may be impaired due to the thermal expansion of the Invar itself.

(3) Sensitivity to dimensional fluctuations caused by low thermal expansion

While Invar's low thermal expansion coefficient is its most important feature, it is also a challenging factor in machining. During processing, even the slightest temperature change causes dimensional changes, making precise processing extremely challenging.

For these reasons, cutting invar materials requires advanced technology and extensive experience. High machining accuracy can be achieved by taking various measures such as appropriate tool selection, optimization of cutting conditions, and cooling methods.

Applications of Invar

Taking advantage of its extremely low thermal expansion coefficient, Invar is used in applications where even minute dimensional fluctuations due to temperature changes must be suppressed. Examples include precision measuring instruments, scientific laboratory equipment, and temperature control components for large structures.

In manufacturing and assembling precision measuring instruments, the dimensional stability of each component is crucial. Expansion or contraction caused by temperature fluctuations can affect overall performance and measurement accuracy. Due to its low thermal expansion, Invar prevents problems caused by temperature differences between dissimilar materials and ensures proper coordination between components.

Invar is particularly reliable in applications where dimensional fluctuations are unacceptable, such as astronomical telescope mounts, precision watch parts, frames for laser interferometers and high-precision measuring devices, and structural components for LNG-related equipment.