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Invar steel, also known simply as Invar, is a unique iron-nickel alloy famous for its extremely low coefficient of thermal expansion. This property makes it ideal for applications that require high dimensional stability across temperature changes, such as aerospace tooling, scientific instruments, and precision measuring devices. The name “Invar” comes from the word “invariable,” referring to its stability under temperature fluctuations.
Chemical Composition of Invar Steel
Invar steel typically consists of approximately 64% iron and 36% nickel. Some variants may include additional elements like cobalt or chromium to fine-tune its thermal expansion and magnetic properties.
| Element | Typical Percentage (%) | Function |
| Nickel (Ni) | 36 | Reduces thermal expansion, enhances corrosion resistance |
| Iron (Fe) | Balance | Base metal, provides strength and structure |
| Cobalt (Co) | Optional 5-6 (in Super Invar) | Further reduces thermal expansion |
| Carbon (C) | <0.05 | Minimized to avoid brittleness |
Physical Properties of Invar Steel
One of the defining characteristics of Invar is its near-zero expansion when exposed to heat. Below are key physical properties:
| Property | Value |
| Density | 8.1 g/cm³ |
| Thermal Expansion Coefficient | ~1.2 x 10⁻⁶ /°C |
| Melting Point | 1425–1440 °C |
| Young’s Modulus | 141 GPa |
| Electrical Resistivity | 75 µΩ·cm |
Why Invar Steel Is Special
Most metals expand when heated, which can cause issues in precision equipment. Invar’s thermal expansion is so low that it maintains nearly the same size even when temperatures change. This makes it invaluable in scientific and aerospace fields where accuracy is critical.
Applications of Invar Steel
Invar steel is used in various high-precision and temperature-sensitive industries. Some of the most common applications include:
| Industry | Application | Purpose |
| Aerospace | Molds, jigs, structural frames | Dimensional stability under temperature fluctuations |
| Scientific Equipment | Measuring devices, cryogenic containers | Maintain shape and accuracy in varying temps |
| Electronics | Shadow masks, CRT components | Prevents distortion from heat |
| Telecommunications | Oscillator components | Frequency stability in temperature shifts |
| Optics | Camera mounts, telescope parts | Prevents misalignment due to expansion |
Grades of Invar Steel
Several grades of Invar are available, each suited to specific applications:
| Grade | Composition | Application |
| Invar 36 | 36% Ni, balance Fe | General low-expansion uses |
| Super Invar | 31% Ni, 5% Co, balance Fe | Extremely precise instruments |
| Invar 42 | 42% Ni, balance Fe | Glass-to-metal sealing in electronics |
Mechanical Properties of Invar Steel
Although Invar is primarily chosen for its thermal properties, its mechanical characteristics are also important:
| Property | Typical Value |
| Tensile Strength | 500–700 MPa |
| Yield Strength | 240 MPa |
| Hardness (Brinell) | 160 HB |
| Elongation at Break | 30–40% |
Machinability and Fabrication
Invar steel is machinable but requires special attention due to its work-hardening tendency. Cold working is possible, and welding can be done using TIG or MIG methods. However, post-weld annealing is often recommended to retain low expansion properties.
Related Questions
What is the main property of Invar steel?
The main property of Invar steel is its extremely low coefficient of thermal expansion, typically around 1.2 x 10⁻⁶ /°C.
What is Invar made of?
Invar is made primarily of 64% iron and 36% nickel, sometimes with added cobalt in high-precision variants.
What is Invar steel used for?
Invar steel is used in aerospace tooling, scientific instruments, electronics, and optical devices where minimal thermal expansion is critical.