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Invar 36, also known as Alloy 36 or UNS K93600, is a nickel-iron alloy renowned for its unique physical and mechanical characteristics, especially its extremely low coefficient of thermal expansion. This property makes Invar 36 the material of choice in high-precision applications such as aerospace, cryogenics, scientific instruments, and electronics. Below is a comprehensive breakdown of the key material properties of Invar 36, organized into physical, mechanical, thermal, and magnetic categories.
Chemical Composition of Invar 36
The stability and properties of Invar 36 are a direct result of its carefully balanced chemical composition:
| Element | Content (%) |
|---|---|
| Nickel (Ni) | 35.0 – 37.0 |
| Iron (Fe) | Balance |
| Cobalt (Co) | ≤ 0.50 |
| Carbon (C) | ≤ 0.05 |
| Manganese (Mn) | ≤ 0.60 |
| Silicon (Si) | ≤ 0.35 |
| Sulfur (S) | ≤ 0.02 |
| Phosphorus (P) | ≤ 0.02 |
| Chromium (Cr) | ≤ 0.25 |
Physical Properties of Invar 36
These properties define the alloy’s behavior under general conditions and are critical for material selection:
| Property | Value | Unit |
|---|---|---|
| Density | 8.1 | g/cm³ |
| Melting Range | 1430 – 1450 | °C |
| Thermal Conductivity | 10.5 | W/m·K |
| Electrical Resistivity (at 20°C) | 0.78 | µΩ·m |
| Curie Temperature | 230 | °C |
Mechanical Properties of Invar 36
These values reflect how Invar 36 performs under mechanical stress and tension:
| Property | Value | Unit |
|---|---|---|
| Tensile Strength | 490 – 620 | MPa |
| Yield Strength | 275 – 450 | MPa |
| Elongation | 30 – 40 | % |
| Hardness (Brinell) | 130 – 200 | HB |
| Modulus of Elasticity | 141 | GPa |
Thermal Expansion of Invar 36
The most famous feature of Invar 36 is its extremely low coefficient of thermal expansion, especially near room temperature:
| Temperature Range (°C) | Thermal Expansion (µm/m·°C) |
|---|---|
| 20 – 100 | ~1.0 |
| 20 – 200 | ~1.3 |
| 20 – 300 | ~1.8 |
This low and stable thermal expansion makes it ideal for use in measuring devices, satellites, and telescope frames where dimensional stability is essential.
Magnetic Properties of Invar 36
Invar 36 is ferromagnetic at room temperature. However, its magnetism reduces sharply above the Curie temperature (~230°C):
| Property | Value |
|---|---|
| Magnetic Behavior | Ferromagnetic below 230°C |
| Curie Temperature | ~230°C |
| Relative Permeability (μr) | ~1000 – 5000 |
Applications Relying on Material Properties
Invar 36 is commonly used in areas where thermal stability and dimensional precision are required:
| Industry | Application |
|---|---|
| Aerospace | Satellite components, composite tooling |
| Electronics | Shadow masks, CRT components |
| Cryogenics | Containers, piping |
| Metrology | Precision instruments, measuring rods |
| Optics | Telescope mounts, interferometry frames |
Related Questions
Is Invar 36 magnetic?
Yes, Invar 36 is ferromagnetic at room temperature but loses magnetism above its Curie temperature of approximately 230°C.
What is the thermal expansion of Invar 36?
The coefficient of thermal expansion for Invar 36 is extremely low, about 1.0 µm/m·°C between 20°C and 100°C, which is its key characteristic.
What are the mechanical properties of Invar 36?
Invar 36 has a tensile strength of 490–620 MPa, yield strength of 275–450 MPa, and elongation between 30–40%, depending on temper and form.