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An Invar rod is a solid cylindrical bar made from Invar 36, a nickel-iron alloy containing approximately 36% nickel. Known for its extremely low coefficient of thermal expansion, Invar rods are engineered to maintain dimensional stability across wide temperature ranges. This makes them indispensable in precision applications like aerospace tooling, measuring devices, optical instruments, and scientific apparatus where even minor dimensional changes can compromise functionality. Invar rods are typically available in various diameters and lengths, either cold-drawn or annealed, to suit both structural and machining requirements.
Chemical Composition of Invar Rod
Invar rods are composed of a specific blend of nickel and iron with tightly controlled impurities:
| Element | Content (%) |
|---|---|
| Nickel (Ni) | 35.0 – 37.0 |
| Iron (Fe) | Balance |
| Carbon (C) | ≤ 0.05 |
| Silicon (Si) | ≤ 0.35 |
| Manganese (Mn) | ≤ 0.60 |
| Phosphorus (P) | ≤ 0.02 |
| Sulfur (S) | ≤ 0.02 |
Key Properties of Invar Rod
The mechanical and thermal stability of Invar rods make them stand out in industrial applications:
| Property | Typical Value |
|---|---|
| Density | 8.1 g/cm³ |
| Thermal Expansion (20–100°C) | ~1.2 × 10⁻⁶ /°C |
| Melting Range | 1425–1430°C |
| Tensile Strength | 485–620 MPa |
| Yield Strength | 240–450 MPa |
| Modulus of Elasticity | ~140 GPa |
| Magnetic Properties | Magnetic at room temperature |
Standard Sizes and Tolerances
Invar rods come in different lengths and diameters depending on usage. They are supplied in standard sizes with tight tolerances for high-precision industries:
| Diameter Range (mm) | Common Lengths (mm) | Form |
|---|---|---|
| 2 – 50 | 1000, 2000, 3000 | Cold-drawn, Annealed |
| 50 – 200 | Cut to length | Forged, Heat-treated |
Applications of Invar Rod
Invar rods are used where dimensional accuracy is critical in environments with changing temperatures:
| Industry | Application |
|---|---|
| Aerospace | Jigs and fixtures for composite layup |
| Metrology | Gauge blocks, measurement bars |
| Optics | Support structures in lens assemblies |
| Electronics | Lead frames and support rods |
| Cryogenics | Structural supports in low-temperature environments |
Workability and Fabrication
Invar rods can be machined and fabricated with conventional tools, though some precautions apply due to their hardness and magnetic properties:
| Process | Guidance |
|---|---|
| Machining | Requires sharp tooling and slow speeds to prevent work hardening |
| Welding | Weldable with compatible filler metals such as Invar 36 wire |
| Annealing | Can be annealed at 830–950°C followed by rapid cooling |
| Surface Treatment | May require passivation or coating to enhance corrosion resistance |
International Standards
Invar rods are produced according to globally recognized material standards to ensure quality:
| Standard | Designation |
|---|---|
| UNS | K93600 |
| ASTM | B753, F1684 |
| AMS | 2241, I23011 |
| ISO | 6208, 10003 |
Packaging and Supply
Invar rods are typically delivered in wooden crates or bundled with protective sleeves to avoid corrosion and deformation during transport. Custom cutting, heat treatment, and certification documentation are often provided by manufacturers to meet project-specific demands.
Related Questions
What are Invar rods used for?
Invar rods are used in tooling, metrology, cryogenic support structures, and optical instruments where thermal expansion must be minimized.
Is an Invar rod magnetic?
Yes, Invar rods are magnetic at room temperature due to their iron content, although they exhibit less magnetism compared to pure iron.
Can Invar rods be welded?
Yes, Invar rods can be welded using compatible Invar 36 filler wire and proper thermal controls to prevent distortion and maintain structural integrity.