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Invar 36 round bar, also known as Alloy 36 or UNS K93600, is a low expansion nickel-iron alloy widely used in industries where dimensional stability is critical. With an outstanding combination of mechanical strength, corrosion resistance, and extremely low coefficient of thermal expansion, Invar 36 round bars are ideal for precision instruments, aerospace tooling, and cryogenic components. This article explores the material's composition, properties, sizes, standards, and applications in detail.
Chemical Composition of Invar 36 Round Bar
The chemical makeup of Invar 36 is designed to maintain its unique low thermal expansion characteristics. Here is a breakdown of its typical composition:
| Element | Content (%) |
|---|---|
| Nickel (Ni) | 35.0 - 37.0 |
| Iron (Fe) | Balance |
| Carbon (C) | ≤ 0.05 |
| Manganese (Mn) | ≤ 0.60 |
| Silicon (Si) | ≤ 0.35 |
| Chromium (Cr) | ≤ 0.25 |
| Phosphorus (P) | ≤ 0.020 |
| Sulfur (S) | ≤ 0.020 |
Mechanical Properties of Invar 36 Round Bar
Invar 36 round bar exhibits stable mechanical properties at both ambient and cryogenic temperatures. Below are the typical mechanical values:
| Property | Value |
|---|---|
| Tensile Strength | 485 MPa (min) |
| Yield Strength (0.2% offset) | 275 MPa (min) |
| Elongation | ≥ 35% |
| Hardness | ≤ 100 HB |
Physical Properties
The outstanding low thermal expansion of Invar 36 is the key reason for its use in sensitive environments. Below is a list of relevant physical properties:
| Property | Value |
|---|---|
| Density | 8.1 g/cm³ |
| Melting Point | 1430°C |
| Coefficient of Thermal Expansion (20–100°C) | 1.3 × 10⁻⁶ /°C |
| Thermal Conductivity | 10.7 W/m·K |
| Electrical Resistivity | 0.78 μΩ·m |
Standard Specifications
Invar 36 round bar is produced according to several global standards:
| Standard | Description |
|---|---|
| ASTM F1684 | Standard specification for Invar round bar and wire |
| AMS 2241 | General tolerances for nickel alloys |
| UNS K93600 | Unified Numbering System designation |
Size Range and Tolerances
Invar 36 round bars are available in a wide range of diameters and lengths:
| Diameter (mm) | Length (mm) | Tolerance (Diameter) |
|---|---|---|
| 6 – 50 | 1000 – 6000 | ±0.05 mm |
| 51 – 100 | 1000 – 6000 | ±0.10 mm |
| Above 100 | 1000 – 6000 | ±0.20 mm |
Applications of Invar 36 Round Bar
Thanks to its exceptional dimensional stability and resistance to thermal distortion, Invar 36 round bar is essential in many high-precision and temperature-sensitive fields:
| Industry | Application |
|---|---|
| Aerospace | Satellite structures, tooling fixtures |
| Electronics | Shadow masks, TV panels |
| Oil & Gas | Cryogenic components, LNG tankers |
| Instrumentation | Precision measuring devices |
| Optics | Telescope and laser system components |
Machinability and Welding
Invar 36 round bars can be machined and welded using standard nickel-iron alloy techniques:
| Operation | Guideline |
|---|---|
| Machining | Use slow speeds and rigid tools to reduce work hardening |
| Welding | Shielded metal arc or TIG welding recommended; post-weld annealing improves properties |
| Forming | Cold forming possible with moderate force; annealing may be required afterward |
Heat Treatment and Stability
To optimize thermal stability and stress relief, Invar 36 round bars are often heat-treated:
| Process | Temperature (°C) | Effect |
|---|---|---|
| Annealing | 830 – 950 | Improves ductility and reduces internal stress |
| Stress Relief | 315 – 370 | Reduces distortion from machining |
Related Questions
What is the thermal expansion coefficient of Invar 36 round bar?
Invar 36 round bar has an extremely low coefficient of thermal expansion—approximately 1.3 × 10⁻⁶ /°C in the 20–100°C range, making it ideal for precision components where thermal stability is essential.
What are the main applications of Invar 36 round bar?
Invar 36 round bars are primarily used in aerospace tooling, cryogenic systems, precision instruments, and optical devices due to their dimensional stability and low thermal expansion.
Is Invar 36 round bar suitable for welding?
Yes, Invar 36 round bar can be welded using TIG or shielded metal arc welding. It is recommended to perform post-weld annealing to restore optimal material performance.