INVAR

1. Extremely low coefficient of thermal expansion (core advantage): Within the temperature range of -100°C to +200°C, the average linear expansion coefficient is ≤ 1.8 × 10⁻⁶ /°C, significantly lower than ordinary carbon steel and stainless steel, enabling near-zero expansion.
2. Excellent dimensional stability: After stabilization treatment, dimensional changes during long-term service are minimal, making it suitable for precision instruments and metrological standards.
3. Good ductility and workability: In the annealed state, the elongation is 30-40%, allowing for various forming operations such as cold rolling, cold drawing, stamping, and machining.
4. Superior low-temperature performance: At -196°C (liquid nitrogen temperature), there is no ductile-to-brittle transition, maintaining good strength and toughness; it is a core material for LNG storage and transportation.
5. Good weldability: Can be joined using methods such as TIG welding, resistance welding, and electron beam welding. Stress-relief annealing after welding can restore low-expansion properties.
6. Complies with multiple international standards: ASTM B753 / ASTM F1684, UNS K93600, GB/T 15018 (4J36), etc.

A:

The low expansion characteristics of INVAR arise from the Invar Effect, a physical phenomenon caused by ferromagnetism:
1. Balance between Magnetostriction and Lattice Thermal Vibrations: Below the Curie temperature (approximately 230°C), INVAR is in a ferromagnetic state. As temperature increases, the magnetization decreases, leading to the 'magnetostriction' effect-spontaneous magnetization in ferromagnets tends to cause volume expansion. Meanwhile, atomic thermal vibrations also result in normal lattice thermal expansion.
2. Mutual Cancellation of Positive and Negative Effects: These two effects exactly cancel each other out in INVAR-when temperature rises, atomic thermal vibrations expand the lattice, but the decrease in magnetic order leads to a contraction from magnetostriction that precisely balances this expansion. Consequently, the alloy exhibits an extremely low thermal expansion coefficient over a wide temperature range from -100°C to +200°C.
3. Failure Above the Curie Point: When the temperature exceeds the Curie point, the alloy transitions from ferromagnetism to paramagnetism, losing its magnetism and reverting to normal positive expansion behavior. The thermal expansion coefficient then jumps to approximately 12 × 10⁻⁶ /°C, comparable to that of ordinary steel.

The heat treatment of INVAR has a decisive impact on the final microstructural stability and dimensional accuracy:
1. Annealing (Stress Relief, Homogenization): Heat to 815-830°C (1525°F), hold for 0.5 hours per inch of thickness, furnace cool at a rate ≤ 200°F per hour, and air cool after cooling below 600°F.
2. Stabilization Treatment (Recommended for High-Precision Parts): Hold at 300-400°C for 10-20 hours, then cool slowly to room temperature to eliminate residual stresses, stabilize the austenitic structure, and minimize dimensional drift during subsequent use.
3. Stress Relief Treatment (After Machining): Hold at 200-300°C for 1-2 hours, then air cool to prevent deformation caused by machining stresses.
Note: INVAR is highly sensitive to the cooling rate during heat treatment. Rapid cooling can lead to austenite supersaturation, resulting in precipitation phase transformation during subsequent use and causing changes in the coefficient of thermal expansion. Therefore, all heat treatments must be performed using a slow cooling method.

Effective Working Temperature (with Stable Low Expansion Performance):
It is recommended to be ≤ 200°C, where it can maintain an extremely low coefficient of thermal expansion and excellent dimensional stability. The optimal temperature range for use is from -100°C to +100°C.
Low-temperature performance:
INVAR maintains good strength and toughness at -196°C (liquid nitrogen temperature) without ductile-to-brittle transition, making it an ideal material for the storage and transportation of LNG (-162°C).
High-Temperature Limitation:
When the temperature exceeds 230°C (Curie point), the alloy transitions from ferromagnetic to paramagnetic, losing its low-expansion properties. The thermal expansion coefficient increases sharply to approximately 12 × 10⁻⁶ /°C, comparable to that of ordinary steel. Therefore, INVAR is not suitable for high-temperature environments above 230°C.
Temperature Cycle Effects:
Any thermal cycle exceeding the Curie point will permanently alter its low-expansion properties, as the magnetic structure cannot fully restore to its original state upon cooling.

INVAR has good welding properties, but the following points should be noted:
1. Recommended welding methods: TIG (Tungsten Inert Gas welding), resistance welding, and electron beam welding.
2. Pre-welding preparation: The welding area must be strictly cleaned to remove oil, scale, and moisture.
3. Protective gas: High-purity argon gas should be used for welding and backside protection to prevent high-temperature oxidation.
4. Post-welding treatment: It is recommended to perform stress-relief annealing (approximately 600-650°F) after welding to eliminate grain growth or precipitated phases that may occur in the heat-affected zone, avoiding changes in local expansion coefficients.
5. Filler metal: It is recommended to use INVAR filler wire of the same material to ensure the low expansion performance of the weld seam matches that of the base material.
6. Key precautions: INVAR has a low thermal conductivity (approximately 10 W/(m·K)), so the heating rate must be controlled during welding to avoid thermal stress concentration.

Partial substitution is possible, but it needs to be evaluated based on specific temperature ranges and precision requirements:
Advantages of INVAR:
1. It has an extremely low coefficient of thermal expansion (α ≤ 1.8 × 10⁻⁶ /°C) within the range of -100°C to +200°C, making it a classic low-expansion alloy with the widest application and most mature technology.
2. It exhibits good workability and weldability, allowing it to be formed into various shapes.
3. It possesses excellent low-temperature properties and is a core material for LNG storage and transportation.
Limitations of INVAR:
Inadequate high-temperature (>230°C) performance:
The coefficient of thermal expansion rises sharply above the Curie point, making it unsuitable for high-temperature environments. For applications between 200-400°C, consider 4J32 (with approximately 32% nickel content, offering a more stable coefficient of thermal expansion over a lower temperature range) or 4J40.
General corrosion resistance:
It is prone to corrosion in salt spray or sulfur-containing environments and requires surface protection. For applications with high corrosion resistance requirements, Stainless Invar (Stainless Invar, 54Co-9Cr-Fe) can be considered.
Long-term dimensional drift:
Slight dimensional changes may occur when stored at room temperature over a long period. Stabilization treatment is required for high-precision applications.
Material Selection Recommendations:
Please provide detailed operating conditions (temperature range, accuracy requirements, medium environment, service life) for specific material selection. We can assist with technical and economic comparison analysis to help you make the optimal choice between INVAR and other low-expansion alloy grades.
For quotations on specific specifications, material test reports, or technical parameter sheets, please contact our sales engineers. We support customizing dimensions, heat treatment conditions, and surface treatments as required, and provide a full set of internationally certified documents.