4J32 Super-Invar Alloy Stainless Steel

4J32 is an Fe-Ni-Co ultra-low expansion precision alloy (National Standard GB/T 15018-2018, where 4 represents expansion alloy and J represents precision alloy), with international standard designations including Super-Invar and Nilo 32-5. It is a highly competitive ultra-low expansion model in the 4J series of expansion alloys. The core advantage of 4J32 Super-Invar alloy is its near-zero coefficient of thermal expansion (≤1.5×10⁻⁶/℃) in the wide temperature range of -60℃ to 80℃, among which the average linear expansion coefficient in the range of 20~100℃ can be precisely controlled to ≤1.0×10⁻⁶/℃. With its excellent dimensional stability, it has become a core material in high-end manufacturing fields such as precision instruments, optical systems, and aerospace, and is widely used in the manufacture of key structural components sensitive to temperature fluctuations.

I. Basic Definition and Core Characteristics of 4J32 Super-Invar Alloy

• Core Positioning: 4J32 Super-Invar alloy is a precision alloy focusing on ultra-low expansion coefficient and high dimensional stability. It is specially adapted to scenarios with large temperature fluctuations but high requirements for dimensional accuracy. It can effectively suppress part size deviations caused by temperature changes and ensure the long-term operational stability of products.

• Microstructural Characteristics: 4J32 Super-Invar alloy has a stable austenitic (face-centered cubic) structure, containing a small amount of ferromagnetic phase, with a Curie point of about 230℃ (ferromagnetic below this temperature and non-magnetic above this temperature). Through precise composition control and professional heat treatment processes, volume expansion caused by martensitic transformation at low temperatures can be effectively avoided, further improving dimensional stability.

• Core Advantages: Compared with traditional expansion alloys, 4J32 Super-Invar alloy has a lower expansion coefficient, good mechanical properties and processability, and balances cost performance. It can meet the dual needs of high-end manufacturing fields for dimensional accuracy and cost control, and is the preferred material for replacing some high-end Invar alloys.

II. Chemical Composition of 4J32 Super-Invar Alloy

III. Core Properties of 4J32 Super-Invar Alloy

1. Thermal Expansion Performance (Core Performance of 4J32 Super-Invar Alloy)

• 20～100℃: The average linear expansion coefficient is ≤1.0×10⁻⁶/℃, the expansion curve is gentle without sudden changes, and the dimensional stability is excellent, which is the most prominent advantage of 4J32 Super-Invar alloy;

• -60℃～80℃: The average linear expansion coefficient is ≤1.5×10⁻⁶/℃, the expansion performance is stable in a wide temperature range, which can adapt to the use requirements in various complex temperature environments;

• Above Curie Point: When the temperature exceeds 230℃ (the Curie point of 4J32 Super-Invar alloy), the expansion coefficient will increase sharply, and it will lose ferromagnetism, so long-term use above this temperature should be avoided.

2. Mechanical Properties

• Tensile Strength: 650～720 MPa, which is higher than that of ordinary Invar alloy. 4J32 Super-Invar alloy has stronger structural bearing capacity and can adapt to more force-bearing scenarios;

• Yield Strength: 380～450 MPa;

• Elongation: ≥20%, with good plasticity, which can meet the various forming and processing needs of 4J32 Super-Invar alloy such as cold rolling, cold drawing, bending, and deep drawing;

• Hardness: 180～220 HB, with moderate hardness, which is convenient for subsequent processing operations such as cutting and bending of 4J32 Super-Invar alloy.

3. Physical and Chemical Properties

• Density: 8.15 g/cm³, which meets the lightweight use requirements of precision alloys;

• Melting Point: About 1450℃, suitable for high-temperature smelting and processing processes;

• Resistivity: 0.42 μΩ·m (20℃), 4J32 Super-Invar alloy has stable electrical conductivity and can be used for some conductive precision components;

• Corrosion Resistance: 4J32 Super-Invar alloy has good corrosion resistance in dry atmosphere, fresh water, and neutral salt solution; in humid or acid-base environments, surface treatment such as electroplating and passivation is required to improve corrosion resistance;

• Thermal Fatigue Performance: Under the condition of ΔT=300℃, the thermal fatigue life of 4J32 Super-Invar alloy reaches 1.2×10⁴ cycles, which is better than that of traditional Invar alloy, but the oxidation rate will increase sharply when the temperature exceeds 550℃, so the service environment temperature should be noted.

IV. Product Forms and Supply Status of 4J32 Super-Invar Alloy

• Plates/Strip: Cold-rolled strip 0.038～3mm×1～250mm, hot-rolled plate 2.5～100mm×100～3000mm, mainly used in the manufacture of 4J32 Super-Invar alloy optical brackets, precision instrument housings, wafer trays and other products;

• Bars: Cold-drawn bars Φ3.5～200mm, hot-forged bars Φ10～400mm, suitable for 4J32 Super-Invar alloy high-end structural components such as aerospace precision components and precision measuring tools;

• Wires: Cold-drawn wires Φ0.025～10mm, which can be used for small 4J32 Super-Invar alloy components such as micro-sensors and precision leads;

• Pipes: Φ1～50mm, mainly used for 4J32 Super-Invar alloy sealing products such as vacuum feedthroughs and low-temperature seals;

• Supply Status: 4J32 Super-Invar alloy is mainly in annealed state (soft state), and cold-worked state can be provided according to customer needs. The annealed state can effectively eliminate processing stress and ensure the dimensional stability of 4J32 Super-Invar alloy products.

V. Key Process Points of 4J32 Super-Invar Alloy

1. Smelting Process

4J32 Super-Invar alloy adopts vacuum induction melting (VIM) process, and vacuum induction + electroslag remelting (ESR) double process can be used in some high-end scenarios to ensure alloy purity. The smelting vacuum degree should be ≤5×10⁻³Pa, the oxygen content should be strictly controlled to ≤30ppm, the pouring temperature should be controlled at 1550～1600℃, and the superheat should not exceed 50℃ to avoid dendrite coarsening, ensure the composition uniformity and structural compactness of 4J32 Super-Invar alloy, and reduce the impact of inclusions on performance.

2. Heat Treatment Process (Core Process of 4J32 Super-Invar Alloy)

• Standard Process: Water quenching at 840℃±10℃ for 1h (solution treatment), then furnace cooling or air cooling at 315℃±10℃ for 1h (aging treatment), which can obtain a uniform and stable austenitic structure, reduce the residual stress of 4J32 Super-Invar alloy to below 35MPa, and ensure its ultra-low expansion performance;

• Homogenizing Annealing: Argon-protected annealing at 1150℃ for 4h to eliminate dendrite segregation, make the Co element distribution uniformity reach 98.5%, and improve the performance consistency of 4J32 Super-Invar alloy;

• Stress Relief Annealing: After cold working, 4J32 Super-Invar alloy needs to be annealed at 470～540℃ for 1～2h to avoid dimensional deformation caused by processing stress. The single-pass deformation rate of cold working is recommended to be ≤15%, and the total deformation rate should not exceed 30%.

3. Processing and Welding Processes

• Hot Working: The hot working temperature of 4J32 Super-Invar alloy is controlled at 1100～1150℃, and the final forging temperature is not lower than 850℃ to avoid low-temperature embrittlement and ensure processing quality;

• Cold Working: 4J32 Super-Invar alloy can be subjected to forming processes such as cold rolling, cold drawing, bending, and deep drawing. Annealing should be performed in time after processing to prevent work hardening;

• Welding: 4J32 Super-Invar alloy can be welded by argon arc welding, resistance welding and other methods. Welding wire of the same material should be selected. The surface oil should be cleaned before welding, the heat input should be strictly controlled, and slow cooling should be performed after welding to ensure that the weld performance is close to the base metal and does not affect the expansion characteristics of 4J32 Super-Invar alloy.

VI. Typical Application Fields of 4J32 Super-Invar Alloy

• Precision Instruments and Measuring Tools: 4J32 Super-Invar alloy can be used for standard linear scales, gauge blocks, optical platforms, laser equipment optical brackets, lithography machine wafer trays, etc., using its ultra-low expansion characteristics to ensure measurement and equipment accuracy;

• Aerospace Field: 4J32 Super-Invar alloy is suitable for satellite optical systems, inertial navigation devices, gyroscopes, aerospace low-temperature seals, etc., which can adapt to temperature fluctuations in the space environment and ensure the dimensional stability of components;

• Electronics and Semiconductor Field: 4J32 Super-Invar alloy can be used for semiconductor manufacturing fixtures, high-precision sensor housings, vacuum feedthroughs, etc., to meet the requirements of precision packaging and dimensional stability;

• Other High-End Fields: 4J32 Super-Invar alloy can also be applied to LNG storage tank auxiliary components, medical precision instruments (such as MRI cavity components), high-end watch movements, etc., adapting to various high-end manufacturing scenarios sensitive to temperature.