Invar alloy is a special alloy primarily composed of iron and nickel, most notably characterized by its extremely low coefficient of thermal expansion. Its name "Invar" comes from the English word "invariable," reflecting its characteristic of exhibiting almost no dimensional expansion or contraction with temperature changes. Invar alloys are mainly used in precision instruments requiring high dimensional stability, aerospace structural components, optical equipment, and liquefied gas storage systems.
Main Components and Structure of Invar Alloy
The typical chemical composition of Invar alloy is approximately 63% iron (Fe), approximately 36% nickel (Ni), with the remainder being trace amounts of carbon, silicon, manganese, and other elements. The alloy's low thermal expansion characteristic stems from its unique iron-nickel atomic arrangement; as temperature increases, changes in the alloy's internal magnetic order counteract the thermal expansion effect of ordinary metals.
Common Invar alloy grades include: Invar 36, Invar 32-5, Super Invar, and Kovar. Invar 36 (also known as 4J36 or UNS K93600) is the most widely used model.
Physical Properties of Invar Alloy
1. Extremely low coefficient of thermal expansion: Within the temperature range of 20℃ to 100℃, its average coefficient of linear expansion is only about 1.2 × 10⁻⁶/℃, far lower than that of ordinary carbon steel (about 12 × 10⁻⁶/℃).
2. Density: Approximately 8.1 g/cm³.
3. Thermal conductivity: Low thermal conductivity, approximately 10–14 W/(m·K).
4. Magnetic properties: Invar alloy is ferromagnetic at room temperature, losing its magnetism at its Curie point of approximately 230℃.
5. Hardness: The Brinell hardness in the annealed state is generally 130–180 HB, which can be improved through cold working or aging treatment.
Advantages and Applications of Invar Alloy
Invar alloy's greatest advantage lies in its extremely high dimensional stability under varying temperatures. It maintains a very low rate of expansion across a range from tens of degrees below zero to hundreds of degrees Celsius, making it ideal for precision structural components and temperature-controlled environments.
Main applications include:
— Optical system supports, lens mounts, interferometer bases;
— Aerospace equipment, satellite structural components, precision gyroscope housings;
— Liquefied gas (LNG) storage tanks and pipeline structures;
— Electronic packaging, instrument scales, gauge blocks, and other measuring tools.
Machining Performance of Invar Alloy
While Invar alloy has relatively stable mechanical properties, its machinability is generally considered "slightly difficult to machine." The main reasons include:
1. Significant work hardening tendency;
2. Poor thermal conductivity, resulting in concentrated cutting heat;
3. Prone to tool sticking and tool wear.
It is generally recommended to use sharp carbide tools, lower cutting speeds, and forced coolant. It has good weldability, but heat input must be strictly controlled to avoid microstructure coarsening.
Comparison of Invar Alloy with Other Materials
Compared to ordinary carbon steel, Invar alloy has a coefficient of thermal expansion that is only 1/10; compared to stainless steel, Invar alloy has slightly lower strength but better thermal stability; compared to titanium alloys, Invar alloy has superior dimensional stability at low temperatures, making it irreplaceable in cryogenic engineering and metrology instruments.
Typical Domestic and International Grades
— Chinese Grades: 4J36, 4J32-5, 4J38;
— US UNS Numbers: K93600 (Invar 36), K93500 (Super Invar);
— German DIN Grade: 1.3912;
— French AFNOR: FeNi36.
Invar alloy is a typical "low-expansion precision alloy" widely used in aerospace, optics, and cryogenic engineering fields due to its excellent thermal stability and weldability. Although it is difficult to process, its dimensional stability is almost irreplaceable, making it one of the essential basic materials in precision engineering.