Invar® Material Properties, Applications, and Specifications

UNS K93603, UNS K93050

EFINEA Metals has supplied Invar to engineers, metallurgists, and procurement teams across aerospace, optics, electronics, and scientific instrumentation since 1965. This page consolidates the technical reference material most often requested for Invar and Free-Cut Invar “36”® Alloy FM, including thermal characteristics, physical properties, mechanical properties, chemistry, specifications, and the applications where the alloy is most commonly specified.

For questions about whether Invar fits your design, our sales engineers and metallurgists are reachable at 800-348-6268. No order quantity is too small. As an Invar 36 supplier, we stock the alloy in plate, sheet, strip, rod, and bar for prompt shipment.

Invar® Properties

Invar properties are best understood across four data sets: thermal expansion behavior, physical properties such as density and conductivity, mechanical properties under tension, and chemistry. The tables below summarize typical values for ASTM F1684 material.

Average Coefficient of Thermal Expansion

Thermal expansion is the property that defines Invar. The coefficient stays remarkably low from room temperature through approximately 200°C, then rises as the temperature approaches and exceeds the Curie point at 279°C. This narrow operating window is what designers must respect when specifying Invar for precision applications.

The data below is taken on material annealed at 871°C for one hour, then furnace-cooled.

To compare the average linear coefficient of thermal expansion across our line of controlled expansion glass and sealing alloys, please refer to the Thermal Expansion Table.

Invar Typical Physical Properties

Physical properties describe how Invar behaves as a bulk material with no load applied. These values are used for calculating heat transfer, sizing current paths, and estimating component weight.

The Curie temperature of 279°C is the key threshold for design work. Invar’s low expansion behavior depends on the magnetic state of the iron-nickel matrix, and once the alloy passes through the Curie point on heating, the matrix becomes paramagnetic, and the expansion coefficient rises significantly. For applications where dimensional stability is critical, keep operating service temperatures well below this threshold.

Invar Typical Mechanical Properties

Mechanical properties indicate how Invar will behave during forming, machining, and service loading.

Invar machines slowly and tends to gum up tooling, which is the reason the free-machining variant was developed in the first place. Standard Invar requires sharp tools, generous coolant flow, and patience. When machining time is a significant cost driver, customers typically move to Free-Cut Invar “36” for round bar applications.

Invar Chemistry

The chemistry of Invar is held to tight tolerances because the low expansion behavior is highly sensitive to nickel content. Deviations from the 36% nickel target shift the inversion temperature and degrade dimensional stability through the working range. 

Every Invar shipment from EFINEA includes mill certifications, and chemistry can be verified with handheld XRF equipment at our warehouses when documentation traceability is critical to the program. 

Invar Specifications

If you are working from an older drawing that references a superseded specification, contact our sales team for assistance with the cross-reference.

Free-Cut Invar “36”® Alloy FM

Free-Cut Invar “36” is a free-machining variant developed for round bar applications that require extensive machining work. The grade contains controlled selenium additions, which improve chip break and tool life without significantly affecting the low expansion behavior that defines the Invar family. The alloy is covered under UNS K93050. 

Free-Cut Invar “36”® Alloy FM Mean Coefficient of Thermal Expansion

The thermal expansion behavior of Free-Cut Invar tracks closely with standard Invar through the working temperature range, with the same low coefficient that defines the alloy family.

Free-Cut Invar “36”® Alloy FM Typical Physical Properties

Physical properties for Free-Cut Invar are nearly identical to standard Invar, which is consistent with the shared base chemistry.

Free-Cut Invar “36”® Alloy FM Typical Mechanical Properties

The selenium addition produces a small reduction in ductility relative to standard Invar, which is expected and acceptable for the round bar applications where Free-Cut is typically used. Cold-drawn material shows substantially higher strength than annealed bars.

Free-Cut Invar “36”® Alloy FM Chemistry

Free-Cut Invar “36”® Alloy FM Specifications

For additional background, the websites of ASTM International and ASM International publish the full technical standards governing both Invar and the Free-Cut variant. As a free-cut Invar 36 supplier, EFINEA carries standard round bar sizes in stock and can quote custom diameters on request.

Invar® Applications

Invar applications have continued to expand for more than a century. The combination of very low thermal expansion, good corrosion resistance, and consistent performance across cryogenic and moderately elevated temperatures keeps the alloy in active service across aerospace, electronics, scientific instrumentation, and precision manufacturing. The sections below cover the most frequently requested uses.

Advanced Composite Molds for Aerospace

The aerospace composite industry is one of the largest current consumers of Invar. Carbon fiber-reinforced polymer (CFRP) parts cure at elevated temperatures, and the tooling must match the thermal expansion of the cured composite to hold dimensional tolerances through the cure cycle. Invar tooling is specified for wing skins, fuselage panels, engine nacelles, and structural components on commercial and military aircraft programs. 

The same approach is used in space launch vehicle manufacturing, where dimensional accuracy on large composite structures depends on tooling that does not grow or shrink with the part during cure.

Semiconductor and Display Manufacturing

CRT shadow masks, deflection clips, and electron gun components historically represented one of the largest Invar markets, and while CRT displays have been largely replaced in consumer applications, the underlying technology remains in service in specialty and industrial equipment. 

Invar is also specified in semiconductor wafer-handling fixtures and certain types of mask substrates, where dimensional stability supports lithographic accuracy.

Optical Benches and Precision Instruments

Optical benches, interferometers, telescope structural components, and laser cavity supports use Invar because optical alignment must hold across the small temperature swings typical of laboratory and field environments. The same dimensional stability requirements drive the use of Invar in metrology equipment, gauge blocks, length standards, and reference structures for coordinate measuring machines.

Scientific Instrumentation and Research

Beyond optical benches and metrology, Invar is specified in mass spectrometer components, vacuum chambers, particle accelerator hardware, satellite instrument structures, and ground-based telescope mounts. Anywhere a research instrument must withstand thermal expansion, Invar is on the short list of candidate materials.

LNG Tankers and Transfer Lines

Liquefied natural gas storage tanks and transfer lines operate at cryogenic temperatures around -162°C, and the structural shell must accommodate enormous thermal contraction as the system cools from ambient to operating temperature. 

Invar membrane construction is the standard solution for LNG containment because the alloy’s low expansion behavior minimizes thermal stress on the containment system and the surrounding hull structure. The same approach extends to cryogenic transfer lines and storage equipment.

Electrical Power Equipment

High voltage transmission lines incorporate Invar reinforcing wires within aluminum conductor cables (ACIR designs) to reduce sag at elevated operating temperatures. The Invar core holds dimensional stability as the aluminum heats up under load, which allows utilities to operate transmission lines closer to their thermal limits without violating clearance requirements. 

Invar also appears in transformer cores, electrical circuit breakers, and other power equipment where thermal cycling must not affect dimensional accuracy.

Bimetal Thermostats and Temperature Controls

Bimetal strips for thermostats and temperature regulators pair Invar with a higher-expansion alloy. As the assembly heats up, the differential expansion between the two materials produces the deflection that triggers a switch or actuator. The predictable, repeatable expansion behavior of Invar is what makes bimetal thermostat performance consistent across millions of cycles.

Precision Timekeeping

Pendulum clocks and clock balance wheels used Invar long before more exotic alloys became available, and the alloy is still specified for high-end mechanical timepieces and certain scientific clock applications. The same dimensional stability that keeps a pendulum’s period constant across temperature is what made Invar the original material of choice for precision timekeeping.

Telecommunications and Microwave Hardware

Radar cavity resonators, microwave filters, echo boxes, and mobile telephone hardware use Invar where the resonant frequency must remain stable across temperature variation. Even small dimensional changes shift the resonant frequency of a cavity, which is unacceptable in precision RF systems.

Other Applications

Additional uses include magnetic shielding, positioning devices, precision condenser blades, seals and spacers for specialized frames, electronic housings, and laser components. The common factor across all of these is a requirement for dimensional stability that ordinary structural alloys cannot meet.

Why Source Invar® from EFINEA Metals

EFINEA Metals has supplied controlled-expansion alloys to high-technology industries since 1965. As an Invar 36 supplier with inventory across four ISO 9001:2015 and AS9100D-certified facilities in New Jersey and California, we ship standard stock within 24 hours of order confirmation. Our in-house precision cutting, shearing, slitting, and waterjet services support prototype quantities and production runs. Every shipment includes mill certifications, and our metallurgists are available to consult on grade selection, specification interpretation, and fabrication guidance.

To request a quote, download the EFINEA Invar® or Free-Cut Invar “36”® data sheets from our Technical Library, or to speak with a sales engineer, call 800-348-6268.

Frequently Asked Questions About Invar®