Alloy 36 (Invar): Low Thermal Expansion Properties & Applications

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Overview

Alloy 36, universally known by its trade name Invar (UNS K93600 / W.Nr. 1.3912), is a binary nickel-iron alloy containing approximately 36% nickel that exhibits an extraordinarily low coefficient of thermal expansion (CTE) near room temperature. Discovered in 1896 by Swiss physicist Charles Edouard Guillaume — a discovery that earned him the 1920 Nobel Prize in Physics — Invar's CTE of approximately 1.2 × 10^-6/K between 20°C and 100°C is roughly one-tenth that of carbon steel and one-twentieth that of aluminum. This unique property arises from a magnetostrictive volume change that counteracts normal thermal expansion, a phenomenon known as the Invar effect.

What makes Alloy 36 remarkable is that its low expansion is not merely a laboratory curiosity — it is a robust, repeatable, and industrially scalable property. For over a century, engineers have relied on Invar wherever dimensional stability under temperature change is non-negotiable. Modern applications span from the massive LNG carrier membrane tanks that transport natural gas across oceans, to the micron-level precision of optical interferometer mounts, to the composite curing tools that shape carbon-fiber aircraft wings.

The alloy's composition is deceptively simple: approximately 64% iron and 36% nickel, with tight controls on trace elements. The Invar effect peaks at precisely this nickel content; deviations as small as 1% can increase the CTE by 50% or more. Carbon, manganese, and silicon are kept low to avoid forming phases that disrupt the magnetostrictive mechanism. This compositional sensitivity is both a blessing — the alloy is straightforward to melt and process — and a challenge, as manufacturing requires precise chemistry control to guarantee the specified expansion properties.

In the annealed condition, Alloy 36 is relatively soft and ductile, with tensile strength around 450-550 MPa. Cold working can substantially increase strength, though with some sacrifice in the low-expansion behavior. The alloy is fully austenitic and non-magnetic at room temperature, becoming ferromagnetic below its Curie temperature of approximately 280°C. Above this temperature, the Invar effect disappears and the CTE rises to values typical of conventional nickel alloys.

At Hangbo Alloy Group, Alloy 36 is produced under rigorous quality control to meet the demanding specifications of the LNG, aerospace, electronics, and precision engineering industries. We supply round bars, plates, sheets, strips, seamless tubes, forgings, and welding wire, all supported by full material certifications and optional third-party inspection.

Chemical Composition (ASTM F1684)

The chemistry of Alloy 36 is deceptively simple but critically precise. The nickel content must fall within the narrow range of 35.0-37.0% to achieve the Invar effect. Even small deviations from this window dramatically increase the CTE. Impurity elements are tightly restricted because they can form second phases or alter the magnetic behavior that underlies the low-expansion mechanism.

Element	Min %	Max %	Role / Effect
Iron (Fe)	Balance	Balance	Matrix element; forms the austenitic FCC structure with nickel
Nickel (Ni)	35.0	37.0	Critical element; the 36% level produces the Invar effect via magnetostriction
Carbon (C)	—	0.05	Kept low to avoid carbide precipitation that disrupts magnetic behavior
Manganese (Mn)	—	0.35	Deoxidizer; excess can alter Curie temperature and CTE
Silicon (Si)	—	0.35	Deoxidizer; excess increases CTE and promotes sigma phase
Chromium (Cr)	—	0.25	Residual element; not intentionally added; raises CTE
Cobalt (Co)	—	0.50	Residual element; can raise Curie temperature slightly
Phosphorus (P)	—	0.015	Impurity; must be minimized for ductility
Sulfur (S)	—	0.015	Impurity; must be minimized for hot workability

Physical Properties

Alloy 36 has an austenitic face-centered cubic (FCC) crystal structure at room temperature. Its most defining physical characteristic is the extremely low thermal expansion coefficient, but several other properties are important for design and application engineering. The alloy is non-magnetic at room temperature in the fully annealed condition, becoming ferromagnetic upon cooling below the Curie point.

Property	Value	Unit
Density (20°C)	8.05	g/cm³
Melting Range	1425 - 1435	°C
Curie Temperature	280	°C
Specific Heat (20°C)	515	J/kg·K
Thermal Conductivity (20°C)	10.5	W/m·K
Electrical Resistivity (20°C)	0.78	μΩ·m
Modulus of Elasticity (20°C)	141	GPa
Poisson's Ratio	0.29	—
Magnetic Permeability (annealed)	< 1.01	—

Coefficient of Thermal Expansion (CTE)

The CTE of Alloy 36 is not constant across all temperature ranges — it increases with temperature and rises sharply near the Curie point. Below are the mean CTE values for key temperature intervals that are relevant to industrial applications. For critical designs, actual measured CTE data for the specific heat treatment condition and product form should be obtained.

Temperature Range (°C)	Mean CTE (×10-6/K)	Notes
-200 to 20	1.5	Cryogenic range; excellent stability for LNG applications
-50 to 20	1.2	Sub-zero applications
20 to 100	1.2 - 1.5	Room temperature range; lowest CTE region
20 to 150	1.8 - 2.2	Slight increase above 100°C
20 to 200	2.5 - 3.0	Moderate increase; approaching Curie point
20 to 250	4.0 - 5.0	Significant increase near Curie temperature
20 to 300	6.0 - 7.0	Above Curie point; Invar effect essentially lost
20 to 400	10.0 - 11.0	Comparable to conventional austenitic alloys

For comparison, the CTE of common materials at room temperature: carbon steel ~11.7, stainless steel 304 ~17.3, aluminum 6061 ~23.6, and copper ~16.5 (all ×10^-6/K). Alloy 36's value of 1.2 is dramatically lower, making it the material of choice when dimensional changes with temperature must be minimized.

Mechanical Properties at Room Temperature

Alloy 36 is a moderately strong, highly ductile material in the annealed condition. Unlike precipitation-hardening alloys, its strength is derived primarily from solid-solution strengthening and work hardening. Cold working can substantially increase tensile and yield strength, but with some effect on the CTE — heavily cold-worked material may show slightly higher expansion. The values below represent typical properties for annealed and cold-worked conditions.

Property	Annealed	25% Cold Worked	50% Cold Worked
Tensile Strength	450 - 550 MPa (65 - 80 ksi)	600 - 680 MPa (87 - 99 ksi)	700 - 800 MPa (102 - 116 ksi)
Yield Strength (0.2% offset)	240 - 310 MPa (35 - 45 ksi)	480 - 550 MPa (70 - 80 ksi)	580 - 680 MPa (84 - 99 ksi)
Elongation in 2 inches	30 - 40%	12 - 18%	5 - 10%
Reduction of Area	60 - 70%	45 - 55%	35 - 45%
Hardness	80 - 90 HRB	90 - 98 HRB	22 - 28 HRC
Charpy V-notch Impact	> 100 J	60 - 80 J	30 - 50 J

Elevated Temperature Behavior

Alloy 36 is not a high-temperature alloy in the conventional sense. Its primary value — the low CTE — diminishes as temperature rises and effectively disappears above the Curie point of ~280°C. However, understanding its behavior across the full temperature spectrum is essential for proper application engineering.

Key Temperature Thresholds:

• -200 to 100°C: Optimal performance zone. CTE remains below 2.0 × 10^-6/K. Excellent for cryogenic and ambient-temperature precision applications.
• 100 to 200°C: Transition zone. CTE gradually increases to 2.5-3.0 × 10^-6/K. Still significantly lower than conventional metals, but designers must account for the non-linear expansion curve.
• 200 to 280°C: Near-Curie zone. CTE rises rapidly to 4-5 × 10^-6/K. Not recommended for precision dimensional stability applications in this range.
• Above 280°C: Above Curie temperature. The Invar effect is lost. CTE approaches 10 × 10^-6/K, comparable to conventional nickel alloys. The material retains reasonable strength up to ~450°C but without any expansion advantage.

Corrosion Resistance

Alloy 36 provides moderate atmospheric corrosion resistance but is not intended for aggressive chemical environments. Because the alloy contains no chromium (unlike stainless steels or nickel-chromium superalloys), it does not form a passive chromium oxide protective layer. In practice, this means:

Corrosion Behavior in Various Environments:

• Indoor / Dry Atmosphere: Excellent. No significant corrosion over extended periods. Suitable for precision instruments and electronics in controlled environments.
• Rural Outdoor Atmosphere: Fair to good. A thin oxide film forms but corrosion rates are low. Surface discoloration may occur over years of exposure.
• Marine / Humid Atmosphere: Poor without protection. Surface rusting can occur. Protective coatings (painting, electroplating, electroless nickel plating) are recommended for outdoor or high-humidity service.
• Fresh Water: Moderate. Pitting may occur in stagnant conditions. Flowing fresh water is less aggressive.
• Acids and Alkalis: Not recommended. The alloy is attacked by most mineral acids. It should not be used in chemical process environments.
• Cryogenic Environments: Excellent. At LNG temperatures (-162°C), corrosion is negligible. This is one reason Invar is ideal for LNG containment — the environment is both cryogenic and non-corrosive.

For applications requiring corrosion resistance in addition to low expansion, consider surface treatments or alternative alloys. Electroless nickel plating, chromium plating, or epoxy coatings are common protective measures for Invar components used in humid or mildly corrosive environments.

Applications

Alloy 36's unique low-expansion characteristic has made it indispensable across industries where dimensional stability under temperature variation is critical. From the largest LNG carriers to the smallest scientific instruments, Invar solves a fundamental engineering challenge: how to keep things the same size when the temperature changes.

LNG Containment and Cryogenics

• LNG Carrier Membrane Tanks: Alloy 36 is the primary material for the GTT Mark III and NO96 membrane containment systems used in over 70% of the world's LNG carrier fleet. Invar membranes, typically 0.7 mm thick, line the cargo holds and accommodate the extreme thermal contraction from ambient to -162°C without cracking or buckling. The alloy's low CTE minimizes thermal stresses at the welded joints between membrane panels.
• LNG Storage Tanks: Land-based LNG storage facilities use Invar as the primary barrier material in full-containment and membrane-type tanks.
• Cryogenic Transfer Lines: Invar pipelines and expansion joints maintain dimensional stability during the extreme temperature swings of cryogenic fluid transfer.

Precision Instruments and Metrology

• Seismographs and Geophysical Instruments: The low CTE ensures that pendulum lengths and sensor positions remain stable despite ambient temperature changes.
• Laser Interferometer Components: Optical mounts, reference flats, and metrology frames use Invar to maintain sub-micron alignment.
• Precision Length Standards: Surveying tapes, calibration bars, and gauge blocks made from Invar maintain accurate dimensions across normal temperature ranges.
• Clock Pendulums: Historical use in precision chronometers and pendulum clocks where length stability with temperature is essential for accuracy.

Aerospace Composite Tooling

• Composite Curing Molds: Invar tooling is the industry standard for manufacturing carbon-fiber reinforced polymer (CFRP) components for aircraft. During autoclave curing at 180°C, the mold's low CTE closely matches that of the carbon fiber composite, minimizing residual stresses and ensuring part accuracy.
• Wing Skin and Fuselage Panel Tools: Large Invar molds for Airbus A350 and Boeing 787 composite structures.

Electronics and Semiconductor

• Shadow Masks: Historically used in CRT display manufacturing where thermal expansion during the deposition process must be minimized.
• Lead Frames: Some specialized semiconductor packages use Invar or Invar-clad materials for thermal expansion matching.
• Substrates and Carriers: For precision electronic assemblies requiring thermal stability.

Other Applications

• Bimetallic Strips and Thermostats: Invar is the low-expansion side of bimetallic strips paired with high-expansion alloys like manganese-copper-nickel for temperature-sensing and control devices.
• Glass-to-Metal Seals: Where the expansion of Invar can be matched to specific glass compositions.
• Magnetic Shielding: High magnetic permeability at low temperatures makes it useful for cryogenic magnetic shielding in scientific instruments.

Available Product Forms

Hangbo Alloy Group manufactures and supplies Alloy 36 (Invar) in a comprehensive range of product forms, conforming to ASTM F1684 and other applicable standards. All material is supplied with full mill test certificates and is available with third-party inspection (SGS, TUV, BV, DNV) upon request.

• Round Bars: Diameters from 6 mm to 350 mm, hot-rolled, forged, or cold-drawn, in annealed or cold-worked condition. Standard lengths 3-6 meters; custom lengths available.
• Plates & Sheets: Thickness 0.5 mm to 50 mm, width up to 2500 mm, length up to 6000 mm. Hot-rolled or cold-rolled, annealed and pickled or bright annealed finish. Invar membrane sheets for LNG applications produced to GTT specifications.
• Precision Strips: Thickness 0.05 mm to 3.0 mm, width up to 650 mm. Tight thickness tolerance (±0.005 mm) for electronic and precision applications. Available in coil form.
• Seamless Tubes & Pipes: OD 6 mm to 219 mm, various wall thicknesses per customer specification. Cold-drawn or cold-rolled, annealed.
• Forgings: Custom open-die and ring-rolled forgings including flanges, discs, rings, shafts, and complex shapes per customer drawings. Maximum forging weight up to 5000 kg.
• Welding Wire: Matching composition filler wire for GTAW (TIG) and GMAW (MIG) welding of Invar components. Diameters 0.8 mm to 5.0 mm, precision layer-wound on spools. Conforms to AWS A5.14 ERNiFe-1 classification.

Welding and Fabrication

Alloy 36 can be successfully welded using standard arc welding processes, but careful attention to filler metal selection and heat input is essential to preserve the low-expansion properties in the weld zone.

Recommended Welding Processes:

• Gas Tungsten Arc Welding (GTAW / TIG): Preferred for thin sections and precision work. Provides excellent control of weld pool chemistry. Use AWS A5.14 ERNiFe-1 filler metal.
• Gas Metal Arc Welding (GMAW / MIG): Suitable for thicker sections and production welding. Use matching ERNiFe-1 filler wire.
• Resistance Welding: Spot and seam welding are feasible for thin sheet assemblies.
• Laser Welding: Used for precision Invar components where minimal heat input is critical.

Key Welding Considerations:

• Filler Metal: Always use matching composition filler (ERNiFe-1). The nickel content of the filler must closely match the base metal to maintain the Invar effect across the weld. Dilution with non-matching filler will locally increase the CTE.
• Heat Input: Keep heat input low to moderate. Excessive heat can cause grain growth in the heat-affected zone and promote segregation.
• Preheat and Interpass Temperature: Generally not required for thin sections. For thick sections, a modest preheat of 50-100°C may be beneficial. Keep interpass temperature below 150°C.
• Post-Weld Heat Treatment: Not normally required for Alloy 36. Stress relief at 600-700°C may be applied if residual stresses are a concern, but verify that the CTE is not adversely affected.
• Cleaning: Thoroughly clean all surfaces before welding. Remove oils, greases, and oxides. Use stainless steel wire brushes dedicated to nickel alloys to avoid iron contamination.

Machining Recommendations

Alloy 36 is machinable but presents some challenges due to its high nickel content and tendency to work-harden. In the annealed condition, the alloy is relatively soft and ductile, which can lead to built-up edge on cutting tools and stringy chip formation. The following practices are recommended for optimal results:

Operation	Tool Material	Speed (m/min)	Feed (mm/rev)	Depth of Cut (mm)
Turning (rough)	Cemented Carbide (C2/C3)	25 - 40	0.25 - 0.50	2.0 - 5.0
Turning (finish)	Cemented Carbide (C2/C3)	35 - 55	0.10 - 0.20	0.2 - 1.0
Milling	Cemented Carbide	15 - 25	0.05 - 0.15 per tooth	0.5 - 2.0
Drilling	HSS-Co or Carbide	10 - 18	0.05 - 0.15	—
Tapping	HSS-Co	3 - 6	—	—

Use generous amounts of water-soluble coolant or cutting oil. Rigid tool setups are essential to avoid chatter. Cold-drawn material generally machines better than annealed material due to reduced ductility. Chip breakers on cutting inserts help manage the continuous chips typical of nickel alloys.

Related Standards

Standard	Description
ASTM F1684	Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
ASTM B753	Standard Specification for Thermostat Component Alloys (covers Invar for bimetallic strips)
AMS I-23011	Military Specification for Invar Alloy (Nickel-Iron Alloy, Low Expansion)
EN 10095	Heat resisting steels and nickel alloys — European standard
DIN 1715	German standard for thermostat metals (historical, now partially superseded by EN)
GB/T 4339	Chinese standard for test method of thermal expansion characteristic parameters
JIS H2501	Japanese standard for phosphor bronze and nickel silver sheets and plates
AWS A5.14	Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods (ERNiFe-1)

Comparison with Related Alloys

Property	Alloy 36 (Invar)	Alloy 42	Alloy 48	Alloy 52 (Kovar)
Nickel Content	36%	42%	48%	29% Ni + 17% Co
UNS Number	K93600	K94100	K94800	K94610
CTE (20-100°C)	1.2	5.3	8.5	5.5
CTE (20-400°C)	10.0	7.0	9.5	5.1
Density (g/cm³)	8.05	8.12	8.20	8.36
Curie Temp (°C)	280	380	470	435
Primary Use	Lowest expansion; precision, LNG	Glass/silicon seal; lead frames	Glass seal; reed switches	Glass/ceramic seal; hermetic packages

CTE values in ×10^-6/K. Alloy 36 provides the lowest room-temperature expansion of any commercial metal, while Alloy 42 and Kovar are optimized for expansion matching with specific glasses and ceramics in electronic packaging.

Frequently Asked Questions (FAQ)

The following questions address the most common inquiries from engineers, buyers, and technical professionals considering Alloy 36 for their applications.

Contact Us for Alloy 36 (Invar)

Hangbo Alloy Group maintains mill-direct supply of Alloy 36 (Invar) round bars, plates, sheets, precision strips, seamless tubes, forgings, and welding wire. Our material conforms to ASTM F1684 and is supported by complete mill test certificates. We serve customers in the LNG, aerospace, precision instrument, and electronics industries with reliable quality and competitive pricing.

For quotations, material certifications, or technical consultation, contact our sales team or call +86-136-1165-6360. We typically respond within 10 minutes.