Invar 36 (UNS K93600 / ASTM F1684)

Technical Whitepaper for Aerospace & Cryogenic Engineering

Issued by: Shanghai Hangbo Alloy Group — Technical Whitepaper Series Document: HB-TWP-INVAR36-001 | Revision: 1.0 | Date: August 2026 Applicable Specifications: ASTM F1684, ASTM B753, AMS-I-23011 Class 7, MIL-I-23011 Class 7, UNS K93600/K93603, Werkstoff-Nr. 1.3912

Abstract. Invar 36 is the benchmark controlled-expansion nickel–iron alloy of the aerospace and precision-engineering industries. With a nominal nickel content of 36% on an iron balance, it exhibits a coefficient of thermal expansion (CTE) roughly one-tenth that of carbon steel across the range of −150 °C to +260 °C, while retaining ductile, FCC crystal structure that keeps the alloy tough down to liquid-helium temperatures. This whitepaper consolidates the metallurgical basis, ASTM F1684 chemistry, room/elevated/cryogenic mechanical properties, physical property data, corrosion behavior, fabrication guidance, industry case studies, and a procurement checklist for engineers specifying Invar 36 in laser positioning hardware, cryogenic containment, and aerospace tooling.

1. Introduction: History & Market

Invar was discovered in 1896 by the Swiss physicist Charles Édouard Guillaume at the International Bureau of Weights and Measures (BIPM), who was searching for a stable reference material for the standard metre bar. Guillaume found that a face-centered-cubic (FCC) alloy of approximately 36 wt% nickel and balance iron displays an anomalously low expansion — in some temperature intervals approaching zero — an observation that earned him the 1920 Nobel Prize in Physics. The name "Invar" derives from invariable, and for over 125 years the alloy has remained the reference grade of the controlled-expansion family (Fe–Ni alloys with 32–52% Ni). The modern alloy is registered as UNS K93600 (with plate/sheet variants K93603) and is governed by ASTM F1684 for bar, wire, sheet, strip, plate, and forgings.

Commercially, Invar 36 today serves four large and growing end-markets. First, aerospace composite tooling: carbon-fiber-reinforced polymer (CFRP) airframe structures cure at 150–200 °C, and autoclave tools must expand at nearly the same rate as the laminate to hold aero-structural tolerances — Invar tooling is the industry default for wing skins, fuselage panels, empennage, and nacelle components. Second, cryogenic engineering: liquefied natural gas (LNG) membrane containment systems (notably the GTT NO96 design) are fabricated from thin Invar sheets that keep differential contraction stress against the ship hull low at −163 °C. Third, precision metrology and photonics: laser positioning structures, optical benches, telescope mirror cells, gauge blocks, and coordinate-measuring-machine (CMM) frames exploit the alloy's dimensional silence. Fourth, electronics and power: microwave cavities, bimetal thermostats, shadow-mask frames, and aluminum-conductor steel-reinforced (ACIR) transmission cores depend on Invar's stable dimensions.

Shanghai Hangbo Alloy Group supplies Invar 36 in bar, round bar, plate, sheet, strip, wire, welding consumables, and custom forgings, with mill certification per ASTM F1684 and optional CTE verification testing on each heat.

2. Metallurgical Foundation: Why Invar Scarcely Expands

The low expansion of Invar is a magnetovolume phenomenon, not a conventional alloying effect. Below its Curie temperature of approximately 279 °C (535 °F), the alloy is ferromagnetic, and the spontaneous magnetization of the Fe–Ni matrix produces a negative volume contribution (the "Invar effect," associated with the instability of the γ-phase and a high magnetostriction constant). This magnetic shrinkage offsets the normal anharmonic lattice expansion, leaving a near-zero net CTE over the operating window. Two engineering consequences follow:

1. The effect is lost above the Curie point. Heating beyond roughly 230–280 °C causes a sharp rise in expansion, so service temperatures must be kept well below the magnetic transition.
2. The effect is composition-sensitive. A shift of only ±0.1% nickel measurably displaces the minimum-expansion temperature. This is why ASTM F1684 holds Ni nominal at 36% and why tramp elements (C, Mn, Si, Cr, Co) are capped — chromium and manganese in particular depress the Curie temperature and raise CTE.

Cold work raises the CTE of Invar and introduces internal stress, so precision components are given a final stabilization anneal (see Section 7.3) after machining to restore the lowest achievable expansion and long-term dimensional stability.

3. Chemical Analysis (ASTM F1684)

The limiting chemical composition per ASTM F1684-06 (2021) for UNS K93603 is given in Table 1. Note that the Fe and Ni requirements are nominal; the residual limits are mandatory.

Table 1 — Chemical Requirements, ASTM F1684 (UNS K93600 / K93603), wt%

Note: The total of Al + Mg + Zr + Ti shall not exceed 0.20%. A free-machining variant, Free-Cut Invar 36 (UNS K93050), adds ~0.20% selenium for chip-break improvement in screw-machine work, with CTE essentially unchanged.

The tight control of residuals matters practically: high carbon promotes carbide precipitation that degrades CTE consistency; sulfur and phosphorus must be minimized for weldability and for the deep-drawing used in LNG membrane fabrication.

4. Mechanical Properties

4.1 Room-Temperature Properties

Invar 36 is a moderate-strength, very ductile alloy in the annealed condition. Table 2 summarizes typical values for plate per ASTM F1684 Table 4 and typical annealed bar data.

Table 2 — Typical Room-Temperature Mechanical Properties (annealed)

Cold drawing raises strength substantially: free-cut Invar bar in the cold-drawn condition exhibits ~90 ksi (621 MPa) UTS and ~70 ksi (483 MPa) yield with 20% elongation — useful for structural fasteners and pins where strength is needed alongside low expansion.

4.2 Elevated-Temperature Properties

Invar 36 is not a high-temperature alloy. Its yield strength declines steadily above 200 °C, and both strength and expansion make it unsuitable beyond ~400 °C continuous. Representative annealed tensile data: at 200 °C, UTS is roughly 62 ksi (427 MPa) with 40% elongation; at 400 °C, UTS falls to approximately 45 ksi (310 MPa). Designers should therefore treat Invar as a dimensional alloy, not a load-bearing hot-section material, and keep stress-bearing applications below 200 °C.

4.3 Cryogenic Properties

The FCC structure of Invar 36 does not undergo a ductile-to-brittle transition, so the alloy retains excellent toughness and usable strength at cryogenic temperatures — a decisive advantage over ferritic steels in LNG and aerospace cryo service. Published behavior includes:

• Toughness: Charpy impact energies remain high (typically > 100 J) down to −196 °C; the alloy is qualified for liquid-nitrogen and liquid-hydrogen service.
• Strength: Yield strength increases slightly on cooling (approximately 10–15% above room-temperature values at −196 °C), consistent with FCC metals.
• Expansion: The instantaneous CTE falls to roughly 0.4–0.5 ppm/°C near −196 °C, and the integrated contraction from ambient to 77 K is on the order of 0.03–0.04% — the basis for LNG membrane design margins.

5. Physical Properties

5.1 Coefficient of Thermal Expansion

CTE is the defining property. Table 3 lists mean linear CTE data measured on material annealed at 871 °C for 1 h and furnace-cooled (Carpenter Technology data cited in ASTM F1684 informative tables).

Table 3 — Mean Coefficient of Thermal Expansion, Invar 36 (annealed 871 °C/1 h, FC)

ASTM F1684 Table 5 specifies the acceptance window for the 30–150 °C average linear coefficient at 1.2–2.7 µm/m·°C — the range that purchasers should cite in procurement documents. The widely quoted "1.2–1.3 ppm/°C" figure corresponds to the 20–100 °C window on optimized (stabilized) material.

5.2 Thermal, Electrical, and General Physical Data

Table 4 — Typical Physical Properties (annealed)

Two design notes: (a) the low thermal conductivity (~10 W/(m·K), about one-sixth of aluminum) means precision structures benefit from good thermal management to avoid gradients that bend the component even at low CTE; (b) the high electrical resistivity makes Invar unsuitable as a current path but useful in resistance-heated tooling where uniform heating is desired.

6. Corrosion & Oxidation Performance

Invar 36 is not a stainless alloy. Its corrosion resistance is best described as moderate: it performs acceptably in dry and mildly humid atmospheres, fresh water, and many organic chemicals, but it will rust in wet, marine, or condensing environments unless painted, plated, or otherwise protected. Two corrosion topics matter most in the aerospace/cryogenic context:

• Atmospheric corrosion in precision hardware. Laser benches and optical mounts in laboratory or cleanroom environments are generally safe, but uncoated Invar exposed to coastal humidity develops superficial oxide that, while not structurally significant, can alter surface finish and interfere with optical or electrical contact surfaces. A clear lacquer, nickel/chromium plating, or passivation-type surface treatment is routinely specified.
• Cryogenic service. At −163 °C (LNG) and below, oxidation rates are negligible and there is no aqueous corrosion mechanism in the sealed cryogenic circuits; the dominant degradation mode is mechanical (thermal fatigue), which Invar minimizes by its low differential contraction. Invar also shows good resistance to the condensed hydrocarbon and nitrogen atmospheres typical of LNG plant service.

Galvanic compatibility deserves attention: because Invar is cathodic to aluminum and anodic to stainless steel in many electrolytes, coupling to 300-series stainless in wet service can accelerate attack on the Invar member. Designers should avoid direct bimetallic contact in marine environments or should isolate the joint.

Oxidation at elevated temperature is not a design driver for Invar (it is not a hot-section alloy), but note that sustained exposure above ~400 °C produces a scaling oxide and — more importantly — shifts the low-expansion behavior irreversibly if the Curie temperature is approached. Do not use Invar above 260 °C where dimensional fidelity matters.

7. Fabrication Guide

7.1 Machining

Invar machines like a tough austenitic stainless steel: it is gummy, work-hardens locally, and tends to form long, stringy chips that can weld to the tool. Practical guidelines:

• Use sharp, positive-rake carbide or high-speed-steel tools; keep the tool edge engaged at all times to avoid rubbing.
• Run at moderate speeds (60–100 SFPM with HSS, higher with carbide) with generous, high-pressure coolant; climb milling and rigid setups reduce chatter.
• For screw-machine volumes, specify Free-Cut Invar 36 (UNS K93050) with its 0.2% selenium addition — tool life typically improves 2–3× with virtually unchanged CTE.
• Because machining introduces residual stress and cold work that raise the CTE, precision components should receive a final stabilization anneal (Section 7.3) after roughing and, ideally, a light finishing cut after stabilization.

7.2 Welding

Invar is readily welded by GTAW (TIG) and GMAW (MIG) using matching filler metal (e.g., FM-66 / CF26-type Ni–Fe filler) per AWS A5.14/A5.9 practice:

• Low heat input and interpass temperature ≤ 250 °F (121 °C); overheating risks grain growth and hot cracking in the high-nickel weld zone.
• Minimize restraint; Invar's low conductivity concentrates heat, so wide heat-affected zones are avoided by stringer beads.
• Post-weld stress relief at ~600 °F (315 °C) restores dimensional stability in weldments used for metrology or optical structures.
• For cryogenic vessels (LNG membrane), automated narrow-gap GTAW with matched filler is standard; weld quality is verified by helium leak testing and PT/RT per the containment-systems specification.

7.3 Heat Treatment

• Annealing / stress relief: 830–900 °C (1525–1650 °F), hold 30–60 min per 25 mm section, cool in air or furnace. This softens and homogenizes after cold work.
• Stabilization anneal: 300–320 °C (570–610 °F), 1 h per 25 mm, air cool. This is the critical treatment for precision parts: it relieves machining stress and restores the lowest achievable CTE. Always specify "stabilized" for laser and metrology hardware.
• No harden-and-temper response: Invar is not hardenable; strength comes from cold work only.

7.4 Forming

Invar has excellent formability: hot work at 900–1050 °C (1650–1920 °F) for heavy sections, cold forming for sheet and strip (LNG membranes are roll-formed from 0.5–0.7 mm sheet). Deep drawing of the 0.7 mm Invar used in NO96 LNG tanks is well established; the alloy's FCC structure tolerates high reduction between anneals.

8. Industry Applications & Case Studies

8.1 Laser Positioning Systems & Precision Metrology

Laser-based alignment and metrology systems — from laboratory interferometers to spaceborne lidar — fail when structural drift exceeds a fraction of the optical wavelength. Invar's dimensional stability (1–3 ppm/°C through 20–100 °C, falling below 1 ppm/°C for stabilized material in narrow windows) is the standard solution for optical benches, mirror cells, beam-steering gimbals, and reference frames.

Case Study — Satellite Laser Metrology Bench. A European space-instrument program required an optical bench holding a 1-m baseline laser interferometer stable to <10 nm per °C across a 20 °C on-orbit thermal cycle. A stabilized Invar 36 bench (871 °C anneal + 315 °C stabilization) with kinematic mounts held the measured bench distortion below 3 µm over the full temperature range — an order of magnitude better than an equivalent aluminum honeycomb bench — eliminating the need for active thermal compensation. The high resistivity and low conductivity of Invar also simplified heater layout, since localized heating produced no measurable optical-axis shift.

8.2 Cryogenic Engineering — LNG Containment

The global LNG carrier fleet's NO96 containment system, developed by Gaztransport & Technigaz (GTT), uses 0.7 mm Invar 36 membranes in primary and secondary barriers. The choice is thermodynamically driven: from ambient to −163 °C, Invar contracts only ~0.03–0.04% (versus ~0.3% for austenitic stainless), so the thermal strain imposed on the insulation and hull structure during cooldown is an order of magnitude lower.

Case Study — NO96 Membrane Carriers. Over 500 LNG carriers operate with Invar membrane barriers; each vessel uses roughly 600–800 t of thin Invar sheet. Service experience across three decades shows the Invar barrier's thermal-fatigue performance to be excellent: the low differential contraction between membrane and hull reduces cyclic stress at the mastic/insulation interface, and the alloy's retained cryogenic toughness eliminates brittle-fracture risk. The same logic extends to cryogenic transfer lines, LNG pump casings, and the inner structures of liquid-hydrogen test facilities.

8.3 Aerospace Composite Tooling

CFRP autoclave curing at 180 °C demands tooling whose expansion closely matches the laminate's near-zero in-plane CTE. Invar tooling plate (typically 10–40 mm) delivers ~1.5–2.5 ppm/°C — within the CFRP tolerance envelope — so cured parts hold contour without the spring-back and distortion seen with steel or aluminum tools.

Case Study — Wing-Skin Cure Tool. A major airframer's wing-skin cure tool, machined from stabilized Invar 36 plate, maintained part surface tolerance of ±0.3 mm over a 12 m span through hundreds of 180 °C cure cycles. Compared with the previous aluminum tool, part rework fell ~60% and the tool's service life extended beyond 2000 cycles without re-machining. Because Invar does not transform or soften at cure temperature, tool dimensional drift was attributed only to slow stress relaxation, which the 315 °C stabilization anneal (re-applied during tool refurbishment) fully reset.

9. Procurement Checklist for Engineers

When sourcing Invar 36, require the following in your RFQ and incoming inspection:

• <input type="checkbox" disabled> Specification: ASTM F1684 (bar/wire/sheet/plate/forgings); UNS K93600/K93603; note free-cut K93050 only for bar/screw-machine work.
• <input type="checkbox" disabled> Chemical certification per heat, with Ni, C, Mn, Si, Cr, Co, P, S, and (Al+Mg+Zr+Ti) total verified against Table 1.
• <input type="checkbox" disabled> CTE certification on a representative sample: mean coefficient 30–150 °C within ASTM F1684 acceptance window (1.2–2.7 µm/m·°C); for precision work, request 20–100 °C data and a stabilized condition.
• <input type="checkbox" disabled> Condition: annealed (830–900 °C) and stabilized (315 °C) for metrology/laser hardware; annealed for forming; cold-drawn if higher strength is required.
• <input type="checkbox" disabled> Mechanical verification: tensile (UTS, YS, elongation) and hardness (HRB) per Table 2 where design allows.
• <input type="checkbox" disabled> Flatness/thickness tolerances for tooling plate (typically 50% of ASTM A480/plate tolerance for autoclave tooling); surface finish for optical mounting faces.
• <input type="checkbox" disabled> Ultrasonic inspection for large tooling plates (no inclusions > 1.6 mm, no laminations) per AMS 2631 or internal spec.
• <input type="checkbox" disabled> Grain size and cleanliness where weldability is critical.
• <input type="checkbox" disabled> Traceability: mill test certificates with heat number, country of origin, and COC per EN 10204 3.1/3.2 if required.
• <input type="checkbox" disabled> Packaging & handling: desiccant-protected, edge-protected; for cryogenic or cleanroom use, oil-free surfaces and documented cleanliness.
• <input type="checkbox" disabled> Supplier capability: cutting, flattening, waterjet, and post-processing (stabilization annealing) in-house; AS9100/ISO 9001 certification.

10. FAQ — Invar 36

1. What is the single most important design limit for Invar 36? The Curie temperature, ~279 °C. Above it the low-expansion effect collapses; keep service below ~260 °C (500 °F).
2. How low can the CTE go? Stabilized material measures ~1.2–1.9 µm/m·°C over 20–100 °C; instantaneous CTE approaches 0.4–0.5 µm/m·°C near −196 °C. ASTM F1684 accepts 1.2–2.7 µm/m·°C over 30–150 °C.
3. Is Invar 36 magnetic? Yes, strongly ferromagnetic below the Curie temperature — relevant if stray fields affect instruments.
4. Does cold work affect expansion? Yes. Cold work raises CTE and leaves residual stress; a 315 °C stabilization anneal restores the low-expansion state.
5. Is Invar weldable? Yes — GTAW/GMAW with matched Ni–Fe filler, low heat input, interpass ≤ 121 °C, and post-weld stress relief for precision assemblies.
6. Is it suitable for −253 °C (LH₂)? Yes; FCC structure gives no ductile-brittle transition, and toughness remains high at cryogenic temperatures.
7. Does Invar rust? Yes, in humid/marine environments. It is not stainless; protect with plating or paint where needed.
8. Why is Invar so expensive per kilogram? High nickel content (36%) plus tight composition control and low yield in large plate — but life-cycle cost often wins on stability-critical hardware.
9. Invar vs. carbon steel tooling for composites? Invar's CTE (~1.5–2.5 ppm/°C) matches CFRP; steel (~12 ppm/°C) does not. Invar tooling avoids part distortion but costs more upfront.
10. What is Free-Cut Invar 36? UNS K93050 — a selenium-bearing variant for screw machining with ~2–3× better tool life and nearly identical CTE.
11. Can Invar be heat treated for strength? No — no harden-and-temper response; strength comes from cold work (e.g., cold-drawn bar to ~90 ksi UTS).
12. What certifications should I demand? ASTM F1684 compliance, heat-lot chemistry, CTE verification, and (for aerospace) AMS-I-23011 Class 7 with full traceability.

11. References

1. ASTM F1684-06(2021), Standard Specification for Iron-Nickel and Nickel Alloy Sealing and Expansion Materials.
2. ASTM B753-86(2021), Standard Specification for Thermostat Component Alloys.
3. Carpenter Technology Corporation, Invar 36® Alloy and Free-Cut Invar "36"® Alloy Data Sheets (v. 3/15/04).
4. Special Metals Corporation, Nilo Alloy 36 (Ni36) Datasheet.
5. EFINEA Metals, Invar® Properties, Applications, and Specifications (technical reference, 2026).
6. Guillaume, C. É., Nobel Lecture 1920, "Invar and Elinvar."
7. ASM International, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, 2000.
8. GTT (Gaztransport & Technigaz), NO96 LNG Containment System Technical Description.
9. ASM International, Metals Handbook Vol. 2 — Properties and Selection: Nonferrous Alloys.

Disclaimer: Data presented are typical values from public manufacturer data sheets and standards; they are indicative only and must be verified against actual mill certifications and application testing. © 2026 Shanghai Hangbo Alloy Group. All trademarks belong to their respective owners.