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Invar 36 / (1.3912) / UNS K93600

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Invar 36, also widely known as Nilo Alloy 36, is a nickel-iron alloy. The material consists essentially of 36% nickel and 64% iron. The fundamental property of this alloy is its very low thermal expansion, which is why it is used in many measuring and control instruments. Because it expands so little it is frequently used in fields such as aerospace. It is also very often used in various composite materials, in reference lengths, in measuring and metrology instruments, in parts requiring high precision and in thermostat shafts. The material is also designated UNS K93600 and UNS K93601.

Machinability: Moderate difficulty can be expected in terms of machinability. Machining, welding and cold forming are generally possible, but they require particular care.​‌​​‌​

Machining: It can be machined by conventional techniques such as milling, turning and drilling. It is important, however, to use suitable cutting fluids in order to minimise the effects of temperature change and to obtain a good surface finish.

Welding: Welding can be carried out, although Invar 36 can present some difficulties. Heat treatment may be required after welding in order to preserve the original properties of the alloy.​‌​​‌​

Cold forming: Cold forming can be applied, but care should be taken in operations requiring high precision.

Hot forming: Hot working and forming can be carried out, but care and proper temperature control are important during the operation.​‌​​‌​

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What Invar 36 Is — and Why “Zero Expansion” Is the Wrong Phrase

Invar 36 (UNS K93600 / K93601 / K93603 · W.Nr. 1.3912 · EN designation Ni36 · FeNi36 · Chinese 4J36) is a binary iron-nickel alloy, nominally 36 % nickel, balance iron. It contains no meaningful chromium, no meaningful molybdenum, does not precipitation harden, and is never bought for strength. It exists for one property: an extraordinarily low coefficient of thermal expansion near room temperature. Charles Édouard Guillaume discovered it in 1895 and received the 1920 Nobel Prize in Physics for it.​‌​​‌​

The physics of the effect also explains every one of the alloy’s limits. The Invar effect arises from spontaneous volume magnetostriction: magnetic ordering swells the lattice in the direction opposite to normal thermal expansion, and the two effects roughly cancel. This is a magnetic phenomenon. As magnetic order weakens the cancellation degrades, and above the Curie temperature it disappears entirely — the alloy then expands like any ordinary iron-nickel. Low expansion is therefore not a material constant but a behaviour valid inside a narrow temperature window.

This is the warning that must be published. The 1.2–1.5 × 10⁻⁶/K figure datasheets put on the cover belongs to the 20–100 °C range. The same alloy gives 5.5 × 10⁻⁶/K over 20–300 °C and 10.1 × 10⁻⁶/K over 20–500 °C — at 500 °C Invar 36 is practically indistinguishable from ordinary steel. Publishing a single expansion number for this alloy is a fatal error.​‌​​‌​

Trade names — not all of them are the same material​‌​​‌​

Invar® is a registered trademark of ArcelorMittal/Aperam; Nilo® 36 is Special Metals, Pernifer® 36 / VDM® Alloy 36 is VDM Metals, Invar® M93 is Aperam’s cryogenic grade. The Chinese equivalent is 4J36. All belong to the Fe–36 Ni family, but carbon and impurity ceilings differ from mill to mill — and that is exactly what governs dimensional stability. LNG membrane grades such as Aperam Invar M93 are produced and qualified as 0.7 mm strip through a separate route; they are not the same product as general-purpose Invar 36 plate.

Standards by Product Form — and What Happened in 2024​‌​​‌​

Do not quote Invar 36 without reading this section. The alloy’s position in the ASTM system changed fundamentally in 2024, and the large majority of distributor pages still publish the old picture.​‌​​‌​

What the withdrawal of F1684 means in practice​‌​​‌​

Withdrawal does not ban the alloy; it removes the specification basis. The consequences are concrete: (1) “conforms to ASTM F1684” is no longer a reference to an in-force standard — the purchase order must cite it with its edition year (“F1684-06(2021), withdrawn edition”); (2) mills are moving to their own internal specifications and carbon, sulphur and silicon ceilings are diverging between mills; (3) third-party inspection is now performed against your order text, not against an ASTM table. Practical advice: write the chemical ranges, the heat treatment route and the expansion acceptance band EXPLICITLY into the order confirmation. The same reasoning applies to ASTM F15 (Kovar) and ASTM F30 (Fe-Ni sealing alloys) — all three were withdrawn together in 2024.

Code Acceptance and Temperature Limits — Here the Limit Is Physics, Not Code​‌​​‌​

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No published ASME pressure-vessel allowable stress table for Invar 36 could be verified. Unlike the Ni-Cr-Mo corrosion alloys, this material is not marketed as an ASME Section VIII material; do not look for a maximum code temperature — there isn’t one. Invar 36’s real limits are metallurgical, and they are collected below.

Product Forms With NO Standard — the Commercially Valuable Section

After the withdrawal of F1684 this section covers nearly the whole alloy. These are the sentences a sales engineer should have memorised.​‌​​‌​

Chemical Composition​‌​​‌​

Invar 36 is a binary alloy: nickel and iron. Everything else is an impurity under a ceiling — and those impurities are exactly what governs dimensional stability.

The one sentence of chemistry that reaches purchasing

In Invar 36 the purpose of chemistry is not corrosion but dimension. Carbon and interstitial impurities redistribute over time, and this produces a measurable dimensional drift. The published range is striking: commercial Invar materials have been reported to change dimension by 1.5 to 27 ppm per year, and worst cases as high as +11.0 ppm per day at 20 to 70 °C have been reported. The same literature shows that with ultra-low carbon and impurities plus the correct stabilisation route, 1 ppm per year is achievable. If you are buying a metrology frame or a lithography stage, demand the CARBON value and the stabilisation records, not just the chemical range.​‌​​‌​

Mechanical Properties

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Invar 36 is a soft alloy and is never bought for strength. Do not mix the two tables below: the first is typical annealed data versus temperature, the second is cold-worked tempers.

Physical Properties

Heat Treatment and Dimensional Stability​‌​​‌​

In Invar 36 heat treatment is done for DIMENSION, not for strength. In this alloy this section replaces the solution-annealing section of a corrosion alloy as the critical one.

Temporal instability — with numbers

Invar 36 changes dimension on its own over time, and that is not a defect but the nature of the alloy. The published range: commercial Invar 36 materials have shown 1.5 to 27 ppm per year of dimensional change, and in the worst reported cases values as high as 11.0 ppm per day at 20 to 70 °C. The change is a carbon-dependent phenomenon. When carbon and the Mn/Si/P/S/Al impurities are each held below 0.01 % and the three-step route above is applied, the literature reports 1 ppm per year stability together with <1 × 10⁻⁶/°C expansion. Commercial Invar 36 is not that material. On a one-metre optical bench, 27 ppm per year means 27 micrometres per year — unacceptable for most precision optics. For dimensionally critical work, demand the stabilisation records and the carbon value on the certificate.​‌​​‌​

A second and less known effect: magnetic history. Invar is ferromagnetic and has a positive saturation magnetostriction of about 4 ppm. An Invar part that has passed through a strong magnetic field, or been machined on a magnetic chuck, can show a measurable permanent dimensional change after the field is removed. Magnetic chucks, magnetic lifters and magnetic particle inspection should not be used on dimensionally critical Invar parts; if they have been, the part should be demagnetised and re-stabilised.

Welding​‌​​‌​

Invar 36 is weldable — but “weldable” carries an unusual meaning here. The problem is not strength; it is the EXPANSION of the weld bead.

Machining — Invar Is Difficult to Machine; Say So Up Front

Invar 36 is NOT an easy material to machine, and saying so at the quotation stage beats saying so at the delivery stage. The alloy work-hardens aggressively like austenitic stainless steel, produces stringy, gummy chips that birdnest around the tool and block coolant from the cutting zone. With one fifth the thermal conductivity of steel, heat accumulates at the tool tip.​‌​​‌​

The single most common mistake is letting the tool dwell or ride on the surface. That creates a glazed, work-hardened surface, and restarting a cut in it is very difficult. Interrupted cuts, dwelling and successive thin cuts are the three things to avoid absolutely.

Corrosion — Invar 36 Is NOT Stainless

This is the most frequently skipped and most expensive section on this page. Invar 36 is not a corrosion alloy. It rusts. Having nickel in the name does not change that.​‌​​‌​

Why it rusts — the metallurgy in one sentence

Invar 36 contains no meaningful chromium (ceiling ≤0.50 %, at most mills ≤0.25 %, and it is an impurity ceiling, not a deliberate addition). It forms no passive oxide film. The nickel content is 36 %, enough to give an austenitic lattice but nowhere near enough to passivate. For corrosion purposes, think of Invar 36 as a low-alloy steel — not mechanically, but as regards rust.​‌​​‌​

Where it is GOOD

It is good in dry atmosphere at room temperature (mill statement). In controlled environments — a climate-controlled metrology lab, an optical bench under dry nitrogen, a spacecraft structure in vacuum — Invar 36 poses no corrosion problem. It is good in cryogenic service, in dry LNG and in dry nitrogen: the liquid hydrocarbon is not corrosive and there is no moisture. One mill rates humidity resistance as “good” on a four-level comparative scale — but that is a relative rating, not an absolute claim of resistance.​‌​​‌​

WHERE IT FAILS — the list that must be published

1. Humid or wet atmosphere. The mill statement is explicit: “in humid or moist atmospheres, corrosion can occur in the form of rust.” Condensation in storage, rain in transit, sweating in the shop — all produce surface rust.
2. Seawater and chlorides. No resistance whatsoever. Do not expose Invar 36 to seawater, salt spray or chloride-bearing process fluid.
3. Acids. With no chromium and no molybdenum it is unprotected in both reducing and oxidising acids. This is not an acid alloy.
4. Fingerprint corrosion. On precision Invar surfaces bare-hand contact creates measurable pitting. Hand perspiration contains chloride and organic acids. Invar must not be handled without gloves.
5. Composite tooling — the most common commercial failure. Autoclave steam, release agents and volatiles from prepreg resin produce rust and staining on the Invar surface. A rusted tool face is copied directly onto the part surface. Invar composite tools do not last without coating or continuous maintenance.
6. Long outdoor storage. Left in the open, Invar plate rusts within weeks. VCI packaging, dry storage and a protective oil film are mandatory.
7. Galvanic couples. In a wet environment, Invar 36 in contact with stainless steel, titanium or carbon-fibre composite becomes the anode. The Invar–carbon-fibre couple is a known problem in composite tooling; use an insulating interlayer.​‌​​‌​

Protection methods

Published and field-proven methods: electrolytic or electroless nickel plating (the most common; it also eases welding and soldering), chromium plating (for tool faces), paint and resin coatings, dry nitrogen blanketing and VCI packaging. A caution when choosing a coating: the coating thickness and the coating’s own expansion are a measurable error source on thin, precision sections, and hydrogen from the plating bath can cause trouble in a part that has not been stress relieved. No numerical pitting or crevice corrosion data (PREN, critical pitting temperature, etc.) could be found for Invar 36 — do not publish such a number for this alloy; it would not be meaningful anyway.​‌​​‌​

Frequently Asked Questions

Your datasheet says 1.5 × 10⁻⁶/K. Our part reaches 250 °C. Is that a problem?​‌​​‌​

Yes, a serious one — and it is the single most common design error made with Invar 36.
That 1.5 × 10⁻⁶/K figure is a mean value for the 20–100 °C range. Invar’s low expansion arises from a magnetic phenomenon and disappears as magnetic order weakens. The point where the expansion curve bends — the inflection point — is at 220 °C; the Curie temperature is 279 °C (some publishers put 230 °C on that line, most likely the inflection point mislabelled).
In your range the real numbers are: 2.6 over 20–200 °C, 3.5 over 20–250 °C, 5.5 × 10⁻⁶/K over 20–300 °C. So for a part reaching 250 °C, Invar’s effective expansion is two to three times the catalogue figure. At 300 °C it is 3.7 times. At 500 °C it is 10.1, practically indistinguishable from carbon steel (~12).
What to do: if your range exceeds 200 °C, Invar 36 is the wrong alloy. If you need a match to glass or ceramic, alloy 42 (inflection 370 °C) or Kovar (inflection 450 °C) is the right choice. If you simply need low expansion above 200 °C, no Fe-Ni controlled-expansion alloy will give you what you want, and the design must solve it with expansion compensation. Treating one catalogue number as a design constant is the most expensive mistake in this family.

We bought an Invar 36 composite tool and it rusted in six months. Is the material defective?​‌​​‌​

Almost certainly not. Invar 36 rusts; that is not a material defect but the definition of the alloy.
Invar 36 contains no meaningful chromium: the ceiling is 0.50 %, at most mills 0.25 %, and it is an impurity ceiling, not a deliberate addition. No passive oxide film forms. The mills’ own wording is that it resists dry atmosphere at room temperature and that in humid or wet atmosphere corrosion can occur in the form of rust.
Composite tooling is the most demanding application in this respect: autoclave steam, release chemistry, prepreg volatiles and shop humidity all appear together. On top of that comes the galvanic couple between Invar and carbon fibre — in a wet environment Invar is the anode and Invar is what dissolves.
The right question is not “is the material defective” but “what was the protection plan”. Approaches that work: electroless nickel or chromium plating of the tool face; cleaning and a protective film after every cycle; dry storage and a VCI cover when idle; breaking direct contact with carbon fibre using an insulating interlayer; and never touching the tool face with bare hands (hand perspiration contains chloride and pits Invar).
One more caution: coating thickness and the coating’s own thermal expansion are a measurable error source on thin precision surfaces. Account for the coating when the tool tolerance is calculated, not as an afterthought.

Our specification says “Invar 36 to ASTM F1684”. We hear the standard has been withdrawn. Now what?​‌​​‌​

You heard correctly. ASTM F1684 was withdrawn in 2024 with no replacement published. In the same round ASTM F15 (Kovar) and ASTM F30 (Fe-Ni sealing alloys) were also withdrawn — so the entire controlled-expansion family lost its ASTM basis at once.
This does not mean the alloy is banned. The material is the same material and the mills are the same mills. What changed is what the certificate is issued against.
Do three things in practice. First, cite the standard with its edition year in the purchase text: “chemical requirements per ASTM F1684-06(2021) (withdrawn edition), Table 1”. That is a clear and defensible reference for an inspector. Second, move the work the standard used to do into the order text: chemical ranges (especially carbon), the heat treatment route (annealed, or three-step stabilised), the expansion measurement range and acceptance criterion, temper, grain size and hardness. Third, require an EN 10204 3.1 certificate and, for critical work, add 3.2 independent inspection.
What not to do is accept without question an old supplier certificate that still says “conforms to F1684” and overlook that chemical ranges now differ from mill to mill. After a withdrawal those differences tend to widen, because no document imposes a common ceiling any more.

Invar 36 or Kovar? We are sealing a metal lid to a ceramic package.​‌​​‌​

Kovar. Invar 36 is the wrong alloy here, and the reason is precisely what makes Invar valuable: it expands TOO LITTLE.
In a hermetic glass-to-metal or ceramic-to-metal joint the goal is not low expansion but MATCHED expansion. Alumina expands at ~6–8 × 10⁻⁶/K, borosilicate glass at ~4.6–5.3 × 10⁻⁶/K. Kovar gives 5.1 × 10⁻⁶/K over 20–300 °C and — more importantly — the SHAPE of its curve is tailored to the glass curve. Invar 36 may look like 5.5 over the same range, but it is only 1.5 over 20–100 °C; so during cooldown the glass and the metal follow very different paths and stress accumulates in the joint. The result is a cracked seal or a leak.
The second difference is surface chemistry. For glass sealing, Kovar is given a wet hydrogen decarburising anneal followed by a controlled nickel–cobalt oxide film; that film is what the glass wets and dissolves. The cobalt in Kovar makes that oxide easier to melt and dissolve in the glass — Invar contains no cobalt and the mechanism does not work.
In short: for dimensional stability (optical benches, metrology frames, composite tools, LNG membranes) use Invar 36. For a hermetic seal to glass or ceramic use Kovar (borosilicate) or alloy 42 (silicon and alumina). The two are not interchangeable, and calling them both “low-expansion Fe-Ni alloys” hides the entire difference.

Common Datasheet Errors — Check Before You Order​‌​​‌​

1. Publishing a single expansion number — the most common and most expensive error. “Invar 36 = 1.2 × 10⁻⁶/K” is true for 20–100 °C and hides that it is 5.5 over 20–300 °C and 10.1 over 20–500 °C. In this alloy expansion is a table, not a number.
2. Confusing the Curie temperature with the inflection point. The inflection point is 220 °C (where the curve bends); the Curie temperature is 279 °C (where magnetic order is lost). Some publishers put 230 °C on the Curie line; that is most likely the inflection point mislabelled. For design, 220 °C is the number that matters — expansion starts climbing from there.
3. Confusing “Invar 42” with Invar 36. Alloy 42 (UNS K94100 · W.Nr. 1.3917 · NILO 42) is sometimes sold as “Invar 42”. It is not the same alloy: 42 % nickel, 4.0–4.7 × 10⁻⁶/K over 30–300 °C, inflection point 370 °C. Its use is different too — it is a glass/ceramic matching and lead-frame alloy, not a dimensional stability alloy. Read the UNS number on the order and the certificate, not the trade name.
4. Missing the ASTM withdrawals. F1684, F15 and F30 were all withdrawn in 2024 with no replacement. Most pages that still say “conforms to ASTM F1684” do not know this. Cite the edition year.
5. UNS number confusion. K93600 and K93601 are general Invar 36; K93603 is the grade the withdrawn ASTM F1684 covered; K93050 is the free-machining grade, covered in F1684 for bar only; K93500 is Fe–32 Ni–5 Co — not Invar 36. One mill states “K93600 to K93603 depending on application”; read the number on the certificate.
6. Material number confusion. 1.3912 = Invar 36 (Ni36), 1.3917 = alloy 42, 1.3981 = Kovar. All three are called “low-expansion Fe-Ni” and all three do entirely different jobs.
7. Density conflict. Published values range over 8.05 · 8.11 · 8.13 g/cm³ — an 80 kg difference per cubic metre. If you price by weight, state which density you used in the quotation.
8. Melting point row shift. 1427–1430 °C is correct for Invar 36. The 1450 °C that appears on some pages is the Kovar and alloy 48 value.
9. Decimal error in the carbon ceiling. One European mill sheet shows carbon at ≤0.4 %; every other source gives ≤0.10 % or lower. This is almost certainly a decimal error, and carbon is the element that governs dimensional stability in this alloy — question the number.
10. Unit error in a heat-treatment temperature. A widely mirrored Invar page gives the full anneal as “1550 °F or 2 hours at 1350 °C”. 1350 °C is just below the melting point and is plainly a value that should read °F. Do not anneal Invar at 1350 °C.
11. Confusing annealing with stabilisation. 790 °C / air cool is a softening anneal and gives no dimensional stability. Dimensional stability comes from the three-step route: 815–830 °C + WATER QUENCH + 315 °C for 1 h + 93–100 °C for 24–48 h. Any datasheet that puts both routes on the same line is misleading.
12. Assuming Invar is “stainless”. Because nickel is in the name it is assumed not to rust. It has no chromium (a ≤0.50 % impurity ceiling), forms no passive film and rusts in a humid environment.
13. Omitting the magnetic behaviour. Invar 36 is ferromagnetic with ~4 ppm saturation magnetostriction. A part machined on a magnetic chuck or exposed to a strong field can show a permanent dimensional change. Do not use magnetic workholding on dimensionally critical Invar.
14. Omitting the filler’s expansion. “Invar is weldable” is true but incomplete. There is no AWS class; nickel-base fillers (FM 61, FM 82, FM 92) will make a sound weld but do not match expansion. On a dimensionally critical joint use a matched Fe–36 Ni filler and demand its expansion data.
15. The phrase “zero expansion”. It is zero at no temperature. The lowest published mean is 1.3 × 10⁻⁶/K over −100 to 20 °C. Do not sell a zero-expansion material.

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