Invar 36: Low-Expansion Alloy Properties & Product Forms
Basic Description
Sagiant Piping Industries supports enquiries for Invar 36 / FeNi36 (UNS K93600, K93601, K93603; Werkstoff 1.3912) as bar, sheet, plate, strip, wire and forgings. Invar 36 is a ~36% nickel-iron alloy whose coefficient of thermal expansion is very low near room temperature but rises with temperature, so the useful low-expansion range is confirmed for the actual interval rather than assumed flat. It is used for precision instruments, optical supports, composite tooling, electronic components and LNG membranes where the required CTE interval and documentation are defined. Material is offered by form and condition against supported references, with acceptance limits and editions confirmed per order and EN 10204 3.1 or 3.2 documentation; expansion testing is agreed as routine, optional or project-specific with a stated interval and method. Invar and NILO are trademarks of their owners; Sagiant is an independent supplier and stockist, and the verified range is mainly bar and flat products rather than seamless pipe.
Invar 36 (FeNi36): chemistry, thermal expansion, properties and RFQ data
Invar 36 (FeNi36) composition (nominal vs limiting), thermal expansion by temperature interval, condition-specific mechanical and physical reference data, standards status by form and fabrication/stabilisation guidance. Producer reference values are shown separately from specification acceptance limits confirmed on the certificate; a mean CTE applies only to its stated interval.
Technical Specification
| Parameter | Specification |
|---|---|
| Material | Invar 36 / FeNi36 (UNS K93600, K93601, K93603; Werkstoff 1.3912); ~36% nickel-iron controlled-expansion alloy |
| Key property | Very low coefficient of thermal expansion near room temperature (interval-dependent) |
| Product forms | Bar, sheet, strip/foil, plate, wire, forgings and tube — by actual route |
| Conditions | Annealed, stress-relieved or dimensionally stabilised, per specification |
| Basis | Producer reference data; specification acceptance limits, form, condition and edition confirmed per order |
“Invar 36” is a ~36% nickel-iron alloy, not a family of every controlled-expansion alloy; Super Invar and free-machining variants are distinct records. Invar and NILO are trademarks of their owners; Sagiant is an independent supplier and stockist, and the verified range is mainly bar and flat products rather than seamless pipe.
Chemical Composition (wt %)
| Element | wt % |
|---|---|
| Nickel, Ni | 35.0–38.0 |
| Iron, Fe | Remainder |
| Carbon, C | ≤0.10 (a narrower purchase limit such as ≤0.05 may apply) |
| Manganese, Mn | ≤0.60 |
| Silicon, Si | ≤0.35 |
| Phosphorus, P | ≤0.025 |
| Sulfur, S | ≤0.025 |
| Chromium, Cr | ≤0.50 |
| Molybdenum, Mo | ≤0.50 |
| Cobalt, Co | ≤1.0 |
Phosphorus and sulfur are shown separately rather than as a combined limit, and “not specified” is distinguished from zero. Free-machining and Super Invar variants carry their own chemistry tables. Specification limits, nominal composition and heat analysis are different datasets; the certificate governs.
Grade & designation cross-reference
| Record | Treatment |
|---|---|
| Conventional Invar 36 / FeNi36 | Main grade; full form- and condition-specific data |
| UNS K93600 / K93601 / K93603 | UNS identity and melt route stated separately — K93603 is not simply “the vacuum-melted grade”; producer associations vary |
| NILO 36 | Producer trade-name cross-reference, with the producer’s own limits and documentation |
| INVAR M93 | Separate producer variant if supplied; cryogenic qualification & form identified |
| Free-Cut / free-machining Invar | Separate free-machining material, not an ordinary temper; exact current designation & chemistry verified |
| Super Invar 32-5 | Related Fe-Ni-Co alloy, not an equivalent grade of FeNi36 |
| Alloy 39 / 42 / 48, Kovar | Related controlled-expansion alloys — comparison/link only, not Invar 36 |
A matching designation does not establish identical melt practice, heat treatment, product form, CTE acceptance interval or code/customer approval; each equivalent carries its own source, chemistry comparison and substitution restrictions.
Thermal Expansion
| Temperature interval | Mean CTE (×10⁻⁶/°C) |
|---|---|
| ~20–100°C | ~1.2–2.0 (very low) |
| 25–260°C | ~4.18 |
| 25–371°C | ~7.60 |
| Cryogenic (to reference) | Contraction stated as total relative length change for the actual interval |
Values are mean (not instantaneous) coefficients, direction- and stabilisation-dependent. Full length-change data are used where the thermal response is nonlinear, with reference temperature, endpoint, cycle history, specimen orientation, test method and typical-vs-acceptance status recorded. The useful low-expansion range is checked over the actual application interval — it does not stay flat to a universal cutoff.
Physical Properties
| Property | Value |
|---|---|
| Density | ~8.05 g/cm³ |
| Curie temperature | ~279°C (producer-specific) — distinct from the expansion-curve inflection |
| Magnetic response | Ferromagnetic below the Curie temperature (not treated as an expansion cutoff) |
| Modulus / resistivity / conductivity | By condition and temperature, per producer series |
| Melting range & specific heat | Per producer datasheet |
The former claim that “low expansion holds to the Curie point (~230°C)” is removed: the Curie temperature (~279°C) and the expansion-curve inflection are different concepts, and low-expansion performance is verified over the actual interval. Long-term dimensional drift, stabilisation history and thermal hysteresis are considered — low CTE alone does not establish long-term stability.
Mechanical Properties
| Property (typical, annealed) | Value |
|---|---|
| Tensile strength | ~450 MPa |
| 0.2% yield strength | ~275 MPa |
| Elongation | ~30% |
| Hardness | ~160 HB (scale/load stated) |
Cold-worked, stress-relieved/stabilised and welded material are separate rows where supported. Reduction of area, impact toughness, fatigue and cryogenic/welded-joint data are supplied with the applicable qualified dataset, otherwise identified as unavailable; typical results are not converted into minima, and hardness is not converted to tensile strength without a valid material-specific basis.
Standards by Product Form
| Reference | Status / scope |
|---|---|
| ASTM F1684 | Withdrawn (2024) — retained only as a clearly identified legacy contractual reference where explicitly accepted, not an unqualified current specification |
| ASTM B753 | Active, but scoped to thermostat-metal component alloys (bonded thermostat sheet/strip) — not a general Invar bar/plate/tube specification; state the alloy designation, edition & ordering conditions |
| AMS 5223 | Removed — the AMS 5223 series is a 42Ni-5Cr-Ti alloy strip, not FeNi36; no substitute is guessed |
| Melt route | Air-melted vs vacuum-melted stated separately from the UNS identity |
| Other references | Historical cryogenic references (e.g. A658/SA-658) treated as legacy until current status & project-code acceptance are established |
Blanket “ASTM/ASME/ANSI/API/MSS/EN” availability is not claimed; only supported references are listed, and a dimensional standard does not certify the material.
Available Forms, Sizes & Dimensions
| Form | Basis |
|---|---|
| Bar / rod | Diameter, length, tolerances, finish & condition |
| Sheet / strip / foil | Thickness, width, length/coil, thickness tolerance, flatness, surface & orientation |
| Plate | Thickness, width, length & tolerances |
| Wire | Diameter, temper & condition |
| Tube (where offered) | Actual OD × wall, length, straightness, ovality, seamless/welded, seam treatment, NDE/leak test & procurement spec — not a stainless pipe schedule |
| Forgings / fabricated parts | By drawing/route with dimensional acceptance |
No generic stainless schedule chart is presented as an Invar specification; a schedule is not a pressure rating, and where a contract adopts a dimensional schedule the standard, sizes and walls are stated separately from material and design requirements. Unsupported forms are marked “custom dimensions subject to technical review”.
Applications & Limitations
| Application | Selection reason / limit |
|---|---|
| Precision metrology & optical supports | Dimensional stability over a defined interval; dimensional aging may matter more than a large strength table |
| Composite / aerospace tooling | Low expansion at cure temperature — use the CTE for the actual tooling interval, not the 20–100°C value |
| Electronic & thermostat components | Controlled expansion matched to the assembly |
| LNG membranes / cryogenic | Low contraction at cryogenic temperature; LNG use does not establish hydrogen-system suitability or pressure-piping acceptance |
Generic boiler/refinery/HVAC applications are removed. Invar is not positioned as a general replacement for stainless steel or corrosion-resistant nickel alloys; storage/rust prevention and surface protection are advised, and corrosion data are given with medium, concentration, temperature and exposure where relevant.
Fabrication & Heat Treatment
| Operation | Guidance |
|---|---|
| Machining | Addresses gummy cutting behaviour with rigid, sharp tooling; surface finish & induced stress controlled |
| Forming & anneal | Intermediate anneals appropriate to thickness and reduction |
| Heat treatment | Separate softening anneal, stress relief and dimensional stabilisation cycles (atmosphere, time, cooling, section-size limits); rough-machine → stabilise → finish-machine where justified |
| Welding | Qualified process & filler (e.g. CF36 for NILO 36, distinct from filler 365) — matching “36% nickel” alone is insufficient; a generic stainless filler is not equivalent; distortion & cracking controlled |
| Verification | Weld-metal expansion compatibility checked, and final dimensional/CTE testing after processing steps that could alter performance |
Low expansion is verified after the fabrication and heat-treatment sequence, not assumed from base-metal data alone.
Testing & Documentation
| Item | Scope |
|---|---|
| Thermal expansion (CTE) | Stated as routine, optional or project-specific, with the interval, sampling, test method (e.g. ASTM E228) and certification arrangement identified |
| Mechanical | Room-temperature tensile and hardness per contracted method & acceptance criteria |
| PMI | Confirms alloy identity within the instrument/method capability; does not establish complete chemistry |
| Certification | EN 10204 3.1 (manufacturer) vs 3.2 (independent/third-party); document type does not inherently require every test |
Contracted tests, methods, intervals and acceptance criteria are agreed at ordering with heat/lot traceability rather than promised unconditionally.
Product Range
The invar 36 range can be supplied with related pipes, tubes, butt weld fittings, forged fittings, flanges, fasteners and custom fabricated items where required by the BOQ.
Applications
For dimensional references, see the Pipe Schedule & Fittings Guide. For related supply, review Pipes & Tubes, Butt Weld Fittings, Forged Fittings and Flanges.
Types of Products
| Type | Description |
|---|---|
| Pipes & Tubes | Seamless, welded and large-diameter supply |
| Fittings | Butt weld, forged and fabricated fittings |
| Flanges | Forged flanges and special machined items |
| Plates / Bars / Round Bars | Subject to grade and project availability |
Available Forms
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Frequently Asked Questions
What is the thermal expansion coefficient of Invar 36?
It depends on the temperature interval, and a mean coefficient applies only to its stated range. Near room temperature (~20–100°C) the mean CTE is very low (~1.2–2.0 ×10⁻⁶/°C), but it rises with temperature — for example about 4.18 ×10⁻⁶/°C over 25–260°C and about 7.60 ×10⁻⁶/°C over 25–371°C. The low near-room-temperature value must not be extrapolated to tooling temperatures; the useful interval is confirmed for the actual application, by dilatometry (e.g. ASTM E228) with stated uncertainty.
Does the low expansion hold up to the Curie temperature?
No — that conflates two different things. The expansion-curve inflection and the Curie temperature (about 279°C for a producer datasheet) are separate concepts, and expansion does not stay flat up to a universal cutoff. Low-expansion performance is checked over the actual temperature interval rather than assumed constant to the Curie point.
Is Invar 36 magnetic?
Yes — Invar 36 is ferromagnetic below its Curie temperature. Magnetic permeability, coercivity and saturation are given per condition where they matter to instrument design; magnetism is not described as an expansion cutoff.
What is the chemistry of Invar 36, and are K93600/K93603 interchangeable?
A producer limiting example (NILO 36) is roughly Ni 35–38, C ≤0.10, Mn ≤0.60, Si ≤0.35, P ≤0.025, S ≤0.025, with Fe as remainder — but that is not a universal acceptance table, and a narrower purchase limit (for example C ≤0.05) may apply. K93600, K93601 and K93603 are separate UNS identifiers with their own applicable requirements and melt routes; K93603 is not simply “the vacuum-melted grade”, and they are not automatic substitutes.
Which specifications apply to Invar 36?
Carefully, by scope: ASTM F1684 was withdrawn in 2024 and is kept only as an identified legacy reference where explicitly accepted; ASTM B753 is active but scoped to thermostat-metal (bonded bimetal) components, not general Invar bar/plate/tube; and AMS 5223 does not apply to Invar 36 at all (it is a 42Ni-5Cr-Ti alloy strip). Blanket ASTM/ASME/EN availability is not claimed — the applicable designation, edition and ordering conditions are confirmed per order.
Do you supply Invar 36 tube or pipe?
Tube can be offered where supported, described by actual OD × wall, length, construction (seamless/welded), seam treatment and testing — not a stainless pipe schedule. We do not publish a generic schedule chart as an Invar specification; a schedule is not a pressure rating, and unsupported tubular forms are marked “subject to technical review”. The verified range is mainly bar and flat products.
What information is needed for a quotation?
Provide the designation/producer variant, form and dimensions, the service temperature interval, any CTE limit and test requirement, final processing and dimensional-stability target, surface condition, the exact specification and edition, certification and traceability, and the applicable drawing or code. Availability and the accepted specification route are confirmed with the quotation.
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