Invar 36 Forgings Seamless rolled rings, flanges, round bars, discs, sleeves and shafts in FeNi36 controlled-expansion alloy. Open-die forged to your drawing in Jiangyin, China.

1. What Invar 36 is, and why the expansion coefficient is so low

Invar 36 is a binary iron-nickel alloy, nominally 36 % nickel with the balance iron, that expands roughly one tenth as much as carbon steel when heated. Its mean coefficient of thermal expansion is about 1.30 µm/m·°C between 20 °C and 93 °C, against roughly 12 for plain carbon steel and 17 for 304 stainless.

The effect is magnetic, not structural. In the ferromagnetic state, a magnetic volume contraction runs opposite to ordinary lattice expansion and cancels most of it. Charles Édouard Guillaume identified this in 1897 and won the 1920 Nobel Prize in Physics for it. The cancellation weakens as temperature rises and disappears entirely at the Curie point of 279 °C (535 °F), above which the alloy becomes paramagnetic and expands like any ordinary metal.

Two things follow from this, and both matter when you specify the alloy. First, the useful window is narrow: keep service temperature well below 200 °C if dimensional stability is the reason you chose the material. Second, nickel content has to be held tightly. A drift of a few tenths of a percent away from 36 % moves the inversion temperature and raises the expansion coefficient measurably. That is why the alloy is melted under close control and why the mill certificate matters more here than on a structural grade.

2. Designations, trade names and equivalents

Invar 36 is sold under many names. If a drawing calls for any of the following, Jiangyin Jiangnan Metal can forge it from the same material:

Invar® is a registered trademark of Aperam Alloys Imphy; Nilo® of Special Metals; Pernifer® of VDM Metals. Jiangyin Jiangnan Metal is not affiliated with these companies and supplies the generic ASTM F1684 alloy.

3. Chemical composition of Invar 36

Invar 36 is melted by EAF + VOD and remelted by ESR (electroslag remelting) to keep the nickel tight and the inclusion content low. Ladle and product analysis are both reported on the certificate.

Limits vary slightly between standard editions, mill practices and customer specifications. The values above are the ASTM F1684 baseline we work to. Certified heat analysis is issued on the EN 10204 3.1 or 3.2 certificate; if your drawing imposes tighter limits, state them on the enquiry and we will confirm before accepting the order.

4. Physical properties of Invar 36

5. Thermal expansion data: the number you are buying

Mean coefficient of thermal expansion, measured from 20 °C to the stated temperature, on material annealed at 871 °C and furnace cooled. This is the dataset plotted at the top of the page.

What that means in a real part. A 2000 mm Invar 36 optical bench warming from 20 °C to 40 °C grows about 0.05 mm. The same bench in carbon steel grows about 0.47 mm, and in 304 stainless about 0.69 mm. On an instrument holding micron-level alignment, that difference is the entire reason the alloy exists.

6. Mechanical properties of Invar 36 forgings

Invar 36 behaves mechanically like a mild structural steel. It is not a strength alloy, and nobody selects it for load capacity, but it is ductile, tough and readily formed. It retains good strength and toughness at cryogenic temperature, which is why it is used for LNG membrane containment and cryogenic transfer lines down to −196 °C. Impact testing at low temperature can be included in the test plan; state the temperature and acceptance criteria on the enquiry.

Cold work raises strength considerably but also raises the expansion coefficient and introduces residual stress that will relax later and move the part. For dimensionally critical work, forge, anneal and stabilise rather than cold work.

7. Invar 36 forms and sizes we forge

Everything below is made to order against a drawing or a dimension list. There is no standard catalogue. Open-die forging exists so you do not have to design around stock sizes.

Ranges are indicative of routine work, not hard limits. Send the drawing. Pieces outside these ranges are often still possible, and we will say so rather than quote something we cannot make.

Supply condition options: as-forged with machining allowance, rough-machined (turned and bored), or finish-machined to drawing. Heat-treatment condition, surface protection and marking are agreed at order.

8. How Invar 36 forgings are produced

Six stages from ingot to certified part. The two that decide whether the forging is any good are the heating step and the stabilising heat treatment.

1. Melting: EAF + VOD, then ESR. Nickel is held tightly to 36 % because a drift of a few tenths of a percent shifts the expansion coefficient. Electroslag remelting improves soundness and reduces segregation, which matters for large sections.
2. Heating: 1090–1180 °C (2000–2150 °F), fast, no soaking. This is the critical precaution with Invar 36. Long soaking lets sulphur from the furnace atmosphere penetrate the surface and causes checking and oxide penetration. Billets go in hot and come out working.
3. Open-die forging or ring rolling. Upsetting and drawing on the press for bars, shafts, discs and blocks; piercing and ring rolling for seamless rolled rings. Forging finishes above roughly 900 °C so the structure stays worked and the grain flow follows the part contour.
4. Heat treatment. Full anneal, then stabilising or stress relief depending on how tight the dimensional requirement is. See section 9.
5. Rough machining. Turning, boring and facing to the agreed allowance. A stress relief between roughing and finishing is standard on precision work.
6. Inspection and release. Chemistry, mechanical tests, hardness, dimensional report and ultrasonic inspection to EN 10228-3, SEP 1921 or ASTM A388, then the EN 10204 3.1 or 3.2 certificate.

9. Heat treatment of Invar 36

Two routes, chosen by how stable the part has to be. Both are available on order; state which one your drawing requires.

Route A: general forged parts

Add approximately one hour of soak per additional 25 mm (1 in) of section thickness above 25 mm. A 100 mm thick plate therefore needs about 5 hours at stress-relief temperature. Stress relief is performed between rough and finish machining, and again after any weld repair.

Route B: minimum expansivity, instrument work

The classic three-stage stabilisation treatment, used where the part must hold dimension to the micron over years:

1. Heat to 830 °C (1525 °F), hold 30 minutes, water quench.
2. Re-heat to 300 °C (570 °F), hold 1 hour, air cool.
3. Re-heat to 100 °C (212 °F), hold 48 hours, air cool.

10. Machining and welding Invar 36

Invar 36 has a reputation for being unpleasant to machine, and it is deserved. The alloy work hardens, its low thermal conductivity keeps heat at the cutting edge instead of carrying it away in the chip, and the chip is stringy and welds to the tool. Tool life is short and parts distort if the process is not controlled.

What works

• Sharp tools, always. Any wire edge or feathered edge causes galling immediately. Inspect and re-grind often.
• Rigid setups, positive rake. Short tools, minimum overhang, no chatter.
• Flood coolant. Controlling heat build-up is the biggest single factor in preventing warpage.
• Turning, HSS tooling: roughly 18–20 m/min (60–65 SFM), feed 0.075–0.11 mm/rev. Carbide runs 2–3× faster with 50–100 % more feed.
• Drilling: around 13 m/min (43 SFM); short drills, thinned web, 118–120° point, frequent peck to clear the galling chip.
• Stress relieve between roughing and finishing and take light finishing cuts. This is not optional on precision parts.

A free-machining variant with a controlled selenium addition (nominally 0.2 %) exists and greatly improves chip break and tool life, with little effect on the Invar behaviour. It is a valid choice when the part is mostly machining work. Ask if it suits your drawing.

Welding

Invar 36 welds by conventional GTAW, GMAW and SMAW methods. The main caution is not to overheat the weld pool, which causes spatter and porosity. A matching Invar filler is used where the joint must share the base metal's expansion behaviour. Full anneal after extensive welding, and stress relief after minor weld repair.

11. Where Invar 36 forgings are used

Every application below shares one requirement: a part that must not change length when the temperature changes.

• LNG and cryogenics

  Membrane containment for LNG carriers and storage tanks, transfer lines, cryogenic piping and vessel internals. Invar 36 keeps its toughness down to −196 °C and barely contracts on cooldown.

• Aerospace composite tooling

  Lay-up moulds and curing tools for CFRP structures. The tool must match the low expansion of the composite through the autoclave cycle or the part comes out wrong.

• Optics and laser systems

  Optical benches, laser cavity structures, mirror mounts, telescope trusses and interferometer frames where micron alignment must hold across the ambient range.

• Metrology and standards

  Gauge blocks, length standards, geodetic and seismic instruments, comparator frames, calibration fixtures and scientific instrument bases.

• Semiconductor and precision motion

  Positioning stages, lithography and inspection equipment frames, precision spindles, and structures carrying wafer-scale tolerances.

• Electronics and RF

  Radar and microwave cavity resonators, waveguides, echo boxes and filters for mobile telephony, magnetic shielding, small transformer cores and specialised housings.

• Thermostatic elements

  Bimetal thermostats and temperature regulators, where Invar 36 is bonded to a high-expansion alloy so that a temperature change produces controlled, repeatable motion.

• Valves and process equipment

  Valve bodies, seats, stems and blocks for ball, gate, globe, check and plug valves; forged tube sheets and flanges for heat exchangers and pressure vessels.

• Horology and instruments

  Pendulum rods, clock balance wheels, circuit-breaker components, precision condenser blades and dimensionally stable frames, seals and spacers.

12. Limitations you have to design around

Invar 36 is a specialist material with real drawbacks. Knowing them before you order saves a rejected forging later.

13. Invar 36 compared with the usual alternatives

If Invar 36 does not fit, one of these usually does. Coefficients are approximate mean values near room temperature.

Jiangyin Jiangnan Metal forges most of these grades. If you are still choosing between them, send the operating temperature range, the tolerance you need to hold and the load case, and we will tell you which one we would use, including when the answer is a cheaper alloy than Invar 36.

14. Testing, inspection and certification

Every Invar 36 forging leaves the factory with a document package. The scope below is standard; anything additional is agreed at order and priced in the quotation.

Certification. Forgings are released with an EN 10204 3.1 certificate issued against the mill heat, or EN 10204 3.2 witnessed by a third party such as TÜV, SGS, BV, Lloyd's Register or DNV when the order calls for it. Third-party inspection visits are arranged on request and the inspector's release is a condition of shipment on those orders.

Marking, heat traceability, packing specification and shipping documents follow the order requirements. Standard export packing is oiled, VCI-wrapped and crated for sea freight, which matters on an alloy with no corrosion resistance of its own.

15. How to request an Invar 36 quotation

Send a drawing to sales@steelforgepieces.com. A quotation normally comes back within 24 hours on working days. The more of the list below you include, the firmer the price and the fewer the follow-up emails.

1. Drawing or dimensions: PDF, DWG or STEP, or just OD × ID × thickness / length.
2. Grade and standard: Invar 36, or the equivalent designation your drawing uses (ASTM F1684, UNS K93600, 1.3912, 4J36).
3. Quantity, and whether repeat orders are expected.
4. Supply condition: as-forged, rough-machined or finish-machined.
5. Heat-treatment condition, and any guaranteed CTE band.
6. Test and certification requirements: EN 10204 3.1 or 3.2, UT class, any additional tests.
7. Delivery terms and destination port: EXW, FOB, CIF or DAP.
8. Required delivery date.

If you are early in design and do not have a drawing yet, describe the part, the temperature range and the tolerance you need to hold. We would rather help you get the geometry right than quote something that will not work.

16. Frequently asked questions about Invar 36

17. About the factory supplying these forgings

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory located at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The works produce forged rings, seamless rolled rings, flanges, round bars, discs, sleeves, tube sheets, shafts and near-net forged shapes in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, including controlled-expansion grades such as Invar 36.

Jiangyin sits in the Yangtze River delta between Shanghai and Nanjing, in the centre of China's forging and special-steel industry, with the ports of Shanghai and Zhangjiagang within easy reach for export shipment. Work is quoted against customer drawings and released with EN 10204 3.1 or 3.2 certification.

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