Controlled Expansion Alloys · Iron-Nickel · Forged to Drawing

Invar 42 Forgings: Alloy 42 / UNS K94100 / ASTM F30 / 4J42

• UNS K94100
• ASTM F30
• DIN 1.3917
• NiFe42 · Ni42
• 4J42 (GB/T 15018)
• NF A54-301
• 42H

Published: 12 May 2022 · Last updated: 20 August 2026 · Technically reviewed by the Jiangyin Jiangnan Metal Co., Ltd. Metallurgical Engineering Team · Reading time approx. 14 minutes

Nickel

41–42wt %

CTE 20–100 °C

5.3×10⁻⁶ /°C

CTE 20–300 °C

4.5–6.5×10⁻⁶ /°C

Curie point

≈360°C

Density

8.11g/cm³

Melting

1435°C

Anneal

850–1000°C

Max ring OD

2500mm

Max piece

8000kg

Quote in

24hours

What is Invar 42?

Invar 42 is a binary iron-nickel alloy of nominally 41–42% nickel, balance iron, bought for one property: a low, predictable coefficient of thermal expansion that matches the materials it is sealed, bonded or clamped to. It sits in the controlled-expansion family that begins with Invar 36 and rises through Invar 42, Alloy 46, Alloy 48 and Alloy 52 as nickel content increases.

The behaviour comes from the Invar effect. In face-centred-cubic iron-nickel alloys near 36% nickel, the normal thermal expansion of the crystal lattice is almost cancelled by a magnetostrictive contraction that occurs as ferromagnetic ordering weakens with rising temperature. The cancellation is strongest at 36% Ni and weakens in a controlled, repeatable way as nickel is added. At 41–42% Ni the residual expansion settles at roughly 5.3 ppm/°C. That is deliberately not the minimum available; it is the value that matches silicon, alumina and hard sealing glasses.

Two consequences follow from that mechanism, and between them they explain most field failures with this grade.

• The low expansion disappears above the inflection point (≈360 °C). The magnetostrictive contraction stops once the alloy passes its Curie temperature and turns paramagnetic. Above that, expansion climbs toward that of an ordinary austenitic alloy. The useful controlled-expansion range is therefore quoted as room temperature to about 300 °C. Assuming the low CTE still holds at 500 °C is a design error, not a tolerance issue.
• Cold work distorts the expansion curve. Residual strain from drawing, straightening or heavy machining shifts the measured coefficient away from the datasheet value. Invar 42 is supplied and used annealed for this reason, and precision parts receive a stabilising treatment after final machining.

There is essentially no chromium in the alloy, so oxidation resistance is modest and corrosion resistance is poor. Where a controlled-expansion part also has to survive an aggressive environment, the answer is plating (nickel or gold on lead frames), a coating, or a different grade. Changing the heat treatment will not fix it.

Why "Invar 42" Is a Name, Not a Standard

There is no standard called "Invar 42". Invar® is a trademark associated with the original 36% nickel alloy discovered by Charles Édouard Guillaume in 1896, work that won the 1920 Nobel Prize in Physics. Over a century of industrial use, "Invar" drifted into a generic shop word for the whole iron-nickel controlled-expansion family, and producers attached numbers to it to indicate nickel content. "Invar 42" is the result: a widely used, widely understood, and formally undefined name.

That matters commercially, because a purchase order carrying only a trade name cannot be certified against anything. The generic designations can:

In practice, when a drawing says "Invar 42" it means UNS K94100 in about 95% of cases. The residual 5%, where the designer meant the lowest-expansion grade and wrote the wrong number, or copied a legacy note without checking, is where the money is lost. The grade resolver below exists to close that gap in under a minute.

Invar 42 vs Invar 36: The Real Difference

Invar 36 and Invar 42 are different alloys with different nickel contents, different expansion coefficients, different Curie points and different applications. They are not grades of the same thing and they are not interchangeable. The headline number most engineers know is the 20–100 °C coefficient: 1.3 ppm/°C for Invar 36 against 5.3 ppm/°C for Invar 42, a factor of four.

What far fewer engineers know is that the gap closes as temperature rises, and above about 250 °C it very nearly disappears. It is a point that changes grade selection on any part that runs warm, and it is missing from most published datasheets.

Where each grade genuinely belongs

Invar 36 is specified where the least possible movement is wanted at or near ambient temperature: LNG containment membranes and transfer lines, large aerospace composite cure tooling, metrology frames, optical benches, precision instrument structures and shadow masks. Its problem is that it is soft, difficult to machine to a stable dimension, and loses its advantage as soon as the part runs warm. See our Invar 36 forgings page for that grade.

Invar 42 is specified where the expansion has to match something rather than be minimised: semiconductor lead frames matched to silicon, hard-glass and alumina seals, bimetal thermostat elements, thermostat rods, and dimensionally stable hardware that runs between 100 °C and 300 °C. Its higher Curie point makes it the more reliable choice across a wide operating range, and it is the cheaper of the two.

Kovar is specified when a borosilicate glass seal has to survive to 450 °C. It costs two to three times as much as Invar 42 because of the 17% cobalt, and below 300 °C it buys nothing that Invar 42 does not already provide.

What Are the Equivalent Designations of Invar 42?

Engineers reach this grade through at least fifteen different names, depending on the standards body, the producer and the decade the drawing was written. Every designation below refers to the same nominal Fe-42Ni controlled-expansion chemistry. Jiangyin Jiangnan Metal Co., Ltd. accepts purchase orders under all of them and supplies material certified to UNS K94100 / ASTM F30 with the equivalents cross-listed on the certificate.

What Is the Chemical Composition of Invar 42?

The chemistry is simple by design. Nickel sets the expansion coefficient; every other element is held low because residuals shift the expansion curve, degrade magnetic behaviour or interfere with glass wetting during sealing. Carbon is capped tightly because carbides both embrittle the alloy and disturb the expansion match.

There is one point of real practical importance here: the American and Chinese specifications set nickel differently. ASTM F30 states 41.0% nominal and explicitly allows the producer to adjust nickel so that the alloy meets the expansion requirement. YB/T 5235 sets a tighter band of 41.5–42.5% for 4J42 and tightens phosphorus and sulfur. Material certified to 4J42 therefore satisfies most drawings written to ASTM F30, but the reverse is not automatically true.

Our melting practice. Jiangyin Jiangnan Metal Co., Ltd. melts Invar 42 by EAF + VOD followed by ESR (electroslag remelting). VOD lowers carbon and dissolved gases; ESR refines the inclusion population and gives the directional solidification structure that produces a clean, uniform forging. For sealing and lead-frame work where surface oxide quality governs glass adhesion, VIM + VAR double-vacuum stock can be sourced instead. Specify this at RFQ stage, since it changes both price and lead time. Full ladle and product analyses are reported on the EN 10204 certificate.

What Is the Coefficient of Thermal Expansion of Invar 42?

The mean coefficient of thermal expansion of Invar 42 is approximately 5.3 × 10⁻⁶ /°C over 20–100 °C, and 4.5–6.5 × 10⁻⁶ /°C over 20–300 °C under ASTM F30 practice. In imperial units that is about 2.9 × 10⁻⁶ /°F over 70–212 °F. This is the number the alloy is bought for, and the number to verify on the certificate.

Four points follow from this table, all of them practical.

1. Mean is not instantaneous. Datasheet values are mean coefficients between room temperature and the stated upper temperature. If a part cycles between 150 °C and 250 °C, the mean coefficient over that interval is not the 20–300 °C figure. For tight seals, request the measured expansion curve rather than a single number.
2. The specification you order to changes the number you get. A drawing written to ASTM F30 accepts anything from 4.5 to 6.5 over 20–300 °C, a ±18% band. A drawing written to YB/T 5235 accepts 4.0 to 5.0. If your design cannot absorb that spread, state a tolerance on the coefficient explicitly rather than relying on either standard's default.
3. Heat-to-heat variation is real. A 0.5% shift in nickel moves the coefficient measurably. For precision assemblies, order every part from a single heat and state that requirement on the purchase order. We will reserve material accordingly.
4. Condition governs the value. A cold-drawn or heavily machined part will not measure at the annealed value. Anneal, then finish, then stabilise, in that order.

Physical and Mechanical Properties of Invar 42

Invar 42 is not a structural alloy. It is austenitic, cannot be hardened by heat treatment, and is used at modest stress levels. Mechanical properties matter mainly for handling, machining and forming rather than load capacity. Strength can only be raised by cold work, and cold work is precisely what destroys expansion accuracy, so in practice the annealed properties are the ones that apply to a finished part.

How Is Invar 42 Annealed and Heat Treated?

Invar 42 has no hardening transformation. The only heat treatments that matter remove cold work and stabilise dimensions, and both exist to protect the expansion coefficient.

What Invar 42 Forged Products Are Available?

Jiangyin Jiangnan Metal produces Invar 42 through three routes, selected by geometry and quantity. Open-die forging covers long shafts, blocks, tube sheets and large discs, wherever single-piece size matters more than repeatability. Seamless ring rolling produces rings from 200 mm to 2,500 mm outside diameter and is the normal choice for sealing rings, flange blanks, positioning rings and instrument frames. Near-net-shape forging is used where the die profile can remove 30–50% of the rough machining, a saving worth more on this grade than most, because Invar 42 is expensive per kilogram and slow to machine.

How Do You Forge, Machine and Weld Invar 42?

Forging

Invar 42 is hot-worked from approximately 1,100–1,200 °C, with the finishing temperature held above roughly 900 °C. It is single-phase austenitic with no transformation to work around, but it is sensitive to sulfur and to overheating. Sulfur pick-up from fuel or die lubricant causes hot shortness at the grain boundaries; soaking too close to the solidus coarsens the grain irreversibly. Furnaces are run clean and neutral to slightly reducing.

Forging reduction of at least 4:1 from the ingot breaks down the as-cast structure. For seamless rolled rings, the pierced blank goes through radial-axial rolling so grain flow follows the circumference. After the final blow the piece is slow-cooled and given a full anneal at 850–1,000 °C. That anneal, not the forging, is what establishes the expansion coefficient the customer will measure.

Machining

Machining behaviour is closest to austenitic stainless steel. The alloy is gummy, work-hardens under a rubbing tool, and produces long stringy chips. The rules are the ones used on 304 and 316:

• Sharp, positive-rake carbide tooling. Replace at the first sign of edge rounding rather than running the insert out.
• Turning speeds of roughly 25–45 m/min with coated carbide; heavy, positive, uninterrupted feed of 0.15–0.35 mm/rev.
• Never dwell. A tool that stops feeding while still in contact glazes the surface and work-hardens a layer the next pass has to cut through.
• Rigid setups and generous flood coolant. The alloy's low thermal conductivity concentrates heat at the cutting edge.
• Leave 2–4 mm of stock after roughing, apply an intermediate stress relief, then finish with light depths of cut to avoid re-introducing the cold work the anneal removed.

Welding and brazing

Invar 42 welds readily by GTAW (TIG), electron beam, laser and resistance methods using matching Fe-42Ni filler. Joint faces must be scrupulously clean, because sulfur, lead and low-melting-point contamination cause cracking. Preheat is not normally required. Where the joint must hold the same expansion behaviour as the parent metal, a post-weld anneal at 850–1,000 °C in a protective atmosphere is recommended; without it the weld and heat-affected zone expand along a different curve and the assembly distorts on thermal cycling.

For ceramic-to-metal assemblies, Invar 42 is normally joined with an active braze (Ti-containing Ag-Cu) or to a pre-metallised ceramic with a conventional Ag-Cu eutectic. The braze cycle usually exceeds the alloy's inflection point, so plan fixturing and the cooling ramp on the basis that the metal expands at the higher above-Curie rate on the way up and at the low rate on the way back down.

Where Is Invar 42 Used?

Every application below relies on the same property: the alloy moves by a small, known and repeatable amount when temperature changes.

Invar 42 Production Capability at Jiangyin Jiangnan Metal

Jiangyin Jiangnan Metal Co., Ltd. operates an open-die forging and ring-rolling plant in Jiangyin, Jiangsu Province, China, employing approximately 460 people including 9 senior engineers and 32 intermediate engineers. Invar 42 is produced alongside the rest of our controlled-expansion range (Invar 36, Alloy 42 and Alloy 48) on the same equipment used for nickel alloys and precipitation-hardening stainless grades.

Standards, Testing and Certification

Invar 42 orders at Jiangyin Jiangnan Metal Co., Ltd. are produced and certified against the specifications below. The chemistry specification is normally ASTM F30, DIN 1.3917 or YB/T 5235; the inspection-document type is normally EN 10204 3.1.

• ASTM F30
• ASTM F29
• ASTM B753
• UNS K94100
• DIN 1.3917 / NiFe42
• DIN 17745
• SEW 385
• NF A54-301
• GB/T 15018
• YB/T 5235
• EN 10204 3.1
• EN 10204 3.2
• EN 10228-3 (UT)
• SEP 1921 (UT)
• ASTM A388 (UT)
• ISO 9001:2015

What appears on the certificate

• Heat number, with full ladle and product chemical analysis
• Melting route (EAF + VOD + ESR, or VIM + VAR where specified)
• Mechanical test results (tensile, yield, elongation, hardness) on coupons from the delivered heat
• Heat-treatment records: anneal temperature, hold time, atmosphere, cooling method, and the stabilising cycle where applied
• Ultrasonic examination report to the ordered standard and acceptance class
• Dimensional inspection report
• Measured coefficient of thermal expansion over the ordered temperature range, added on request and strongly recommended for sealing and metrology parts
• Cross-listed equivalent designations (UNS K94100 / ASTM F30 / DIN 1.3917 / NiFe42 / 4J42 / NF A54-301)

Quality gates and non-conformance handling

Every Invar 42 order passes six mandatory hold points at which production cannot continue without QA sign-off: raw-material chemistry verification, forging temperature compliance, post-forging ultrasonic examination, heat-treatment chart approval, mechanical and expansion test acceptance, and final NDE plus dimensional inspection. Customer-witnessed hold points can be added at no charge. Any out-of-specification finding raises a formal non-conformance report within 24 hours, with root-cause analysis inside five working days and the proposed disposition sent to the customer before any rework is carried out.

How to Specify an Invar 42 Forging Order

Invar 42 carries two specification decisions that most grades do not: the name has to be disambiguated, and the expansion requirement has to be stated explicitly because it is the property being bought and it is not implied by chemistry alone. The steps below remove the ambiguity that causes most disputes on this grade.

1. Confirm which alloy you actually needCheck the required coefficient of thermal expansion, not the name. Near 1.3 ppm/°C means Invar 36 (UNS K93600). Near 5.3 ppm/°C means Invar 42 (UNS K94100). Grade resolver →
2. Name the grade generically"Alloy 42 / UNS K94100 / ASTM F30", cross-referenced to DIN 1.3917 / NiFe42 and 4J42 / YB/T 5235 where relevant. Never order against a trademarked brand name alone.
3. State the CTE requirementValue, temperature range, tolerance, and whether it must be measured on the delivered heat and reported on the certificate.
4. Send the drawingDimensions, tolerances, surface finish, required grain-flow direction.
5. Specify conditionAnnealed 850–1,000 °C as standard; add the 300–350 °C stabilising treatment for precision parts.
6. Specify the forged routeSeamless rolled ring with circumferential grain flow where stability matters, and state whether machined-from-plate is permitted.
7. Add a single-heat clauseRequired whenever several parts of one assembly must expand identically.
8. Define NDE and certificationUT to EN 10228-3, SEP 1921 or ASTM A388 with acceptance class; EN 10204 3.1 or 3.2 with the nominated third party.

Top 10 Mistakes When Ordering Invar 42 Forgings

1. Ordering "Invar 42" and receiving Invar 36, or the reverse. A factor of four in expansion coefficient. Always confirm against the required CTE value, never against the name on the drawing.
2. Assuming the low expansion holds at high temperature. Above the ≈360 °C inflection point the effect is gone. Design and qualify inside the useful range, or change grade.
3. Choosing Invar 36 for a part that runs hot. Above 300 °C, Invar 36 has passed its Curie point and Invar 42 is the more stable alloy, at lower cost. See Table 1.
4. Treating Invar 42 and Kovar as interchangeable. Similar at room temperature, different above 360 °C, and different in cost by a factor of two and a half.
5. Ordering the chemistry but not the expansion coefficient. ASTM F30 chemistry alone does not guarantee a specific CTE. If it matters, specify it and require it on the certificate.
6. Mixing ASTM and YB/T certificates within one assembly. The two standards define different nickel bands and different expansion acceptance ranges. Pick one and state it.
7. Accepting parts machined from plate in place of forgings. Grain flow and residual stress differ; the machined part moves more through thermal cycling.
8. Skipping the stabilising treatment after final machining. Residual stress redistributes in service and the part drifts out of tolerance, often weeks after acceptance.
9. Mixing heats within a single assembly. Small nickel differences between heats produce measurable expansion differences. Specify single heat.
10. Ignoring corrosion. Invar 42 has essentially no chromium. Unprotected outdoor or humid service will rust. Specify plating or a coating.

Frequently Asked Questions: Invar 42

Glossary

Invar 42

Common trade-style name for the iron-42% nickel controlled-expansion alloy; generically Alloy 42 / UNS K94100 / ASTM F30 / DIN 1.3917 / 4J42. Not the same alloy as Invar 36.

Invar 36

The original Invar alloy at 36% nickel, UNS K93600, ASTM F1684, with the lowest expansion of the iron-nickel family at about 1.3 × 10⁻⁶ /°C over 20–100 °C.

UNS K94100

Unified Numbering System designation for the Invar 42 / Alloy 42 chemistry. The generic, brand-free name to use on purchase orders.

4J42

Chinese designation for the same iron-42% nickel sealing alloy, classified under GB/T 15018 and supplied to the technical requirements of YB/T 5235.

Invar effect

The anomalous near-cancellation of thermal expansion in face-centred-cubic iron-nickel alloys near 36% nickel, caused by a magnetostrictive contraction offsetting normal lattice expansion.

Inflection point

The temperature at which the expansion curve bends sharply upward, coincident with the Curie point at about 360–370 °C for Invar 42. The practical ceiling for controlled-expansion service.

Curie temperature

The temperature above which a ferromagnetic material becomes paramagnetic. In the iron-nickel controlled-expansion alloys, the Curie point and the expansion inflection point are the same physical event.

Mean CTE

The average coefficient of thermal expansion between two stated temperatures, as distinct from the instantaneous coefficient at a single temperature. Datasheet figures are almost always mean values referenced to room temperature.

Matched seal

A glass- or ceramic-to-metal seal in which both materials have nearly identical expansion coefficients, so the joint cools to near-zero residual stress.

Compression seal

A seal designed so the metal contracts slightly more than the glass, leaving the glass in compression, which is the stress state glass tolerates best.

Lead frame

The stamped or etched metal frame that carries the electrical connections and mechanical support of a semiconductor package. Invar 42 / Alloy 42 is one of the two dominant lead-frame materials, alongside copper alloys.

ESR

Electroslag remelting. A secondary melting process that refines inclusion content and produces a directionally solidified ingot suited to forging.

VOD

Vacuum oxygen decarburisation. A secondary refining step that lowers carbon and dissolved gases.

Stabilising treatment

A low-temperature soak, typically 300–350 °C, applied after final machining to relieve residual stress and lock dimensions before service.

Seamless rolled ring

A ring produced by piercing a forged billet and expanding it on a radial-axial ring mill, giving continuous circumferential grain flow and better dimensional stability than a ring machined from plate.

EN 10204 3.1 / 3.2

Inspection document types. 3.1 is a mill certificate issued by the manufacturer's own independent inspection department; 3.2 is countersigned by an independent third party nominated by the purchaser.

Technical References

Chemistry, expansion, physical-property and heat-treatment data on this page are drawn from the published standards and engineering references below. Test results reported on our material certificates are independent and traceable to calibrated laboratory equipment.

1. ASTM F30, Standard Specification for Iron-Nickel Sealing Alloys, ASTM International, West Conshohocken, PA. Primary chemistry specification for 42 Alloy / UNS K94100.
2. ASTM F29, Standard Specification for Dilute Nickel-Iron Sealing Alloys, ASTM International.
3. ASTM F15, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy, ASTM International. Kovar / UNS K94610 comparison data.
4. ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications, ASTM International. Invar 36 comparison data.
5. ASTM B753, Standard Specification for Thermostat Component Alloys, ASTM International.
6. ASTM A388, Standard Practice for Ultrasonic Examination of Steel Forgings, ASTM International.
7. DIN 17745, Wrought alloys of nickel and iron, Deutsches Institut für Normung.
8. SEW 385, Stahl-Eisen-Werkstoffblatt: nickel-iron alloys, sheet, strip and bar, Verein Deutscher Eisenhüttenleute.
9. AFNOR NF A54-301, Nickel-iron controlled expansion alloys, Association Française de Normalisation.
10. GB/T 15018, Precision alloys: classification, designation and general chemical composition, Standardization Administration of China.
11. YB/T 5235, Technical requirements for iron-nickel-chromium and iron-nickel sealing alloys, China. This is the specification under which 4J42 is supplied, including the 20–300 °C and 20–450 °C expansion bands and the 900 °C ± 20 °C test-piece treatment.
12. EN 10204:2004, Metallic products: types of inspection documents, CEN, Brussels.
13. EN 10228-3, Non-destructive testing of steel forgings, Part 3: ultrasonic testing, CEN.
14. SEP 1921, Ultrasonic testing of steel forgings, Stahl-Eisen-Prüfblatt.
15. ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International. See the section on low-expansion alloys.
16. ASM Specialty Handbook: Nickel, Cobalt and Their Alloys, J.R. Davis (ed.), ASM International.
17. Guillaume, C.É., Recherches sur les aciers au nickel, Comptes Rendus de l'Académie des Sciences, 1897. The original work on the Invar effect (Nobel Prize in Physics, 1920).
18. Wachtel, E. and Bakonyi, I., Magnetism and thermal expansion anomalies in Fe-Ni alloys. Standard reference literature on the Invar mechanism.
19. Espe, W., Materials of High Vacuum Technology, Pergamon Press. Glass-to-metal and ceramic-to-metal sealing practice.

Standards cited are the revisions known to us at the time of the last page review. For procurement, always reference the revision in force at the contract date. All trademarks referenced belong to their respective owners.

Cite This Page

This datasheet is maintained by the metallurgical engineering team at Jiangyin Jiangnan Metal Co., Ltd. and is free to quote, reference or link to. If you use the data in a specification, report or article, please attribute it as follows.

Jiangyin Jiangnan Metal Co., Ltd. (2026). Invar 42 Forgings: Alloy 42 / UNS K94100 / ASTM F30 / 4J42. Designation, Properties and Ordering Guide. Jiangyin, Jiangsu, China. Retrieved from https://www.steelforgepieces.com/Nickel-Alloy/Invar-42.html. Last updated 20 August 2026.

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