Controlled Expansion Alloys · Nickel-Iron

Alloy 42 / UNS K94100 / ASTM F30 / DIN 1.3917 Forging Parts

Published: 12 May 2022  |  Last updated: 12 August 2026  |  Technically reviewed by the Jiangyin Jiangnan Metal Co., Ltd. Metallurgical Engineering Team

UNS

K94100

Werkstoff

1.3917

Spec

ASTM
F30

Nickel

41wt %

CTE 20–100 °C

5.3×10⁻⁶ /°C

Density

8.11g/cm³

Inflection pt

370°C

Melting pt

1435°C

What is Alloy 42 (UNS K94100)?

Alloy 42 is a binary nickel-iron alloy of nominally 41% nickel and balance iron, specified for one reason: its coefficient of thermal expansion is low, predictable, and close to that of the materials it is sealed or bonded to. It belongs to the controlled-expansion (or low-expansion) family that begins with Invar 36 and runs upward through Alloy 42, Alloy 46, Alloy 48 and Alloy 52 as nickel content increases.

What produces this behaviour is the Invar effect. In face-centred-cubic iron-nickel alloys near 36% nickel, 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 adding nickel reduces it in a controlled, repeatable way. At 41–42% Ni the residual expansion settles at roughly 5.3 ppm/°C, which is close to the values for silicon, alumina and several hard sealing glasses. The nickel level in this grade is chosen for that match rather than for minimum expansion.

Two consequences follow from this mechanism, and between them they account for most field problems with the grade.

• The low expansion disappears above the inflection point (~370 °C). The magnetostrictive contraction that produces the effect stops once the alloy passes its Curie temperature and becomes paramagnetic. Above that point the expansion coefficient climbs toward that of an ordinary austenitic alloy. The useful controlled-expansion range of Alloy 42 is therefore quoted as room temperature to about 300 °C, and assuming the low CTE holds at 500 °C is a design error.
• Cold work distorts the expansion curve. Residual strain from drawing, straightening or heavy machining shifts the measured coefficient away from the datasheet value. Alloy 42 is supplied and used in the annealed condition for this reason, and precision parts usually receive a stabilising treatment after final machining.

There is essentially no chromium in the alloy, so it has only modest oxidation resistance and little corrosion resistance. Where a controlled-expansion part also has to survive an aggressive environment, the usual answer is protective plating (nickel or gold on lead frames) or a change of grade.

What Forged Products Are Available in Alloy 42?

Jiangyin Jiangnan Metal produces Alloy 42 through three routes, selected by geometry and quantity. Open-die forging covers long shafts, blocks, tube sheets and large discs, and is used 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. That saving is worth more on this grade than on most, because Alloy 42 is expensive per kilogram and slow to machine.

One route-selection point applies specifically to this grade. Since Alloy 42 is bought for dimensional behaviour, grain flow and residual stress matter more than peak strength. A rolled ring with continuous circumferential grain flow holds its diameter through thermal cycling better than the same ring machined out of plate, even though both meet the same ASTM F30 chemistry. For precision frames and rings, specify the forged route rather than accepting a machined-from-solid substitute.

• Seamless rolled rings
• Forged rings
• Forged flanges
• Forged round bars
• Forged flat bars & blocks
• Forged discs & blanks
• Forged shafts & spindles
• Forged sleeves & bushings
• Forged tube sheets
• Forged tubes & hollows
• Forged gear blanks
• Custom near-net-shape parts

What Are the Equivalent Designations of Alloy 42?

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

What Is the Chemical Composition of Alloy 42?

The composition below reflects ASTM F30 practice for the 42% nickel sealing alloy. The chemistry is simple by design: nickel sets the expansion coefficient, and every other element is held low because residuals shift the expansion curve, degrade the magnetic behaviour or interfere with glass wetting during sealing. Carbon is capped tightly because it embrittles the alloy and forms carbides that disturb the expansion match.

What Is the Coefficient of Thermal Expansion of Alloy 42?

The mean coefficient of thermal expansion of Alloy 42 is approximately 5.3 × 10⁻⁶ /°C over 20–100 °C, and falls in the range 4.5–6.5 × 10⁻⁶ /°C over 20–300 °C (about 2.9 × 10⁻⁶ /°F over 70–212 °F, and 2.5–3.6 × 10⁻⁶ /°F over 70–572 °F). This is the number the alloy is bought for, and it is the number to verify on the certificate.

Three practical points follow from this.

1. Mean versus 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. Heat-to-heat variation is significant. A 0.5% shift in nickel moves the coefficient measurably. For precision work, order all parts of an assembly from a single heat and state that requirement on the purchase order. We will reserve material accordingly.
3. Condition governs the value. A cold-drawn or heavily machined part will not measure at the annealed value. Anneal, then finish, then stabilise.

What Are the Physical Properties of Alloy 42?

What Are the Mechanical Properties of Alloy 42?

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

How Is Alloy 42 Annealed and Heat Treated?

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

Processing sequence matters on precision parts. The correct order is: forge, anneal, rough machine, intermediate stress relief, finish machine, stabilise at 300–350 °C, then measure. Skipping the intermediate relief on a part with heavy stock removal lets residual stress redistribute after finishing, and the part moves. On a 500 mm ring the movement is typically tens of microns, which is enough to fail a metrology or sealing application.

Glass-to-Metal and Ceramic-to-Metal Sealing with Alloy 42

A hermetic seal works when the metal and the glass or ceramic contract along nearly the same path as the assembly cools from the sealing temperature to room temperature. If the glass contracts more than the metal it ends in tension and cracks, usually radially and usually within days. If the metal contracts slightly more in a controlled way around a pin, the glass ends in compression, which is the stress state glass carries well. That small controlled mismatch is the basis of the compression seal, while a near-zero mismatch is the basis of the matched seal.

The 5.3 ppm/°C figure for Alloy 42 sits in the useful window for matched seals to hard sealing glasses (the Corning 7052 and 7056 family), for ceramic-to-metal seals with alumina, and for direct expansion matching to silicon. That last point is the main reason it became the standard semiconductor lead-frame material. It is not the right partner for soda-lime soft glass at 9 ppm/°C, where Alloy 48 or Alloy 52 is used instead.

Alloy 42 vs Invar 36, Alloy 46, Alloy 48, Alloy 52 and Kovar

The controlled-expansion family is chosen by expansion coefficient, not by strength or corrosion resistance. The table below is the practical selection chart: find the CTE you need to match, and the grade follows.

Alloy 42 against Kovar is the comparison that comes up most often. Their room-temperature coefficients are almost identical, so the two are often assumed to be interchangeable. They are not. The 17% cobalt in Kovar holds the expansion curve flat and matched to borosilicate glass up to about 450 °C, whereas Alloy 42 starts to diverge above its 370 °C inflection point. Where a seal is made at or cycles near 450 °C, the cost premium for Kovar is justified. Below 300 °C, which covers lead frames, thermostats, positioning hardware and most alumina seals, Alloy 42 does the same job without cobalt in the bill of materials.

How Do You Forge, Machine and Weld Alloy 42?

Forging

Alloy 42 is hot-worked from approximately 1,100–1,200 °C, with the finishing temperature held above roughly 900 °C. The alloy 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, and soaking too close to the solidus coarsens the grain irreversibly. Furnaces should be run clean and neutral to slightly reducing.

Forging reduction of at least 4:1 from the ingot is used to break down the as-cast structure. For seamless rolled rings, the pierced blank goes through radial-axial rolling so that grain flow follows the circumference. This is the structural reason a rolled Alloy 42 ring holds diameter through thermal cycling better than a ring machined from plate. After the final blow the piece is slow-cooled and then given a full anneal at 850–1,000 °C, and that anneal 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 practical rules are the same 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 will glaze the surface and work-harden a layer the next pass has to cut through.
• Rigid setups and generous flood coolant. The low thermal conductivity of the alloy concentrates heat at the cutting edge.
• Leave 2–4 mm of stock after rough machining, apply an intermediate stress relief, then finish. On precision parts, finish-machine with light depths of cut to avoid re-introducing the cold work the anneal removed.

Welding

Alloy 42 welds readily by GTAW (TIG), electron beam, laser and resistance methods, using matching Fe-42Ni filler. Joint faces must be scrupulously clean, since sulfur, lead and low-melting-point contamination cause cracking. Preheat is not normally required. Where the joint has to 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 from the base material and the assembly distorts on thermal cycling.

Where Is Alloy 42 Used?

All of the applications below rely on the same property: the alloy moves by a small, known and repeatable amount when temperature changes.

Alloy 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. Alloy 42 is produced alongside the rest of our controlled-expansion range (Invar 36, Invar 42 and Alloy 48) on the same equipment used for nickel alloys and precipitation-hardening stainless grades.

Standards, Testing and Certification

Alloy 42 orders at Jiangyin Jiangnan Metal Co., Ltd. are produced and certified against the specifications below. The chemistry specification is normally ASTM F30 or DIN 1.3917; 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
• AFNOR NF A54-301
• 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 and full ladle plus 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; 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 / NF A54-301)

Quality gates and non-conformance handling

Every Alloy 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 Alloy 42 Forging Order

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

Recommended drawing callout

Top 10 Mistakes When Ordering Alloy 42 Forgings

1. Assuming the low expansion holds at high temperature. Above the 370 °C inflection point the effect is gone. Design and qualify inside the useful range, or change grade.
2. Confusing "Invar 42" with Invar 36. A four-fold difference in expansion coefficient. Always confirm against the required CTE value, never against the name on the drawing.
3. Treating Alloy 42 and Kovar as interchangeable. Similar at room temperature, different above 370 °C, and different in cost by a factor of two and a half.
4. 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.
5. Accepting parts machined from plate in place of forgings. Grain flow and residual stress differ; the machined part will move more through thermal cycling.
6. Skipping the stabilising treatment after final machining. Residual stress redistributes in service and the part drifts out of tolerance, often weeks after acceptance.
7. Mixing heats within a single assembly. Small nickel differences between heats produce measurable expansion differences. Specify single heat.
8. Specifying high strength alongside tight expansion control. These conflict for this grade. Resolve the requirement, or move to a hardenable controlled-expansion alloy.
9. Ignoring corrosion. Alloy 42 has essentially no chromium. Unprotected outdoor or humid service will rust. Specify plating or a coating.
10. Leaving surface-oxide condition unspecified on sealing parts. Glass adhesion depends on the oxide the part arrives with. State the required surface condition and pre-oxidation treatment at RFQ stage.

Glossary

Alloy 42

Controlled-expansion nickel-iron alloy of nominally 41% Ni, balance Fe. UNS K94100, ASTM F30, DIN 1.3917.

UNS K94100

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

ASTM F30

Standard Specification for Iron-Nickel Sealing Alloys, covering the composition and property requirements of Alloy 42 and its neighbours in sheet, strip, rod, bar, tube and wire.

CTE

Coefficient of thermal expansion. The fractional change in length per degree of temperature change, quoted here in units of 10⁻⁶ per °C, equivalently parts per million per °C (ppm/°C).

Mean CTE

The average coefficient 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.

Invar effect

The anomalous near-cancellation of thermal expansion in face-centred-cubic Fe-Ni 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 370 °C for Alloy 42. The practical ceiling for controlled-expansion service.

Curie temperature

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

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. 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.

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.

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.

Frequently Asked Questions: Alloy 42 / UNS K94100

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.
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. DIN 17745, Wrought alloys of nickel and iron, Deutsches Institut für Normung.
7. SEW 385, Stahl-Eisen-Werkstoffblatt, nickel-iron alloys, sheet, strip and bar, Verein Deutscher Eisenhüttenleute.
8. AFNOR NF A54-301, Nickel-iron controlled expansion alloys, Association Française de Normalisation.
9. EN 10204:2004, Metallic products. Types of inspection documents, CEN, Brussels.
10. EN 10228-3, Non-destructive testing of steel forgings, Part 3: Ultrasonic testing of ferritic or martensitic steel forgings, CEN.
11. SEP 1921, Ultrasonic testing of steel forgings, Stahl-Eisen-Prüfblatt.
12. ASTM A388, Standard Practice for Ultrasonic Examination of Steel Forgings, ASTM International.
13. ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International, section on low-expansion alloys.
14. ASM Specialty Handbook: Nickel, Cobalt and Their Alloys, J.R. Davis (ed.), ASM International.
15. Guillaume, C.É., Recherches sur les aciers au nickel, the original work on the Invar effect, Comptes Rendus de l'Académie des Sciences, 1897 (Nobel Prize in Physics, 1920).
16. Wachtel, E. and Bakonyi, I., Magnetism and thermal expansion anomalies in Fe-Ni alloys, standard reference literature on the Invar mechanism.
17. Espe, W., Materials of High Vacuum Technology, Pergamon Press. Glass-to-metal and ceramic-to-metal sealing practice.
18. Corning Incorporated, published technical data on sealing-glass expansion coefficients (7052, 7056, 7740 families).

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.

Related Grades and Forged Products

• Invar 36
• Invar 42
• Alloy 48
• Ni-Span-C Alloy 902
• Incoloy A-286
• A286
• Incoloy DS
• AISI 330
• Inconel 706
• Monel 400
• Forged & rolled rings
• Forged disks
• Forged tubes
• Forged spindles
• Open die forgings