Controlled-Expansion Alloys · Nickel-Iron · DIN designation

NI42 (DIN Ni 42 / W.-Nr. 1.3917 / NiFe42) Forging Parts

Published: 12 May 2022 · Last updated: 22 August 2026 · Technically reviewed by the Jiangyin Jiangnan Metal Co., Ltd. Metallurgical Engineering Team

Werkstoff

1.3917

DIN spec

17745

UNS

K94100

Nickel

41–43 wt %

CTE 20–100 °C

5.3 ×10⁻⁶/°C

Density

8.11 g/cm³

Rm

440–640 N/mm²

Inflection pt

≈370 °C

What is NI42?

NI42 is a binary nickel-iron alloy of nominally 42 % nickel and balance iron, bought for one reason: its thermal expansion is low, predictable and matched to the glass, ceramic or silicon it will be joined to. In the German system it appears as Ni 42 or NiFe42 under DIN 17745, and as material number 1.3917. It belongs to the controlled-expansion family that starts at Invar 36 and climbs through NI42, Alloy 46, Alloy 48 and Alloy 52 as nickel content rises.

The behaviour comes from the Invar effect. In face-centred-cubic iron-nickel alloys near 36 % nickel, ordinary lattice expansion is very nearly cancelled by a magnetostrictive contraction that occurs as ferromagnetic ordering weakens with rising temperature. The cancellation peaks at 36 % Ni; adding nickel weakens it in a controlled, repeatable way. At 41–43 % Ni the residual expansion settles near 5.3 ppm/°C, a level chosen to sit close to silicon, alumina and the hard sealing glasses rather than to reach the minimum.

Two things follow from this, and they account for most of the trouble reported on the grade in service.

• The low expansion stops at the inflection point (≈370 °C). The magnetostrictive contraction ends once the alloy passes its Curie temperature and turns paramagnetic. Above that, the coefficient climbs toward that of an ordinary austenitic alloy. The dependable controlled-expansion window for NI42 is room temperature to about 300 °C, and assuming the low coefficient 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. NI42 is therefore supplied and used annealed, and precision parts normally get a low-temperature 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 a wet or aggressive environment, the answer is protective plating (nickel or gold on electronic parts) or a change of grade.

NI42 forgings: supplier quick facts

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China, producing NI42 (W.-Nr. 1.3917 / NiFe42 / Alloy 42 / UNS K94100) forged rings, seamless rolled rings, flanges, shafts, discs, sleeves, bushings, tube sheets and bars to customer drawings.

Capability figures are for NI42 specifically and are the limits we forge to on this grade, not general shop maxima.

What are the equivalent designations of NI42?

Engineers reach this grade through at least a dozen names, depending on which standards body wrote the drawing, which mill supplied the last batch and which decade the specification dates 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 and cross-lists the equivalents on the material certificate.

What is the chemical composition of NI42?

NI42 to DIN 17745 / W.-Nr. 1.3917 contains 41–43 % nickel with the balance iron, and holds carbon to 0.05 % maximum, manganese to 1.0 % maximum and silicon to 0.3 % maximum. The chemistry is deliberately simple: nickel sets the expansion coefficient, and everything else is held low because residual elements shift the expansion curve, degrade the soft-magnetic behaviour or interfere with glass wetting during sealing.

Nickel, carbon, manganese and silicon limits follow the DIN 17745 / SEW 385 band for 1.3917. Residual-element caps reflect ASTM F30 practice and our own mill limits, and are reported on every certificate whether or not the drawing calls for them.

What are the mechanical properties of NI42?

NI42 in the annealed condition has a tensile strength of 440–640 N/mm², a 0.2 % proof strength of 230–330 N/mm² and elongation of 30–40 %. These are the DIN/SEW acceptance bands and they are what we certify against on a European order. NI42 is not a structural alloy: it is austenitic, cannot be hardened by heat treatment, and is used at modest stress. Strength can only be raised by cold work, and cold work is what destroys the expansion accuracy the alloy was bought for.

What are the physical properties and thermal expansion of NI42?

The mean coefficient of thermal expansion of NI42 is approximately 5.3 × 10⁻⁶ /°C over 20–100 °C, and stays low and nominally constant over the range 20–300 °C (85–570 °F). That is the number the alloy is purchased for, and it is the number to verify on the certificate, because chemistry conformance alone does not guarantee it.

Density, melting point, inflection point and the 20–100 °C coefficient are well established for this chemistry and can be used directly. Values marked "typical" vary with heat, section size and condition and are indicative for screening only. Where any physical value is contractually important, state it on the purchase order and Jiangyin Jiangnan Metal Co., Ltd. will report the measured result on the certificate.

Practical notes on the expansion coefficient

1. Mean is not instantaneous. Datasheet figures 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, ask for the measured expansion curve rather than a single number.
2. 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 on the purchase order and 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.

How is NI42 annealed and heat treated?

NI42 is annealed between 850 °C and 1000 °C (1560–1830 °F) in a protective atmosphere. The alloy has no hardening transformation, so the only heat treatments that matter are the ones that remove cold work and stabilise dimensions, and both exist to protect the expansion coefficient.

1. MeltEAF + VOD + ESR, Ni to target ±0.25 %
2. ForgeStart 1,100–1,200 °C, finish above 900 °C
3. Anneal850–1,000 °C, protective atmosphere
4. Rough machineLeave 2–4 mm stock
5. Stress relief300–350 °C
6. Finish machineLight cuts, sharp tools
7. Stabilise300–350 °C, slow cool
8. Test & certifyCTE, UT, chemistry, EN 10204 3.1 / 3.2

Standard NI42 process route at Jiangyin Jiangnan Metal Co., Ltd. Steps 5 and 7 are added for precision and metrology parts and are omitted on general forging stock unless the order calls for them.

What NI42 forged products are available?

Jiangyin Jiangnan Metal produces NI42 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 a die profile can remove 30–50 % of the rough machining. That saving is worth more on NI42 than on most grades, because the alloy is expensive per kilogram and slow to cut.

One point about route selection applies particularly to controlled-expansion work. Since NI42 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 identical chemistry. For precision frames and sealing rings, specify the forged route explicitly and refuse 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 open-die forgings

How is NI42 melted and forged?

NI42 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 picked 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 are run clean and neutral to slightly reducing.

A 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. That is the structural reason a rolled NI42 ring holds diameter through thermal cycling better than a ring cut from plate. After the last blow the piece is slow-cooled and given a full anneal at 850–1,000 °C, and it is that anneal, not the forging, which establishes the expansion coefficient the customer will measure.

How is NI42 machined and welded?

Machining

Machining behaviour is closest to austenitic stainless steel: gummy, work-hardening under a rubbing tool, and producing long stringy chips. The practical 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, since the 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 so you do not re-introduce the cold work the anneal removed.

Welding

NI42 welds readily by GTAW (TIG), electron beam, laser and resistance methods using matching Fe-42Ni filler. Joint faces must be scrupulously clean: sulfur, lead and other low-melting 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, weld and heat-affected zone expand along a different curve from the base material and the assembly distorts on thermal cycling.

Where is NI42 used?

Every application below rests on the same property: the alloy moves by a small, known and repeatable amount when temperature changes. The forged product forms listed are those Jiangyin Jiangnan Metal Co., Ltd. supplies into each sector.

NI42 production capability at Jiangyin Jiangnan Metal

Jiangyin Jiangnan Metal Co., Ltd. operates an open-die forging and ring-rolling plant at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, employing approximately 460 people including 9 senior engineers and 32 intermediate engineers. NI42 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.

What testing and certification comes with NI42 forgings?

NI42 orders at Jiangyin Jiangnan Metal Co., Ltd. are produced and certified against the specifications below. The chemistry specification is normally DIN 17745 / W.-Nr. 1.3917 or ASTM F30; the inspection document is normally EN 10204 3.1, with 3.2 third-party witnessed certification available on request.

• DIN 17745
• W.-Nr. 1.3917
• SEW 385
• ASTM F30
• ASTM F29
• ASTM B753
• UNS K94100
• 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, with full ladle analysis and product analysis
• Melting route (EAF + VOD + ESR, or VIM + VAR where specified)
• Mechanical test results (tensile, proof strength, 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 (NI42 / 1.3917 / NiFe42 / UNS K94100 / ASTM F30 / NF A54-301) so one certificate satisfies a DIN drawing and an ASTM purchase order

Quality gates and non-conformance handling

Every NI42 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 do you specify an NI42 forging order?

NI42 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.

1. Name the grade"NI42 / W.-Nr. 1.3917 / NiFe42 (= UNS K94100)". Avoid brand names alone
2. State the CTE requirementValue, temperature range, and whether it must be measured and certified
3. Send the drawingDimensions, tolerances, surface finish, grain-flow direction
4. Specify conditionAnnealed as standard; add stabilising treatment for precision parts
5. Define NDEUT to EN 10228-3, SEP 1921 or ASTM A388, with acceptance class
6. Specify certificationEN 10204 3.1 or 3.2; name the third party for 3.2
7. Single-heat requirementState it if several parts must expand identically
8. Quantity & deliveryPieces, target date, port, Incoterms

Copy-ready drawing callout

NI42 compared with Invar 36, Alloy 48, Alloy 52 and Kovar

The controlled-expansion family is chosen by expansion coefficient, not by strength or corrosion resistance. Find the coefficient you have to match, and the grade follows.

NI42 against Kovar is the comparison that comes up most. Their room-temperature coefficients are almost identical, so the two get treated as interchangeable. They are not. The 17 % cobalt in Kovar keeps the expansion curve flat and matched to borosilicate glass to about 450 °C, whereas NI42 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, NI42 does the same job without cobalt in the bill of materials.

Nine common mistakes when ordering NI42 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. Confirm against the required CTE value, never against the name on the drawing.
3. Treating NI42 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. DIN 17745 or 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 moves 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 on this grade. Resolve the requirement, or move to a hardenable controlled-expansion alloy.
9. Ignoring corrosion. NI42 has essentially no chromium. Unprotected humid or outdoor service will rust it. Specify plating or coating.

Glossary

NI42 (Ni 42, NiFe42)

The DIN designation of the controlled-expansion nickel-iron alloy of nominally 41–43 % Ni, balance Fe. Material number 1.3917; equivalent to Alloy 42 / UNS K94100 / ASTM F30.

W.-Nr. 1.3917

The German Werkstoffnummer (material number) for NiFe42. The most precise short designation to place on a European drawing.

DIN 17745

German standard covering wrought nickel-iron alloys, including the composition of NI42 / NiFe42.

SEW 385

Stahl-Eisen-Werkstoffblatt 385, covering nickel-iron alloys in sheet, strip and bar, including property requirements for 1.3917.

UNS K94100

Unified Numbering System designation for the same chemistry. The generic, brand-free name for an international purchase order.

ASTM F30

Standard Specification for Iron-Nickel Sealing Alloys, the American chemistry and property specification covering this grade in bar, rod, tube, sheet and strip.

4J42

The Chinese designation for the Fe-42Ni precision expansion alloy, from the 4J precision-alloy series.

CTE

Coefficient of thermal expansion. The fractional change in length per degree, quoted here in 10⁻⁶ per °C, equivalently ppm/°C.

Mean CTE

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

Invar effect

The near-cancellation of thermal expansion in face-centred-cubic Fe-Ni alloys close to 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 NI42. The practical ceiling for controlled-expansion service.

Dehnung

German for elongation. The ductility figure (A) reported on European mill certificates, 30–40 % for annealed NI42.

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 made 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 about NI42 / W.-Nr. 1.3917

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. DIN 17745, Wrought alloys of nickel and iron, Deutsches Institut für Normung. Composition of NiFe42 / W.-Nr. 1.3917.
2. SEW 385, Stahl-Eisen-Werkstoffblatt: nickel-iron alloys, sheet, strip and bar, Verein Deutscher Eisenhüttenleute.
3. ASTM F30, Standard Specification for Iron-Nickel Sealing Alloys, ASTM International, West Conshohocken, PA.
4. ASTM F29, Standard Specification for Dilute Nickel-Iron Sealing Alloys, ASTM International.
5. ASTM F15, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy, ASTM International (Kovar comparison data).
6. ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications, ASTM International (Invar 36 comparison data).
7. ASTM B753, Standard Specification for Thermostat Component Alloys, ASTM International.
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, 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, Comptes Rendus de l'Académie des Sciences, 1897. The original work on the Invar effect (Nobel Prize in Physics, 1920).
16. 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.

Related grades and forged products

• Alloy 42 / UNS K94100
• Invar 42
• Invar 36
• Alloy 48
• Ni-Span-C Alloy 902
• Incoloy A-286
• A286
• Incoloy DS
• AISI 330
• Inconel 706
• Nimonic 81
• Alloy 31
• Monel 400
• Monel K-500
• Hastelloy C-276
• Waspaloy
• Forged & rolled rings
• Forged disks
• Forged tubes
• Forged spindles
• Open die forgings