What is Alloy 42-6 (UNS K94760)?
Alloy 42-6 is a controlled-expansion iron-nickel-chromium sealing alloy containing nominally 42 % nickel and 6 % chromium with the balance iron, designated UNS K94760, GB 4J6 and formerly ASTM F31. Its average linear coefficient of thermal expansion is 8.5–9.5 × 10⁻⁶/°C between 30 °C and 350 °C and 9.7–10.7 × 10⁻⁶/°C between 30 °C and 425 °C, which matches soft (soda-lime and lead) sealing glasses such as Corning 0120. The chromium addition allows the alloy to grow a tightly adherent dark-green chromium oxide film during wet-hydrogen annealing; that oxide dissolves into the molten glass and is what makes the seal vacuum-tight. Jiangyin Jiangnan Metal Co., Ltd. supplies Alloy 42-6 as open-die forgings: rings, bars, discs, flanges, sleeves and tube sheets, with EN 10204 3.1 or 3.2 certification.
Ordinary structural metals expand roughly two to three times faster than glass. Weld a stainless steel pin into a glass bead and the seal will craze or shatter the first time it is heated and cooled. Sealing alloys solve this by exploiting the Invar effect: in iron-nickel alloys around 36–50 % nickel, magnetostriction partly cancels normal thermal expansion, so the metal expands abnormally slowly until it passes its magnetic transition, where the expansion curve bends sharply upward.
Alloy 42-6 sits at the high end of that family. Plain Alloy 42 (Fe-42Ni) is used for harder glasses; adding about 6 % chromium raises the expansion curve so it tracks soft glass instead, and it also supplies the chromium needed to form the sealing oxide. That is the whole design intent of the grade: expansion matched to 0120-type glass, plus a chromium oxide that fluxes into the glass. [1][3]
Expansion match
≈8.5–9.5 × 10⁻⁶/°C over 30–350 °C, close to soft soda-lime and lead sealing glasses and far below stainless steel (≈16 × 10⁻⁶/°C).
Dark-green sealing oxide
Wet-hydrogen anneal at 1038–1204 °C grows an adherent Cr-rich oxide that dissolves into molten glass and forms a hermetic bond.
Ferromagnetic below ≈350 °C
The expansion inflection follows the magnetic transition, which is why the specified expansion limits stop at 425 °C and why service above that point loses the match.
What are the equivalent designations and trade names for Alloy 42-6?
Buyers meet this alloy under at least a dozen names. All of the entries below describe the same nominal Fe-42Ni-6Cr chemistry, and Jiangyin Jiangnan Metal accepts purchase orders written against any of them:
| Body / region | Designation | Notes |
|---|---|---|
| USA · UNS | K94760 | The primary generic identifier. Use this on drawings and POs. |
| USA · ASTM | ASTM F31 | Nickel-Chromium-Iron Sealing Alloys. Covered K94760 (42Ni-6Cr) and K95150 (47Ni-6Cr). Withdrawn 2024. See §4. |
| China · GB | 4J6 | Chinese precision-alloy designation, Ni 41.5–42.5 %, Cr 5.4–6.3 %. |
| Generic industry | Alloy 426 · 42-6 · 42Ni-6Cr · H42X6 | Common shop and catalogue shorthand. |
| Trade name (Carpenter) | Glass Sealing “42-6” | Carpenter Technology Corporation product name. |
| Trade name (Vacuumschmelze) | Vacovit 426 | VACUUMSCHMELZE GmbH product name. |
| Trade name (others) | Sealmet HC-4 | Third-party product name for the same chemistry. |
| Related, not equivalent | UNS K95150 / 47Ni-6Cr | The 47 % nickel sister grade in the same standard, with higher expansion. Do not substitute without checking the seal. |
| Related, not equivalent | ASTM F30 grades: Alloy 42, 46, 48, 52 | Chromium-free Fe-Ni sealing alloys. Different expansion, no Cr oxide. |
Trademark notice. Glass Sealing “42-6” is a product name of Carpenter Technology Corporation; Vacovit is a trademark of VACUUMSCHMELZE GmbH & Co. KG; Sealmet is a trademark of its respective owner. Material produced by those companies under those names is theirs. Material we produce is correctly described as UNS K94760 / 42Ni-6Cr / GB 4J6, the same generic chemistry, manufactured independently by Jiangyin Jiangnan Metal Co., Ltd. We are not affiliated with, sponsored by or endorsed by any of those trademark holders.
Designation lookup
Type any name (42-6, K94760, F31, 4J6, Vacovit, 426) and see everything it maps to.
All matched names refer to the same nominal Fe-42Ni-6Cr chemistry. Confirm the exact chemistry band and expansion limits on your order, because producers differ slightly inside the nominal range.
What is the chemical composition of Alloy 42-6?
The table compares the limits Jiangyin Jiangnan Metal works to for forged product against the withdrawn ASTM F31 specification and the Chinese 4J6 band. Nickel and chromium set the expansion behaviour; carbon, silicon, manganese, phosphorus and sulphur are held low because they degrade the sealing oxide and the vacuum tightness of the finished seal.
| Element | Jiangnan forging limits | ASTM F31 (K94760) | GB 4J6 | Why it is controlled |
|---|---|---|---|---|
| Nickel (Ni) | 41.00–43.00 | 42.0 nominal | 41.5–42.5 | Sets the Invar-effect expansion curve and the magnetic transition temperature |
| Chromium (Cr) | 5.60–6.40 | 5.6 nominal | 5.4–6.3 | Raises expansion to match soft glass; forms the adherent sealing oxide |
| Carbon (C) | 0.020 max | 0.07 max | 0.05 max | Carbide formation disturbs expansion stability and gases the seal |
| Silicon (Si) | 0.30 max | 0.30 max | 0.30 max | Deoxidiser; excess silica embrittles the glass interface |
| Manganese (Mn) | 0.25 max | 0.25 max | 0.25 max | Deoxidiser; volatilises in vacuum service if too high |
| Phosphorus (P) | 0.025 max | 0.025 max | 0.02 max | Impurity, causes grain-boundary segregation |
| Sulphur (S) | 0.025 max | 0.025 max | 0.02 max | Impurity, causes hot-shortness during forging and poor oxide adhesion |
| Aluminium (Al) | 0.20 max | 0.20 max | 0.20 max | Residual deoxidiser; excess forms a non-wetting alumina layer |
| Iron (Fe) | Balance | Remainder | Balance | Matrix |
Why our carbon limit is tighter than the standard. ASTM F31 allowed up to 0.07 % C because it was written for thin wire and ribbon. Forged sections are far thicker, so we hold carbon to 0.020 % max to keep the expansion curve uniform through the section and to avoid carbide stringers that show up as UT indications. If your specification demands the full F31 band instead, say so on the enquiry and we will supply to it.
Is ASTM F31 still an active standard?
No. ASTM F31 was withdrawn in 2024, together with the rest of the ASTM Committee F01 glass-sealing family: F30 (iron-nickel sealing alloys), F15 (Fe-Ni-Co, “Kovar” type), F29 (Dumet wire) and F1684 (low-expansion Fe-Ni and Fe-Ni-Co alloys). The last active revision of F31 was F31-21. Withdrawal does not change the alloy, and the withdrawn document remains purchasable as a historical standard. [2]
Practically, this means a purchase order that says only “per ASTM F31” now references a document that is no longer maintained. Three ways to write the requirement so it is unambiguous:
- Cite the withdrawn edition explicitly Write “UNS K94760, chemistry and expansion per ASTM F31-21 (withdrawn 2024), historical edition”. This is what most electronics buyers are doing today and it is the clearest option.
- Specify the properties directly on the drawing State the chemistry band and the required mean CTE and range, for example “mean CTE 30–350 °C = 8.5–9.5 × 10⁻⁶/°C”, so the requirement stands on its own without any standard.
- Use the Chinese or in-house equivalent GB 4J6 remains active, as do most OEM in-house sealing-alloy specifications. We can cross-certify one heat against several of these on a single certificate.
Note on ASTM F30. That standard covered the chromium-free iron-nickel sealing alloys: Alloy 42, 46, 48 and 52, never the chromium-bearing 42-6. If your existing documentation calls for “Alloy 42-6 per ASTM F30”, the intended reference is almost certainly F31. We flag mismatches like this at the enquiry stage rather than after the forging is cut.
What is the coefficient of thermal expansion of Alloy 42-6?
Expansion is the property the alloy exists for, so it is the one to specify and the one to test. ASTM F31 set the mean linear coefficient over two temperature ranges, measured on material given a stabilising anneal of 15 minutes at 1100 °C in hydrogen or cracked ammonia followed by a controlled furnace cool. [2]
| Temperature range | Mean CTE (× 10⁻⁶/°C) | × 10⁻⁷/°C | Status |
|---|---|---|---|
| 30 to 350 °C | 8.5 – 9.5 | 85 – 95 | Specified band |
| 30 to 425 °C | 9.7 – 10.7 | 97 – 107 | Specified band |
| 30 to 300 °C | ≈ 7.9 | ≈ 79 | Typical |
| 30 to 400 °C | ≈ 9.8 | ≈ 98 | Typical |
| 30 to 500 °C | ≈ 11.2 | ≈ 112 | Typical |
| 30 to 600 °C | ≈ 12.1 | ≈ 121 | Typical |
| 30 to 700 °C | ≈ 13.0 | ≈ 130 | Typical |
Specified bands are the acceptance limits of ASTM F31-21 (withdrawn 2024). Typical values above 425 °C are informative only. They are not acceptance criteria, and above the magnetic transition the alloy no longer matches sealing glass.
Expansion curve: where the match holds and where it stops
Thermal expansion calculator
How much will your Alloy 42-6 part grow? Enter a length and a temperature rise.
Uses the mean coefficient over the whole range, which is how sealing-alloy expansion is specified and measured. For a seal, what matters is not the absolute growth but the difference between the metal and the glass over the same range.
Glass-match screening
Enter the expansion coefficient of your glass and see how well Alloy 42-6 tracks it.
First-pass screening only. A real seal design also depends on the shape of both expansion curves below the glass set point, the seal geometry (matched, compression or housekeeper), the glass transformation temperature and the annealing schedule. Send us the glass grade and we will review it with you.
What are the mechanical properties of Alloy 42-6?
Alloy 42-6 is a soft, ductile, single-phase alloy. It is not a structural grade and cannot be strengthened by heat treatment. Room-temperature properties for forged and annealed product:
| Property | Minimum (SI) | Minimum (imperial) | Test method |
|---|---|---|---|
| Tensile strength, Rm | 552 MPa | 80 ksi | ASTM E8 / ISO 6892-1 |
| Yield strength, Rp0.2 | 276 MPa | 40 ksi | ASTM E8 / ISO 6892-1 |
| Elongation, A | 30 % | 30 % | ASTM E8, 4D gauge |
| Hardness, typical | 140–180 HB | ≈ 75–90 HRB | ASTM E10 / E18 |
Do not order a hardness or strength “condition”. Unlike precipitation-hardening grades such as 17-4PH, Alloy 42-6 has no aged conditions. Every heat treatment applied to it exists to serve the seal: to relieve forging stress, recrystallise the grain structure, stabilise the expansion curve and grow the oxide. Specify the expansion and the anneal, not a strength class.
What are the physical properties of Alloy 42-6?
| Property | Value | Unit | Condition / note |
|---|---|---|---|
| Density | 8.15 | g/cm³ | Use this for forging weight, see the calculator |
| Melting temperature | ≈ 1430 | °C | Approximate liquidus |
| Electrical resistivity at 20 °C | 0.92 | µΩ·m | = 92 µΩ·cm |
| Thermal conductivity | 13.4 | W/m·K | Room temperature |
| Specific heat | 504 | J/kg·K | Room temperature |
| Magnetic behaviour | Ferromagnetic | n/a | Below the magnetic transition; the transition is what bends the expansion curve |
| Forging temperature | ≈ 1204 | °C | Upper working temperature, see §10 |
| Annealing | ≈ 788 | °C | Then air cool; a stabilising hydrogen anneal is used before sealing |
Typical published values for the 42Ni-6Cr chemistry [3][4][5]. Values on your certificate are measured on your heat and take precedence.
Alloy 42-6 vs Alloy 42, 46, 48, 52 and Invar 36: which sealing alloy do you need?
Choosing a sealing alloy is choosing an expansion curve. Pick the alloy whose curve tracks your glass or ceramic over the sealing and service range, not the one with the best strength or corrosion numbers.
| Alloy | Nominal chemistry | Designation | Typical seal partner | Choose it when… |
|---|---|---|---|---|
| Alloy 42-6 | Fe-42Ni-6Cr | K94760 · 4J6 | Soft lead and soda-lime glass (0120 type) | You need a soft-glass match and a self-forming chromium sealing oxide |
| Alloy 42 | Fe-42Ni | K94100 | Harder sealing glasses, lead frames | Lower expansion is required; no chromium wanted |
| Alloy 46 | Fe-46Ni | K94600 | Soft glass, vitreous enamel, ceramic | Enamelling or ceramic sealing without chromium |
| Alloy 48 | Fe-48Ni | K94800 | Soda-lime glass, enamel | A slightly higher expansion than 46 is needed |
| Alloy 52 | Fe-51Ni | N14052 | Soft glass, reed switches | The highest expansion of the Fe-Ni sealing family is required |
| Invar 36 | Fe-36Ni | K93600 | Not a sealing alloy, used for dimensional stability | You need minimum expansion: tooling, moulds, metrology, LNG |
| Kovar type (Fe-Ni-Co) | Fe-29Ni-17Co | K94610 | Hard borosilicate glass, alumina | Hard-glass and ceramic packages, a different curve entirely |
Rule of thumb. Soft glass with a chromium oxide bond calls for Alloy 42-6. Hard borosilicate or alumina calls for a Fe-Ni-Co Kovar-type alloy. Pure dimensional stability with no seal calls for Invar 36. If you are not sure, send the glass or ceramic grade and the service temperature range and we will match it.
How do you make a glass-to-metal seal with Alloy 42-6?
The seal is made by the oxide, not by the metal. Alloy 42-6 is given a wet-hydrogen anneal that grows a thin, tightly adherent dark-green chromium-rich oxide on the surface. When the glass is melted against it, that oxide dissolves into the glass and produces a continuous chemical transition from metal to glass, and that is what makes the joint hermetic. Too little oxide and the glass will not wet; too much and the layer fails inside itself. [1][3]
- Clean the surface Remove drawing lubricant, machining oil and handling contamination. Pickling in hydrochloric acid followed by thorough rinsing and drying is the classic route; the surface must be chemically clean before the oxide is grown.
- Grow the oxide in a wet-hydrogen anneal Typically about one hour at 1038–1204 °C (1900–2200 °F) in hydrogen of controlled dew point. The dew point sets how oxidising the atmosphere is and therefore how thick the oxide grows; it is the main process variable to control.
- Stabilise the expansion The expansion-test anneal used by ASTM F31, 15 minutes at 1100 °C in hydrogen or cracked ammonia at about −40 °C dew point followed by a furnace cool at under 5 °C per minute to 200 °C, also serves as a practical stabilising treatment for production parts.
- Seal Melt the glass against the oxidised metal at the glass working temperature and hold long enough for the oxide to dissolve. Taper or radius the metal edge inside the seal, because a sharp edge concentrates stress in the glass.
- Anneal the finished seal Cool through the glass annealing range slowly to relieve residual stress. Inspect for stress with polarised light, then leak test, using helium mass-spectrometer testing for vacuum work.
Matched seal
Metal and glass expansions are close over the whole range, so residual stress stays low. This is the normal use of Alloy 42-6 with soft sealing glass.
Compression seal
A higher-expansion outer housing (for example a mild-steel grommet) shrinks onto glass around an Alloy 42-6 pin, putting the glass in compression. Strong, and forgiving of a rough expansion match. Common in headers and terminals.
Machining after the oxide anneal. Any turning, grinding, blasting or handling that removes the oxide leaves bare metal that the glass will not wet, producing a leak path that only shows up at the helium test. Finish machine first, then oxidise, then seal.
How is Alloy 42-6 forged and heat treated?
Alloy 42-6 forges and cold works readily. The difficulty is not deformation but keeping the chemistry clean and the structure uniform, because both control the expansion curve.
| Operation | Parameter | Notes |
|---|---|---|
| Melting route | EAF + VOD, then ESR | Electroslag remelting for low sulphur, low gas content and freedom from inclusions that disturb the seal |
| Forging start temperature | ≈ 1204 °C (2200 °F) | Upper working temperature; soak thoroughly before the first blow |
| Finish forging | Above ≈ 900 °C | Finishing too cold leaves residual strain that shifts the expansion curve until it is annealed out |
| Forging reduction | ≥ 4 : 1 | Breaks down the as-cast structure and gives uniform grain flow |
| Annealing | ≈ 788 °C (1450 °F), air cool | Softening and stress-relief anneal for machining |
| Stabilising / oxide anneal | 1038–1204 °C, wet hydrogen, ≈1 h | Grows the sealing oxide and stabilises the expansion curve |
| Cooling after stabilising | < 5 °C/min to 200 °C | Per the ASTM F31 expansion-test practice; slow cooling gives a reproducible curve |
| Machining | Gummy, work-hardens | Sharp positive-rake tooling, rigid setup, generous coolant, no dwelling. Similar practice to austenitic stainless |
| Welding | Conventional processes | GTAW with matching filler; keep the weld area clean and re-anneal before oxidising for a seal |
What forged forms and sizes of Alloy 42-6 can you supply?
Everything is made to the customer's drawing or dimensional list. The forms we produce most often in this grade:
- Forged rings
- Seamless rolled rings
- Forged round bars
- Forged flanges
- Forged discs and disks
- Forged sleeves
- Forged bushings
- Forged tube sheets
- Forged shafts and spindles
- Forged pipes and tubes
- Forged blocks and blanks
- Forged valve stems and seat rings
- Near-net-shape forgings to drawing
| Capability | Range |
|---|---|
| Outside diameter | 100 – 6,000 mm |
| Length | 100 – 12,000 mm |
| Single-piece weight | 10 – 15,000 kg |
| Ring rolling | 3 m and 6 m radial-axial ring mills |
| Forging hammers | 1 t, 3 t, 6 t, 9 t |
| Hydraulic press | 4,500 t |
| Heat treatment | Annealing, normalising, quench and temper, stabilising anneal |
| Machining | Rough or finish machined to drawing |
These are the works limits across all grades. Alloy 42-6 orders are usually much smaller. Most sealing-alloy forgings fall between a few kilograms and a few hundred kilograms. Send your size and we will confirm feasibility before quoting.
Alloy 42-6 forging weight calculator
Density 8.15 g/cm³. Use the result to fill in your enquiry.
Result is the finished net weight. Allow roughly 20–35 % extra for machining stock to get the rough forging weight, more on complex profiles.
Where is Alloy 42-6 used?
Almost every application comes back to one requirement: a metal part that must stay dimensionally married to glass through repeated heating and cooling.
Lamps and lighting
Lead-in wires, ferrules and sealing rings in incandescent, halogen and discharge lamp envelopes.
Electron and vacuum tubes
Tube hardware, headers and vacuum-tight windows where the seal must hold a hard vacuum for years.
Telecommunications hardware
Hermetic feedthroughs, connector bodies and housings for RF and microwave assemblies.
Hermetic headers and terminals
Compression seals in pressure and gas terminals, sensor headers and instrument feedthroughs.
Sensors and instruments
Bodies, sleeves and rings for pressure sensors, thermocouple heads and laboratory instrumentation.
Thermostatic components
The controlled-expansion half of bimetal strip and temperature-regulating assemblies.
Choosing the right grade for the job. Alloy 42-6 is a sealing alloy, chosen for its match to soft glass. If your requirement is minimum expansion for dimensional stability (composite moulds, metrology frames, precision tooling), use Invar 36 instead. If the seal partner is a hard borosilicate glass or an alumina ceramic, a Fe-Ni-Co Kovar-type alloy is the correct choice. Tell us the application and we will confirm the grade before you commit to a forging.
What testing and certification comes with an Alloy 42-6 forging?
Chemistry and cleanliness
Full elemental analysis by optical emission spectrometry on every heat, reported against the ordered chemistry band. ESR remelting keeps sulphur, phosphorus and gas content low, and all three degrade seal quality.
Thermal expansion verification
Mean CTE measured by dilatometer on a stabilised sample and reported on the certificate. It is the single test that matters most for this grade. Specify the temperature ranges you want verified.
Ultrasonic testing
To EN 10228-3, SEP 1921 or ASTM A388, to the class stated on your order. Surface inspection by magnetic particle or dye penetrant on request.
Mechanical and metallographic
Tensile, hardness and impact testing on our universal, hardness and impact machines; grain size and structure checked on the metallographic microscope.
Certification
EN 10204 3.1 as standard. EN 10204 3.2 with third-party witness (Lloyd's, DNV, BV, ABS, TÜV or SGS) on request at the order stage.
Quality system
ISO 9001:2015. Customers may witness any production stage, including melting, forging, heat treatment and final testing. Heat numbers are traceable from billet to shipped part.
How do you specify an Alloy 42-6 forging order?
Sealing-alloy orders go wrong in predictable ways. Seven lines on the enquiry prevent almost all of them:
- Name the material unambiguously Write “Alloy 42-6, UNS K94760 (42Ni-6Cr), GB 4J6”. If you reference ASTM F31, add the edition and note that it was withdrawn in 2024.
- State the expansion requirement Give the mean CTE and the temperature range you need verified, for example “mean CTE 30–350 °C = 8.5–9.5 × 10⁻⁶/°C, dilatometer report required”. This is the acceptance criterion for the grade.
- Send the drawing or the dimensional list A 2D drawing or 3D model with tolerances and surface finish, or simply OD × ID × height for rings and bars. Say whether you want as-forged, rough machined or finish machined.
- Specify the delivery condition Annealed for machining, stabilised, or oxidised and ready to seal. If you want the wet-hydrogen oxide grown here, say so, and remember that no machining may follow it.
- Define the testing Chemistry, tensile, hardness, expansion test, ultrasonic class per EN 10228-3 / SEP 1921 / ASTM A388, and any surface NDT.
- Choose the certificate EN 10204 3.1, or 3.2 with a named third-party inspection body.
- Give quantity, delivery target and destination Quantity, required date, and the delivery term: EXW Jiangyin, FOB Shanghai, CIF or DDP to your port.
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Eight mistakes to avoid when ordering Alloy 42-6
1. Specifying strength instead of expansion
A hardness or tensile class tells the mill nothing useful about a sealing alloy. State the mean CTE and the temperature range instead.
2. Citing ASTM F30 for a chromium-bearing grade
F30 covered Alloy 42, 46, 48 and 52, none of which contain chromium. For 42-6 the reference was F31, withdrawn in 2024.
3. Machining after the oxide anneal
It removes the layer the seal depends on. Finish machine first, then oxidise.
4. Using the alloy above its match range
Past roughly 425 °C the expansion curve climbs steeply and the glass match is gone. Check your peak service and processing temperatures, not just the nominal one.
5. Substituting the 47Ni-6Cr sister grade
UNS K95150 sits in the same standard but expands differently. It is not a drop-in replacement.
6. Leaving the delivery condition blank
Annealed, stabilised and oxidised parts are not interchangeable. If it is not written down, it will be wrong for someone.
7. Ordering no expansion test
Chemistry inside the band does not guarantee the curve, because thermal history matters. Ask for the dilatometer result on the certificate.
8. Ignoring residual stress in thick sections
A forging finished cold and never annealed will shift dimensionally when the seal is made. Specify the stabilising anneal.
Frequently asked questions about Alloy 42-6
What is Alloy 42-6?
Alloy 42-6 is a controlled-expansion iron-nickel-chromium sealing alloy containing nominally 42 % nickel and 6 % chromium with the balance iron. It is designated UNS K94760, GB 4J6, and was covered by ASTM F31 until that standard was withdrawn in 2024. It is used where metal must be sealed hermetically to soft glass.
What do the numbers “42-6” mean?
42 is the nominal nickel content in percent and 6 is the nominal chromium content. The rest is iron. The nickel level sets the Invar-effect expansion behaviour; the chromium raises the expansion curve to match soft glass and forms the oxide that bonds to it.
Are Alloy 42-6, UNS K94760, 4J6, Alloy 426 and ASTM F31 alloy the same material?
Yes. All describe the same nominal Fe-42Ni-6Cr chemistry. Trade names for it include Carpenter Glass Sealing “42-6”, Vacovit 426 and Sealmet HC-4; those brands belong to their respective owners. Material produced by Jiangyin Jiangnan Metal Co., Ltd. is correctly described as UNS K94760 / 42Ni-6Cr / GB 4J6.
Is ASTM F31 still an active standard?
No. ASTM withdrew F31 in 2024, along with F30, F15, F29 and F1684 from the same committee. The last active revision was F31-21. The alloy itself is unchanged and the withdrawn document is still available as a historical standard, so the usual practice now is to cite “ASTM F31-21 (withdrawn 2024)” or to state the chemistry and expansion limits directly on the drawing.
What is the coefficient of thermal expansion of Alloy 42-6?
The mean linear coefficient is 8.5–9.5 × 10⁻⁶/°C from 30 to 350 °C and 9.7–10.7 × 10⁻⁶/°C from 30 to 425 °C. Typical informative values continue to about 11.2 × 10⁻⁶/°C at 500 °C and 13.0 × 10⁻⁶/°C at 700 °C, but above the magnetic transition the glass match no longer holds.
Which glass does Alloy 42-6 seal to?
Soft sealing glasses: lead and soda-lime types, classically Corning 0120. The alloy was developed to follow the expansion of that family. For hard borosilicate glass or alumina ceramic, an iron-nickel-cobalt Kovar-type alloy is the correct choice instead.
What is the difference between Alloy 42 and Alloy 42-6?
Alloy 42 is Fe-42Ni with no chromium and a lower expansion, used with harder glasses and in lead frames. Alloy 42-6 adds about 6 % chromium, which raises the expansion curve to match soft glass and lets the surface grow the adherent chromium oxide that makes a hermetic bond. They are not interchangeable.
Is Alloy 42-6 magnetic?
Yes, it is ferromagnetic at room temperature. The loss of ferromagnetism at the magnetic transition is exactly what causes the sharp upward bend in the expansion curve, which is why the specified expansion limits stop at 425 °C.
What is the density of Alloy 42-6?
About 8.15 g/cm³. Melting temperature is approximately 1430 °C, electrical resistivity 0.92 µΩ·m at 20 °C, thermal conductivity 13.4 W/m·K and specific heat 504 J/kg·K.
What forged shapes of Alloy 42-6 can you supply?
Forged and seamless rolled rings, round bars, discs, flanges, sleeves, bushings, tube sheets, shafts, pipes, blocks and near-net-shape parts to drawing. Works capability spans 100–6,000 mm diameter, up to 12,000 mm long and 10–15,000 kg per piece, although sealing-alloy forgings are usually far smaller.
Can Alloy 42-6 be welded and machined?
Yes to both. Conventional welding processes apply, GTAW with matching filler being normal, and the weld area should be re-annealed and re-oxidised before sealing. Machining behaves like an austenitic stainless: the alloy is gummy and work-hardens, so use sharp positive-rake tools, a rigid setup, positive feed and plenty of coolant.
Why does Alloy 42-6 turn dark green during annealing?
That colour is the chromium-rich oxide grown during the wet-hydrogen anneal, and it is deliberate. The oxide dissolves into the molten glass during sealing and produces the chemical bond that makes the joint vacuum-tight. Removing it by machining or blasting after annealing destroys the seal.
What is the maximum service temperature of Alloy 42-6?
For sealing purposes, roughly 425 °C, the upper limit of the specified expansion bands. Beyond that the alloy passes its magnetic transition, expansion accelerates and the match to glass is lost. The metal itself survives much higher temperatures, but the seal does not.
How do I get a price and lead time for Alloy 42-6 forgings?
Send the drawing or dimensions, quantity, required expansion band, delivery condition and certificate level to sales@steelforgepieces.com or WhatsApp +86 189 2135 9659. We reply within 24 hours. The RFQ generator above assembles the enquiry text for you.
Glossary
- Alloy 42-6
- Controlled-expansion sealing alloy of nominally 42 % nickel, 6 % chromium, balance iron, for glass-to-metal seals with soft glass.
- UNS K94760
- The Unified Numbering System designation for the 42Ni-6Cr sealing alloy, and the least ambiguous name to use on a purchase order.
- GB 4J6
- Chinese precision-alloy designation for the same chemistry, nickel 41.5–42.5 % and chromium 5.4–6.3 %.
- ASTM F31
- Standard Specification for Nickel-Chromium-Iron Sealing Alloys, covering UNS K94760 and UNS K95150. Withdrawn by ASTM in 2024; last revision F31-21.
- Controlled-expansion alloy
- An alloy whose thermal expansion is deliberately tuned, usually by nickel content, to match another material such as glass or ceramic.
- Invar effect
- The anomalously low thermal expansion of certain iron-nickel alloys, caused by magnetostriction partly offsetting normal lattice expansion below the magnetic transition.
- Mean coefficient of thermal expansion (CTE)
- The average fractional length change per degree over a stated temperature range, quoted here in × 10⁻⁶/°C. Glass datasheets often use × 10⁻⁷/°C.
- Wet-hydrogen anneal
- Annealing in hydrogen of controlled dew point, used to grow the adherent chromium oxide that bonds to molten glass.
- Matched seal
- A glass-to-metal seal in which both materials have nearly the same expansion, so residual stress stays low.
- Compression seal
- A seal in which a higher-expansion outer housing shrinks onto the glass, holding it in compression. Used in headers and terminals.
- EN 10204 3.1 / 3.2
- Inspection certificate types. 3.1 is issued by the manufacturer's independent quality department; 3.2 is countersigned by a third-party inspector.
References
- ASTM F31-21, Standard Specification for Nickel-Chromium-Iron Sealing Alloys (withdrawn 2024), ASTM International, Committee F01.
- ASTM International, standard status records for F31, F30, F15, F29 and F1684, all withdrawn in 2024. store.astm.org/f0031-21.html
- Alloy Digest FE-137, “Carpenter Glass Sealing 42-6”, ASM International. Composition, physical, elastic and tensile properties of UNS K94760.
- Ulbrich Stainless Steels & Special Metals, Alloy 42-6 (UNS K94760) technical data.
- NiWire Industries Co., Ltd., Alloy 42-6 / K94760 / ASTM F31 datasheet. Chemistry, physical constants and annealing practice.
- Carpenter Technology Corporation, Glass Sealing Alloys technical brochure. Seal types, expansion curves of common glasses and sealing alloys.
- GB/T standards for iron-nickel and iron-nickel-chromium precision alloys (4J series).
- EN 10204:2004, Metallic products. Types of inspection documents, CEN.
- EN 10228-3, SEP 1921 and ASTM A388. Ultrasonic examination of steel and alloy forgings.
Standards cited are the revisions known at the date of last review. Always reference the revision in force at your contract date. Values on the certificate issued with your order are measured on your heat and take precedence over any published typical data.
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