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Nimonic 80A / UNS N07080 / ASTM B637

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Nimonic 80A is a nickel-based superalloy, alloyed with chromium and hardened by ageing. Within the nickel alloy group it stands out for holding its mechanical strength over long periods in the 650-850 °C range. Its UNS designation is N07080.

Its hardening mechanism rests on the γ′ (Ni₃(Al,Ti)) precipitates formed by the titanium and aluminium additions. This precipitate phase gives high resistance to the softening that comes with temperature. The combined Ti and Al content is normally held in the 3.5-4% range, which is the critical threshold for forming the γ′ phase.​‌​​‌​

Oxidation resistance is very high at service temperatures up to 900 °C. Resistance to thermal fatigue under repeated thermal cycling, and easier weldability than other nickel superalloys, are the alloy’s outstanding practical advantages.

It is used for gas turbine blades, seals, brackets and fastening components; in nuclear power for in-reactor components and high temperature bolting; and in automotive and motorsport for turbocharger wheels and exhaust systems. It carries NACE MR0175 / ISO 15156-3 approval for H₂S-bearing environments.​‌​​‌​

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What Nimonic 80A Is — Nichrome With Titanium and Aluminium Added

Nimonic 80A (UNS N07080 / W.Nr. 2.4952 and 2.4631 / EN chemical name NiCr20TiAl / AFNOR NC 20 TA / BS NA20) is a precipitation-hardenable (age-hardenable) nickel-chromium alloy made by adding 1.8–2.7 % titanium and 1.0–1.8 % aluminium to a nominal 20 % chromium, balance nickel matrix. Metallurgically the description is that simple: 80/20 nichrome, plus just enough Ti and Al to form γ′. The hardening mechanism is coherent precipitation of the ordered intermetallic γ′ — Ni₃(Ti,Al).​‌​​‌​

The single sentence that distinguishes this alloy: among γ′-hardened nickel alloys it is the simplest, the cheapest and the highest in chromium — no cobalt, no molybdenum, no niobium, effectively no iron. 18–21 % chromium gives it the best oxidation and combustion-deposit resistance in the γ′ family; γ′ gives it hot hardness and fatigue strength at valve temperature. The price is the temperature ceiling: about 815 °C under load. Above that γ′ coarsens and begins to dissolve, and the alloy starts behaving like a plain solid-solution nichrome. Nimonic 80 was developed in 1941 at Wiggin in Birmingham for the first jet engine turbine blades and 80A followed in 1945; it lost the turbine-blade job decades ago and the surviving commercial application, which carries the overwhelming majority of today’s tonnage, is exhaust valves in internal combustion engines.

Standards by Product Form

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Read this section carefully: the standards map for 80A is markedly WEAKER than for most nickel alloys. This is a bar and forging alloy: there is no ASTM equivalent for plate, pipe or wire.

Where 80A sits inside EN 10090 — and its relation to the X-series

ASME Code Acceptance and Code Temperatures — Where We Have To Be Honest

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This is the section most distributor pages quietly skip, and that is exactly why it is commercially valuable.​‌​​‌​

Product Forms With NO Standard — the Commercially Valuable Section​‌​​‌​

Chemical Composition — ASTM and EN Describe the Same Alloy With Different Limits​‌​​‌​

The ASTM and EN composition tables for 80A are NOT the same, and the difference genuinely matters on a certificate. Three elements diverge: aluminium, iron and cobalt. Unless the order states which table governs, a heat that passes ASTM can fail EN.

γ′ — the metallurgical core of this page

All of 80A’s strength comes from γ′, the ordered intermetallic Ni₃(Ti,Al). The phase precipitates coherently with the FCC γ matrix — its lattice planes continue those of the matrix. Dislocations can pass a coherent particle only by cutting it or by looping around it; both cost energy, and the alloy hardens.
The Ti + Al sum is the critical number. Across the specification band it is 2.8 % to 4.5 % (Ti 1.8–2.7 + Al 1.0–1.8). That single sum fixes three things at once: the γ′ volume fraction (strength), the γ′ solvus (service ceiling) and susceptibility to strain-age cracking (weldability). They cannot be separated — nobody can sell you an 80A that is both stronger and easier to weld.
The Ti/Al ratio matters too. A titanium-rich γ′ coarsens faster than an aluminium-rich one, so long-term high-temperature stability does not like Ti near the top of the band. That is the second reason the ASTM aluminium floor of 0.50 is risky.​‌​​‌​

Mechanical Properties — Do Not Confuse Minimum With Typical

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Three separate sets of minima are published for 80A, and mixing them is the most common error in the trade. Read the tables below separately.​‌​​‌​

Physical Properties​‌​​‌​

Heat Treatment and Thermal Stability — THERE ARE TWO SEPARATE ROUTES​‌​​‌​

There is no single “standard heat treatment” for 80A, and this is the most critical information on the page. The alloy is processed by two different routes for two different jobs: the creep route (turbines, bolting, long-term static load) and the valve route (endurance-strength driven). Same chemistry, different part.

Thermal stability — where 80A degrades

80A is not expected to show an embrittling ordering reaction or a “window to avoid” of the kind seen in solid-solution alloys. There is one degradation mechanism and it is simple: γ′ COARSENS and eventually DISSOLVES. By Ostwald ripening the large γ′ particles consume the small ones, the spacing between them opens up, and dislocations begin to loop rather than cut — the alloy softens. The process becomes measurable above about 700 °C and reaches a commercially unacceptable rate around 815 °C. The physical measurement is the modulus: 161 GPa at 700 °C, 130 GPa at 800 °C — 19 % lost in a single 100 °C step. The rule is single: do not exceed 815 °C under load, and design long-term service from the creep table. Numerical data on 80A’s loss of ductility and toughness after tens of thousands of hours of exposure could not be independently verified — for a critical application ask the producer for post-exposure data.​‌​​‌​

Welding — Strain-Age Cracking Is the Most Important Warning on This Page

80A is a γ′-hardened alloy, and for welding that means one thing: the risk of STRAIN-AGE CRACKING (reheat cracking). A fabricator who does not understand this risk produces parts that crack after welding — and the cracks usually appear not during welding but on the heating ramp of the post-weld heat treatment.​‌​​‌​

What actually goes wrong​‌​​‌​

1. Welding aged material. The most frequent and most expensive mistake. The part may not crack during welding; the crack appears during post-weld heat treatment or on the first thermal cycle. A pre-weld solution anneal is not negotiable.
2. Applying a “stress-relief anneal”. On 80A a “stress relief” at 600–750 °C is exactly the ageing cycle — it does not relieve stress, it hardens the matrix and triggers the crack. Either a full solution anneal is performed, or nothing is.
3. A load-carrying joint made with the wrong filler. A joint made with ENiCrFe-3 leaves weld metal markedly weaker than the parent after ageing. Acceptable for a repair; not for a design joint.
4. Trusting the sentence “80A is a weldable alloy”. Some distributor pages print it with no conditions attached. The truth is: 80A can be welded in the right condition and with the right heat treatment cycle; if those conditions are not met, it cracks.

Machining​‌​​‌​

A caveat that must be stated plainly: the parameters below are starting values compiled from secondary sources, not the producer’s own 80A-specific table. Verify them with your own trials. The producer’s qualitative rule is clear: select a lower cutting speed and keep the tool engaged in the cut at all times.

Corrosion — Where It Is Good, and WHERE IT FAILS

80A is a high-temperature oxidation alloy. It is NOT an aqueous corrosion alloy. This distinction is the most frequently violated part of the page, because the alloy’s 18–21 % chromium makes it look like something better than a stainless steel. It is not: there is no molybdenum, no nitrogen, no copper.​‌​​‌​

Where it is good — high-temperature oxidation

80A forms a tightly adherent, protective scale based on Cr₂O₃ (chromium oxide). The published scaling-resistance limit is 1000 °C. Wear studies report a layered oxide developing in the sequence Cr₂O₃ / TiO₂ / Al₂O₃ / substrate at the interface — so titanium and aluminium do more than form γ′, they also work beneath the scale. Against iron-base heat-resisting steels of the same chromium this is a clear advantage: the nickel matrix gives fast re-passivation when the scale cracks and reduces spalling under thermal cycling.​‌​​‌​

Where it is good — the exhaust-gas environment (the real application)

This is 80A’s commercial reason to exist. Producer data states that the alloy has proven its resistance to vanadium pentoxide (V₂O₅), sodium compounds and sulphur compounds as an exhaust valve in heavy-oil-fired engines. That trio is the classic valve-killer of heavy-fuel marine and stationary diesels:
Vanadium pentoxide is a low-melting oxide (melting around 675 °C) which in the molten state dissolves and fluxes away the protective Cr₂O₃ scale — the classic catastrophic oxidation mechanism.
Sodium sulphate (Na₂SO₄), formed from sodium in marine air and sulphur in the fuel, drives hot corrosion.
80A survives this environment because of its high chromium — the stronger but lower-chromium members of the γ′ family, for example X-750 at 14–17 % Cr, do not substitute for it here. This is the strongest technical argument you have when selling 80A.​‌​​‌​

WHERE IT FAILS — 1: aqueous corrosion

80A contains NO molybdenum. That single fact explains every one of its aqueous weaknesses. Molybdenum is the element that prevents local breakdown of the passive film (pitting and crevice corrosion) in chloride-bearing media. Calculated roughly, with Mo = 0 and N ≈ 0 the alloy’s pitting resistance equivalent is only its chromium: about 18–21 — below 316L (about 24) and not comparable with 2507 super duplex or C-276.
Practical consequences: do not use it in seawater. Do not use it in chloride process solutions. Do not use it in reducing acids (HCl, H₂SO₄) — that is where B-3 and C-276 live. No published aqueous corrosion rate tables for 80A could be found, and that is not a gap but a message: producers do not sell this alloy for that duty.​‌​​‌​

WHERE IT FAILS — 2: reducing, sulphidising atmospheres

This is the classic weakness of high-nickel alloys, and 80A is about 75 % nickel. In a reducing atmosphere low in oxygen and rich in sulphur — incomplete combustion products, sulphurous process gases, petrochemical regeneration environments — the protective Cr₂O₃ scale cannot remain stable and nickel sulphide forms. The Ni–Ni₃S₂ eutectic is widely reported in the general literature to melt at about 645 °C — that is a liquid phase far below valve temperature. The liquid sulphide penetrates along grain boundaries and destroys the alloy from the inside; the damage is fast and irreversible.
Caution — do not conflate two separate claims. Producer data says 80A resists the sulphur compounds in exhaust gas; that environment is oxidising (large excess air) and the Cr₂O₃ scale is stable. It does not follow that the same alloy will survive in a reducing sulphidising process atmosphere. A quotation that misses this distinction ends in a field failure. For such an environment look at higher-chromium, lower-nickel choices such as 800H or 601 — and still have the sulphur partial pressure evaluated.​‌​​‌​

WHERE IT FAILS — 3: mechanical collapse above 815 °C

This is not a corrosion failure, but in the field it looks like one. Above 815 °C the γ′ coarsens and dissolves; the part begins to creep, the valve head distorts, the seat face loses contact and hot gas leakage starts. The escaping gas burns the seat (guttering) and the failure report calls it “corrosion”. The real cause is excess temperature. Look at the creep table: the 10⁵ h rupture strength at 800 °C is 20 MPa — engineering-wise, close to zero.​‌​​‌​

Frequently Asked Questions

Our exhaust valve is 21-4N (1.4882). Is it worth paying to move to Nimonic 80A?​‌​​‌​

The answer depends entirely on the real valve-head temperature and the loading regime, and deciding without measuring those two numbers is burning money.
Both sit in the same standard. EN 10090 covers 1.4882 / X50CrMnNiNbN21-9 and 2.4952 / NiCr20TiAl alike, and both are delivered in the +AT+P condition. There is no standards difference; they are two rungs of the same ladder.
What 21-4N does: an austenitic matrix stabilised with manganese and nitrogen, plus niobium carbides and nitrides. It is sufficient for an ordinary petrol engine’s exhaust valve and costs an order of magnitude less per kilogram. Its weakness is that as temperature rises it loses hot hardness and fatigue strength quickly; its matrix is supported by solid solution and dispersed carbides, not by a coherent precipitate.
What 80A does: γ′ — Ni₃(Ti,Al) — keeps the yield strength remarkably flat against temperature: a room-temperature yield of about 600 MPa is still about 500 MPa at 600 °C. Add 18–21 % chromium and you also get resistance to vanadium, sodium and sulphur compounds, which the producer publishes as proven in heavy-fuel engines.
So the decision rule: if your valve-head temperature is inside the comfortable range of the austenitic steel and your failures are wear or seating related, moving to 80A will not fix your problem, only make it expensive. But if your failures look like head distortion, seat face loss, gas leakage (guttering) or stem fracture — that is, hot-strength and fatigue failures — 80A makes a real, measurable difference. Turbocharged high-output engines and heavy-fuel diesels historically moved to 80A for exactly this reason.
One final warning: if you do move, specify the valve-route heat treatment (solution anneal 1010–1050 °C), not the creep route. 80A delivered on the wrong route will not give you the fatigue life you paid for.

The datasheet says “maximum operating temperature 1000 °C”. Can we use it at 900 °C?​‌​​‌​

No. And this is the most common and most expensive misreading of 80A.
1000 °C is an OXIDATION number, not a STRENGTH number. The producer’s wording is “resistance to scaling up to 1000 °C” — meaning the surface is not rapidly consumed at that temperature. The same producer publishes the maximum service temperature under load as 815 °C (1500 °F).
Look at the numbers. The EN 10302 route creep data gives a 10⁵ h rupture strength of 75 MPa at 700 °C, 37 MPa at 750 °C and 20 MPa at 800 °C. The curve is not published beyond 800 °C — because there is no meaningful strength left to publish. At 900 °C γ′ is largely dissolved and the alloy behaves like a γ′-free nichrome, i.e. at the level of alloy 600 or Nimonic 75. Everything you paid extra for buys you nothing there.
Physical confirmation: modulus of elasticity 161 GPa at 700 °C and 130 GPa at 800 °C — 19 % lost in a single 100 °C step. That is a direct measurement of the collapse of γ′.
What to do at 900 °C: if the load is low and the requirement is really oxidation resistance, solid-solution alloys such as Nimonic 75 or alloy 601 are already sufficient and far cheaper. If the load is high you need Nimonic 90 (cobalt-bearing, rupture strength published to about 920 °C) or a cobalt-base alloy. What you must not do is convert an oxidation number into a design temperature.

The certificate says W.Nr. 2.4631 but we ordered 2.4952. Do we reject it?​‌​​‌​

Not automatically — but this is a case where you must check the chemistry table line by line.
First the fact: 2.4952 and 2.4631 name the same alloy, and the overwhelming majority of producers publish them together, without distinction — a German producer, a French mill and several distributors print the two numbers side by side. They are not two different alloys.
Then the caveat: no authoritative statement distinguishing the two numbers could be found. The only observable pattern is this: 2.4952 is quoted together with the EN 10090 / EN 10269 / EN 10302 / DIN 17742 table (Al 1.0–1.8 % · Fe ≤1.5 % · Co ≤1.0 % · C ≥0.04 %), while 2.4631 is commonly quoted together with the N07080 / NA20 / ASTM B637 band (Al 0.50–1.80 % · Fe ≤3.0 %). That is an observation, not a proven rule, and it is presented as such on this page.
What to do is simple: look not at the number but at the actual values on the analysis certificate. Is aluminium below 1.0 %? Is iron above 1.5 %? Is carbon below 0.04 %? The answers to those three questions tell you whether you meet the EN specification you ordered far more reliably than the number at the top of the page. If all three are inside the EN band, the number is a labelling matter and you can ask the supplier to correct it. If any one of them falls outside, the material is not compliant no matter what number is printed.
Write this into the order: “NiCr20TiAl, W.Nr. 2.4952, to the EN 10269 chemistry table” — so that which band governs is never a matter for debate.

Common Datasheet Errors and Traps — Check Before You Order​‌​​‌​

1. Yield and tensile strength swapped. A widely mirrored 80A page prints “yield 1000 MPa, tensile 621 MPa”. Yield cannot exceed tensile. The truth is the reverse: 621 MPa corresponds to the yield (about 90 ksi, the ASTM B637 minimum) and 1000 MPa to the tensile. Reject on sight any table where yield exceeds tensile.
2. A factor-of-ten error in electrical resistivity. At least one publisher prints 12.4 µΩ·m. The correct figure is 1.24 µΩ·m = 124 µΩ·cm = 1.24 Ω·mm²/m. In a heating-element or current-carrying calculation that error is off by ten times.
3. “Tensile 145 ksi / 1000 MPa minimum per ASTM B637.” WRONG. The B637 minimum is 135 ksi / 930 MPa. The 1000 MPa figure comes from the floor of the EN 10269 band or from the BS HR 1 / MSRR route. No standard contains a table combining the two systems in one row.
4. “Solution annealed: Rp0.2 600 MPa.” One producer labels its table that way, but those values are almost identical to ASTM B637’s FULLY HEAT TREATED minima. An 80A with no γ′ precipitated is not expected to yield at 600 MPa. If you are buying un-aged material, have it tested on the heat.
5. Three different cobalt limits. Cobalt is NOT LISTED in the ASTM B637 N07080 row; the EN route says ≤1.0 % and BS/commercial practice ≤2.0 %. In nuclear work that difference is directly an activation question. Put it in the order.
6. Nimonic 90’s chemistry printed as 80A. Any 80A table showing Co 15–21 % is WRONG — that is Nimonic 90 (N07090 / 2.4632). In 80A cobalt is an impurity ceiling, not an alloying element. Similarly, research literature sometimes describes 80A as “nichrome plus Ti/Al” using a nominal containing Fe 5.0 %; that exceeds even the ASTM ceiling of 3.0 % and is three times the EN ceiling of 1.5 %. Do not use it as a purchasing limit.
7. BS HR numbers confused with product form. The correct mapping is: BS HR 1 = rod/bar, BS HR 201 = plate/sheet/strip, BS HR 401 = pipe/tube, BS HR 601 = forgings. If a distributor page says “BS HR 601” and sells plate, either the number or the product is wrong.
8. “Maximum operating temperature 1000 °C.” That figure is the scaling resistance limit. The maximum service temperature under load is 815 °C. A third number, about 550 °C, is the relaxation limit some wire and spring makers publish. Three different numbers, three different meanings — label each one.
9. Writing a single “heat treatment”. 80A has two routes: the creep route (solution anneal 1050–1080 °C) and the valve route (solution anneal 1010–1050 °C). Any recipe that omits the 849 °C / 24 h stabilising step is incomplete. A separate sheet route (1150 °C / 2–3 minutes) is published and must not be confused with the bar route. Note too that majority practice and the ASTM route call for air cooling, while a single source writes “water quench”.
10. Saying “80A welds easily”. With Al + Ti = 2.8–4.5 % the alloy sits in the strain-age cracking risk band. Welding is done only in the solution-annealed condition and the full post-weld heat treatment cycle is mandatory. A simple stress-relief anneal is not an option.
11. Implying NACE MR0175 compliance. Listing of N07080 in ISO 15156-3 Annex A could not be verified and the alloy contains no molybdenum. Never make that declaration.
12. Treating 2.4952 and 2.4631 as two different alloys. They are the same alloy. But the chemistry bands published alongside the two numbers are not identical — look at the Al, Fe and C values on the certificate, not at the number.

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