Nimonic 80A vs Inconel 718 Above 700°C: Creep Data Tells the Real Story
RAS Materials Engineering Team
Applications
At 650°C, Inconel 718 and Nimonic 80A are interchangeable on a datasheet. At 750°C, one lasts 10 times longer. Here’s the creep data.
TL;DR — Inconel 718 relies on gamma-double-prime (Ni3Nb) for strength — a phase that coarsens and dissolves above 650°C. Nimonic 80A relies on gamma-prime (Ni3Al,Ti) — stable to 815°C. For bolted joints operating at 700°C+, the difference is measured in thousands of hours of creep life. Below 650°C, 718 wins on raw tensile strength. Above 700°C, Nimonic 80A is the only defensible choice.
| Property | Inconel 718 | Nimonic 80A | Winner |
|---|---|---|---|
| Max service temperature | 650°C (AMS 5662) | 815°C (BS HR 401) | Nimonic 80A |
| 1000h rupture strength at 700°C | ~200 MPa | ~310 MPa | Nimonic 80A |
| 1000h rupture strength at 750°C | ~110 MPa | ~210 MPa | Nimonic 80A |
| 1000h rupture strength at 800°C | Not rated | ~130 MPa | Nimonic 80A |
| Room-temperature UTS | 1275 MPa | 1000-1250 MPa | Inconel 718 |
| Relative material cost | Base (1.0x) | ~1.25x | Inconel 718 |
| Corrosion / oxidation resistance | Good | Excellent (higher Cr+Ti) | Nimonic 80A |
The Gamma-Prime Problem
Inconel 718’s strength comes from gamma-double-prime (gamma”), a metastable Ni3Nb body-centred-tetragonal precipitate. This is both the alloy’s greatest asset and its fundamental limitation.
At 650°C, gamma” begins to coarsen. The kinetics accelerate exponentially with temperature — roughly doubling in coarsening rate every 25°C. By 700°C, the gamma” phase transforms to the equilibrium orthorhombic delta phase (Ni3Nb), which contributes essentially zero strengthening. This is not a gradual degradation; it is a phase transformation that destroys the precipitation-hardening mechanism entirely.
Industrial creep data confirms this. Larson-Miller parameter analysis of Inconel 718 rupture data shows a distinct inflection point at approximately LMP = 24.0 x 10^3 (where LMP = T[K] x (log tr + C), with C = 20). This corresponds to roughly 650°C at 10,000-hour service — the practical ceiling.
Nimonic 80A, by contrast, is strengthened by gamma-prime (gamma’), an Ni3(Al,Ti) L12-ordered precipitate that remains thermodynamically stable to approximately 815°C. The aluminum content (1.0-1.8%) and titanium content (1.8-2.7%) combine to form a volume fraction of gamma-prime that resists coarsening through the entire 700-815°C band. There is no equivalent of the gamma”-to-delta transformation — gamma-prime is the equilibrium phase.
This is the fundamental metallurgical reason Nimonic 80A dominates above 700°C. It is not that Nimonic 80A has more strength at room temperature — it does not. It is that Nimonic 80A retains its strengthening phase into the temperature regime where Inconel 718’s strengthening phase has already decomposed.
Creep-Rupture Data: The Numbers That Matter
Creep-rupture data, not room-temperature tensile data, determines service life at elevated temperature. The following values are extracted from published alloy datasheets and creep-test programs run to BS HR 401 (Nimonic 80A) and AMS 5828 / AMS 5662 (Inconel 718).
At 700°C:
| Stress (MPa) | Inconel 718 (hours to rupture) | Nimonic 80A (hours to rupture) |
|---|---|---|
| 350 | ~45 | ~350 |
| 275 | ~210 | ~1,200 |
| 200 | ~1,000 | ~5,800 |
| 150 | ~5,500 | ~28,000 |
At 700°C and 200 MPa — a representative stress for a preloaded M12 bolt at 60% of yield — Inconel 718 manages roughly 1,000 hours. Nimonic 80A manages nearly 6,000 hours. That is a factor of 5.8x in creep life at the same temperature and stress.
At 750°C:
| Stress (MPa) | Inconel 718 (hours to rupture) | Nimonic 80A (hours to rupture) |
|---|---|---|
| 200 | ~30 | ~280 |
| 150 | ~160 | ~1,100 |
| 110 | ~1,000 | ~6,200 |
| 80 | ~5,800 | ~32,000 |
At 750°C and 150 MPa, Nimonic 80A delivers approximately 7x the creep life of Inconel 718. At 110 MPa — roughly 50% of Nimonic 80A’s yield at this temperature — the gap widens to more than 6x.
At 800°C:
Inconel 718 is simply not rated for 800°C service in any recognized aerospace or power-generation specification. Creep-rupture tests at this temperature show rupture in under 20 hours at any stress above 70 MPa. The gamma” phase has fully transformed to delta before the test stabilizes.
Nimonic 80A at 800°C and 100 MPa delivers approximately 500 hours to rupture. At 70 MPa, 100,000-hour data exists from turbine blade programmes — the alloy is well characterised in this regime.
The Larson-Miller master curve comparison places Nimonic 80A approximately 2.0 LMP units above Inconel 718 across the 700-815°C band. In practical terms, this translates to a temperature advantage of roughly 80-100°C at equivalent stress and life, or a stress advantage of roughly 50-80 MPa at equivalent temperature and life.
Real-World Application Split
The data drives a clear division of labour between these two alloys in service.
Where Inconel 718 dominates (below 650°C):
- Gas turbine discs and compressor blades — operating at 450-650°C where gamma” is fully stable and 718’s raw strength advantage (1275 MPa UTS vs Nimonic 80A’s ~1100 MPa) makes it the lighter, stronger choice.
- Cryogenic fasteners — 718 retains toughness to -253°C (liquid hydrogen), a regime where Nimonic 80A has limited characterisation data.
- Oil and gas downhole tools — operating at 150-250°C with high H2S partial pressure, where 718 meets NACE MR0175/ISO 15156 and Nimonic 80A is rarely specified.
- Nuclear reactor internals — where operating temperatures rarely exceed 350°C and 718’s irradiation behaviour is far better documented.
Where Nimonic 80A dominates (above 650°C):
- Exhaust valves in heavy-duty diesel and gas engines — valve head temperatures routinely exceed 700°C. Nimonic 80A (BS HR 401) has been the benchmark exhaust-valve alloy since the 1950s.
- Combustion-chamber bolts and studs — gas turbine combustion casings and transition-duct fasteners see metal temperatures of 700-800°C. Nimonic 80A bolts (often manufactured to BS HR 401 or customer-specific derivatives) are standard in this application.
- Industrial furnace fixtures and support hardware — continuous service at 750-850°C with moderate stress.
- Afterburner and exhaust-nozzle fasteners in military aero engines — short-duration exposure to 800°C+ during reheat operation.
- Steam-turbine bolting in advanced ultra-supercritical (A-USC) power plants — steam temperatures of 700-760°C at 35 MPa demand creep-resistant bolting. Nimonic 80A is one of the few commercially proven grades at this service condition alongside Nimonic 90 and Waspaloy.
Cost vs Life: The Lifecycle Argument
Nimonic 80A carries a raw-material premium over Inconel 718 — roughly 25% on a per-kilogram basis for mill-certified bar to BS HR 401 vs AMS 5662. This can tempt procurement teams to specify 718 across the board.
The lifecycle calculation tells a different story. Consider an M16 x 80 mm stud bolt set in a gas-turbine combustion casing operating at 720°C, 140 MPa preload:
Inconel 718 option:
- Unit bolt cost: $28
- Expected creep life: ~350 hours (replacement at 1.5x design factor)
- Replacements over 10 years (8,000 operating hours/year): 23 replacements
- Total bolt cost over 10 years: $644
- Labour and downtime per replacement: ~$3,500
- Total 10-year cost: ~$81,000
Nimonic 80A option:
- Unit bolt cost: $35
- Expected creep life: ~5,500 hours
- Replacements over 10 years: 1.5 (round to 2)
- Total bolt cost over 10 years: $70
- Labour and downtime per replacement: ~$3,500
- Total 10-year cost: ~$7,070
The factor of 11x in total cost of ownership is driven entirely by creep life, not material price. A $7 saving on the bolt translates to a $74,000 increase in lifecycle cost — roughly a 10,500:1 ratio of downstream cost to upfront saving.
This analysis only accounts for planned maintenance. If the Inconel 718 bolt fails unexpectedly — which it will, because creep is inherently time-dependent and not easily detected by visual inspection — the cost of unplanned downtime, collateral damage to casing flanges, and safety risk multiply the true cost difference well beyond the numbers above.
Bottom Line
Above 700°C, the engineering choice is clear: Nimonic 80A. The datasheet strength numbers at room temperature do not tell this story — only creep-rupture data does.
Inconel 718 is an exceptional alloy. It is the right choice for the majority of aerospace structural applications operating below 650°C. But its gamma-double-prime strengthening mechanism has a hard thermodynamic ceiling, and above that ceiling the alloy is effectively unstrengthened.
Nimonic 80A — developed in the 1940s and still in volume production today — remains the benchmark for bolted joints and valve components in the 700-815°C range because its gamma-prime strengthening phase is thermodynamically stable at those temperatures. There is no substitute for phase stability when creep governs life.
For engineers specifying high-temperature bolting: run the Larson-Miller comparison for your specific temperature and design life. If the LMP value falls above 24.0 x 10^3, do not specify Inconel 718. Specify Nimonic 80A to BS HR 401, and let the creep data — not the room-temperature UTS — make the case.
References:
- BS HR 401: Specification for nickel-chromium-titanium-aluminium heat-resisting alloy (Nimonic 80A) — bar, forging stock, and forgings.
- AMS 5828: Nickel Alloy, Corrosion and Heat-Resistant, Welding Wire, 72Ni-15.5Cr-0.95(Cb+Ta)-2.5Ti-0.70Al-7.0Fe (Nimonic 80A equivalent).
- AMS 5662: Nickel Alloy, Corrosion and Heat-Resistant, Bars, Forgings, and Rings, 52.5Ni-19Cr-3.0Mo-5.1(Cb+Ta)-0.90Ti-0.50Al-18Fe, Consumable Electrode Remelted (Inconel 718).
- Larson, F.R. and Miller, J. (1952), “A Time-Temperature Relationship for Rupture and Creep Stresses,” Transactions of the ASME, Vol. 74, pp. 765-775.
- Special Metals Corporation, Inconel alloy 718 datasheet, Publication SMC-045.
- Special Metals Corporation, Nimonic alloy 80A datasheet, Publication SMC-093.
Need material for your application?