Selecting a high temperature alloy is not a matter of finding something that "withstands heat". It is a question of matching the alloy's strengthening mechanism to the specific combination of temperature, stress, environment and life that the component actually sees. An alloy that is excellent in a turbine blade may be a poor choice for a furnace fixture, and the reverse is equally common. This guide works through the major duty families and what each one demands of the material.

The Three Things That Actually Limit an Alloy

Before comparing grades, it helps to separate the three independent failure mechanisms. Most alloy selection errors come from confusing them.

  • Creep — slow, permanent deformation under sustained stress at elevated temperature. This is the dominant limit in rotating machinery and any long-duration stressed component. An alloy can be perfectly strong on a tensile test at room temperature and still creep away under load at 700 °C.
  • Oxidation and hot corrosion — reaction with the gas atmosphere. Oxidation consumes metal and forms scale; hot corrosion (sulfidation, carburisation, vanadate attack) can be far faster and is the usual cause of premature failure in gas turbine hot sections.
  • Thermal fatigue — cracking from repeated heating and cooling cycles. A furnace cycling daily, or a turbine start-stop regime, imposes this rather than steady creep.

The alloy families divide roughly along these lines. Solid-solution strengthened alloys (the 800 series, Haynes 188) resist oxidation and hot corrosion well but have relatively low strength. Precipitation hardened alloys (718, Waspaloy, Nimonic grades) get their strength from fine precipitate particles and creep much more slowly, but some are vulnerable to strain-age cracking. Cobalt-based alloys (Haynes 188, Haynes 230) hold up in the hottest, most corrosive atmospheres where nickel-based grades start to struggle.

Understanding which family an alloy belongs to explains most of its behaviour — including why it fails, and how.

Gas Turbine Hot Section: Where the Temperature Is Highest

The gas turbine hot section is the most demanding high temperature application there is, because the peak metal temperature sits close to the material's own melting point while the blade is also spinning under centrifugal load. Different components in that section fail for different reasons, so they take different alloys.

Component Duty and failure mode Typical alloy family
Turbine blades Centrifugal stress plus creep at the highest metal temperature. Single-crystal and directionally solidified castings where creep life is critical. Nickel-based gamma-prime strengthened superalloys; cobalt-based and noble alloys at the very hottest stations
Turbine discs Very high centrifugal and cyclic stress at moderate-to-high temperature. Failure is usually low-cycle fatigue, not creep. 718-type and other gamma-double-prime strengthened alloys, PM-processed
Combustor liner Severe thermal cycling plus hot corrosion from the combustion products. Perforation from burn-through is the classic failure. Solid-solution strengthened nickel or cobalt alloys with high chromium
Hot section fasteners Sustained load at temperature. Critically, these see slow strain ageing during service, which is why alloys prone to strain-age cracking need careful substitution control. Waspaloy, Nimonic and similar gamma-prime grades
Nozzles and transition pieces Hot corrosion and erosion from particulates in the gas path. Solid-solution strengthened nickel alloys with high chromium

For hot section fasteners specifically, Waspaloy (UNS N07001, EN 2.4654) is the classic choice rather than 718. Waspaloy is gamma-prime strengthened with 12–15% cobalt, and it maintains its advantage over 718 above roughly 650–705 °C. The trade-off is strain-age cracking: Waspaloy is susceptible to it, so it cannot be substituted freely into a 718 fastener position, and the service temperature limit depends on the applied stress as well as the metal temperature.

For burner and hot corrosion duty, the solid-solution grades are the answer because resistance to the corrosive gas phase matters more than strength. Haynes 188 (UNS R30188) is a cobalt-based solid-solution alloy with a chromium content that gives good oxidation resistance to around 1095 °C, plus good sulfidation resistance. It is a workhorse for combustor and heat-treatment fixture duty. Be aware of its limitation: it is a solid-solution alloy, so its strength is comparatively low. Where both strength and hot corrosion resistance are needed, those requirements usually point to a different grade entirely.

Steam Turbines: Lower Temperatures, Different Problems

Steam turbines run far cooler than gas turbines — typically up to around 600 °C at the hottest stages, depending on the plant and the steam conditions. The metal temperature is not the challenge, so the alloy choice looks different.

  • Blades and rotor discs — governed by creep and low-cycle fatigue under wet-dry steam cycling. Austenitic stainless steels and 9% to 12% chromium creep-resistant steels such as G91, G92 and Marbnetic (9Cr-1W-V) cover most duties, with nickel alloys used at the highest temperatures.
  • Casing and bolting — creep-resistant steel and high-temperature bolting alloys.
  • Corrosion and erosion — the classic problem in the low-pressure end, where moisture impingement strips the protective oxide from the last stages. Erosion shields and stainless or copper alloys handle this.

The important practical point: do not default to nickel superalloys in a steam turbine. They cost several times more than the creep-resistant steels that actually satisfy the duty, and they bring fabrication difficulties — notably poorer resistance to steam corrosion cracking and more difficult welding — in exchange for strength the application does not need.

Nuclear Reactor Applications

Reactor service is the most qualification-intensive high temperature application, because the design life is measured in decades and the environment combines heat, radiation, water chemistry and mechanical load simultaneously.

Reactor type Typical conditions Material requirements
PWR — pressurised water ~290–325 °C, 155 bar, high neutron flux Low-alloy and stainless steels for pressure boundaries; nickel-based alloys for steam generator tubing, control rod drive parts and large internals where corrosion in primary water governs
BWR — boiling water ~285 °C at saturation, 70 bar, high steam flow Same principle; thinner-wall tubing designs and more demanding flow-accelerated corrosion control
Fission fast reactors ~500–600 °C, sodium or lead coolant High creep-resistant ferritic-martensitic or austenitic steels, with 9–12% chromium grades preferred; swelling and liquid-metal compatibility drive selection
Gas-cooled reactors High temperature, CO₂ or helium coolant Nickel-based alloys where oxidation and creep in the coolant stream dominate; graphite and ceramic fuels change the surrounding matrix requirements
Research reactors Moderate temperature, very high flux in the core Aluminium and aluminium alloys widely used for cladding, beam windows and core plate because of low density, low neutron absorption and good thermal conductivity

Two failure mechanisms are specific to reactor service and worth naming. SCC in primary water — nickel alloys such as Alloy 690 and 825 are used in tubing and core internals precisely because they resist chloride and primary-water stress-corrosion cracking better than 600. And void swelling in irradiated ferritic-martensitic steels, which caps the operating temperature of that family in fast reactors.

At Plus Metals we supply the reactor-relevant alloys including Inconel 718 and Haynes 188; more specialised pressure-boundary and cladding grades are handled as specific enquiries, quoted against the governing code and the mill certification you require.

Furnaces, Heat Treatment and Thermal Systems

Furnace and kiln duty is where high temperature alloy choice is most often made on price-per-kilogram instead of service temperature, and where the waste is largest.

  • Furnace fixtures, trays and baskets — must resist creep at load-carrying temperature while surviving repeated thermal cycling. 718 and 800H-type grades cover typical furnace atmospheres.
  • Radiant tubes, burners and nozzles — high heat flux, internal oxidation, and long service life in the flame path.
  • Heat treatment baskets in carburising and nitriding atmospheres — note that carburising and nitriding atmospheres require specific grades; ordinary 718 and 800 are attacked by these atmospheres at temperature. Use a 600-series or 330-type grade.
  • Muffles, crucibles and thermowell shields — heat resistance rather than load-bearing strength usually governs.

One atmosphere note that causes real failures: high chromium content improves oxidation resistance but makes an alloy more prone to embrittlement in certain reducing or carburising atmospheres. The corrosion resistance and the atmosphere compatibility trade against each other, and the two must be checked together.

Where the Selection Actually Goes Wrong

Four mistakes account for most of the high temperature alloy failures we see specified into service:

  1. Choosing on room-temperature strength. Room temperature tensile strength is a poor predictor of creep life. The alloy with the higher room temperature strength can have the shorter life at temperature.
  2. Ignoring the atmosphere. Air, steam, combustion gas, vacuum, molten salt and hydrofluoric environments are different enough that the same grade cannot serve all of them. Haynes 188 behaves very differently in air versus in a carburising atmosphere.
  3. Free substitution between gamma-prime grades. Waspaloy and Nimonic grades can suffer strain-age cracking during service. Dropping one in as a like-for-like replacement for 718 in a hot section fastener position can introduce a failure mode the original design never had.
  4. Confusing maximum oxidation temperature with service temperature. An alloy that resists oxidation to 1095 °C does not carry structural load at 1095 °C. Oxidation resistance and creep strength are different properties with different limits, and the lower of the two governs.

Matching an Alloy to Its Service Temperature

The number that decides most selections is the actual metal temperature under load, not the maximum temperature the material survives in an oxidation test. These are different properties and the lower one always governs. As a rough orientation:

Service metal temperature Typical material choice Governing concern
Up to ~550 °C Ferritic and martensitic alloy steels, 9–12% chromium creep-resistant grades, 800-type iron-nickel alloys Creep and thermal fatigue
~550–700 °C 718, 725, 800H, Nimonic grades, ferritic-martensitic steels for reactors Creep life; strain-age cracking risk in gamma-prime grades
~700–950 °C Waspaloy, Nimonic 75 and 80, gamma-prime strengthened nickel grades Creep plus hot corrosion; atmosphere becomes critical
Above ~950 °C Cobalt-based alloys (Haynes 188, Haynes 230), nickel-based superalloys, ceramic thermal barrier coatings Oxidation and hot corrosion dominate; load-carrying strength falls away

Read that last row carefully. Above roughly 950 °C, the creep strength of nickel-based alloys falls fast and the practical answer increasingly involves a superalloy substrate with a thermal barrier coating, or a ceramic component. That is a design decision, not a purchasing one, but it is worth establishing early because it changes which alloy is worth specifying.

Alloy Families and Where They Sit

Alloy UNS / EN Strengthening Where it fits
Inconel 718 (Alloy 718) N07718 / 2.4668 Gamma double-prime (Ni3Nb), density about 8.19–8.22 g/cm³ The general-purpose high temperature workhorse. Turbine discs and casings, fasteners, heat treatment fixtures, springs, nuclear internals. Excellent weldability because the precipitate forms slowly.
Waspaloy N07001 / 2.4654 Gamma-prime with 12–15% cobalt, density about 8.19 g/cm³ Hot section fasteners and highly stressed rotating parts above the range where 718 holds its advantage. Susceptible to strain-age cracking.
Haynes 188 R30188 Solid-solution, cobalt-based, density about 8.98 g/cm³ Combustors, heat treatment fixtures, burners and hot corrosion duty. Oxidation resistant to roughly 1095 °C, but low in stressed strength.
Haynes 230 N06230 Solid-solution, cobalt-based, density about 8.97 g/cm³ Burner parts and heat treatment fixtures where severe oxidation resistance to about 1149 °C is required.
Nimonic grades Various, e.g. N06075 for Nimonic 75 Gamma-prime, nickel-based Turbine and combustion components above the 718 range; high-temperature springs and fasteners.
Iron-nickel 800 series N08810 (800H) Solid-solution with titanium and aluminium stabilisation Furnace fixtures, heat treatment baskets, process equipment and expansion joints at moderate temperature. Cheaper than nickel-based alloys.

Supply Forms for Turbine and Reactor Work

Hot section and pressure-retaining components start from specific mill forms, and the specification must match the form as well as the grade:

  • Plate, sheet and strip — combustor liners, transition pieces, heat exchanger shells and cladding. Sheet is usually the practical form for thin-wound components; plate for heavier structural shells.
  • Bar, rod and forging stock — fastener and bolting material, machined components, and disc forging stock. The classic high temperature forms.
  • Rings and forgings — turbine discs, seals and pressure parts, supplied as forging or finished machined rings to the governing AMS or ASTM forging specification.
  • Wire and weld consumables — matching filler for GTAW and SMAW, where the deposited metal must meet the same strength and corrosion requirements as the parent.

For specifications, Inconel 718 bar and forgings are typically supplied to ASTM B637 / SB637, with the AMS 5662 (solution treated) and AMS 5663 (aged) conditions defining the treatment. Sheet is covered separately. Always state the product form and the condition together — 718 solution-treated and 718 aged are different orders with different certificates.

Specifying a High Temperature Alloy

Six pieces of information make the difference between a quotation and a clarification cycle:

  1. Alloy and UNS number, with the specific condition or heat treatment condition.
  2. Product form — plate, sheet, bar, rod, ring, forging or weld consumable.
  3. Dimensions and quantity, with acceptable tolerance.
  4. Governing specification — ASTM, AMS, ASME, EN, or a customer specification, plus the certification type required.
  5. Service conditions — actual metal temperature, applied stress, atmosphere, and any cycling regime.
  6. Certification requirement — EN 10204 3.1 or 3.2, and whether third-party inspection is required.

Item five is the one that determines whether the material is fit for purpose, and it is the one most often left out of a purchase order because it feels like design information rather than purchasing information. A 718 order quoted without the service temperature is a legitimate order — we will supply it — but it cannot be the basis of a recommendation.

Alloys We Supply for High Temperature Service

Our high temperature portfolio covers the alloys most often specified for turbine, reactor and thermal duty. Each links to its full grade detail:

  • Inconel 718 (Alloy 718) — the general-purpose gamma-double-prime grade for discs, casings, fasteners and fixtures.
  • Waspaloy — gamma-prime with cobalt, for hot section fasteners and higher-temperature stressed duty.
  • Haynes 188 — cobalt-based solid solution for combustors, burners and hot corrosion duty.
  • Haynes 230 — for the most severe oxidation environments in burner and fixture duty.
  • Haynes 242 — age-hardenable, intermediate-temperature structural and fastener grade.
  • Nimonic 75 and Nimonic C263 — gamma-prime grades for elevated-temperature components and springs.

For grades outside this list — including Incoloy 800H, Hastelloy X and Inconel 601 — raise the requirement as a specific enquiry and we will confirm availability against the standard and condition you need rather than quoting an approximate equivalent.

Everything is supplied with an EN 10204 3.1 MTC as standard, with 3.2 and third-party inspection available where the contract requires it. See the high temperature alloy range for the wider overview.

Request a Quotation

Send the alloy, product form, dimensions, specification and service conditions, and we will confirm availability, certification scope and lead time in one response. If you are still deciding between grades, tell us the temperature, stress and atmosphere and we will tell you which family fits and what to watch out for.

Enquire Now – Send the alloy, form and service conditions for a quote
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Frequently Asked Questions

Q1. What is the maximum service temperature of a nickel-based high temperature alloy?

It depends on the grade and, critically, on whether the component is carrying structural load. Many nickel-based alloys resist oxidation to around 1000–1100 °C, but their creep strength falls away well below that, so the usable structural service temperature is typically 650–800 °C depending on the grade and the applied stress. Above roughly 950 °C the practical answer usually involves a superalloy substrate with a thermal barrier coating, or a ceramic component. Give us the actual metal temperature, stress and atmosphere and we will tell you which of those limits governs your duty.

Q2. Is Inconel 718 suitable for gas turbine blades?

Not for the aerofoil itself. 718 is used extensively in turbine discs, casings, seals, fasteners and other highly stressed structural parts, and its excellent weldability is why it is so widely fabricated. The turbine blade hot section — the aerofoil that sees the peak metal temperature while spinning under centrifugal load — uses more heavily alloyed gamma-prime superalloys, often as single-crystal or directionally solidified castings, or noble and cobalt-based alloys at the very hottest stations. Waspaloy takes over from 718 for hot section fasteners and very highly stressed parts above the range where 718 holds its advantage.

Q3. What is the difference between Waspaloy and Inconel 718?

Both are nickel-based and both are gamma-prime strengthened, but Waspaloy contains 12–15% cobalt and maintains its strength advantage over 718 above roughly 650–705 °C. The important caveat is that Waspaloy is susceptible to strain-age cracking during service, so it cannot be substituted freely into a 718 position — the substitution itself can introduce a failure mode the original design never had. The usable service limit also depends on applied stress, not metal temperature alone.

Q4. Can I use 718 and 800 series in a furnace atmosphere?

Yes, for ordinary oxidising furnace atmospheres. But not for carburising or nitriding atmospheres — ordinary 718 and 800-type alloys are attacked by those atmospheres at temperature. For carburising, nitriding and carbonitriding work, use a 600-series grade or a 330-type alloy instead. This is a routine and expensive specification error, because the alloy looks entirely appropriate for a furnace until it starts to intergranularly attack.

Q5. Do you supply these alloys with test certificates and third-party inspection?

Every supply is issued with an EN 10204 3.1 MTC reporting chemical analysis and mechanical results against the standard named on your order. EN 10204 3.2 certification and third-party witnessing by agencies such as SGS, BV, Lloyd's or TÜV are available where the contract requires them — specify the certification type on the enquiry and we will quote the right scope. Traceability by heat number is provided as standard.