Incoloy 800 Alloy (UNS N08800): Chemical Composition, Properties And Grade Selection Guide

Incoloy 800 (UNS N08800) is a wrought, solid-solution-strengthened iron-nickel-chromium alloy containing 30.0–35.0% nickel and 19.0–23.0% chromium, with a density of 7.94 g/cm³ and a melting range of 1,357–1,385 °C. This guide consolidates the composition limits, physical and mechanical data, heat-treatment and hot-working windows defined in ASTM B407, B408, B409 and B163, and explains how Incoloy 800 differs from 800H and 800HT in carbon content, Al+Ti total and grain size. It is written for design engineers, procurement specialists and fabricators specifying heating-element sheaths, thermocouple protection tubes and high-temperature corrosion-resistant tubing.

1. What Is Incoloy 800? Grade Definition and Alloy System

Incoloy 800 is a wrought heat- and corrosion-resistant alloy based on iron, nickel and chromium, strengthened by solid solution rather than by martensitic transformation or precipitation hardening. Nickel is specified at 30.0–35.0% and chromium at 19.0–23.0%, with deliberate additions of 0.15–0.60% aluminum and 0.15–0.60% titanium to control structure and precipitation behavior.

Unlike conventional austenitic stainless steels, the alloy is not designed to maximize room-temperature strength. Its purpose is to satisfy four requirements simultaneously under high-temperature service: oxidation resistance, resistance to process-media corrosion, structural stability, and manufacturability. After appropriate heat treatment the microstructure is predominantly austenitic, and the alloy is supplied as tube, plate, sheet, strip, bar and forgings.

This combination of properties accounts for more than six decades of service in industrial heating, heat treatment, petrochemical processing and high-temperature process equipment. Typical uses include sheathing for electrical heating elements, thermocouple protection tubes, furnace internals, heat-treating fixtures, and selected high-temperature process piping and heat-exchanger tubing.

Three engineering characteristics define its position in the material selection map.

Oxidation resistance. Chromium promotes a dense, self-healing chromium-rich oxide scale, which gives the alloy sustained resistance to scaling in air and combustion products. The iron content provides a further benefit: it suppresses internal oxidation in atmospheres that oxidize chromium while reducing nickel — a failure mode to which higher-nickel alloys are more susceptible.

Corrosion resistance and structural stability. The elevated nickel content stabilizes the austenitic matrix and improves resistance to chloride stress-corrosion cracking in aqueous media. That resistance is substantial but not absolute: 30–35% nickel remains below the level associated with immunity in severe chloride environments, and performance still depends on medium composition, temperature, stress and surface condition.

Formability and weldability. The alloy is readily hot- and cold-formed using conventional austenitic practice and can be joined by gas tungsten arc, gas metal arc and shielded metal arc welding with matching nickel-base filler metals.

A qualification is essential at the outset. Incoloy 800 is not a universal high-temperature material. Where a component carries load at temperature over long durations and wall thickness must be set from creep and stress-rupture data, the design should be evaluated against Incoloy 800H or Incoloy 800HT, with reference to the applicable design temperature, design life and code.

2. Incoloy 800 Chemical Composition: Nickel, Chromium and the Al+Ti Balance

The composition is built on an iron-nickel-chromium matrix, with major and minor elements controlled to deliver heat resistance, corrosion resistance and fabricability at the same time. Individual elements should not be read in isolation; their effectiveness depends on the balance between them.

Nickel (Ni), 30.0–35.0%. Nickel stabilizes the austenitic structure, lowers susceptibility to strain-induced martensite, and improves performance across a range of corrosive media. Relative to 300-series stainless steels, this nickel level markedly improves resistance to chloride stress-corrosion cracking. Because it remains below the roughly 42% nickel level generally associated with immunity, boiling magnesium chloride testing will still crack the alloy; the material must not be described as immune to stress-corrosion cracking in all chloride-bearing service.

Chromium (Cr), 19.0–23.0%. Chromium is the basis of both oxidation and corrosion resistance and determines the composition and stability of the protective oxide. In sulfur-bearing, carburizing or low-oxygen-potential atmospheres, scale behavior differs materially from that in air, and suitability must be assessed against the specific process environment.

Aluminum (Al) and titanium (Ti). Both elements participate in structural and precipitation control, forming stable carbides, nitrides and dispersed phases that influence grain-boundary migration and microstructural evolution at temperature. In 800H and 800HT the aluminum and titanium contents, and their combined total, are directly linked to creep and stress-rupture performance. Their role should not, however, be reduced to "preventing harmful phase formation": long-term structural stability also depends on temperature, time, carbon content and heat-treatment practice.

Carbon (C). Standard Incoloy 800 permits carbon up to 0.10% with no lower limit; 800H narrows the window to 0.05–0.10%, and 800HT to 0.06–0.10%. Higher carbon increases the volume fraction of intragranular carbides and therefore high-temperature strength, but it also accelerates sensitization and precipitation kinetics.

It follows that incoming inspection should not stop at nickel and chromium. For pressure-retaining or long-life components, carbon content, Al+Ti total, grain size and heat-treatment records carry equal weight.

3. Incoloy 800 Strengthening, Heat Treatment and Hot-Working Window

3.1 Solid-Solution Strengthening and High-Temperature Capability

Incoloy 800 derives its baseline properties from alloying elements held in solid solution in the austenitic matrix, together with the resulting structural condition — not from quench hardening or age hardening. Annealing or solution treatment brings the material to the structure and properties required by the product specification, and room-temperature strength sits at the normal level for heat- and corrosion-resistant alloys.

One point is frequently overlooked: within this three-grade family, standard 800 has the highest room-temperature and short-time elevated-temperature strength. The reason is metallurgical — 800 is annealed at a lower temperature and therefore retains a finer grain, whereas 800H and 800HT must be solution treated at high temperature to achieve ASTM No. 5 or coarser grain, which lowers room-temperature yield strength. The advantage of the higher grades appears along the time axis: their creep and stress-rupture strength substantially exceeds that of 800, while 800 performs better under room-temperature and short-time loading.

Grade selection therefore cannot rest on a comparison of tensile values alone. The controlling failure mode must be identified first: yield or short-term overload, or long-term creep deformation and stress rupture.

3.2 Annealing and Solution-Heat-Treatment Practice

Heat treatment of Incoloy 800 is determined by product form, applicable specification, fabrication route and supplier documentation, with attention to heating temperature, holding time, cooling method, and the resulting structure and mechanical properties.

The following relationships are useful in practice. Softening of cold-worked material begins at approximately 760 °C; recrystallization is essentially complete after 10–15 minutes at 980 °C; pronounced grain growth occurs above 980 °C; and for thin sections a satisfactory anneal can be obtained in 2–5 minutes at 1,040 °C. Stress relief begins at approximately 540 °C and is virtually complete after 1.5 hours at 870 °C; for plate, allow 1 hour per 25 mm of thickness or 1.5 hours at 870 °C, whichever is greater.

For 800H and 800HT, the high-temperature solution treatment and the associated grain-size requirement must also be verified. Both grades are normally solution treated in the range of approximately 1,120–1,200 °C to develop a coarse grain, and 800HT specifically requires a minimum of 1,149 °C (2,100 °F). Slowly cooled 800H will not deliver the creep performance implied by its chemistry.

A single temperature range should never be adopted as the universal heat-treatment specification for plate, tube, bar and finished parts alike. Production must follow the ordering specification and the governing product standard.

3.3 Hot Working, Cold Working and the 650–870 °C Prohibition

The hot-forming range for Incoloy 800 is 870–1,200 °C (1,600–2,200 °F). Heavy forging should be completed above 1,010 °C, and light working may continue down to 870 °C. No working of any kind should be carried out between 650 and 870 °C — in this interval strength rises while ductility falls, and forging or bending readily initiates cracking. After hot working, cool in air or faster. Where subsequent pickling is required, or where the component will serve in a sensitization-sensitive environment, pass rapidly through 540–760 °C to limit continuous grain-boundary carbide precipitation.

Hot-working temperature, reduction, strain rate and intermediate annealing jointly determine structure and forming quality; specific parameters should follow the supplier's recommendations for the product form concerned. The heating atmosphere must be low in sulfur, and oil, paint, marking crayons and other contaminants containing low-melting-point elements must be removed before charging, to avoid liquid-metal embrittlement. Hot-worked parts should be inspected for surface cracking, dimensional deviation and structural condition.

In cold working, the work-hardening rate is somewhat lower than that of common austenitic stainless steels, and the annealed tensile-to-yield ratio typically exceeds 2, so substantial deformation is possible before an intermediate anneal is needed. Intermediate annealing should still be scheduled according to the amount of cold reduction — generally above approximately 20% — and surfaces inspected after forming. For cold-formed parts whose geometry prevents springback, residual tensile stress can initiate cracking on heating; a fine-grain input stock and controlled cold reduction are preferable.

4. Incoloy 800 Properties: Physical Data and Mechanical Values

4.1 Chemical Composition Limits (UNS N08800)

The table below gives the general composition limits for Incoloy 800. Purchase and acceptance must follow the specific grade, product standard, contract clauses and mill test certificate.

Note: the 0.30–1.20% Al+Ti window applies across the three grades in the family. Certain product standards specify only individual aluminum and titanium limits for N08800, so acceptance must reference the clause of the standard named in the order.

4.2 Physical Properties: Density, Melting Range, Conductivity and Resistivity

All values vary with temperature and material condition. Thermal-stress calculations, heating-element design and high-temperature structural analysis must not extrapolate from room-temperature data.

4.3 Mechanical Properties by Product Form

Mechanical properties must be verified against the specific product form and standard. The same UNS number carries different minimum guaranteed values under different product standards and delivery conditions — a frequent source of dispute at goods-in inspection.

Typical mill values are useful as a process reference but do not replace specification minimums: annealed bar reaches approximately 590 MPa tensile and 250 MPa yield at room temperature, while extruded tube reaches approximately 524 MPa tensile, 183 MPa yield and 60% elongation. Certain product standards provide exemptions from the yield-strength and elongation requirements for thin-gauge strip; these should be confirmed at the ordering stage.

For engineering selection, the following should be verified:

• Room-temperature tensile strength, 0.2% proof strength and elongation;

• Allowable stress at design temperature;

• Creep and stress-rupture strength for long-term high-temperature duty;

• Oxidation, corrosion and thermal-fatigue risk under cycling;

• Suitability of product form, size and the referenced material standard.

For long-term high-temperature components, room-temperature tensile data provide only a baseline reference and cannot substitute for elevated-temperature design data and creep assessment.

5. Incoloy 800 vs 800H vs 800HT: Differences and Grade Selection

The most common procurement error in this family is to treat the three grades as directly interchangeable. Their base compositions are similar and their physical properties essentially identical, but they differ materially in carbon content, Al+Ti total, grain size and long-term load-carrying capability.

Note: the table summarizes general requirements for the family. Specific orders may impose tighter composition or grain-size conditions, and acceptance must follow the ordering specification.

5.1 Incoloy 800 — the Base Grade

Incoloy 800 suits components that must balance oxidation resistance, corrosion resistance and fabricability. Where loading is short-term or not creep-controlled, the grade offers the best combination of cost and processing behavior. Its conventional design ceiling is generally taken as 816 °C (1,500 °F), and certain product standards treat 593 °C as the reference temperature for normal service of UNS N08800 — above that point the failure mechanism shifts from yield to creep, and the selection logic changes with it.

5.2 Incoloy 800H — Long-Term High-Temperature Loading

800H combines a controlled carbon window with an ASTM No. 5 or coarser grain-size requirement to raise creep and stress-rupture performance. Under the ASME code system its allowable design stresses extend to 982 °C (1,800 °F). Furnace tubes, reformer tubes, pigtails, headers and ethylene cracking coils — all long-term high-temperature loaded parts — should be evaluated in this grade.

5.3 Incoloy 800HT — Tightened Chemistry

800HT narrows the 800H window further, restricting carbon to 0.06–0.10% and Al+Ti to 0.85–1.20%, and requires solution treatment at not less than 1,149 °C. The purpose is to hold creep and stress-rupture performance within a tighter band, which yields allowable stresses above those of 800H in the 593–899 °C range. Material meeting 800HT necessarily meets 800H; the converse does not hold.

As a design reference, stress-rupture strength for 800H and 800HT is of the following order at 100,000 hours: approximately 90 MPa at 650 °C, 37 MPa at 760 °C, 17 MPa at 870 °C and 5.5 MPa at 980 °C. These values vary with heat, product form and test conditions; they support concept comparison only and must not be used directly as design allowables.

Selection principle. Where duty is governed by long-term stress, creep deformation or stress rupture, the choice should be made between 800H and 800HT on the basis of design temperature, design life and the governing code — not by raising the temperature limit of standard 800. Conversely, where short-term loading, thermal fatigue or media corrosion controls, the higher grades yield no corresponding benefit and their coarser grain reduces room-temperature strength and toughness reserve.

6. Incoloy 800 Applications: Heating-Element Sheaths and Corrosion-Resistant Tube

The value of Incoloy 800 lies in the balance it strikes between heat resistance, corrosion resistance and manufacturability. Suitability of a specific part must be judged against working temperature, process medium, stress state and design life.

6.1 Heating-Element Sheaths

Incoloy 800 is used for the metallic sheathing of industrial electrical heating elements. Such service demands oxidation resistance, elevated-temperature strength, formability and weld soundness in equal measure. The sheath also acts as the dielectric and heat-transfer interface, so its electrical resistivity, thermal conductivity and expansion coefficient directly influence power distribution and element life. The alloy performs stably in continuous oxidizing atmospheres below approximately 800 °C and is an established choice for tubular element sheathing. Where sheath surface temperature exceeds 850 °C for sustained periods, or where large cyclic gradients are imposed, 800H or a higher grade should be evaluated.

6.2 Thermocouple Protection Tubes

The alloy is used for protection tubes in a range of high-temperature measurement duties, providing mechanical protection together with heat and corrosion resistance. Selection must consider atmosphere (oxidizing, reducing, carburizing, sulfur-bearing), temperature fluctuation, contaminant deposition and cycling frequency. In strongly carburizing or reducing atmospheres, protection tubes typically fail through carburization and embrittlement of the inner wall rather than external oxidation.

6.3 Heat-Treatment Furnaces and Industrial Heating Equipment

Applications include furnace internals, baskets, trays, fixtures, retorts, radiant tubes and supports. These parts cycle repeatedly between heat-up and cool-down, and thermal fatigue and oxide spalling are the dominant failure modes. Where components carry load at temperature for extended periods, the creep and stress-rupture behavior of 800H or 800HT should be evaluated.

6.4 Petrochemical and High-Temperature Process Equipment

The alloy serves in heat exchangers, process piping and high-temperature equipment parts, including reformer tubes, pigtails and headers in hydrogen and synthesis-gas units, and exchanger and piping systems in nitric acid service where resistance to chloride stress-corrosion cracking is required. Where the medium contains hydrogen, sulfur, hydrocarbons or other corrosive species, a dedicated material-compatibility assessment is mandatory; grade and nickel content alone are not sufficient evidence of suitability. For sour oil and gas service, the applicability of NACE MR0175 / ISO 15156 must also be confirmed.

6.5 High-Temperature Tube and Structural Components

Where temperature, medium and mechanical load all fall within the alloy's envelope, Incoloy 800 can be used for a wide range of industrial tube and structural parts. For pressure vessels and code piping, grade, product standard, allowable stress and fabrication requirements must all be shown to satisfy the design documents; where welded joints are involved, welding procedure qualification and the service-temperature capability of the filler metal must be verified as well.

7. Incoloy 800 Procurement FAQ

Q1. What is the difference between Incoloy 800 and 800H, and how do I confirm I have received 800H?

Verify the UNS designation, carbon content, grain size and mill test certificate. Standard 800 permits carbon up to 0.10% with no minimum; 800H requires 0.05–0.10% carbon together with an ASTM No. 5 or coarser grain size. Identical nickel and chromium contents are not sufficient evidence of 800H — grain size is an equally decisive criterion, and a fine-grain product cannot deliver 800H stress-rupture performance even with compliant chemistry.

The purchase order should state UNS N08810, the product standard, the heat-treatment condition and the grain-size acceptance requirement, measured to ASTM E112 and reported lot by lot. For long-term high-temperature components, conformity with the governing design code or allowable-stress basis should also be confirmed.

Q2. How should the hot-working temperature for Incoloy 800 be determined?

The hot-forming range is 870–1,200 °C. Heavy forging should be completed above 1,010 °C, light working may continue to 870 °C, and working between 650 and 870 °C is prohibited. Specific parameters must account for product form, deformation process, heating conditions and supplier recommendations; an unverified temperature range should never be adopted as a universal operating specification.

The heating atmosphere must be low in sulfur and fuel sulfur content controlled. Oil, paint, marking compounds and other contaminants containing low-melting-point elements must be removed before charging to prevent liquid-metal embrittlement. Cool in air or faster after hot working, passing rapidly through 540–760 °C.

Q3. Can Incoloy 800 be welded, and is post-weld heat treatment required?

Yes. The alloy has good weldability and can be joined by gas tungsten arc, gas metal arc or shielded metal arc welding according to joint configuration, thickness and service environment. Nickel-chromium fillers such as ERNiCr-3 (AWS A5.14) or ENiCrFe-3 (AWS A5.11) are normally specified. Matching-composition fillers are generally not recommended, because their higher carbon content reduces elevated-temperature ductility.

Key controls include low heat input, stringer beads, interpass temperature below 150 °C, complete interpass cleaning and joint faces free of oil and marking paint. Post-weld heat treatment is normally not required for this family. Where a design code mandates stress relief, a full solution anneal with rapid cooling is required rather than a low-temperature soak, since the low-temperature annealing range is precisely where carbide and sigma-phase precipitation is most active.

Q4. How does Incoloy 800 compare with 316 stainless steel?

The two alloys belong to different systems and are optimized for different duty. Type 316/316L contains 16–18% chromium, 10–14% nickel and 2.0–3.0% molybdenum; the molybdenum addition gives it excellent pitting and crevice-corrosion resistance. Incoloy 800, with 30–35% nickel and 19–23% chromium, offers advantages in high-temperature oxidation, carburization and sulfidation resistance and in resistance to chloride stress-corrosion cracking, but its low molybdenum content leaves it inferior to 316L in pitting resistance in chloride-bearing water.

In high-temperature chloride environments, neither alloy can be judged by grade designation alone. Selection must weigh chloride concentration, temperature, dissolved oxygen, pH, residual stress and actual load, supported by corrosion data or test results. Incoloy 800 should not be claimed to resist cracking in all chloride environments, nor treated as an unconditional substitute for 316 stainless steel.

Q5. What information must be specified when purchasing Incoloy 800?

1. Grade and material identification: Incoloy 800, 800H or 800HT, with the corresponding UNS designation (N08800 / N08810 / N08811).

2. Product form and size: seamless tube, welded tube, heat-exchanger tube, plate, sheet, strip, bar or forging, with dimensions, tolerances and quantity.

3. Governing standard: plate, sheet and strip to ASTM B409 (ASME SB409); bar to ASTM B408; seamless pipe and tube to ASTM B407; heat-exchanger tube to ASTM B163; welded pipe and tube to ASTM B514 / B515; forgings to ASTM B564; fittings to ASTM B366.

4. Service conditions: design temperature, process medium, working pressure, mechanical load, design life and corrosion environment.

5. Delivery condition: annealed or solution-treated condition, surface finish (pickled, bright, ground) and other agreed requirements; for 800H and 800HT, the solution-treatment temperature and grain size must be stated.

6. Inspection and documentation: chemical analysis, mechanical properties, grain size, heat-treatment records and mill test certificate (EN 10204 3.1 recommended), with non-destructive testing and retest requirements where applicable.

7. High-temperature design basis: for creep- or stress-rupture-sensitive components, the governing design code, allowable-stress basis and grade requirement.

One further point deserves emphasis. Plate, strip, bar, seamless tube and welded tube are governed by separate ASTM standards and cannot share a single technical condition. An order that specifies "ASTM B409" while actually procuring tube creates a mismatch between standard scope and product form — one of the most common causes of rejection at goods-in inspection.

Conclusion: Understanding the Incoloy 800 Selection Envelope

Incoloy 800 is an iron-nickel-chromium heat- and corrosion-resistant alloy with a mature industrial base, valued for the balance it achieves between high-temperature oxidation protection, useful corrosion resistance and reliable fabrication.

For heating-element sheaths, thermocouple protection tubes and a wide range of industrial heating components, Incoloy 800 warrants first evaluation. For long-term high-temperature loading, creep-controlled duty or elevated-temperature pressure service, 800H and 800HT should be compared directly, with grade selection and wall-thickness calculation carried out against the governing design code.

Sound selection is not a matter of pursuing higher nickel content or a higher temperature rating. It is a matter of matching grade, product standard, heat-treatment condition, grain size, service environment and design life to one another. Where any one of these is mismatched, the performance advantage of the alloy will not be realized over the life of the equipment.