Incoloy 800 vs 800H vs 800HT: Temperature Limits and Creep Resistance

Parameter	Incoloy 800	Incoloy 800H	Incoloy 800HT
UNS Number	N08800	N08810	N08811
Carbon Content (wt%)	≤ 0.10	0.05-0.10	0.06-0.10
Al + Ti Control	Not Controlled	Not Controlled	Tightly Controlled (0.30-0.70)
Annealing Temp	~980°C	≥ 1,143°C	1,143-1,177°C
ASTM Grain Size	Fine (7-8)	5 or Coarser	5 or Coarser (Rec. 2.5-5)
ASME Max Temp	593°C (1,100°F)	~1,095°C	~1,150°C*
700°C / 10⁵h Rupture	~25 MPa	~45 MPa	~55 MPa

* ASME Section I permits 800HT to 1,095°C; Division 1 allows up to 1,150°C. Practical sustained service rarely exceeds 900°C due to oxidation limits.

Incoloy 800 vs 800H vs 800HT

Incoloy 800, 800H, and 800HT are three nickel-iron-chromium alloys that share the same base chemical composition. The difference comes down to three factors: carbon content control, aluminum-plus-titanium (Al+Ti) ratio, and solution annealing temperature. These three factors together determine grain size, which in turn governs creep resistance and high-temperature strength.

If your service temperature is below 600°C and creep is not a design consideration, Incoloy 800 is sufficient and more economical. If your component operates above 600°C under sustained load, you need 800H or 800HT. And if you are targeting maximum creep life in the 700–900°C range, 800HT is the definitive choice, offering up to 60% higher allowable stress at 800°C compared to 800H.

Incoloy 800, 800H, and 800HT are high-temperature nickel-iron-chromium alloys designed for oxidation-resistant service at elevated temperatures. All three belong to the same alloy family and contain approximately 30–35% nickel, 19–23% chromium, and minimum 39.5% iron. The primary distinction is not in base composition but in carbon content control, Al+Ti ratio optimization, and heat treatment, which collectively determine their creep performance.

The 800 series alloys were developed by Special Metals Corporation (originally Inco) to fill a gap in the materials market: a material that could resist oxidation and carburization at high temperatures while offering better economics than straight nickel-based alloys like Inconel 600. The chromium content of approximately 20% provides oxidation resistance, while the nickel content of approximately 32% ensures structural stability at elevated temperatures.

Over time, engineers discovered that by tightening the carbon specification, controlling the aluminum-to-titanium ratio, and applying a high-temperature solution anneal, the creep and stress-rupture properties could be dramatically improved. This led to the evolution from 800 to 800H, and finally to 800HT.

All three grades share nearly identical base chemistry ranges for nickel (30–35%), chromium (19–23%), and iron (balance, ≥39.5%). The critical difference lies in carbon content specification and Al+Ti ratio control. Incoloy 800 allows carbon up to 0.10% without a lower limit, 800H restricts carbon to 0.05–0.10%, and 800HT further tightens carbon to 0.06–0.10% while imposing strict control over the combined aluminum and titanium content (0.30–0.70%, with Al:Ti ≥ 1).

What Is the Key Chemical Difference Between These Three Grades

■ Carbon Content: Incoloy 800 allows up to 0.10% carbon; however, low-carbon heats (below 0.05%) are permitted. Incoloy 800H mandates 0.05–0.10% carbon to ensure sufficient carbide precipitation at grain boundaries during the high-temperature anneal. Incoloy 800HT uses 0.06–0.10%, optimized to balance grain boundary pinning with ductility.

■ Al + Ti Control: Incoloy 800 and 800H do not tightly control the combined Al+Ti content, which can range from 0.30% to 1.20%. Incoloy 800HT restricts Al+Ti to a narrow window of 0.30–0.70%, with the additional requirement that the aluminum-to-titanium ratio be at least 1. This prevents the formation of the detrimental eta (η) phase during long-term exposure at elevated temperatures.

■ Practical Implication: These subtle chemistry differences, combined with the appropriate heat treatment (see below), determine whether the alloy will have fine grains (low creep strength) or coarse grains (high creep strength).

Incoloy 800 is limited to 593°C (1,100°F) for pressure-retaining service per ASME Section VIII Division 1 and has no ASME-listed allowable stress above this temperature. Incoloy 800H extends this limit to approximately 1,095°C for ASME Section I and Section VIII applications. Incoloy 800HT achieves the highest ASME coverage, reaching 1,150°C under Division 1, with sustained practical use recommended below 900°C where oxidation becomes the lifetime-limiting mechanism.

The ASME Boiler and Pressure Vessel Code provides the authoritative reference for design temperature limits. The underlying reason Incoloy 800 drops out above 593°C is not that the alloy melts or fails catastrophically; instead, its fine-grained microstructure lacks the creep strength required for sustained pressure containment. ASME deems it unsafe to list an allowable stress when the material's long-term creep data cannot support a reliable design.

■ Incoloy 800 (UNS N08800): ASME Section VIII Div 1, Table UHA-23 lists allowable stresses only up to 593°C (1,100°F). At 500°C, the allowable stress is 72 MPa; at 300°C, it is 86 MPa. No values are listed for 600°C or above.

■ Incoloy 800H (UNS N08810): ASME Section II Part D lists allowable stress values spanning from ambient to 1,095°C. At 600°C, the allowable stress is 64 MPa; at 700°C, 44 MPa; at 800°C, 26 MPa; at 900°C, 14 MPa.

■ Incoloy 800HT (UNS N08811): ASME Section II Part D assigns higher allowable stresses than 800H at elevated temperatures. At 700°C, the allowable stress is 52 MPa (versus 44 MPa for 800H); at 800°C, 32 MPa (versus 26 MPa); at 900°C, 18 MPa (versus 14 MPa). This represents a 20–23% improvement in design margin at 700–800°C.

Creep resistance in the 800-series is governed primarily by grain size. Incoloy 800 has a fine-grain structure (ASTM 7–8), which offers good room-temperature tensile properties but poor creep strength at elevated temperatures. Incoloy 800H and 800HT are solution-annealed at much higher temperatures (≥1,143°C versus approximately 980°C for 800), which produces a coarse-grain structure (ASTM 5 or coarser).

Coarse grains reduce grain boundary sliding, the primary creep mechanism, resulting in substantially longer creep life.

Why Do 800H and 800HT Outperform 800 in Creep Resistance

Creep is a time-dependent deformation that occurs when a metal is subjected to stress at high temperature. At the microstructural level, creep proceeds primarily by grain boundary sliding: individual grains slide past each other along their boundaries. Fine-grained materials have more grain boundary area, which means more pathways for sliding. Coarse-grained materials have fewer grain boundaries, which naturally limits this mechanism.

Quantitatively, the difference is dramatic. At 700°C, the 10,000-hour creep strength at 1% strain for Incoloy 800 is approximately 28 MPa, while 800H achieves approximately 46 MPa, and 800HT reaches approximately 52 MPa-an 86% increase from 800 to 800HT. Looking at 100,000-hour rupture strength at the same temperature: 800 holds approximately 25 MPa, 800H approximately 45 MPa, and 800HT approximately 55 MPa.

■ At 800°C / 100,000 hours rupture: Incoloy 800 offers approximately 12 MPa, 800H approximately 22 MPa, and 800HT approximately 30 MPa-a 150% improvement from 800 to 800HT.

■ At 900°C / 100,000 hours rupture: Incoloy 800 provides approximately 4 MPa, 800H approximately 9 MPa, and 800HT approximately 13 MPa. At these temperatures, even 800HT faces significant design constraints, which is why practical service rarely exceeds 900°C.

■ Metallurgical mechanism: The high-temperature anneal (1,143–1,177°C) dissolves the majority of chromium carbides, then the controlled cooling allows them to re-precipitate primarily at grain boundaries, pinning them against sliding. The carefully controlled Al+Ti content in 800HT ensures formation of stable gamma-prime (γ') precipitates (Ni₃Al,Ti) that provide additional intragranular strengthening without the risk of forming brittle η-phase at service temperatures.

Grain size is the single most important metallurgical variable distinguishing these three alloys. For high-temperature creep service, coarse grain (ASTM 5 or coarser) is always preferred because it minimizes grain boundary area and therefore minimizes grain boundary sliding, the dominant creep mechanism. Incoloy 800H and 800HT mandate a minimum grain size of ASTM 5; 800HT further recommends ASTM 2.5–5 for optimal creep life. In contrast, Incoloy 800 is typically supplied in a fine-grain condition (ASTM 7–8) and is not suitable for applications where creep governs the design.

The relationship between grain size and creep resistance is well-documented across nickel-based alloys. A fine-grain structure (ASTM ≥8) can reduce creep life by more than 50% compared to a coarse-grain structure (ASTM 2.5–5) under identical temperature and stress conditions. This is why the materials certification for 800H and 800HT must include the ASTM grain size number; it is not optional.

■ Any post-fabrication heat treatment performed below 1,100°C can inadvertently refine the grain structure, compromising creep performance. Field welding or hot forming operations must be carefully controlled to preserve the coarse-grain condition.

■ The grain size requirement for 800H is specified in ASME Code Case 1325, which mandates ASTM No. 5 or coarser. For 800HT, the recommended range of ASTM 2.5–5 is based on extensive creep testing data showing optimal balance between strength and ductility.

■ A practical consequence: when procuring 800H or 800HT, the purchase specification must explicitly call out grain size requirements, and the material test report (MTR) must document the measured ASTM grain size number.

Each grade has a unique UNS number and is covered by overlapping sets of ASTM product standards and ASME pressure vessel code cases. Incoloy 800 (N08800) is governed by ASTM B163 (tube), B407 (pipe), B408 (bar), B409 (plate), and similar product forms under ASME Section VIII Div 1. Incoloy 800H (N08810) and 800HT (N08811) share the same ASTM standards but require additional certification: grain size (ASTM E112), and for 800HT, the actual Al+Ti content and annealing temperature must be reported on the material test certificate.

■ Incoloy 800 (UNS N08800): ASTM B163 (seamless tube), B407 (pipe), B408 (rod/bar), B409 (plate/sheet), B514 (welded pipe), B515 (welded tube). ASME Section VIII Div 1, use limited to 593°C.

■ Incoloy 800H (UNS N08810): Same ASTM standards as above, plus ASME Code Case 1325 requiring ASTM grain size 5 or coarser. ASME Section I and Section VIII listed to ~1,095°C.

■ Incoloy 800HT (UNS N08811): Same ASTM standards, plus additional certification requirements: Al+Ti must be reported, annealing temperature and duration must be reported, grain size recommended at ASTM 2.5–5. ASME Section I to 1,095°C; Section VIII Div 1 to 1,150°C.

■ Dual certification: It is common industry practice for mills to supply material dual-certified as 800H/800HT. This means the material meets the more restrictive requirements of 800HT (carbon, Al+Ti, grain size, and anneal) and can be used interchangeably for 800H applications.

The selection is driven by three project inputs: service temperature, creep requirements, and budget. If your design temperature is below 593°C and creep is not a governing failure mode, choose Incoloy 800 for cost efficiency. If your component operates between 600 and 700°C with moderate creep loading, select Incoloy 800H. If you need maximum design margin above 700°C, or your application targets 20+ years of continuous service, invest in Incoloy 800HT.

Which Grade Should You Choose for Your Application

Below is a practical decision framework based on real-world industrial experience:

■ Design temperature ≤ 593°C, no creep requirement: Choose Incoloy 800. Typical applications include chemical process equipment, nitric acid preheaters, and nuclear steam generator tubing. The fine-grain structure provides good tensile strength at moderate temperatures without the cost premium of controlled chemistry and high-temperature annealing.

■ Design temperature 600–700°C, creep governs design: Choose Incoloy 800H. Typical applications include ethylene furnace tubes, steam superheater tubing, and hydrocarbon processing equipment. The coarse-grain structure provides reliable creep life and ASME design allowables are available across this temperature range.

■ Design temperature 700–900°C, long-term service: Choose Incoloy 800HT. Typical applications include ammonia reformer tubes, methanol plant outlet headers, and syngas cooler tubesheets. The additional 20–23% allowable stress advantage at 800°C (32 MPa versus 26 MPa for 800H) translates directly into reduced wall thickness and lower material cost.

■ Special case: cycling conditions: If the service involves frequent thermal cycling, Incoloy 800 with its finer grain structure may offer better thermal fatigue resistance at moderate temperatures. However, this must be carefully analyzed against creep requirements.

The 800-series family finds its broadest application in petrochemical processing, power generation, and industrial heating equipment, where the combination of oxidation resistance, carburization resistance, and high-temperature strength is essential. The choice of grade maps directly to the operating temperature and failure mode of each specific application.

■ Petrochemical: Ethylene cracking furnace tubes (800H), steam methane reformer (SMR) outlet pigtails and manifolds (800HT), methanol synthesis converter internals (800H/800HT), ammonia plant secondary reformer outlet systems (800HT), and hydrogen production equipment.

■ Power generation: Superheater and reheater tubing in conventional and combined-cycle power plants (800H/800HT), heat recovery steam generator (HRSG) components, and nuclear steam generator tubing (800).

■ Industrial heating: Radiant tubes for heat treatment furnaces, bell-type annealing furnace covers, muffle tubes, and thermocouple protection sheaths.

■ Chemical processing: Nitric acid preheater tubes, hydrofluoric acid alkylation unit internals, and components exposed to chloride-bearing environments where stress corrosion cracking is a concern.

All three grades are weldable using matching or over-matching nickel-based filler metals. However, the critical concern for 800H and 800HT is preserving the coarse-grain structure after welding and any post-weld heat treatment. Inappropriate heat treatment below the original solution annealing temperature can refine the grain structure and destroy the creep properties that distinguish these grades from standard 800.

Incoloy 800 vs 800H vs 800HT Welding and Fabrication

■ Filler metal selection: For 800H/800HT, use ERNiCr-3 (Inconel 82) or ERNiCrCoMo-1 (Inconel 617) filler metals when room-temperature tensile properties of the weld must match the base metal. ERNiCr-3 is the most common choice. For 800, ERNiCr-3 is also widely used.

■ Post-weld heat treatment: If PWHT is required by code, it must be performed at a temperature that does not refine the grain structure. For 800H/800HT, this means the PWHT temperature must be at or above 1,100°C to avoid grain refinement. If such a high-temperature PWHT is impractical, the alternative is to accept a slightly reduced creep strength in the heat-affected zone.

■ Hot forming: 800H and 800HT should be hot worked in the range of 870–1,200°C. Hot working below 870°C may cause grain refinement. If cold forming exceeds 5% strain and the component will see service above 600°C, re-solution annealing is recommended.

In modern practice, most mills supply material that is dual-certified to both 800H (N08810) and 800HT (N08811) standards. This means the material meets the more restrictive chemistry and grain size requirements of 800HT and can be used for either 800H or 800HT applications. However, you should always verify the material test report to confirm that Al+Ti content and grain size are within 800HT limits if you intend to use ASME 800HT design allowables.

Can Incoloy 800 be upgraded to 800H or 800HT by re-heat-treating?

No. The chemistry of Incoloy 800 (specifically the carbon content, which may be below 0.05%) does not necessarily meet the minimum carbon requirement of 0.05% for 800H or 0.06% for 800HT. Moreover, the grain structure may not coarsen sufficiently without the controlled carbide precipitation that carbon enables. Re-heat-treating an 800 material to 800H temperatures without the correct chemistry can produce inconsistent and unreliable results. The material should be procured as 800H or 800HT from the mill.

How does Incoloy 800HT compare to Inconel 600 at high temperature?

Incoloy 800HT and Inconel 600 serve different temperature and cost niches. Inconel 600 (UNS N06600) has higher nickel content (~72%) and can operate at temperatures up to 1,093°C with good oxidation resistance. At 700°C, Incoloy 800HT (100,000-hour rupture at approximately 55 MPa) is only about 10% lower in rupture strength than Inconel 600, but at a substantially lower material cost due to the lower nickel content. For applications below 900°C where economics matter, 800HT is often the preferred choice.

Does Incoloy 800 resist chloride stress corrosion cracking?

Yes. The high nickel content (30–35%) of all 800-series alloys provides excellent resistance to chloride stress corrosion cracking (SCC), which is a common failure mode in austenitic stainless steels like 304 and 316. This is one of the key reasons 800 is selected for heat exchanger tubing in services where chlorides may be present, such as cooling water or process streams with chloride contamination.

What is the maximum practical service life for 800HT components?

Incoloy 800HT has been used in ammonia and methanol plant reformer outlet systems for over 25 years of continuous service at temperatures between 850 and 920°C. The limiting factor at these temperatures is typically oxidation and carburization attack rather than creep rupture. With proper design (allowing for oxidation allowance in wall thickness), service lives exceeding 200,000 hours (approximately 23 years) are achievable at temperatures up to 900°C.