Incoloy 800 vs 800H vs 800HT: Carbon Content, Grain Size, and Creep Strength Differences

Introduction

Incoloy 800, 800H, and 800HT belong to the same nickel-iron-chromium alloy family, developed by Special Metals Corporation beginning in the 1950s to deliver heat and corrosion resistance without relying on the large nickel content that made INCONEL alloy 600 a strategically scarce material at the time. All three share the same nominal nickel, chromium, and iron ranges. What separates them - and what a specifying engineer or procurement manager actually needs to understand - is a small set of compositional and metallurgical controls: carbon content, aluminum-plus-titanium content, and the grain size achieved by the final anneal. Together, these controls determine how much creep and stress-rupture strength the material can deliver during years of continuous high-temperature service.

This article compares all three grades directly, using the limiting chemical compositions and long-term rupture-strength data published by Special Metals Corporation in its INCOLOY alloy 800 (SMC-046) and INCOLOY alloy 800H & 800HT (SMC-047) technical bulletins, so that buyers can match the correct grade to the correct service temperature and design life.

What Is the Fundamental Difference Between Alloy 800, 800H, and 800HT?

All three alloys share the same 30–35% nickel / 19–23% chromium / 39.5% min. iron base, but 800H and 800HT are metallurgically upgraded versions of 800 - produced to tighter carbon and aluminum-plus-titanium limits and given a high-temperature solution anneal that grows a coarser grain structure, specifically to raise creep and stress-rupture strength for long-term elevated-temperature service.

Special Metals' own technical literature is explicit on this point: the base elements in all three alloys are the same, and the alloys' physical and thermal properties (density, thermal expansion, electrical resistivity, modulus of elasticity) are essentially identical because their overall compositional ranges overlap. The differences that matter are metallurgical, not chemical in the broad sense - they concern how tightly three specific parameters are controlled: carbon content, aluminum-plus-titanium content, and final grain size.

Because of this, alloy 800 is generally the higher-strength choice for short-term loading at room temperature or moderate temperature, while 800H and 800HT are the correct choice whenever a component must resist time-dependent deformation (creep) during years of continuous exposure above roughly 1,000°F (540°C).

How Does Carbon Content Differ Among the Three Grades, and Why Does It Matter?

Alloy 800 allows carbon up to 0.10% with no specified minimum; alloy 800H requires 0.05–0.10% carbon; and alloy 800HT tightens that range further to 0.06–0.10% carbon - a progressively narrower, higher floor that is the single biggest lever for creep-rupture strength in this alloy family.

Special Metals' bulletins note that it had long been recognized that higher-carbon alloy 800 had higher creep and rupture properties than low-carbon material of the same nominal composition, which is why the company began melting to the 0.05–0.10% range - the upper portion of the 0.10% maximum allowed for standard alloy 800 - well before that range was formalized as its own grade. Carbon in this alloy family precipitates as chromium carbides at grain boundaries during high-temperature exposure; controlled at the right level and grain size, these carbides pin grain boundaries and resist the boundary sliding that drives creep deformation.

Source: Special Metals Corporation, INCOLOY alloy 800H & 800HT technical bulletin, Publication No. SMC-047 (Table 1).

An important nuance for buyers: because alloy 800's carbon ceiling (0.10% max.) overlaps entirely with the 800H and 800HT ranges, a heat of plain alloy 800 can, by chemistry alone, already fall inside the 800H carbon window. Carbon content alone does not make a heat 800H-qualified, however - the grain size requirement discussed below must also be met and verified.

Why Does Grain Size Matter for Creep Strength?

Alloy 800 has no specified grain size requirement, while both 800H and 800HT require an average ASTM grain size of No. 5 or coarser, achieved by solution annealing at 2,100–2,200°F (1,150–1,200°C) - a coarser grain structure that meaningfully increases resistance to creep and stress rupture at elevated temperature, at some cost to post-exposure toughness.

Grain size and creep resistance are directly linked in this alloy system because creep deformation at high temperature is driven substantially by sliding along grain boundaries. A coarser grain structure has proportionally less grain-boundary area per unit volume, which reduces the pathways available for that boundary sliding and, in combination with carbide pinning, raises the stress a component can sustain before it deforms unacceptably or ruptures over a given design life.

Special Metals' bulletin is specific about how this is achieved in production: alloys 800H and 800HT are solution annealed as a final production step at 2,100–2,200°F (1,150–1,200°C), with time at temperature adjusted for section size and furnace characteristics to reach a minimum average grain size of ASTM No. 5.

The bulletin also flags the point of diminishing returns and the associated trade-off directly: temperature and time should be controlled to limit excessive grain growth, since little additional creep strength is gained beyond a certain coarseness, while very coarse grain lowers toughness after high-temperature exposure. For applications where more than about 20% cold forming will follow, or where fine surface finish is required, the same bulletin recommends ordering material in a fine-grain condition instead, underscoring that the coarse-grain treatment is a deliberate service-specific trade-off, not a universal upgrade.

Source: Special Metals Corporation, INCOLOY alloy 800H & 800HT technical bulletin, Publication No. SMC-047 (Table 1 and Annealing - Basic Practice section).

Special (finer or coarser) grain-size ranges - ASTM 1–5 for plate and tube/pipe, and ASTM 2–5 for sheet - can also be agreed for specific orders, so buyers with an unusual combination of forming and service requirements should specify grain size explicitly on the purchase order rather than assuming the standard ASTM 5-or-coarser default.

How Do Aluminum-Plus-Titanium Levels Differ Between 800H and 800HT?

Alloy 800 and 800H both allow 0.30–1.20% combined aluminum plus titanium, but alloy 800HT restricts that combined range to a narrower, higher 0.85–1.20% - the third control, alongside carbon and grain size, that Special Metals uses to push 800HT's creep-rupture strength above 800H's.

Special Metals' bulletins describe this directly as a deliberate strategy: the company maintained aluminum and titanium contents in the upper portion of the specified range for its own 800H production specifically because doing so produced higher creep and stress-rupture properties than competitors' 800H material meeting only the minimum requirements. Rather than leave that upper-range practice as an internal preference, Special Metals formalized it into a distinct, restricted-chemistry grade - 800HT - with a mandatory 0.85–1.20% Al+Ti floor and ceiling, plus the 0.06–0.10% carbon range and 2,100°F (1,149°C) minimum solution-anneal temperature described above.

Because the 800HT chemistry window sits entirely inside the 800H window, any heat produced to 800HT requirements automatically satisfies 800H requirements as well - which is why 800HT material is routinely dual-certified to both UNS N08811 and UNS N08810. The reverse is not true: ordinary 800H material cannot be certified as 800HT unless it independently meets the tighter carbon, Al+Ti, and grain-size requirements.

Full Composition Comparison: Alloy 800 vs 800H vs 800HT

The three grades differ only in carbon, aluminum, titanium, and grain size - every other limiting composition element (nickel, chromium, iron, manganese, sulfur, silicon, copper) is specified identically across all three.

Source: Special Metals Corporation, INCOLOY alloy 800H & 800HT technical bulletin, Publication No. SMC-047, Table 1 - Limiting Chemical Compositions for INCOLOY alloys 800, 800H, and 800HT.

Which Alloy Has the Highest Creep-Rupture Strength?

Alloy 800HT delivers the highest long-term creep-rupture strength of the three grades, followed by 800H; plain alloy 800 is not recommended by Special Metals for applications requiring optimum creep-rupture properties, particularly above 1,500°F (816°C).

Special Metals states plainly that alloy 800H and 800HT have significantly higher creep and rupture strength than alloy 800, and that alloy 800 is not normally used where optimum creep-rupture properties are required - 800H or 800HT should be specified instead. This ranking holds specifically for long-duration, elevated-temperature loading; at room temperature and during short-time exposure, alloy 800 can actually show higher tensile strength than 800H or 800HT, since the coarse-grain, high-temperature-anneal condition of 800H/800HT trades some short-term strength for long-term creep resistance.

The table below reproduces Special Metals' representative 10,000-, 30,000-, 50,000-, and 100,000-hour rupture-strength values for alloys 800H and 800HT, which are treated as a single dataset in the manufacturer's bulletin because both grades share the same solution-anneal grain-size target; 800HT's tighter carbon and Al+Ti chemistry raises its position within this stress range and is reflected separately in ASME's allowable design stresses (discussed in the next section).

Source: Special Metals Corporation, INCOLOY alloy 800H & 800HT technical bulletin, Publication No. SMC-047, Table 7 - Representative Rupture-Strength Values for INCOLOY alloys 800H/800HT.

Read practically: rupture strength falls off sharply with both rising temperature and increasing design life, which is exactly what a creep-rupture mechanism predicts. A component designed for a 100,000-hour (roughly 11.4-year) service life at 1,600°F (870°C) can carry only about 2.5 ksi (17 MPa) - versus 3.5 ksi at the same temperature for a 10,000-hour design life. This is why matching the correct grade and the correct design-life stress value to the actual operating temperature is central to safe, code-compliant equipment design in this alloy family, and why simply reading a single "maximum temperature" number off a datasheet is not an adequate design basis for creep-limited service.

How Do the ASME Code Case Design Stresses Compare?

Alloy 800HT (ASME Code Case 1987) carries higher maximum allowable design stresses than alloy 800H (ASME Code Case 1325) across the 1,100–1,650°F service range, which is the direct, code-recognized consequence of 800HT's tighter carbon and Al+Ti chemistry and is the practical reason many buyers specify 800HT even where 800H would otherwise meet minimum requirements.

Under the ASME Boiler and Pressure Vessel Code, alloy 800 design stresses for Section I and Section VIII, Division 1 construction are listed directly in Section II, Part D and cover service up to 1,500°F (816°C). Alloy 800H's design stresses are also listed in Section II, Part D and are further addressed by Code Case 1325 (to 1,500°F/816°C) and Code Case 1983 for Section VIII, Division 1 service up to 1,800°F (982°C).

Alloy 800HT's maximum allowable stresses, covered by Code Case 1987, are explicitly higher than 800H's across the 1,100–1,650°F range, and material certified as 800HT automatically meets all 800H requirements - which is why 800HT is commonly dual-stamped to both UNS N08811 and UNS N08810 on mill certificates.

• Alloy 800 (N08800) - Section I / Section VIII, Div. 1 & 2 design stresses per ASME II Part D; service to 1,500°F (816°C).
• Alloy 800H (N08810) - ASME Code Case 1325 (to 1,500°F/816°C) and Code Case 1983 for Section VIII, Div. 1 service to 1,800°F (982°C).
• Alloy 800HT (N08811) - ASME Code Case 1987 (latest revision); higher allowable stresses than 800H from 1,100–1,650°F; also covered to 1,800°F (982°C) via Code Case 1983 values.

A procurement note worth flagging honestly: the historical record shows these allowable stresses have moved before and can move again. According to Special Metals' own account, when the ASME Metals Property Council reanalyzed pooled 800H creep-rupture data from 87 heats and 1,052 data points across multiple producers - including material weaker than Special Metals' own heats - the Code's allowable 800H design stresses were revised downward for the 1,100–1,500°F (593–816°C) range. That episode is precisely why 800HT exists as a separate, tighter-chemistry grade: it let Special Metals continue offering the higher design stresses its own material could support, backed by its own data, independent of the pooled 800H revision.

When Should You Specify Alloy 800 vs 800H vs 800HT?

Specify plain alloy 800 for general corrosion- and heat-resistant service below roughly 1,500°F (816°C) where creep is not the governing design condition; specify 800H as the baseline grade wherever creep-rupture strength governs design life above that range; and specify 800HT where the application justifies its higher allowable design stress and dual UNS certification - typically the highest-temperature, longest-design-life pressure-boundary components.

Alloy 800 - furnace baskets, trays, and fixtures; heat exchangers and piping in nitric-acid or chloride-bearing media where resistance to stress-corrosion cracking matters more than creep life; electric heating-element sheathing.

Alloy 800H - steam/hydrocarbon reformer catalyst tubing, convection tubing, pigtails and outlet manifolds; ethylene cracking and convection tubes; radiant tubes, muffles, and retorts in heat-treating furnaces; steam-generator and superheater tubing.

Alloy 800HT - the same creep-critical furnace and reformer applications as 800H, specified instead of 800H when the design calculation needs the higher Code Case 1987 allowable stress, or when the purchaser wants dual N08810/N08811 certification for flexibility across multiple project specifications.

In practice, most mills today produce alloy 800H material to chemistry and grain size tight enough to also qualify as 800HT, and dual-certify it to both UNS numbers on the mill test report - so buyers should always check the actual MTR chemistry and grain-size result against Table 1 above rather than assuming a nominal "800H" order automatically carries 800HT-level design stresses.

Frequently Asked Questions

Q: Is Incoloy 800HT simply a higher grade of 800H?

A: Not exactly a separate alloy family - 800HT's composition is a restricted subset of 800H's, always falling within 800H's limits. The distinction is that 800HT requires narrower carbon (0.06–0.10% vs. 0.05–0.10%) and higher aluminum-plus-titanium (0.85–1.20% vs. 0.30–1.20%) content, plus a 2,100°F (1,149°C) minimum solution anneal, and carries higher ASME allowable design stresses under Code Case 1987.

Q: Can alloy 800H be substituted for 800HT on a project that specifies 800HT?

A: No, not automatically. Alloy 800HT material always meets 800H requirements, so it can be dual-certified, but the reverse is not guaranteed - ordinary 800H material only qualifies as 800HT if its actual chemistry and grain size independently meet the tighter 800HT limits. The mill test report must be checked.

Q: Does a coarser grain size make 800H and 800HT weaker overall?

A: No - it is a deliberate trade-off in the opposite direction of what "weaker" usually implies. Coarser grain (ASTM No. 5 or coarser) improves long-term creep and stress-rupture strength at elevated temperature, which is the property these grades are designed for. The trade-off is reduced toughness after high-temperature exposure and a lower short-term room-temperature strength compared with fine-grain alloy 800, not an overall weakness.

Q: What is the maximum service temperature for each grade?

A: Under ASME Section I and Section VIII, Division 1, alloy 800 is permitted to 1,500°F (816°C). Alloy 800H is also listed to 1,500°F (816°C) under Code Case 1325, with Section VIII, Division 1 service extended to 1,800°F (982°C) under Code Case 1983. Alloy 800HT's Code Case 1987 stresses cover the same 1,100–1,650°F range as 800H, at higher allowable values, with 1,800°F (982°C) service also addressed via Code Case 1983.

Q: Why do all three alloys have essentially the same corrosion resistance?

A: Because nickel, chromium, and iron - the elements that govern oxidation resistance, carburization resistance, and resistance to chloride stress-corrosion cracking - are specified identically across all three grades. The compositional differences between 800, 800H, and 800HT are confined to carbon, aluminum, and titanium, which affect mechanical and creep properties rather than general corrosion behavior.

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