310S vs Incoloy 800: Heat-Resistant Alloy Selection for Furnace and Kiln Applications

Introduction

Furnace, kiln, and thermal-processing engineers routinely face the same material decision: specify a cost-effective heat-resistant stainless steel, or step up to a nickel-iron-chromium alloy built for sustained high-temperature strength. 310S stainless steel and Incoloy 800 (including the 800H and 800HT variants) are the two most commonly compared candidates for this decision. Both resist oxidation at high temperature, both are readily fabricated and welded, and both appear routinely in furnace doors, muffles, radiant tubes, and heat-treat fixtures.

Their performance diverges sharply, however, once carburizing atmospheres, sustained mechanical load, or long service intervals enter the picture. This article compares the two alloys across composition, temperature capability, oxidation and carburization resistance, creep-rupture strength, fabrication requirements, and total cost of ownership, so that engineers and buyers can match the alloy to the application rather than the other way around.

What Distinguishes 310S from Incoloy 800 Metallurgically?

The composition difference is the root cause of every downstream performance difference discussed in this article. 310S carries chromium as its primary alloying element and nickel as a secondary austenite stabilizer, which is why it is classified as a stainless steel rather than a nickel alloy. Incoloy 800 inverts that ratio: nickel is the majority alloying addition, chromium is secondary, and iron makes up the balance rather than being the base metal. The higher nickel content is what gives the 800-series alloys their resistance to carbon diffusion and their retained strength at temperature - but nickel is also the more expensive raw material input, which is why Incoloy 800 series products carry a persistent cost premium over 310S.

Source: ASTM A240/A480 (310S); ASTM B407/B408/B409 and UNS N08810 mill specifications (Incoloy 800H). Ranges are nominal specification limits; verify against the current mill test report.

Two other composition details matter for furnace service. First, 310S is a low-carbon variant of standard grade 310 (which allows up to 0.25% carbon); the lower carbon in 310S improves resistance to sensitization and intergranular corrosion after welding. Second, Incoloy 800H is itself a controlled-carbon, controlled-grain-size variant of base Incoloy 800, and Incoloy 800HT further tightens the aluminum-plus-titanium range - both refinements exist specifically to raise creep-rupture strength for furnace and process-heater service, which is discussed in Section 4.

Which Alloy Withstands Higher Continuous Operating Temperatures?

310S is commonly rated for continuous oxidation resistance up to approximately 2,000°F (1,093°C) and for intermittent or mildly cyclic service to a similar range, owing to its high chromium content. The practical limiter for 310S in furnace and kiln service is not the oxide scale - it is sigma-phase formation. After prolonged exposure between roughly 1,200°F and 1,650°F (650–900°C), 310-family stainless steels can form a brittle intermetallic sigma phase that reduces room-temperature toughness and ductility. Equipment that cycles through this temperature band repeatedly, such as furnace doors and burner components, is more exposed to this effect than equipment that operates continuously at the top of the temperature range.

Incoloy 800H and 800HT are rated for long-term service up to approximately 2,000°F (1,093°C), with short-duration excursions possible above that at the cost of accelerated strength loss and oxidation. Because the 800-series alloys are solution-annealed at a minimum of 2,100°F (1,149°C) to produce a coarser, more creep-resistant grain structure, and because their higher nickel content suppresses the same sigma-phase mechanism that affects 310S, they tend to retain usable ductility and toughness across a wider portion of the furnace operating range, including through repeated thermal cycling.

Which Alloy Offers Better Resistance to Oxidizing and Carburizing Atmospheres?

Oxidation resistance in both alloys comes primarily from chromium, which forms a protective chromium-oxide scale at high temperature. 310S actually carries more chromium than Incoloy 800H (roughly 25% versus roughly 21%), which is one reason 310S remains a strong, economical performer in clean oxidizing atmospheres such as air furnaces, kiln shells, and burner hardware.

Carburization resistance is a different mechanism, and this is where the two alloys diverge most sharply. Carbon transfer into the metal surface from a carbon-rich atmosphere depends heavily on nickel content, because nickel has very low solubility for carbon and slows its inward diffusion. Field data on 800H tubing in carburizing hydrocarbon-processing service shows the alloy retaining ductility and structural integrity after years of exposure, in conditions where lower-nickel 310-family stainless steels carburize measurably faster. This is the primary reason Incoloy 800H is the incumbent material for ethylene pyrolysis tubes, hydrocarbon cracking furnace components, and heat-treat baskets and fixtures that see repeated carburizing cycles.

Published corrosion-rate comparisons in refinery furnace environments illustrate the gap: Incoloy 800H/800HT alloys have been measured at approximately 0.15 mm/year (6.0 mpy), versus approximately 0.23 mm/year (8.9 mpy) for 310-family stainless steel under comparable conditions. A lower corrosion rate translates directly into longer component life and fewer unplanned outages for inspection or replacement.

How Do Creep and Stress-Rupture Strength Compare at Elevated Temperature?

Creep is the slow, time-dependent deformation of metal under sustained load at high temperature, and stress-rupture strength describes how much stress a material can carry before it fractures after a given number of service hours. This property matters for any furnace component that is under load while hot - a radiant tube under internal pressure, a hanger supporting refractory, or a coil in a pyrolysis furnace - as distinct from components that are simply exposed to heat with no significant mechanical load, such as a muffle liner or shield.

310S offers moderate creep-rupture strength, adequate for lightly loaded components. Incoloy 800H and 800HT were specifically developed to extend creep-rupture life beyond what standard 800 or 310-family stainless steels provide, through controlled carbon content (0.05–0.10%) and a coarse-grain solution anneal at a minimum of 2,100°F (1,149°C). Incoloy 800HT tightens the aluminum-and-titanium specification further on top of that, for applications demanding the longest practical service life under sustained high-temperature stress, such as ethylene furnace tubing and pyrolysis coils.

Source: Compiled from ASTM A240/B407/B409 specification data and published high-temperature corrosion-rate and mechanical-property studies for 310/310H and Incoloy 800H/800HT in refinery and furnace service. Verify design allowables against the current ASME Section II Part D or applicable code edition before final specification.

What Is the Cost Difference, and When Is the Premium Justified?

Nickel is consistently the largest single driver of alloy surcharge pricing for both materials, but Incoloy 800H/HT carries roughly 50 percent more nickel by weight than 310S, and nickel typically trades at a significant multiple of the chromium and iron that make up most of 310S's alloy content. This makes the 800-series alloys the more expensive purchase on a per-pound or per-linear-foot basis, before fabrication is even considered.

The decision framework that follows from the technical comparison above is straightforward. Specify 310S when the component sees a clean oxidizing atmosphere, light or no mechanical load, and moderate service-life expectations - furnace shells, doors, fans, and non-load-bearing shields are common fits. Specify Incoloy 800H or 800HT when the component is under sustained load at temperature, exposed to a carburizing or sulfidizing atmosphere, or when the cost of an unplanned shutdown to replace a failed component substantially exceeds the material cost premium - radiant tubes, process-heater components, ethylene cracking coils, and heat-treat fixtures with long carburizing cycles are common fits.

How Do Fabrication, Welding, and Heat Treatment Requirements Differ?

310S is solution-annealed in the range of 1,900–2,100°F (1,030–1,150°C) followed by rapid cooling, which puts carbides back into solution and maximizes corrosion resistance. If 310-family equipment has seen extended service above 1,200°F (650°C) and sigma phase has formed, ductility can generally be restored with the same solution-anneal cycle; a re-anneal after roughly every 1,000 hours of service above 1,200°F is a commonly cited maintenance interval for equipment prone to sigma formation.

Incoloy 800H and 800HT require a solution anneal at a minimum of 2,100°F (1,149°C) - noticeably higher than the anneal used for 310S - specifically to produce a coarser grain structure (typically ASTM grain size No. 5 or coarser) that improves creep-rupture life. Both alloys are weldable by standard gas tungsten arc and shielded metal arc processes with matching or near-matching filler metal, and both are conventionally cold-formed, though the 800-series alloys can display a characteristic surface texture ("orange peel") after heavy cold forming due to their coarser grain size.

Which Alloy Should You Specify for Common Furnace and Kiln Components?

Source: General selection guidance based on documented alloy properties and common industrial furnace/kiln practice. Component-specific selection should account for actual atmosphere chemistry, pressure rating, thermal cycling frequency, and applicable design code.

Frequently Asked Questions

Q: Can 310S and Incoloy 800H be used interchangeably in the same furnace?

A: Not generally. They have different thermal expansion rates and different creep behavior, so mixing them in a single welded assembly under load can create differential stress. Where both alloys appear in the same furnace, they are typically used in separate, independently supported components - for example, a 310S shell with Incoloy 800H internal tubing.

Q: Is Incoloy 800 the same as Incoloy 800H or 800HT?

A: No. Standard Incoloy 800 (UNS N08800) allows a broader carbon range and does not carry the coarse-grain, high-temperature solution anneal that gives 800H (UNS N08810) and 800HT (UNS N08811) their superior creep-rupture strength. In current furnace and process-heater specifications, 800H and 800HT have largely replaced standard 800.

Q: Does 310S require post-weld heat treatment?

A: Not for corrosion resistance in most furnace applications, since 310S is a low-carbon grade with good resistance to weld-related sensitization. If equipment will see extended service in the sigma-phase-forming range, a periodic solution anneal - not a stress relief - is the appropriate corrective treatment, not a routine post-weld requirement.

Q: What is the practical temperature limit for continuous 310S service?

A: Approximately 2,000°F (1,093°C) for continuous oxidizing service, with the caveat that equipment cycling through the 1,200–1,650°F (650–900°C) range repeatedly is exposed to sigma-phase embrittlement over time and should be evaluated for periodic re-annealing.

Q: Why does nickel content matter so much for carburization resistance?

A: Carbon has very low solubility in nickel and diffuses into it more slowly than into iron or chromium. Because Incoloy 800H carries roughly 30–35% nickel versus roughly 19–22% for 310S, carbon penetrates its surface more slowly under identical carburizing conditions, which is the main reason 800H outperforms 310-family stainless steels in carburizing furnace atmospheres.