Inconel 600 vs Incoloy 800: Nickel Alloy Selection for Heat Treatment Furnace Components

What Are Inconel 600 and Incoloy 800, and How Do Their Compositions Differ?

Inconel 600 is a nickel-chromium-iron alloy built on a nickel-rich base (72% minimum), while Incoloy 800 is fundamentally an iron-based alloy with only 30-35% nickel - a deliberate, historically documented design choice that puts the two alloys on opposite ends of the cost-versus-high-temperature-performance spectrum.

Source: Special Metals Corporation technical bulletins SMC-027 (INCONEL alloy 600) and SMC-046 (INCOLOY alloy 800).

Incoloy 800 was introduced in the 1950s specifically to fill the need for a heat- and corrosion-resistant alloy with a relatively low nickel content, since nickel was, at the time, designated a strategic metal. This is not incidental history - it is the design logic that still separates the two alloys today. Inconel 600's high nickel content gives it excellent resistance to reducing conditions and virtually immunizes it against chloride-ion stress-corrosion cracking; Incoloy 800's aluminum and titanium additions (present in 600 in only trace amounts, if at all) form strengthening precipitates that partly compensate for its lower nickel content at elevated temperature.

Which Alloy Has the Higher Maximum Service Temperature for Furnace Components?

Inconel 600 carries a higher headline service-temperature ceiling - continuous use to 2,000°F (1,093°C) - while standard Incoloy 800 is generally used for its strength characteristics up to about 1,500°F (816°C); for furnace duty above that point, Incoloy's own high-temperature variants, 800H and 800HT, close much of the gap.

Special Metals describes Inconel 600's versatility as spanning "temperatures from cryogenic to above 2,000°F (1,095°C)" and states directly that the alloy's strength and oxidation resistance at high temperature make it useful for retorts, muffles, roller hearths, and other furnace components. By contrast, Special Metals states that Incoloy 800 "is used for its strength characteristics at service temperatures up to about 1,500°F (816°C)" and that "for applications that require high creep or rupture strength, INCOLOY alloys 800H or 800HT are used" instead.

Both 800H and 800H T carry a controlled carbon range (0.05-0.10% and 0.06-0.10% respectively) and a coarser minimum grain size (ASTM 5 or coarser) specifically engineered to raise creep and stress-rupture strength beyond what standard 800 delivers, extending the practical furnace-duty temperature range of the 800 family well past the base alloy's 1,500°F design point.

Which Alloy Better Resists Carburization in Furnace Atmospheres?

Inconel 600 absorbs measurably less carbon than Incoloy 800HT under identical carburizing conditions: Special Metals' own 100-hour gas-carburization test in a hydrogen/2% methane atmosphere shows Inconel 600 gaining 2.66-12.30 mg/cm2, compared with 4.94-21.58 mg/cm2 for Incoloy 800HT - roughly half the weight gain at both temperatures tested.

Source: Special Metals Corporation, gas carburization tests (100 h) in hydrogen/2% methane atmosphere (1,700°F test atmosphere also contained 5% argon), INCONEL alloy 600 technical bulletin SMC-027.

This side-by-side test is one of the few places a furnace-component specifier can see Inconel 600 and Incoloy 800HT measured against the same carburizing exposure in a single, controlled experiment. Carburization matters enormously in this application because hydrocarbon-bearing furnace atmospheres (endothermic gas, cracked ammonia, and similar carburizing environments used in case-hardening and gas carburizing furnaces) drive carbon into the surface of furnace fixtures, forming internal carbides that embrittle the metal and shorten fixture life. Lower carbon uptake translates directly into longer service life for baskets, trays, and fixtures cycling repeatedly through carburizing atmospheres.

Which Alloy Better Resists Cyclic Oxidation and Scale Spallation?

Special Metals' own cyclic oxidation testing at 1,800°F (980°C) - using repeated 15-minute heating and 5-minute cooling cycles in air over 1,000 hours - directly compares Inconel 600 against Type 304 stainless steel, Type 309 stainless steel, and Incoloy 800HT, and demonstrates that the nickel-chromium-iron composition of Inconel 600 retains its protective oxide scale far better than the two stainless steels under repeated thermal cycling.

Cyclic, rather than steady-state, oxidation resistance is the more realistic test for furnace fixtures, which heat up and cool down with every production batch rather than sitting at a single soak temperature indefinitely. Repeated thermal cycling causes the protective chromium-oxide scale on any alloy to crack and spall from the underlying metal's thermal expansion and contraction; the alloy that best resists this spallation retains its protective layer, and therefore its corrosion resistance, over more heat-cool cycles.

Special Metals' inclusion of Incoloy 800HT directly in this same comparison - rather than testing it separately - underscores that both nickel-rich alloys were developed with this exact furnace-cycling failure mode in mind, even though the test shows the highest-nickel alloy (600) with the strongest scale retention of the group.

Which Alloy Offers Better Nitriding Resistance?

Inconel 600 is explicitly positioned by its own manufacturer as the standard material for nitriding containers because of its resistance to nitrogen absorption at high temperature, a specific, named application that does not appear in Incoloy 800's own technical literature.

Special Metals states plainly that Inconel 600 "is the standard material for nitriding containers because of its resistance to nitrogen at high temperatures." Nitriding furnaces use ammonia-dissociation atmospheres (roughly 25% nitrogen, 75% hydrogen) to diffuse nitrogen into steel part surfaces, and any furnace fixture exposed to this atmosphere is itself at risk of nitrogen pickup and embrittlement. Because Incoloy 800's own published literature does not make an equivalent nitriding-resistance claim, fixtures and containers for nitriding furnaces specifically should default to Inconel 600 unless field data supports an alternative.

How Do the Two Alloys Compare in Creep and Stress-Rupture Strength?

Inconel 600's published rupture-strength data extends cleanly to 2,000°F (1,093°C), while Special Metals is explicit that standard Incoloy 800 "is not normally used for applications that require optimum creep-rupture properties" - that role belongs to Incoloy 800H and 800HT, whose restricted chemistry and coarser grain size were engineered specifically to raise long-term, high-temperature strength.

Source: Special Metals Corporation rupture-properties table (solution-treated at 2,050°F/1,121°C, air-cooled), INCONEL alloy 600 technical bulletin SMC-027.

For furnace fixtures under sustained load at high temperature - muffle supports, roller-hearth rolls, or radiant-tube hangers - creep-rupture strength, not tensile strength, is usually the governing design property. Standard Incoloy 800 supports design stresses only to roughly 1,500°F under the ASME Boiler and Pressure Vessel Code, while Incoloy 800H and 800HT's controlled 0.05-0.10% (or 0.06-0.10%) carbon content and ASTM 5-or-coarser grain size were specifically approved by the ASME Code Committee for higher design stresses at 1,100-1,500°F (593-816°C) and beyond, following analysis of 87 heats and 1,052 data points by the Metals Property Council. Where sustained high-temperature load-bearing service is the priority, comparing Inconel 600 against 800H or 800HT specifically - not base Incoloy 800 - is the technically correct comparison.

How Do the Two Alloys Compare in Chloride Stress-Corrosion Cracking Resistance?

Inconel 600's minimum 72% nickel content makes it virtually immune to chloride-ion stress-corrosion cracking, while Incoloy 800's lower 30-35% nickel content can allow cracking under severe laboratory conditions - though Incoloy 800's resistance is still high enough that it is commonly used to replace materials that have already failed in service from this exact failure mode.

This matters for furnace equipment that sees intermittent exposure to chloride contamination - quenchant residues, marine or industrial atmospheres, or cleaning chemicals - in addition to high-temperature service. The general metallurgical rule, confirmed in Special Metals' own literature, is that the tendency of austenitic alloys to crack in chloride solutions decreases as nickel content increases. Inconel 600, with 72% minimum nickel, sits at one end of that spectrum; Incoloy 800, with roughly half the nickel content, sits further along it, but its resistance still substantially exceeds that of standard 300-series austenitic stainless steel fixtures it commonly replaces.

Are Both Alloys Prone to Sensitization During Furnace Thermal Cycling?

Yes - both alloys share the same fundamental vulnerability: chromium carbides precipitate at grain boundaries in roughly the same 1,000-1,400°F (540-760°C) range for both Inconel 600 and Incoloy 800, and furnace fixtures that repeatedly heat through this band on every cycle are exposed to the same sensitization risk regardless of which alloy is chosen.

Special Metals' technical literature for both alloys describes essentially the same mechanism: titanium nitrides and carbides (or, in 600's case, cyanonitrides) form independently of heat treatment, while chromium carbides precipitate specifically between 1,000°F and 1,800°F (540-980°C) for Inconel 600, and 1,000-2,000°F (540-1,095°C) for Incoloy 800, with the most aggressive sensitization band for both alloys sitting at 1,000-1,400°F (540-760°C).

Special Metals even publishes time-temperature-sensitization diagrams for Incoloy 800 based on the Huey test (boiling 65% nitric acid) specifically to quantify this risk. In practice, sensitization is described by Special Metals as "not a problem in most high-temperature applications" for either alloy - it matters primarily if the fixture will later be pickled or exposed to an aggressive aqueous environment while still in the sensitized condition, which is uncommon for fixtures that stay in furnace service.

How Do the Alloys Compare in Cost and Material Selection Strategy?

Incoloy 800 exists specifically because Inconel 600's high nickel content made it expensive and, in the 1950s, strategically constrained - Incoloy 800's iron-based chemistry was a deliberate engineering trade-off to deliver adequate heat and corrosion resistance at a fraction of the nickel content, and that cost logic still governs the choice between these two alloy families today.

This is not a marketing narrative; it is the alloy's documented origin. Special Metals' own history of the alloy states that Incoloy 800 "was introduced to the market in the 1950s to fill the need for a heat- and corrosion-resistant alloy with a relatively low nickel content since nickel was, at the time, designated a 'strategic' metal."

For furnace equipment where the largest, heaviest fixtures (roller-hearth rolls, large muffles, radiant tube bundles) dominate the material cost, specifying Incoloy 800 or 800H/800HT wherever its lower service-temperature ceiling and somewhat lower creep-rupture strength are acceptable can meaningfully reduce total fixture cost compared with an all-Inconel-600 design, while reserving 600 specifically for the highest-temperature, most carburizing, or nitriding-exposed components in the same furnace.

How Do the Alloys Compare in Fabrication and Welding for Furnace Fixture Manufacture?

Both alloys are readily hot- and cold-formed, share a nearly identical work-hardening rate, and can both be welded with INCONEL Filler Metal 82 - a genuinely convenient overlap for shops fabricating mixed-alloy furnace fixtures, since a single filler metal can join both base alloys or a dissimilar-metal joint between them.

Source: Special Metals Corporation technical bulletins SMC-027 (INCONEL alloy 600) and SMC-046 (INCOLOY alloy 800).

What Standards Govern Inconel 600 and Incoloy 800 for Furnace Component Fabrication?

Both alloys are approved under the ASME Boiler and Pressure Vessel Code with dedicated Code Cases - Inconel 600 under Code Case 1827 and Incoloy 800/800H/800HT under Code Case 1325 - with parallel but distinct product-form specifications for each alloy family.

Source: Special Metals Corporation "Available Products and Specifications" tables, INCONEL alloy 600 and INCOLOY alloy 800 technical bulletins.

Which Alloy Should You Specify for Heat Treatment Furnace Components?

Specify Inconel 600 for the most severe carburizing and nitriding exposure, the highest furnace temperatures, or any component where chloride stress-corrosion cracking is a realistic risk; specify Incoloy 800 (or 800H/800HT for higher-temperature, load-bearing duty) where furnace temperatures stay near or below roughly 1,500-1,800°F and lower material cost is a priority.

• Choose Inconel 600 for: Nitriding furnace containers and fixtures, where 600 is the named standard material.
• Choose Inconel 600 for: Heavily carburizing atmospheres (endothermic gas, case-hardening furnaces) where minimizing carbon pickup extends fixture life.
• Choose Inconel 600 for: Continuous service above roughly 1,800-2,000°F, at the upper edge of or beyond Incoloy 800H/800HT's practical range.
• Choose Inconel 600 for: Components with any realistic exposure to chloride contamination in addition to high heat.
• Choose Incoloy 800 / 800H / 800HT for: Large, heavy fixtures - muffles, roller-hearth rolls, radiant tube supports - where material cost scales with tonnage and service temperature stays within the 800-family envelope.
• Choose Incoloy 800 / 800H / 800HT for: Sustained, load-bearing service above 1,500°F where 800H or 800HT's ASME-recognized higher design stresses are sufficient for the application.
• Choose Incoloy 800 / 800H / 800HT for: Applications where chloride-driven stress-corrosion cracking is a low but non-zero risk, and Incoloy 800's still-substantial nickel content provides adequate margin over stainless steel alternatives.

In mixed-alloy furnace designs, the two alloys pair naturally: Inconel 600 for the fixtures and containers exposed to the most severe atmosphere or highest local temperature, and Incoloy 800/800H/800HT for the larger structural fixtures where its lower nickel content reduces total material cost without compromising the furnace's overall service life.

Frequently Asked Questions

Q: What is the main difference between Inconel 600 and Incoloy 800?

A: Inconel 600 (UNS N06600) is a nickel-based alloy with 72% minimum nickel, while Incoloy 800 (UNS N08800) is iron-based with only 30-35% nickel. This composition difference gives Inconel 600 better carburization and nitriding resistance and a higher service-temperature ceiling, while Incoloy 800 offers a substantially lower material cost - a trade-off that traces directly back to Incoloy 800's 1950s design goal of conserving nickel.

Q: Which alloy resists carburization better in furnace atmospheres?

A: Inconel 600. Special Metals' own 100-hour carburization test in a hydrogen/2% methane atmosphere shows Inconel 600 gaining roughly half the weight (2.66-12.30 mg/cm2) that Incoloy 800HT gains (4.94-21.58 mg/cm2) under the same exposure.

Q: Can Incoloy 800 be used at the same temperatures as Inconel 600?

A: Not quite. Inconel 600 is used continuously up to 2,000°F (1,093°C), while standard Incoloy 800 is generally limited to about 1,500°F (816°C) for strength-based design. For higher-temperature furnace duty, the 800H and 800HT variants - with controlled carbon content and coarser grain size - extend Incoloy 800's practical temperature range and close much of the gap.

Q: Is Inconel 600 or Incoloy 800 better for nitriding furnace fixtures?

A: Inconel 600. Special Metals explicitly names it as the standard material for nitriding containers due to its resistance to nitrogen absorption at high temperature; Incoloy 800's own published literature does not make an equivalent claim.

Q: Why does Incoloy 800 have such a low nickel content compared to Inconel 600?

A: Incoloy 800 was introduced in the 1950s specifically to reduce reliance on nickel, which was then classified as a strategic metal in limited supply. Its iron-based chemistry (30-35% nickel versus Inconel 600's 72% minimum) was a deliberate cost and supply-driven engineering trade-off, not a performance target in itself.

Q: Are Inconel 600 and Incoloy 800 both resistant to chloride stress-corrosion cracking?

A: Inconel 600 is considered virtually immune due to its high nickel content. Incoloy 800 can be made to crack under severe laboratory conditions because of its lower nickel content, but its resistance is still high enough that it is commonly used to replace materials that have already failed in service from this exact mechanism.

Q: Do Inconel 600 and Incoloy 800 use the same welding filler metal?

A: For GTAW and GMAW, yes - both alloys are commonly joined with INCONEL Filler Metal 82. For shielded metal-arc welding, Inconel 600 typically uses INCONEL Welding Electrode 182, while Incoloy 800 uses INCO-WELD A Electrode.