How do you choose a furnace alloy?
Furnace alloy selection starts from two questions, not one: the maximum continuous service temperature and the furnace atmosphere. Temperature alone tells you which grades resist oxidation; the atmosphere decides whether you actually need a nickel-rich alloy or can stay with a cheaper heat-resistant stainless steel. A muffle running at 1000 °C in clean air and one running at 1000 °C in a carburising gas need completely different materials.
| Service temperature | Air / oxidising | Carburising | Sulphidising |
|---|---|---|---|
| Up to 700 °C | 304, 321, 316 | 309S, 310S | 309S with caution |
| 700–900 °C | 309S, 310S | Inconel 600, 601 | Inconel 600 |
| 900–1000 °C | Inconel 600, 800HT, RA330 | Inconel 601, 617 | Inconel 601 with caution |
| 1000–1100 °C | Inconel 601, 617, Hastelloy X | Inconel 601, Hastelloy X | not normally viable |
| 1100–1250 °C | Inconel 601, Haynes 214, APM | Hastelloy X, 602CA | not normally viable |
What is the best alloy for 800 °C to 1000 °C?
This is the widest and most commercially common band, and most projects land on one of three answers. Inconel 600 (UNS N06600) is the general-purpose choice where chloride stress-corrosion cracking is also a concern. Incoloy 800HT (UNS N08811) offers higher creep strength for structural parts and is widely used for furnace tubes and headers. RA330 is the cost-effective heat-resistant austenitic grade for trays, baskets and fixtures where loads are moderate.
- Inconel 600: 72% Ni, oxidation resistant to about 1095 °C, immune to chloride SCC
- Incoloy 800HT: controlled carbon and grain size for creep resistance above 800 °C
- RA330: 35% Ni, 19% Cr, good for cyclic duty and cheaper than a superalloy
- 310S: the budget option below 900 °C in clean air, with 25% Cr and 20% Ni
What changes above 1000 °C?
Above 1000 °C the protective oxide scale becomes the limiting factor, and the choice narrows quickly. Inconel 601 (UNS N06601) is the workhorse for this band because its aluminium addition forms a dense, adherent alumina-chromia scale that survives thermal cycling. Inconel 617 adds molybdenum and cobalt for creep strength up to about 1100 °C, and Hastelloy X gives the best combination of oxidation resistance and high-temperature strength in the 1100 to 1200 °C range.
| Grade | UNS | Max continuous service | Why it is chosen |
|---|---|---|---|
| Inconel 601 | N06601 | 1150 °C (oxidation) | Alumina scale, excellent spall resistance |
| Inconel 617 | N06617 | 1100 °C | Creep strength for structural components |
| Hastelloy X | N06002 | 1200 °C | Oxidation plus strength at temperature |
| Inconel 690 | N06690 | 1100 °C | High Cr for oxidising sulphur-bearing gas |
| 310S | S31008 | 1050 °C intermittent | Economy, simple parts, clean air |
Why does the atmosphere matter more than the temperature?
Because carburisation and sulphidation attack the alloy by a different mechanism than oxidation, and the answer is almost never the same grade. In a carburising atmosphere, carbon diffuses into the metal, forms chromium carbides at the grain boundaries and destroys ductility. High nickel content resists this. In a sulphidising atmosphere — typically from sulphur-bearing fuels or ores — nickel itself is attacked, which inverts the usual logic: high-nickel superalloys can fail faster than a lower-nickel, high-chromium stainless steel.
- Carburising: favour high nickel — Inconel 601, 617, or 602CA for severe duty
- Nitriding: high nickel, and avoid grades that form unstable nitride layers
- Sulphidising: favour high chromium with controlled nickel — Inconel 690, or high-Cr stainless
- Reducing atmospheres: protect the scale, and consider a protective atmosphere inside the retort
What about strength at temperature rather than corrosion resistance?
A furnace part often needs both, and they rarely peak at the same grade. Creep strength — the ability to hold a load without slowly stretching — is what fails structural components such as radiant tubes, skid rails and supports, and it is quoted as a stress to rupture in 10,000 or 100,000 hours.
- Incoloy 800HT and 617 are the creep-strength picks for structural furnace parts
- Hastelloy X combines creep strength with the best oxidation ceiling in this list
- Cast grades such as HK40 and HP40 offer higher creep strength but limited fabricability
- For pure fixtures and baskets, creep strength is secondary to oxidation resistance
Which standards apply to furnace alloys?
The standards that cover the wrought grades discussed above are the ones to quote on a purchase order:
- ASTM B168 — nickel-chromium-iron alloy plate, sheet and strip (600, 601, 617)
- ASTM B166 — nickel-chromium-iron alloy bar and bar stock (600, 601)
- ASTM B167 — nickel-chromium-iron alloy seamless pipe and tube (600, 601)
- ASTM B435 — nickel-chromium-molybdenum alloy plate, sheet and strip (Hastelloy X, 617)
- ASTM B572 — nickel-chromium-iron alloy UNS N06002 bar and wire (Hastelloy X)
- ASTM A240 / A240M — chromium and nickel stainless steel plate (309S, 310S)
- ASTM E112 — grain size determination, important for creep-resistant grades
Frequently asked questions
Is Inconel 601 better than 310S for a 1000 °C furnace?
It depends on the atmosphere and the part's role. In clean air and for simple, lightly loaded parts, 310S is usually sufficient and far cheaper. Inconel 601 wins when the atmosphere is carburising, when parts are thermally cycled, or when the oxide scale must not spall onto the workload. Most 1000 °C furnaces with carburising gas end up on 601.
What causes a furnace tube to fail early even in the right alloy?
Three causes account for most failures: overheating above the grade's continuous service limit, carburisation from an atmosphere the specification did not anticipate, and grain growth that reduces creep ductility over long service. The alloy choice is only half the answer — the operating temperature and atmosphere must match the assumption the selection was based on.
Does Hangbo Alloy supply furnace alloys in fabricated form?
Hangbo Alloy supplies Inconel 600, 601, 617, 625, 690, Incoloy 800HT, Hastelloy X and 310S as bar, plate, sheet, seamless tube, welding wire and forgings, with EN 10204 3.1 mill test certificates. Third-party inspection by SGS, BV or TUV is available on request.