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Inconel 601 / (2.4851) / UNS N06601 / AMS 5870

Inconel 601 (2.4851), also widely known as Alloy 601, is one of the most widely used of all nickel alloys. The material contains a high proportion of nickel, usually around 60%. Also containing around 24% chromium, it is formed essentially from nickel and chromium.​‌​​‌​

Although its mechanical properties are excellent, Inconel 601 is more difficult to machine than other alloys because of its high hardness and toughness. Care should be taken during machining in order to avoid excessive tool wear and to achieve good surface quality.

It can be welded by the TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) methods. Care should be taken against the risks of oxidation and thermal stress during welding, and post-weld heat treatment may be required.​‌​​‌​

Alloy 601 is very widely used in heat treatment machinery and equipment components. It is used in industrial heaters, heat treatment furnaces, furnace parts working at very high temperature, exhaust tips, resistance heater components and many other very high temperature applications. Another of its most frequent application areas is jet engines and aircraft components.

Standards by Product Form​‌​​‌​

Alloy 601 (UNS N06601 / W.Nr. 2.4851) is bought for one thing: 1.0–1.7 % aluminium. That aluminium builds an oxide scale that stays attached through thermal cycling. The most expensive warning on this page, though, is a paperwork one: N06601 is not in the scope of ASTM B564, the nickel-alloy forging specification — yet it is sold as “SB-564 flanges”.​‌​​‌​

There are two different “5870” documents and they are being conflated. VdTÜV Werkstoffblatt 5870 is a German pressure-equipment material approval; AMS 5870 is an SAE aerospace material specification. Neither substitutes for the other, and a certificate quoting “5870” without the prefix is ambiguous. Similarly, the DIN 17742–17754 series are legacy callouts; the live European standard is EN 10095, which covers plate, sheet, strip, bars, rods and sections together. The DIN documents’ withdrawal status could not be verified.​‌​​‌​

Composition: Ni 58.0–63.0 % · Cr 21.0–25.0 % · Al 1.0–1.7 % · Fe balance · C ≤0.10 % · Mn ≤1.0 % · Si ≤0.50 % · S ≤0.015 % · Cu ≤1.0 %. One European mill runs a tighter house specification (Cu ≤0.5 %, Co ≤1.5 %, Ti ≤0.5 %, P ≤0.02 %, B ≤0.006 % and a 0.03 % carbon minimum) — do not present those tighter values as “the” specification.

What the aluminium does — the whole point of the alloy. The 1–1.7 % Al is not there for strength. It forms an aluminium-rich sub-layer that mechanically anchors the chromia scale; the originator describes it as an “extremely protective and adherent oxide film” with “unique resistance to oxide spalling under cyclic thermal conditions“. Picture a furnace door, a retort, or a basket going in and out: a plain Ni-Cr scale (alloy 600) spalls on every cycle, exposes fresh metal, and the part loses section by repeated re-scaling. 601’s scale stays put. That is the single reason to pay the premium over 600.​‌​​‌​

Welding, Heat Treatment and Damage Windows

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Welding — consumable by service temperature

Up to 980 °C: SMAW with INCO-WELD A or Electrode 117; GMAW with Filler 82, 601 or 617; GTAW with Filler 82 or 617; SAW with Filler 82. 980–1150 °C: SMAW with Electrode 117 only; GMAW with Filler 601 or 617; GTAW with Filler 617; SAW not recommended. Above 1150 °C: GMAW with Filler 601 only; SMAW, GTAW and SAW are not recommended. And read the last row carefully: for service exposed to H₂S or SO₂ no welding product is recommended at any temperature.​‌​​‌​

Heat input and interpass: only one European mill publishes numbers — GTAW and GMAW/MAG max 8 kJ/cm, SMAW max 7 kJ/cm, plasma max 10 kJ/cm, interpass temperature max 120 °C. Preheat: neither source specifies a preheat requirement — that is the absence of a requirement, not a published statement that none is needed. PWHT: no conventional post-weld heat treatment is specified; the 980 °C for about 3 hours given by that same mill is not a general PWHT but a specific mitigation against stress-relaxation cracking.

Cleanliness is a hard requirement: “alloy 601 must be clean before it is heated. All foreign substances such as grease, oil, paint, and shop soil must be removed”, and “the alloy must be heated in a low-sulfur atmosphere. Fuels for open heating must be low in sulfur.” Sulphur from a marker pen, a cutting fluid or a fingerprint causes liquid-metal embrittlement of high-nickel alloys. Note too that the alloy cannot be bright-annealed in a usual industrial furnace and needs a specialised pickling procedure.​‌​​‌​

Heat treatment and the real damage windows

Not strengthened by heat treatment. Broad ranges of strength are achieved only by combining cold work with annealing. Solution annealing 1100–1200 °C (verified in two sources); for optimum creep resistance aim at 1140–1160 °C and an ASTM E112 grain size of 5 or coarser. No stress-relief temperature or procedure is published in either source — do not invent one.​‌​​‌​

Machining, Service Limits and Where 601 Is the Wrong Choice​‌​​‌​

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No cutting speeds are published on this page. The originator’s 601 bulletin publishes no cutting speeds, feeds, depths of cut or tool grades; it defers to a separate machining publication which could not be reached. A cutting speed you see on a distributor page may not be traceable to the mill bulletin. The qualitative guidance is clear: for best machinability the material should be in the solution-treated condition; maintain machine rigidity; use sharp carbide-tipped tools with heavy, constant feeds (never dwell — dwelling work-hardens the surface and the next pass rides on a hardened skin); soluble oils are recommended, especially with carbide. The work-hardening rate is somewhat higher than alloy 600 and Incoloy 800 — a shop used to 600 or 800H will need more interstage annealing than it expects, so quote accordingly. Hot working 870–1230 °C, large deformations 1040–1230 °C; do not work between 650 and 870 °C.

Frequently Asked Questions​‌​​‌​

601, 600, 800H or 625 for high-temperature service — when is each actually required?

These four are not interchangeable, and price is the worst way to choose. Alloy 600 (Ni ≥72 %, Cr 14–17 %, no aluminium) is the baseline Ni-Cr-Fe alloy; without Al its oxide spalls under thermal cycling. Choose 600 for chloride resistance and for steady, non-cycling heat — not for furnace parts that go in and out. Alloy 601 (Ni 58–63 %, Cr 21–25 %, Al 1.0–1.7 %) buys one thing: a scale that stays attached through thermal cycling. The originator’s 1000-hour cyclic test at 1095 °C — 15 minutes hot, 5 minutes cooling — showed 601 outperforming both 600 and 800. Choose 601 for baskets, trays, fixtures, retorts, muffles, radiant tubes and burner components that cycle. 800H / 800HT is the creep alloy: controlled 0.05–0.10 % carbon plus a mandatory ASTM 5 or coarser grain size, and — critically — ASME Section VIII Division 1 design stresses to 982 °C and Section I to 816 °C. If it is a code pressure part carrying load hot, you almost certainly need 800H/800HT, not 601. 625 (Mo 8–10 %, Nb 3.15–4.15 %) is the corrosion alloy: 120–150 ksi annealed tensile, cryogenic-to-982 °C service and documented freedom from sigma. Choose 625 for aqueous and mixed wet/dry corrosion, not for 1100 °C furnace duty.​‌​​‌​

Why do “maximum service temperature” figures diverge so much, and how should a buyer read them?

Because the published figures answer different questions, and almost nobody says which. Across the sources checked, alloy 601’s “maximum” is quoted as 1000, 1093, 1150, 1200, 1205 and 1250 °C — a 250 °C spread. One distributor page states both “up to 1250 °C” and “oxidation resistance to 2000 °F” (1093 °C), contradicting itself in two paragraphs. The divergence is structural, not sloppiness: one mill’s 1200 °C is explicitly “the maximum operating temperature in air according to DIN EN 10095″ — essentially a scaling limit for unstressed material. The originator’s 1205 °C comes from 500-hour oxidation coupon tests — again, how long a test piece survives the atmosphere. A wire producer’s 1000 °C is explicitly qualified “depending on load and environment” — a load-bearing figure. Both kinds are honest; they measure different failure modes. How to read them: the oxidation number tells you when the alloy stops being chemically stable. It does not tell you when the part stops holding its shape. The 1000-hour rupture strength falls from 193 MPa at 650 °C to 6.9 MPa at 1093 °C. Practical rule: ~1100–1200 °C for unstressed parts, ~950–1000 °C where the part carries load — and state which you mean on every enquiry.​‌​​‌​

Our supplier quoted 601 flanges and forgings to ASTM B564 / ASME SB-564. Is that correct?

On the evidence, no — and this is the most common paperwork error on this alloy. ASTM B564, “Standard Specification for Nickel Alloy Forgings” (current edition B564-25) covers 27 nickel alloys. Checking ASTM’s own published scope, the covered Ni-Cr-Fe grades include N06600, N06603 and N06690 — but not N06601. This is not an indexing gap: B564 is correctly cited for N06600 in the alloy 600 bulletin and for N06617 in the alloy 617 bulletin. The specification simply carries other grades and not this one. Yet distributor pages advertise “Forging: AMS 5715, ASTM B564, ASME SB564” for Inconel 601. A certificate to SB-564 for N06601 is a certificate to a specification that does not list the grade — which an ASME code reviewer or third-party inspector can reject. What the originator actually lists for forgings is DIN 17754 and ISO 9725; ASTM B166 covers the forging stock, not the finished forging. On the purchase order: either (1) specify DIN 17754 / ISO 9725, or (2) buy B166 forging stock and write the forging, solution-anneal and testing requirements into the order yourself, or (3) ask the supplier in writing to show you the grade table in the B564 edition they are certifying to. Do not accept “SB-564, N06601” unchallenged.​‌​​‌​

Is there such a grade as “Inconel 601GC”?

It could not be verified as a current product — the originator publishes no 601GC technical bulletin, the grade is absent from both of its indexes, and no reachable distributor page mentions it. What is verified is that grain coarsening is a real, documented lever on this alloy: one mill gives an anneal at 1140–1160 °C producing an ASTM E112 grain size of 5 or coarser for optimum creep resistance, and the originator specifies the solution-treated condition for rupture-limited service above about 540 °C and the annealed condition for tensile-limited service. The mechanism is exactly the reasoning that produced 800H from 800: coarse grain buys creep and stress-rupture strength at the cost of room-temperature ductility, toughness and formability. If a customer asks for “601GC”, do not offer it as a grade. Write into the purchase order (a) the required grain size as an ASTM E112 number, (b) the solution-anneal temperature and (c) which standard the material is certified to. There is no verified separate UNS number, no separate ASTM or EN grade and no current mill bulletin. Handle it as N06601 with a specified grain size and anneal.​‌​​‌​

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