Inconel Machining: Speeds, Tooling, and the Work-Hardening Rules
Inconel 718 machines at 12% of free-machining steel and hardens the moment you let off the feed. Here is how to cut it anyway — condition selection, carbide parameters by operation, coolant pressure, and the mistakes that scrap parts.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 11 min read
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Quick Answer
Machine Inconel 718 in the annealed condition (AMS 5662, 200–250 HB), with sharp coated carbide at 25–40 m/min turning and 25–35 m/min milling, decisive feeds (0.20–0.35 mm/rev roughing), and coolant-through tooling at 500 PSI minimum. The one rule above all others: never dwell and never take a light rubbing pass — Inconel work-hardens from ~36 HRC to 50–55 HRC at the surface, and every pass after a mistake cuts harder material. Aged stock (AMS 5663) costs you another 30–50% of machinability; machine first, age after.
Why Inconel Is the Benchmark “Difficult” Material
Ask a machinist to name the hardest common metal to cut and the answer is usually Inconel 718. The numbers justify the reputation: a machinability rating around 12% of AISI 1112 free-machining steel, with cutting forces roughly double those of medium-carbon alloy steel. Four material properties create the problem together. It work-hardens almost instantly on contact. Its thermal conductivity (~11.4 W/m·K) is about a quarter of steel’s, so heat piles up at the tool edge. It keeps its strength at cut-zone temperatures where steel would soften. And its microstructure carries hard, abrasive carbides. Everything in this guide — the speeds, the coolant rules, the no-dwell discipline — follows from those four facts.
Start With the Right Condition: Annealed vs Aged
The cheapest machining decision happens before the material arrives: order Inconel 718 annealed (AMS 5662), machine it, then age-harden the finished part. Aged material (AMS 5663) arrives at 330–400 HB — strong, hard, and 30–50% slower to cut — and it will burn through inserts on a schedule that wrecks budgets.
| Condition | Specification | Typical Hardness | Carbide Turning Speed | Machinability |
|---|---|---|---|---|
| Annealed (machine in this condition) | AMS 5662 | 200–250 HB | 25–40 m/min (80–130 sfm) | Baseline — the best 718 gets |
| Aged / precipitation-hardened | AMS 5663 | 330–400 HB | 15–25 m/min (50–80 sfm) | 30–50% lower; burns inserts |
The exception: parts with tight tolerances that distort during aging need a light finish pass after heat treatment. Plan that pass at reduced speeds with shorter tool life budgeted, and say so on the RFQ — a shop quoting aged-condition machining is quoting a different job. For grade selection between 718 and 625, our Inconel vs titanium comparison covers the trade-offs.
Speeds and Feeds by Operation
These are starting points for coated micrograin carbide in annealed 718 — prove them out on tool life and finish, then adjust. The direction of error is always the same: too slow and too light is worse than too fast, because rubbing hardens the surface.
| Operation | Cutting Speed (annealed) | Feed | Key Tactic |
|---|---|---|---|
| Rough turning | 25–40 m/min | 0.20–0.35 mm/rev, 2–5 mm DOC | Strong edge, large nose radius, never dwell |
| Finish turning / boring | 30–40 m/min | 0.10–0.20 mm/rev | Keep the tool moving; no rubbing passes |
| Rough milling | 25–35 m/min | 0.08–0.15 mm/tooth, up to 1.5×D axial | Trochoidal / high-feed paths, constant engagement |
| Finish milling | 18–28 m/min | 0.06–0.10 mm/tooth, 0.5–2 mm axial | Climb milling preferred |
| Drilling | 8–18 m/min | 0.05–0.14 mm/rev | Solid carbide 130–140° point, through-coolant mandatory |
| Tapping | Avoid where possible | — | Thread mill instead — a broken tap in 718 is a scrapped part |
Milling deserves special attention because the tool engages and disengages on every revolution, creating thermal shock. Trochoidal and high-feed toolpaths keep engagement constant and protect the corner of the insert — this is where our 5-axis machining capability earns its keep on superalloy work.
Tooling and Coolant: The Non-Negotiables
Tooling: sharp, coated micrograin carbide with a strong edge preparation for roughing — AlTiN or AlCrN coatings on a tough substrate, large nose radius on turning inserts. For finishing on rigid machines with continuous cuts, whisker-reinforced ceramic inserts run several times faster than carbide and are how production aerospace shops make 718 economical. Whatever the substrate, the edge must be sharp and the setup rigid — vibration in Inconel is work hardening in Inconel.
Coolant: coolant-through tooling at 500 PSI is the practical minimum, and deep-hole work runs 2,500–3,000 PSI. Pressure is doing three jobs at once: pulling heat out of the cut zone, breaking and evacuating the hot continuous chip, and keeping the edge from dwelling in contact. Flood coolant pointed at the general area does not reach the edge and does not count.
The Mistakes That Scrap Inconel Parts
Dwelling in the cut. Any pause with the tool in contact — a program stop, a hesitant feed override — work-hardens the wall beneath the edge. Keep the tool moving, or retract it fully clear.
Light “spring” passes. A 0.1 mm cleanup pass that seemed safe in steel hardens 718’s surface to 50–55 HRC. Finish passes need real depth and real feed to get under the hardened layer.
Tapping threaded holes. A tap that binds in work-hardened 718 breaks, and a broken tap in a nearly finished superalloy part usually means a scrapped part. Thread milling is slower per hole and cheaper per part.
Machining aged stock because it was on the shelf. The 30–50% machinability penalty exceeds any schedule saving. Order annealed.
What This Means for Your Part and Your Quote
Inconel parts are expensive twice: the nickel alloy stock is costly, and the machining is slow by design. A quote that looks high is usually reflecting physics, not padding — 25–40 m/min is what the material allows. You can help the quote: specify annealed condition, tolerance only the features that need it, avoid deep small-diameter holes and tight internal corners, and call out which surfaces are critical so the shop spends its slow finishing time where it matters. Our quote preparation guide has the full checklist, and the tolerances capability page shows what we hold in difficult alloys.
FAQ: Inconel Machining
Why is Inconel so hard to machine?
Four properties stack against the tool. It work-hardens instantly — a light or rubbing pass can harden the surface of Inconel 718 from ~36 HRC to 50–55 HRC in a 50–200 µm layer, so the next pass cuts harder material. Its thermal conductivity is terrible (~11.4 W/m·K, a quarter of steel’s), so cutting heat concentrates at the tool edge instead of leaving with the chip. It retains its strength at the temperatures found in the cut zone, so the tool never gets a soft material to work against. And its microstructure contains hard, abrasive carbides. The result: a machinability rating around 12% of AISI 1112 free-machining steel, cutting forces roughly double those of medium-carbon steel, and tool life measured in minutes.
What cutting speed should I use for Inconel 718?
With carbide in annealed (AMS 5662) stock: start at 25–40 m/min (80–130 sfm) for turning, 25–35 m/min for milling, and only 8–18 m/min for drilling. In aged (AMS 5663) stock at 330–400 HB, drop everything 30–50% — 15–25 m/min turning. These are starting points, not targets: run them until tool life and finish prove out, then adjust. Ceramic or whisker-reinforced ceramic inserts are the exception — on stable machines with continuous finishing cuts they can run several times faster than carbide, which is how production aerospace shops make 718 economical.
Should Inconel 718 be machined annealed or aged?
Annealed, whenever the process allows. Annealed stock (AMS 5662, 200–250 HB) machines at the full parameter range; aged stock (AMS 5663, 330–400 HB) cuts machinability by 30–50% and burns through inserts. The standard aerospace sequence is to machine to near-final size in the annealed condition, then age-harden the part. The exception is parts with very tight tolerances that distort during aging — those get a light finish pass after aging, at reduced speeds, with shorter tool life planned in. Specify the condition you want on the RFQ; it changes the quote.
What coolant does Inconel machining need?
High-pressure, coolant-through tooling is not optional. 500 PSI is the practical minimum through the spindle; deep-hole drilling in Inconel 718 runs 2,500–3,000 PSI. The pressure does three jobs: it blasts heat out of the cut zone, it breaks and evacuates the hot continuous chips that would otherwise wrap and scratch the part, and it keeps the cutting zone from dwelling in contact — the condition that triggers work hardening. Flood coolant from a distance is not a substitute; the jet has to reach the edge.
Why can’t you dwell or take light passes in Inconel?
Because Inconel work-hardens faster than almost any other engineering metal. Any dwell, rubbing, or spring pass lets the tool press and polish the surface instead of shearing it, and that surface layer hardens from ~36 HRC toward 50–55 HRC — harder than the tooling would like to cut. Every following pass then fights work-hardened material, accelerating flank wear in a spiral. The rules follow directly: keep the tool moving under constant feed, never let it sit in contact, take decisive depths of cut, and if the tool must stop, lift it completely clear of the wall first.
Is Inconel 625 easier to machine than 718?
Slightly. Inconel 625 rates around 17% machinability versus roughly 12% for 718 (both against the AISI 1112 = 100% scale), because 625 is solid-solution strengthened and softer (414–655 MPa yield versus 1,034–1,120 MPa for aged 718). The same rules apply to both — no dwelling, high-pressure coolant, constant feed — but 625 is more forgiving of imperfect technique. Both are covered in our Inconel vs titanium comparison, which also maps when you need a superalloy at all.
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