Inconel 718 machining: the numbers, and why they are so low
Inconel 718 rates around 12 % on the AISI machinability index, where free-cutting 1112 steel is 100. That single figure sets expectations for everything below: you are working at a fifth of the speed you would use on stainless, and the process discipline matters more than the machine’s horsepower.
Why it fights back
Three properties do the damage, and they compound.
Thermal conductivity around 11 W/m·K — roughly a sixth of carbon steel. The heat the cut generates stays at the edge instead of leaving in the chip.
Cutting forces 1.5 to 2 times carbon steel. Everything in the setup deflects more: the tool, the fixture, the part. Rigidity you would get away with in aluminium produces chatter here.
Work hardening that happens inside one pass. The surface can go from about 36 HRC to over 50 HRC ahead of the tool. A cutter that rubs instead of cutting creates the hard layer that then destroys the next pass. See work hardening in superalloys for what that does to a process plan.
Starting parameters
Two different jobs depending on condition. Solution-treated (roughly 36 HRC) machines like a difficult stainless. Aged to AMS 5663 (331–400 HB) is a hard-material job.
Solution treated, coated carbide
| Operation | Surface speed | Feed | Depth of cut |
|---|---|---|---|
| Turning, roughing | 20–35 m/min | 0.15–0.25 mm/rev | 1.5–3 mm |
| Turning, finishing | 30–45 m/min | 0.08–0.15 mm/rev | 0.25–0.5 mm |
| Milling, roughing | 20–30 m/min | 0.08–0.15 mm/tooth | 10–20 % radial |
| Milling, finishing | 30–40 m/min | 0.05–0.08 mm/tooth | light axial |
| Drilling | 8–15 m/min | 0.05–0.10 mm/rev | peck |
Aged condition, coated carbide
Drop roughly 30 % from the figures above. Ceramic inserts change the picture entirely — SiAlON and whisker-reinforced grades run roughing at 150–300 m/min because they work hot by design — but they need a rigid machine, real spindle power and an operator who knows the failure modes. That trade-off is covered in ceramic, carbide or CBN.
Tooling that survives
PVD AlTiN or TiAlN coated carbide, positive rake, sharp edge, and a chipbreaker meant for superalloys. A general-purpose steel insert will weld material to the edge within minutes.
Round inserts and high-feed geometries help in roughing because they spread the notch wear that otherwise concentrates at one depth-of-cut line. Notch wear is the characteristic 718 failure: a groove worn exactly where the insert meets the top of the cut, caused by the work-hardened skin from the previous pass.
Flood coolant is the minimum. Through-tool at 70 bar or more is what production shops use, and it is the difference between breaking chips and stringing them. Why coolant pressure matters goes into the mechanism.
The decision that sets your price
The biggest cost lever on a 718 drawing is not the cutting data. It is whether you machine soft and age afterwards, or buy aged bar and machine hard.
Machine then age gives faster cycles and longer tool life, but the aging cycle — roughly 720 °C then 620 °C — moves the part. Expect 0.1 to 0.3 % dimensional change, which means grind stock on critical features and a second operation after heat treatment.
Buy aged, machine hard gives one operation and predictable dimensions, at slower cycles and higher tooling cost.
Few features and tight tolerances usually favour machining aged material. Many features on a large part usually favour soft machining plus finishing. A shop that runs nickel alloys weekly can tell you which side your part falls on from the drawing alone.
What to put on the drawing
- Condition, by specification. “Inconel 718” is incomplete. Write “solution treated and aged per AMS 5663” or “solution treated per AMS 5662”.
- Where the tolerance matters. A blanket tight callout across a 718 part is the most expensive line most buyers write without meaning to.
- Whether you require the heat lot traced. Normal for nickel alloys, cheap to ask for upfront, expensive to reconstruct later. See material certificates and traceability.
- The real date. Aged bar in common sizes is usually available; unusual sections can be months from the mill.
If you are still choosing between 718 and a corrosion alloy, the trade-off is in Inconel 625 or 718.
Background on the alloy family: nickel superalloys.
Have a 718 part that came back expensive or late? Send the drawing and we will tell you which of the decisions above was the cause.