CNC Machining Inconel 718: Benefits and Challenges

• 1
  What makes Inconel 718 hard to cut
• 2
  Benefits that justify CNC machining Inconel 718
• 3
  Challenges you meet on the shop floor
• 4
  When Inconel 718 is the wrong choice
• 5
  Holding tolerance in a hard alloy
• 6
  What our equipment covers
• 7
  How we set up a 718 job
• 8
  Inconel 718 compared with common CNC alloys

Inconel 718 is roughly 50 to 55% nickel, 17 to 21% chromium, with niobium, molybdenum and titanium added. The nickel matrix is austenitic and face-centered cubic, so it does not undergo the phase change that makes steel chips break cleanly. Chips come off stringy and stay tough, which pushes the cutting force back into the insert.

The bigger problem is heat. Thermal conductivity sits near 11 W/m·K, roughly a third of 316L and less than half of mild steel. Heat generated in the shear zone cannot escape through the chip or the workpiece quickly, so it concentrates at the cutting edge. Edge temperatures of 1,000 °C and above are normal in roughing.

Notch wear at the depth-of-cut line is the signature failure. The tool rubs the machined surface just below the active edge, the protective coating breaks down, and a groove forms within minutes. Once that groove deepens, the insert fails suddenly rather than gradually, which is why Inconel 718 rewards conservative parameters and frequent edge changes.

Abrasive carbides add a third mechanism. Primary carbides of niobium and titanium sit hard in the matrix and scrub the flank. Coated carbide grades handle this for a while, but ceramic and CBN inserts only pay off in specific continuous-cut turning situations.

Strength retention is the headline. Age-hardened 718 holds roughly 1,300 MPa tensile at room temperature and still delivers meaningful strength at 650 to 700 °C. Aluminum alloys lose most of their stiffness well below 200 °C, and 316L starts to creep in the 500 °C range. For a turbine bracket or exhaust-side component, that gap decides the design.

Fatigue life under thermal cycling is the second reason. The alloy resists oxidation at high temperature and keeps a stable microstructure through repeated heating and cooling. Parts that see 500 thermal cycles in service are usually specified in 718 rather than a precipitation-hardened stainless that would crack at the grain boundaries.

Corrosion resistance covers a broad range. 718 resists chloride stress corrosion cracking, seawater, and many reducing acids, which is why it turns up in subsea valves and chemical process hardware alongside aerospace parts. Stress-relieved bar stock can be machined to ±0.005 mm on critical bores without the part moving after heat treatment.

Machining also allows geometry that casting and additive routes struggle with. Thin walls, deep bores, sealing faces and threaded ports come off a five-axis center in one setup, which is why prototypes and low-volume production often start as machined parts even when the final product will be cast.

Tool life is the first cost driver. A coated carbide insert that runs 30 minutes in 4140 might last 8 to 12 minutes in 718 at comparable feed. That ratio drives both consumable cost and spindle time, because every edge change costs a pause. Shops that treat 718 like stainless steel burn through inserts and miss tolerances.

Surface finish is the second. Because chips are stringy and the material work hardens at the surface, light finishing passes can rub rather than cut. A 0.3 mm depth of cut on an already machined surface can raise hardness and push Ra from 1.6 μm to 3.2 μm. The fix is to keep the finishing pass deep enough to stay under the hardened layer.

Thin features distort. Roughing stresses release as material is removed, and a 2 mm wall on a 150 mm ring will move more than the tolerance allows. Stress-relieved stock and a rough, stress-relieve, finish sequence cost more but hold the drawing. Skipping the intermediate step usually means scrap.

Hardness and work hardening also punish tapping and drilling. A 6 mm tap in aged 718 can break the moment the feed hesitates, because the tap rubs, the surface hardens, and the torque spikes. Peck drilling with through-coolant at 12 to 18 m/min and a rigid setup is the difference between a finished hole and a broken drill.

If the service temperature stays under 300 °C and the loads are moderate, 17-4PH or 316L will do the job for a fraction of the cost. Machining time in 718 runs two to three times longer than the same part in stainless, and tooling adds to that. There is no reason to pay for high-temperature strength a part never uses.

Deep small holes are another boundary. A 3 mm hole at 10:1 depth in aged 718 is a drill-breaking exercise with limited payoff. If the design can use a larger bore, a shallower depth, or an EDM step, the part gets cheaper and more predictable.

Very thin walls under 1 mm over a large span are hard to hold. The material resists cutting, so every pass pushes the wall. If the function allows a ribbed or thicker section, the tolerance gets easier without changing the alloy's thermal performance.

Finally, consider whether the part needs to be monolithic. A machined 718 body with a bolted-on cover in 316L sometimes meets the same requirements as one solid block of 718, at lower cost and shorter lead time.

Thermal growth is a real factor on 718. A 300 mm bore can move 0.02 mm between a warm machine and a 20 °C inspection room. We rough, let the part equalize, then finish, and we measure at the same temperature the drawing assumes.

Tool wear compensation matters more here than in aluminum. A worn insert cuts undersize, and the drift between the first and twentieth part in a batch can exceed the tolerance band. Operators check the first part, then a mid-batch part, and adjust the offset rather than trusting the program.

Inspection covers raw material certificates, in-process checks and a final dimensional report. Parts run to ±0.005 mm on critical features with surface finish between Ra 0.2 and 0.8 μm when the drawing calls for it. Reports are available on request.

For aerospace and medical work, the paperwork matters as much as the cut. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022, and uploads stay confidential with an NDA available on request.

Machining 718 needs rigidity more than it needs speed. Our 127 high-precision CNC machines include 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, which lets us keep heavy cuts on the right platform instead of forcing them onto a light machine.

Size range goes up to 4,000 mm maximum processing size, with travels of 4,000 × 400 × 150 mm on the large platform, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium machines, and 500 × 500 × 450 mm or 500 × 310 × 200 mm on the compact cells. A Ø400 mm rotary table handles round features in one setup.

We machine 718 from bar, plate and near-net forgings. Related alloys in the same family, including other Inconel grades and titanium TA1, TA2 and TC4, run through the same cells, so a mixed-alloy assembly can ship together.

Aged material is the default for structural parts. If your drawing does not call out temper, we will ask before cutting, because the difference between annealed and aged stock changes speeds, tool life and sometimes the process plan.