Inconel 718 is the workhorse nickel superalloy of aerospace, oil and gas, and turbine engineering for one reason above all: it keeps roughly 1,200 MPa of strength after exposure to 650°C, and it ages hardenable so you can machine it soft and strengthen it afterwards. That makes Inconel 718 machining services a specialist discipline with its own tooling, parameters, and quality requirements. This guide explains how to get 718 parts right the first time.
Why Inconel 718 Dominates Demanding Service
Inconel 718 is a precipitation-hardening nickel-chromium superalloy with niobium and titanium as the hardening agents. Its combination of high strength, creep resistance to about 650°C, and good corrosion and fatigue performance explains its presence in turbine discs, compressor cases, downhole tools, and exhaust hardware. It is also more machinable in the solution-treated condition than 625 or Waspaloy, which is why it is the default superalloy when a design can tolerate its density.
The alloy is usually supplied in the solution-annealed or solution-annealed-plus-aged condition. For machining, the soft solution-treated state (roughly 30 HRC) is far easier to cut, and many shops rough-machine in that condition then send the part for aging. Our Inconel machining guide covers the wider family of nickel alloys and how 718 compares with 625.
The Core Machining Challenge
Like all nickel superalloys, 718 work-hardens aggressively, has low thermal conductivity that traps heat at the tool tip, and develops high cutting forces. The classic failure mode is taking a light finishing cut that rides in the work-hardened layer, smears the surface, and destroys tool life. The corrective discipline is the same across superalloys: positive-rake sharp tooling, moderate surface speeds, steady aggressive feeds that always exceed the prior hardened layer, and high-pressure coolant pointed at the cut.
We run 718 on rigid CNC turning and CNC milling centres, with 5-axis CNC machining for the complex turbine and manifold geometry that defines much of the work. Deep or interrupted features go to wire EDM to remove tool-wear variables on slots and keyways.
Tooling and Parameters That Hold
Sharp uncoated or lightly PVD-coated carbide with a strong edge is the baseline. Ceramic and CBN have niche roles in high-volume rough turning but are unforgiving. Surface speeds for 718 turning sit well below steel values, typically 20 to 40 m/min depending on hardness and tooling, with feed held steady rather than reduced. Rigidity and through-tool coolant are not optional.
Tool life is managed on a scheduled index rather than run to failure, because a degrading edge multiplies work hardening and can push a near-finished feature out of tolerance. On sealing and bearing diameters we finish by CNC grinding to hold geometry the mill cannot. Our Hastelloy machining services page describes the parallel approach for the corrosion-grade superalloys.
Heat Treatment and Its Effect on Machining
The defining advantage of 718 is age hardening. A part can be machined in the soft solution-treated condition, then aged to roughly 35 to 45 HRC, gaining strength without the distortion that would follow machining an already-hard part. The catch is that aging also shifts dimensions slightly, so final features that must hold tight tolerance are often machined after aging, or the shrinkage is pre-compensated.
Our heat treatment capability handles solution anneal and age cycles in line with machining, and we report hardness and distortion data with the part. Where residual stress from roughing could move a thin feature, a stress-relief between rough and finish cuts keeps the final dimension stable.
Surface Finish, Inspection, and Documentation
718 parts usually carry stringent inspection. Sealing and bearing surfaces are finished to specified roughness and geometry, and critical features are verified on a CMM against the drawing. Because the alloy is common in flight and pressure service, documentation is part of the deliverable: material certification traceable to heat lot, chemical and hardness verification, CMM report, and a certificate of conformance.
Surface contamination matters too. Embedded iron or abrasive from fixturing can seed corrosion, so a final clean and passivation-style decontamination protects the surface. Our stainless steel machining page covers the alloy pairing and cleaning rules that keep superalloy assemblies reliable in service.
How to Specify an Inconel 718 Part
State the material specification (AMS 5662/5663 or equivalent), the heat-treat condition you want delivered (solution treated, or solution treated and aged), the hardness target, and any non-destructive test requirement such as fluorescent penetrant inspection. If you can supply the part in the soft condition for machining and age it yourself, lead time and cost both drop. For flight-critical work, name the inspection level and the documentation package up front so the shop quotes the quality system, not just the chip-making.
Frequently Asked Questions
Should Inconel 718 be machined soft or hard?
Usually rough-machined in the soft solution-treated condition, then aged to gain strength, with final features cut after aging if tolerance is tight.
What surface speed should I expect?
Roughly 20 to 40 m/min for turning with coated carbide; far below steel values because heat concentrates at the edge.
Is wire EDM useful on 718?
Yes, for slots, keyways, and deep interrupted features where tool wear would otherwise be uncontrollable.
What documentation comes with the part?
Material certification, hardness and chemical verification, CMM report, and certificate of conformance as standard.
How does 718 differ from Inconel 625?
718 is age hardenable and generally easier to machine; 625 relies on solid-solution strengthening and is tougher to cut.
Can you age-harden in house?
Yes, solution anneal and age cycles are run in line with machining with hardness reporting.
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