Inconel CNC Machining: 718 & 625, Tolerances & Cutting Rules

Inconel is the best-known trade name for a family of nickel-chromium superalloys, and the two grades a machine shop actually cuts are Inconel 718 and Inconel 625. These alloys hold their strength and resist oxidation at temperatures where steel has long since softened and titanium has given up strength: 718 serves at about 700°C, and 625 is rated for service up to 982°C (1800°F). That is why Inconel parts sit in the hottest zones of jet engines, furnaces, and downhole assemblies.

The same heat strength makes Inconel one of the most difficult metals to machine. It work-hardens quickly, it conducts cutting heat poorly, and its microstructure contains hard carbide particles that abrade the tool, so machinability sits at only 10 to 20 percent of free-machining brass. Shops that succeed use rigid machines, sharp carbide or ceramic tooling, low speeds with positive feed, and, on 718, a machining plan built around the aging heat treatment. This page, part of the CNC machining guide, covers the two grades, why they resist cutting, the playbook that works, and the tolerances and costs that follow.

What Inconel is, and how 718 and 625 differ

Inconel is a registered trademark of Special Metals Corporation covering a family of nickel-chromium alloys; other makers sell the same chemistries under other names, and engineers often use the word loosely for any nickel superalloy. “Superalloy” is the operative term: these alloys are built to keep useful strength at high temperature, not to be easy to cut. The grade choice matters more than with most metals, because 718 and 625 reach their properties by different metallurgical routes, and those routes change both the part and the machining plan.

Inconel 718: the precipitation-hardened workhorse

Alloy 718 (UNS N07718) is the high-strength grade and the one most often specified for machined parts. It is strengthened by heat treatment: the material is solution annealed, then aged so that secondary phases precipitate throughout the metal matrix. Aged 718 reaches tensile strengths of 130 to 185 ksi (895 to 1275 MPa) with yield strengths of 105 to 160 ksi. Its datasheet lists aerospace turbine components, rocket motors, and oil field hardware as its core uses, including a special downhole variant produced to NACE MR0175 with a hardness cap of 40 HRC.

Inconel 625: the corrosion-first grade

Alloy 625 (UNS N06625) gets its strength from molybdenum and niobium stiffening the nickel-chromium matrix, so it needs no precipitation-hardening treatment at all. It is the corrosion specialist: its maker documents freedom from pitting and crevice attack in seawater, along with chemical plant, exhaust system, and marine applications, and it serves up to 982°C. Annealed bar and plate carry tensile strengths of 120 to 150 ksi (827 to 1034 MPa), solid strength without any aging step.

Why the grade changes the whole machining job

The two grades machine differently. Annealed 625 and solution-annealed 718 are tough but cuttable with disciplined carbide work; fully aged 718 is dramatically harder on tooling, which is why the machining sequence is planned around the aging treatment. The drawing must call out not just the alloy but its condition and heat-treatment stage, or the shop cannot quote the job honestly.

Key properties, and what they mean at the machine

The table above collects the numbers that matter. Density and melting range explain the alloy’s job: at 8.19 to 8.44 g/cm3 these are heavy metals, denser than steel, and their melting ranges starting near 1260°C underwrite the high-temperature service. The strength rows are condition-dependent, which is the single most important nuance in the table.

Strength depends on condition

A 718 part is softest in the solution-annealed state, hardest after aging, and the standard heat treatment is specific: solution anneal at 1700 to 1850°F, then age at 1325°F for 8 hours, furnace cool to 1150°F and hold for a total aging time of 18 hours. Special Metals’ specification tables show aged material meeting tensile minimums of 180 to 185 ksi with 150 ksi yield minimums, and aged sheet testing around 205 ksi tensile. 625 shows the same logic in reverse: solution-treated material is actually softer (105 to 130 ksi) than annealed stock, and the datasheet recommends solution-treated material where creep resistance matters most.

The machining difficulty indicators

Three table entries read as warnings. Machinability of 10 to 20 percent of free-machining brass compares with 20 to 30 percent for titanium Ti-6Al-4V and roughly 45 percent for stainless steels, so Inconel sits at the bottom of the machinability scale among commonly specified metals. The strengthening route row warns that aged 718 will behave nothing like annealed stock. And the service temperature row explains why shops accept all of this: no cheaper alloy holds load at 700 to 982°C.

Why Inconel is so hard to machine

Four mechanisms stack up, and each one attacks the cutting edge in a different way. Understanding them separately makes the playbook make sense, because every rule below answers one of them.

Work hardening under the tool

Inconel hardens as it deforms, faster than austenitic stainless. A tool that rubs instead of shearing, a feed too light to get under the skin, or a dwell at the bottom of a hole leaves a hardened layer that the next pass must cut through. Cutting forces climb, heat climbs, and tool life collapses. The remedy is unglamorous and strict: keep tools sharp, keep feed heavy enough to cut into soft material beneath the hardened surface, and never let an edge dwell.

Heat with nowhere to go

Like titanium, Inconel conducts heat poorly, so the heat of cutting stays at the edge instead of flowing out with the chip. Unlike titanium, Inconel keeps its strength at high temperature, which is exactly the property that makes it valuable in a turbine and miserable in a milling machine: the chip stays hard and abrasive even when it is glowing. Carbide substrates soften, coatings break down, and speeds must stay low to keep edge temperatures survivable.

Abrasive carbides and notch wear

The alloy’s niobium, titanium, and chromium form hard carbide and carbonitride particles, so every cut works like a grinding pass against the edge. The visible result is notch wear, a concentrated groove at the depth-of-cut line where the work surface drags across the edge. Tooling suppliers recommend round inserts and low depths of cut specifically to spread that contact and dodge the notch, and notching is a main reason tool life on Inconel is measured in minutes, not hours.

Stacked on top is force. Cutting a metal that stays strong at temperature means high cutting forces at room temperature, so any sloppiness in the setup shows up as deflection, chatter, and rubbing, and rubbing triggers the work hardening above.

The machining playbook

The shops that succeed on Inconel run a disciplined system rather than a collection of tricks. The elements below are standard practice across the aerospace machining world.

Machine 718 soft, then age it

The classic sequence roughs and semi-finishes 718 in the solution-annealed condition, ages the part, then takes light finishing passes to final dimension. Cutting fully aged material is possible but slow and punishing, so the roughing stock should come out before the heat treatment. The trade-off to plan for is distortion: the aging cycle can move features, so finish stock must be left on critical surfaces and the tolerance scheme must survive a post-aging pass.

Tooling: carbide first, ceramics for volume

Sharp, tough coated-carbide tools with positive geometry are the default for finishing and for all work on 625. For roughing at volume, whisker-reinforced and Sialon ceramic inserts change the economics. Ceramics keep their hot hardness and do not react with the workpiece, and they accept speeds carbide cannot. Tooling-maker guidance is specific: round inserts, short overhangs, smooth entries and exits, and no full slotting in ceramic milling.

Speeds, feeds, and chip control

Carbide runs in the tens of meters per minute. A published aerospace case machined an Inconel component with a high-feed carbide cutter at 38 m/min. Ceramic inserts lift that ceiling dramatically: turning tops out around 300 m/min, and ceramic milling can reach 1000 m/min, with feeds held low, around 0.05 to 0.08 mm per tooth, so the heat softens the material ahead of the edge and small red-hot chips carry it away. Climb (down) milling with positive geometries is the recommended approach, face milling is preferred over edging and waterline passes, and drill cycles should peck to clear chips rather than dwelling in work-hardening material.

Coolant strategy follows the tool material

With carbide, generous or high-pressure coolant delivered right at the cut manages edge temperature and clears chips. With ceramics the rules split: ceramic turning needs uninterrupted, abundant coolant flow, while ceramic milling wants no coolant at all, because a coolant stream cycling a hot ceramic edge creates thermal stress and cracking. Getting this backwards destroys inserts quickly, so the coolant plan is set per operation, not per part.

Rigidity and workholding

Everything above assumes a stiff system. Keep overhangs short, use the largest shank and holder that fits, and fix the part so the cut pushes it into the support rather than away from it. Structural walls below about 1.5mm (0.060in) invite chatter and deflection on high-force metals like this one, so thin features need support or redesign, and deep pockets should be opened up to let a short, stiff tool reach.

Example: an Inconel component roughed with a ceramic face mill at 800 m/min, 0.13 mm per tooth, and 1.5mm depth of cut removed metal at 132.3 cm3 per minute, versus 19.3 cm3 per minute for the carbide baseline at 38 m/min in the same tooling-maker case. The ceramic route cut 42 hours of machine time per year on that one component, a 469 percent productivity gain as Sandvik reports it for the case, which is why tooling makers position ceramics as the roughing route for Inconel.

Example: a downhole tool body machined from 718 produced to NACE MR0175. The shop roughs in the solution-annealed condition, ages the part (the oil-field variant ages around 1450°F with a 40 HRC hardness cap), then finishes critical bores and threads with fresh carbide and flood coolant. The sequence keeps heavy cuts out of fully aged metal and keeps the hardness under the specification ceiling.

Tolerances and surface finish

Inconel does not hold tighter tolerances than other metals; it holds similar tolerances at higher cost. The precision limit is set by tool wear and the metal’s habit of work hardening, both of which push shops toward conservative, consistent passes.

What to specify

General features follow ISO 2768-1 class m, about plus or minus 0.10mm on small dimensions, with class f, plus or minus 0.05mm, applied to critical features. Grinding and careful finishing can reach the plus or minus 0.025mm (0.001in) class on specific features, the same precision band quoted for titanium and stainless on this site, but on Inconel every tighter band buys significant machine time. As-machined surfaces run Ra 1.6 to 3.2µm (63 to 125µin), and finishing cuts bring critical faces into the Ra 0.8 to 1.6µm (32 to 63µin) range; finer finishes need grinding, and on this metal that is a cost decision, not an afterthought.

Tolerance strategy for heat-treated parts

On aged 718, call out which dimensions are final after heat treatment and leave finish stock accordingly, because tight tolerances machined before aging invite scrap when the part moves in the furnace. The economical scheme holds general tolerances everywhere except the few features that truly need class f or better, then protects those with a post-aging finishing pass.

Inconel against titanium and stainless

Inconel rarely competes on price or ease; it competes on surviving conditions other metals cannot. The comparisons below are about choosing the right metal for the environment, not about finding a cheaper Inconel.

Against titanium

Titanium machines at 20 to 30 percent of free-machining brass, roughly double Inconel’s best case, and at 4.43 g/cm3 it is barely half as dense. Where a part needs strength-to-weight at moderate temperatures, titanium wins outright. Inconel takes over where temperature does: beyond the range where titanium keeps its strength, 718 and 625 keep working, which is why turbine hot sections are nickel alloy and fan and structural parts are titanium or aluminum.

Against stainless

Stainless 316 handles chloride service at moderate temperature and machines at roughly 45 percent of free-machining brass, several times faster than Inconel, from stock any shop can buy. Stainless is the right call for most corrosion duties. 625 earns its place when seawater pitting, hot acids, or temperatures past stainless limits are in play, and its excellent weldability suits fabricated chemical and marine assemblies.

Cost drivers

No single number prices an Inconel part, but the drivers are predictable. Cycle time dominates: at 10 to 20 percent of free-machining-brass removal rates, a cut that takes an hour in free-machining brass takes five to ten hours in Inconel, and even titanium leaves roughly twice the removal rate. Tooling cost per part runs high, with carbide edges consumed in minutes on heavy cuts. Raw nickel-alloy stock is expensive and held by a limited supplier pool, so non-standard sizes add lead time, and deep holes past a 4:1 depth-to-diameter ratio or multi-axis setups multiply cost further. The design levers mirror those drivers: standard stock sizes, fewer setups, generous internal radii, open pockets a stiff tool can reach, and tolerances tightened only where function demands.

Applications

Inconel parts cluster in three worlds: hot gas paths, aggressive chemistry, and the oilfield.

Aerospace turbines

Aged 718 is a standard material for turbine discs, shafts, casings, compressor hardware, and rocket-motor components, parts that carry load while hot for thousands of hours. Most machined 718 lands in the aerospace machining world, and it is why shops that specialize in this metal build their whole process around it.

Chemical processing and marine

625 appears in heat exchangers, distillation columns, reaction vessels, transfer piping, and valves in chemical plants, and in seawater service from mooring cables to submarine fittings, applications its maker documents directly. Exhaust systems and turbine housings in both marine and land-based engines use it for oxidation resistance at temperature.

Energy and downhole

718 produced to NACE MR0175 serves in sour-gas downhole tools, where the hardness cap guards against sulfide stress cracking, and 625 appears in reactor-core components and other nuclear duty. These parts are machined in volumes that suit CNC well: complex, high-value, and made in quantities from ones to hundreds.

When not to use Inconel

If the part never sees high temperature, aggressive chemistry, or high pressure, Inconel is the wrong metal, because every difficulty on this page becomes pure cost. A bracket at room temperature belongs in steel; a corrosion part in mild service belongs in stainless; a part that needs strength-to-weight belongs in titanium or aluminum. Even in hot service, check whether the section really needs nickel alloy or a cheaper material with a coating will do. Inconel is specified when the environment demands it, then designed efficiently around its machining economics.

File format guidance

• State the alloy and its condition (718 solution annealed, 718 aged, 625 annealed) and which machining happens before or after aging; the whole quote depends on it.
• Name the governing specification, for example the AMS 5662/5663 family for 718 bar and forgings or NACE MR0175 for downhole service, so material traceability is unambiguous.
• Keep features within stock bar and plate sizes; nickel-alloy stock is not stocked as widely as steel or aluminum and odd sizes add lead time.
• Prefer generous internal radii and open pockets, and flag any wall under 1.5mm or hole deeper than 4:1 so the shop can plan tooling and fixturing honestly.
• Always specify units in the file or filename. Files without explicit units are read against a supplier default and can come out at the wrong scale, a 25.4x error.