Inconel 718 is the workhorse nickel-based superalloy of the aerospace, gas turbine and energy industries, and it is also one of the most punishing materials a cutting tool can face. Heat, work hardening and an abrasive microstructure combine to destroy ordinary carbide grades within minutes. Choosing the right turning insert — the correct substrate, coating and chipbreaker combination — is the single biggest lever for reducing cost per part. This guide compares the leading coated carbide solutions from Sandvik Coromant and Iscar, explains how to match chipbreaker geometry to the operation, and provides starting cutting parameters that can be safely applied on the shop floor.
Why Inconel 718 Defeats Conventional Tooling
Before comparing grades it is worth being precise about the enemy. Inconel 718 (UNS N07718) is a nickel-iron superalloy strengthened by gamma-prime Ni3(Al,Ti) and gamma-double-prime Ni3Nb precipitates. In the aged condition it typically measures 36–44 HRC, retains a yield strength near 1000 MPa at 650 °C, and reacts to cutting heat by hardening instead of softening. Five characteristics drive almost every tool life problem:
- Severe work hardening. The surface layer hardens by 20–30% during cutting, and the affected zone reaches 0.05–0.15 mm deep. A light feed cuts inside this layer and works it repeatedly, which is why feed must stay above roughly 0.10 mm/rev.
- Extremely poor thermal conductivity. At roughly 11 W/m·K — about a quarter that of medium-carbon steel — heat has nowhere to go but into the cutting edge. Up to 80% of the generated heat ends up in the tool.
- Abrasive microstructure. Hard carbides and nitrides in the matrix cause rapid flank wear even at modest speeds.
- Strength at cutting temperature. The chip does not soften ahead of the edge the way steel does, so mechanical load on the edge stays high and depth-of-cut notching appears quickly.
- Chemical affinity and BUE. Nickel bonds readily with cobalt and iron at moderate temperatures, producing built-up edge in the 20–30 m/min dead zone.
Every good insert choice for 718 is a response to this list: fine-grain substrates for toughness against notch wear, Al-rich PVD coatings for oxidation and crater resistance, sharp but reinforced edges, and high-pressure coolant to drag heat out of the cut zone.
The Coated Carbide Baseline: Sandvik vs Iscar
Both manufacturers anchor their superalloy turning offer in fine-grain (sub-micron to ultra-fine) carbide substrates with PVD coatings. The logic is identical: a fine grain structure resists micro-chipping along the depth-of-cut line, and a thinner PVD coating allows a sharper edge than CVD grades — critical in a material where rubbing generates heat faster than any coating can survive. The differences lie in grade positioning, edge preparation and post-treatment technology.
Sandvik Coromant: GC1115 and the S-Grade Family
GC1115 has long been the reference first-choice grade for turning heat-resistant superalloys and titanium. It pairs a fine-grain substrate with a thin multi-layer TiAlN-based PVD coating, and it holds up well in both continuous and lightly interrupted cuts. For shops running dedicated superalloy production, Sandvik complements it with the S-grade family: S05F for fine finishing, S10T as the all-round medium-operation grade, and S20T for heavier roughing. These grades are optimized specifically against notch wear at the depth-of-cut line — the dominant failure mode in 718 — and are worth specifying when the application is 100% superalloy rather than a mixed-alloy job shop.
Iscar: IC806, IC830 and SUMO TEC Post-Treatment
Iscar’s counterpart to GC1115 is IC806, a sub-micron grain carbide with a high-aluminum PVD coating aimed at heat-resistant superalloys and titanium. It is positioned as a universal grade for roughing and semi-finishing, with good resistance to both notch and flank wear. IC830 covers the medium operation window in stainless steels and superalloys where edge toughness matters more than maximum wear resistance. Iscar’s differentiator is the SUMO TEC post-coating treatment, which induces compressive stresses in the coating and suppresses micro-crack propagation; in practice this shows up as fewer random edge chippings in interrupted cuts and more consistent wear from edge to edge.
Head-to-Head Grade Comparison
| Attribute | Sandvik GC1115 | Sandvik S10T | Iscar IC806 | Iscar IC830 |
|---|---|---|---|---|
| Coating | Thin multi-layer PVD, TiAlN-based | PVD, notch-wear optimized | High-Al PVD | PVD, medium thickness |
| Substrate | Fine grain, tough | Fine grain, tough | Sub-micron grain | Medium-fine grain |
| Edge condition | Light honing | Light T-land plus hone | Light honing | Moderate honing |
| Best suited | Universal HRSA turning, interrupted cuts | Dedicated 718 medium operations | Roughing and semi-finishing 718 and titanium | Tough medium operations, mixed batches |
| Typical Vc range | 25–45 m/min | 30–50 m/min | 30–50 m/min | 25–45 m/min |
Practical read: for a job shop cutting 718 occasionally among other alloys, GC1115 or IC806 are near-equivalent safe choices and interchangeable in most programs. For a dedicated superalloy cell where every second of cycle time counts, the specialized grades (S10T class) and post-treated substrates (SUMO TEC class) buy measurable extra edge life at the same speed.
Chipbreaker and Insert Geometry Selection
In 718, geometry failures cost more than grade failures. A negative-rake 80° diamond (CNMG) is the standard roughing choice on rigid lathes; a 55° or 35° diamond (DNMG, VNMG) with positive rake suits finishing and thin-walled aerospace parts. Edge preparation must be aggressive enough to resist chipping but not so heavy that it generates excess heat: a light honing of 0.03–0.05 mm for finishing edges and up to 0.06–0.10 mm for roughing edges is a sensible window.
| Operation | Insert shape | Geometry notes |
|---|---|---|
| Roughing, continuous | CNMG, negative rake | Reinforced edge, hone 0.06–0.10 mm, chipbreaker tuned for tough, stringy chips |
| Semi-finishing | DNMG, 55° | Neutral to positive rake, hone up to 0.05 mm |
| Finishing | DNMG / VNMG, positive rake | Sharp edge, hone up to 0.03 mm, wiper geometry acceptable at low feeds |
| Interrupted cuts | CNMG with reinforced edge, or round | Maximize edge safety, reduce feed 20% on entry, rigid clamping essential |
Recommended Starting Cutting Parameters for Inconel 718
The table below gives conservative starting windows for aged Inconel 718 (around 40 HRC) with coated carbide and with the ceramic alternatives. Always ramp up: start at the lower Vc bound, confirm tool life and surface integrity, then increase in 10% steps.
| Operation | Tool | Vc (m/min) | Feed (mm/rev) | ap (mm) | Coolant |
|---|---|---|---|---|---|
| Roughing, continuous | Coated carbide | 30–45 | 0.20–0.30 | 2.0–4.0 | HPC 15–20 bar |
| Semi-finishing | Coated carbide | 35–50 | 0.15–0.25 | 1.0–2.0 | HPC or flood |
| Finishing | Coated carbide | 40–55 | 0.10–0.18 | 0.3–1.0 | Consistent flood |
| Interrupted roughing | Coated carbide | 25–35 | 0.15–0.25 | 1.5–3.0 | HPC 15–20 bar |
| Roughing, high speed | Whisker-reinforced ceramic | 200–320 | 0.10–0.20 | 1.0–3.0 | Dry |
| Finishing, high speed | SiAlON ceramic | 180–280 | 0.10–0.18 | 0.5–2.0 | Dry |
| Hard finishing | Low-content PCBN | 150–240 | 0.08–0.15 | 0.1–0.5 | Dry or MQL |
Three rules matter more than any single number in this table. First, keep feed above the work-hardening depth — never let a finishing pass rub at feed below 0.08 mm/rev. Second, direct high-pressure coolant (15–20 bar) precisely at the notch zone; a general flood does roughly half the job. Third, dwell time is fatal: on diameter changes or retracts, pull the insert out of contact rather than letting it spin inside the cut.
Ceramic and PCBN Options When Cycle Time Dominates
Coated carbide tops out near 50 m/min in 718; ceramics change the economics entirely. Whisker-reinforced alumina grades (such as the Sandvik CC6080 class) sustain 200–300+ m/min in stable continuous cuts, and SiAlON grades (such as the Sandvik CC6202 class, or Iscar’s SiAlON ceramic line) add the chemical stability needed for semi-continuous profiles. Ceramics demand what carbide does not: a very rigid machine, negative geometry, absolutely consistent engagement, and dry cutting to avoid thermal shock. Used correctly, they cut cycle times by 60–80% on shafts, rings and flanges. Low-content PCBN fills the finishing niche, delivering 150–240 m/min with excellent surface integrity on aged material, at the cost of sensitivity to interruption.
Troubleshooting Common Failure Modes
| Symptom | Root cause | Corrective action |
|---|---|---|
| Deep notch at depth-of-cut line | Work-hardened surface plus oxidation wear | Vary ap between passes (for example 2.6/2.0 mm), reduce Vc 10–15%, use an Al-rich PVD grade, aim HPC jet at the notch |
| Built-up edge | Vc too low or feed below the hardened layer | Raise Vc into the 35–50 m/min window, keep feed at 0.12 mm/rev or higher |
| Edge chipping | Interrupted engagement or excessive edge hone | Move to a tougher fine-grain grade, reduce feed 20% on entry, verify clamping rigidity |
| Rapid uniform flank wear | Abrasive carbides, Vc too high | Step Vc down 10%, confirm the edge is not running in hardened skin from the previous pass |
| Size drift or taper in finishing | Progressive flank wear | Reduce ap, add wiper geometry, index edges at 0.25–0.30 mm flank wear |
| Thermal cracks perpendicular to edge | Intermittent coolant, thermal shock | Use consistent high-pressure flood or run fully dry; never spray intermittently |
Practical Selection Checklist
- Confirm material condition: aged 718 needs 15–25% lower Vc than solution-treated stock.
- Default to a fine-grain PVD grade — GC1115 or IC806 class — as the baseline; add specialized grades only when running 718 daily.
- Match honing to operation: up to 0.03 mm for finishing, up to 0.10 mm for roughing and interrupted work.
- Feed floor of 0.10 mm/rev, Vc window of 30–50 m/min, adjustments in 10% increments — never 50%.
- Aim high-pressure coolant at the depth-of-cut line, not at the chip curl.
- Track flank wear per edge and index at 0.25–0.30 mm — running edges to failure destroys surface integrity and the next insert.
- For stable continuous profiles above 100 pieces, trial whisker ceramic or SiAlON before adding machines.
Inconel 718 rewards discipline more than experimentation. A fine-grain PVD carbide from either Sandvik or Iscar, run inside a 30–50 m/min window with feeds above the hardened layer and coolant aimed at the notch, will deliver predictable cost per part. Specialized S-grades and SUMO TEC post-treatment squeeze out the last 10–20% of edge life for dedicated production, and ceramics take over entirely when cycle time — not tool cost — is the constraint.
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