Appearance
An aerospace component manufacturer produces heat exchanger tube sheets from Inconel 625 (annealed, 200 HB) and turbine shaft couplings from age-hardened Inconel 718 (45 HRC). Both require drilling 6 mm diameter cooling holes at L/D 30:1. Using TiAlN-coated carbide gun drills at 25 m/min and 0.025 mm/rev with 100 bar coolant, the 625 holes achieve 25 holes per edge with Ra 0.4 µm. The 718 holes under identical parameters achieve only 8 holes per edge with Ra 0.6 µm — the gamma prime precipitates cause rapid flank and notch wear. After optimising separately: for 718, speed reduces to 18 m/min, coolant increases to 140 bar, and a peck cycle is introduced, raising tool life to 14 holes. A PCBN gun drill trial at 50 m/min in 718 achieves 40 holes per edge with Ra 0.25 µm — 5× the carbide tool life at nearly 3× the metal removal rate.
Material Comparison
Chemical Composition
| Element | Inconel 625 (wt%) | Inconel 718 (wt%) |
|---|---|---|
| Nickel | 58+ | 50–55 |
| Chromium | 20–23 | 17–21 |
| Iron | ≤5 | 15–20 |
| Molybdenum | 8–10 | 2.8–3.3 |
| Niobium + Tantalum | 3.15–4.15 | 4.75–5.50 |
| Titanium | ≤0.4 | 0.65–1.15 |
| Aluminium | ≤0.4 | 0.2–0.8 |
The key difference: 718 contains significant iron (15–20%) and higher aluminium + titanium for precipitation hardening, while 625 relies primarily on molybdenum and niobium for solid-solution strengthening.
Mechanical Properties
| Property | Inconel 625 (Annealed) | Inconel 718 (Age-Hardened) |
|---|---|---|
| Hardness | 90–95 HRB (~200 HB) | 40–48 HRC (~400–470 HB) |
| Tensile strength | 760–930 MPa | 1,270–1,550 MPa |
| Yield strength | 310–620 MPa | 760–1,310 MPa |
| Elongation | 30–40% | 12–25% |
| Thermal conductivity | 10.8 W/m·K | 14.7 W/m·K |
| Machinability rating | ~25% of free-cutting steel | ~12–15% of free-cutting steel |
Why Inconel 718 Is Harder to Deep Hole Drill
| Factor | Inconel 625 | Inconel 718 | Impact on Drilling |
|---|---|---|---|
| Strengthening mechanism | Solid-solution | Precipitation (γ′ + γ″) | 718's precipitates are abrasive to cutting tools |
| Hardness (typical) | 200 HB (machinable) | 45 HRC (very hard) | 718 requires 50–70% lower cutting speeds |
| Work-hardening rate | Moderate | Very rapid | 718 work-hardens instantly — dwell or light cuts cause rapid notch wear |
| Chip formation | Continuous, ductile | Segmented at age-hardened condition | 718 chips are more abrasive during evacuation |
| Cutting forces | Moderate | High (60–80% higher) | Higher deflection, more power required |
| Adhesion tendency | Moderate | High — BUE common | 718 tends to weld to cutting edges |
Gun Drilling Parameters
Speed and Feed Comparison
| Parameter | Inconel 625 (Annealed) | Inconel 718 (Annealed) | Inconel 718 (Age-Hardened, 40–48 HRC) |
|---|---|---|---|
| Cutting speed (Vc) | 20–50 m/min | 18–40 m/min | 15–30 m/min |
| Recommended starting Vc | 25 m/min | 22 m/min | 18 m/min |
| Feed range | 0.020–0.080 mm/rev | 0.015–0.060 mm/rev | 0.010–0.050 mm/rev |
| Recommended starting feed | 0.030 mm/rev | 0.025 mm/rev | 0.020 mm/rev |
| PCBN cutting speed | — | — | 50–80 m/min |
| PCBN feed | — | — | 0.015–0.040 mm/rev |
Tip: Inconel 625 in the annealed condition is considerably more forgiving than age-hardened 718. For 625, you can use feeds approximately 30–50% higher than for 718 at the same cutting speed. The softer matrix generates lower cutting forces and less heat, allowing more aggressive parameters.
Feed by Diameter
Inconel 625 (Annealed)
| Drill Diameter (mm) | Feed Range (mm/rev) | Speed at 25 m/min (RPM) |
|---|---|---|
| 3–5 | 0.015–0.040 | 1,590–2,650 |
| 6–8 | 0.020–0.060 | 1,000–1,330 |
| 10–12 | 0.025–0.070 | 660–800 |
| 14–18 | 0.030–0.080 | 440–570 |
Inconel 718 (Age-Hardened, 40–48 HRC)
| Drill Diameter (mm) | Feed Range (mm/rev) | Speed at 18 m/min (RPM) |
|---|---|---|
| 3–5 | 0.008–0.030 | 1,150–1,910 |
| 6–8 | 0.012–0.040 | 720–960 |
| 10–12 | 0.015–0.050 | 480–570 |
| 14–18 | 0.020–0.055 | 320–410 |
Coatings for Gun Drilling
| Coating | Inconel 625 | Inconel 718 | Notes |
|---|---|---|---|
| TiAlN PVD | Recommended | Recommended | Standard choice for both alloys |
| AlTiN PVD | Excellent | Excellent | Higher oxidation resistance for 718 at high speed |
| TiCN PVD | Adequate | Not recommended | Insufficient thermal stability for 718 |
| AlCrN PVD | Very good | Very good | Best for high-temperature applications |
| DLC | Not recommended | Not recommended | Degrades at cutting temperatures in both alloys |
| PCBN (tool material) | Over-specification | Recommended | Economically justified for production 718 drilling |
BTA Drilling Parameters
Speed and Feed by Alloy
| Parameter | Inconel 625 | Inconel 718 (Annealed) | Inconel 718 (Age-Hardened) |
|---|---|---|---|
| Cutting speed (Vc) | 30–60 m/min | 25–50 m/min | 20–40 m/min |
| Feed (mm/rev) | 0.06–0.18 | 0.05–0.15 | 0.04–0.12 |
| Coolant pressure | 3–6 MPa | 4–8 MPa | 6–10 MPa |
| Coolant flow | 4.5–5.5 × D L/min | 5–6 × D L/min | 5.5–6.5 × D L/min |
BTA Parameters by Diameter
Inconel 625
| Diameter (mm) | Vc (m/min) | Speed (RPM) | Feed (mm/rev) | Coolant Flow (L/min) |
|---|---|---|---|---|
| 12–16 | 35–55 | 700–1,460 | 0.06–0.14 | 55–90 |
| 18–22 | 32–50 | 460–880 | 0.08–0.16 | 80–120 |
| 25–35 | 30–45 | 270–570 | 0.08–0.18 | 110–190 |
| 40–50 | 25–40 | 160–320 | 0.10–0.18 | 180–280 |
Inconel 718 (Age-Hardened)
| Diameter (mm) | Vc (m/min) | Speed (RPM) | Feed (mm/rev) | Coolant Flow (L/min) |
|---|---|---|---|---|
| 12–16 | 22–38 | 580–1,010 | 0.04–0.10 | 65–105 |
| 18–22 | 20–35 | 350–620 | 0.05–0.12 | 100–140 |
| 25–35 | 18–30 | 160–380 | 0.06–0.14 | 140–230 |
| 40–50 | 15–25 | 95–200 | 0.06–0.14 | 220–330 |
Warning: Never use the same BTA parameters for Inconel 718 as for 625. The higher strength and work-hardening rate of age-hardened 718 will cause rapid tool failure — typically within 2–5 holes — if 625-optimised speeds and feeds are applied. Reduce speed by 30–40% and feed by 20–30% when switching from 625 to age-hardened 718.
Tool Selection
Insert Grade Comparison
| Material | Inconel 625 | Inconel 718 (Age-Hardened) | Notes |
|---|---|---|---|
| Coated carbide — general | IC806, AH725 | IC806, AH8015 | 718 needs more wear-resistant grades |
| Coated carbide — finishing | IC520 | IC520 | Both alloys can use same finishing grades |
| PCBN (high productivity) | Over-specification | IB85, IB90 (Iscar) | PCBN justified for 718 production |
| Ceramic | Not recommended | Limited use | Not recommended for deep hole drilling |
| Uncoated carbide | Not recommended | Not recommended | Both alloys require coated tools |
Guide Pads
| Material | Inconel 625 | Inconel 718 |
|---|---|---|
| Standard carbide | Adequate | Marginal — may gall |
| Coated carbide (TiAlN) | Recommended | Recommended |
| CBN-tipped | Over-specification | Recommended for production |
| PCD-tipped | Not for steel content | Not for steel content |
Inconel 718's higher hardness and abrasiveness make CBN-tipped guide pads economically justified for production volumes. Inconel 625 can be drilled successfully with standard coated carbide pads.
Tool Geometry
| Feature | Inconel 625 | Inconel 718 |
|---|---|---|
| Rake angle | +6 to +10° | +4 to +8° (more robust edge) |
| Clearance angle | 8–12° | 6–10° |
| Point angle (gun drill) | 120–130° | 130–140° |
| Corner radius | 0.2–0.4 mm | 0.4–0.8 mm |
| Edge preparation | Sharp, light hone | T-land 0.05–0.10 mm |
The more aggressive work-hardening of 718 requires a stronger cutting edge with larger corner radius and T-land to resist notch wear.
Coolant Requirements
Pressure Comparison
| Parameter | Inconel 625 | Inconel 718 |
|---|---|---|
| Gun drilling — minimum | 50 bar | 70 bar |
| Gun drilling — recommended | 70–120 bar | 100–160 bar |
| BTA drilling — minimum | 3 MPa (30 bar) | 4 MPa (40 bar) |
| BTA drilling — recommended | 4–6 MPa (40–60 bar) | 6–10 MPa (60–100 bar) |
| High-productivity (gun drill) | 100–140 bar | 140–200 bar |
Why 718 Needs Higher Pressure
Inconel 718 generates higher cutting forces and produces more abrasive chips. Higher coolant pressure is needed to:
- Evacuate chips — 718's harder, more abrasive chips require higher flow velocity to clear the flute
- Control temperature — despite similar thermal conductivity, the higher cutting forces in 718 generate more heat
- Maintain shaft rigidity — research (Ahmed et al., 2019) demonstrates that higher coolant pressure increases the dynamic stiffness of the gun drill shaft, improving hole straightness in 718
Research on Inconel 718 gun drilling found that 137.9 bar (2,000 PSI) coolant pressure combined with 1,600 RPM spindle speed produced optimal hole straightness (0.19 mm deviation over 350 mm depth). Lower pressures resulted in measurable reductions in straightness.
Tip: Inconel 625 can be successfully gun drilled at 70–100 bar coolant pressure for most applications. Inconel 718 requires a minimum of 100 bar, and 140 bar is recommended for production. If you are limited to below 100 bar for 718, reduce cutting speed by an additional 20% to compensate for reduced chip evacuation and cooling.
Tool Life Comparison
Expected Tool Life (6 mm Gun Drill, L/D 30:1)
| Tool Material | Inconel 625 (Annealed) | Inconel 718 (45 HRC) | Improvement Ratio |
|---|---|---|---|
| TiAlN-coated carbide (standard) | 20–30 holes | 6–12 holes | 2.5–3× for 625 |
| AlTiN-coated carbide (premium) | 25–40 holes | 10–18 holes | 2–2.5× for 625 |
| PCBN-tipped | — | 30–50 holes | — |
| PCBN (at 50 m/min) | — | 35–45 holes | — |
Expected Tool Life (BTA, 25 mm Diameter)
| Insert Grade | Inconel 625 | Inconel 718 (45 HRC) |
|---|---|---|
| General carbide (IC908) | 8–15 m | 3–6 m |
| Premium carbide (IC806, AH8015) | 12–20 m | 5–10 m |
| PCBN inserts | — | 15–30 m |
Dominant Wear Mechanisms
| Wear Mechanism | Inconel 625 | Inconel 718 | Difference |
|---|---|---|---|
| Flank wear | Moderate — gradual abrasive | Rapid — abrasive + adhesive | 718 wears 2–3× faster at same parameters |
| Notch wear | Moderate at depth-of-cut | Severe — from work hardening | Dominant failure mode in 718 |
| Crater wear | Limited at recommended speeds | Moderate at higher speeds | Activate above 30 m/min in 718 |
| Built-up edge | Moderate | Severe — material adhesion | 718 more prone to BUE |
| Edge chipping | Rare | Occasional | 718's segmented chips cause cyclic loading |
Surface Finish Comparison
| Parameter | Inconel 625 | Inconel 718 | Notes |
|---|---|---|---|
| Typical Ra (carbide gun drill) | 0.3–0.6 µm | 0.4–0.8 µm | 625 achieves better finish at same parameters |
| Ra with PCBN gun drill | — | <0.25 µm | PCBN produces superior finish in 718 |
| Best achievable Ra | 0.2–0.4 µm | 0.25–0.5 µm | Both can meet aerospace requirements |
| Burr height | 0.02–0.05 mm | 0.03–0.08 mm | 718 produces larger burrs due toductility |
Troubleshooting
| Problem | Inconel 625 | Inconel 718 |
|---|---|---|
| Rapid flank wear | Speed too high — reduce to 20 m/min | Switch to AlTiN coating or PCBN; reduce speed to 15 m/min |
| Notch wear at depth-of-cut | Increase feed to avoid rubbing | Increase feed, use larger corner radius; dominant issue in 718 |
| Built-up edge | Use polished rake face, increase speed 10% | Use polished + coated inserts, increase coolant pressure |
| Chip jamming | Check coolant flow, reduce peck depth | Increase coolant pressure, verify chip breaker geometry |
| Tool breaks in bore | Reduce feed, check chip evacuation | Reduce peck depth, increase dwell; 718 more prone to torque spikes |
| Poor surface finish | Replace worn insert, check coolant flow | Replace insert sooner — 718 degrades finish as tool wears |
| Oversize bore | Tool deflection from high feed | Check guide pad condition, reduce feed |
| Burr at exit | Maintain feed through breakthrough | Reduce speed in final 2 mm; 718 burrs are harder to control |
| Coolant pressure drop | Check filtration, clean coolant holes | More critical in 718 — any drop causes chip jamming |
| Work hardening at re-entry | Keep peck depth >0.5× diameter | Critical in 718 — avoid shallow pecks |
FAQ
Which is harder to deep hole drill — Inconel 625 or 718?
Inconel 718 (age-hardened) is significantly harder to drill than Inconel 625. At 40–48 HRC with gamma prime precipitates, 718 requires 30–50% lower cutting speeds, produces 2–3× faster tool wear, and is more prone to work hardening and built-up edge formation.
What cutting speed should I use for gun drilling Inconel 625?
Start at 25 m/min with TiAlN-coated carbide gun drills. The range is 20–50 m/min depending on diameter and coolant capability. Higher speeds are possible with ample coolant pressure (100+ bar).
What cutting speed should I use for gun drilling age-hardened Inconel 718?
Start at 15–20 m/min with TiAlN-coated carbide. Maximum with carbide is approximately 30 m/min. For higher productivity, PCBN-tipped gun drills can operate at 50–80 m/min.
Can I use the same tooling for both 625 and 718?
Yes — TiAlN-coated carbide gun drills work well for both alloys. However, the parameters must differ: reduce speed by 30–40% and feed by 20–30% when switching from 625 to age-hardened 718. PCBN tooling is economically justified for 718 production but unnecessary for 625.
What coolant pressure is needed for Inconel 625 deep hole drilling?
Gun drilling: 70–120 bar minimum, 100+ bar recommended for production. BTA drilling: 3–6 MPa (40–60 bar) recommended.
What coolant pressure is needed for Inconel 718 deep hole drilling?
Gun drilling: 100–160 bar recommended (research shows optimal at 138 bar / 2,000 PSI). BTA drilling: 6–10 MPa (60–100 bar) recommended. 718 requires 30–50% higher pressure than 625.
How much more tool life can I expect in 625 compared to 718?
At identical cutting parameters, expect 2.5–3× longer tool life in annealed 625 compared to age-hardened 718. With optimised parameters for each alloy, expect approximately 2× tool life in 625.
Is PCBN worth the cost for Inconel 718 deep hole drilling?
For production volumes, yes. PCBN gun drills can operate at 50 m/min (vs. 18 m/min for carbide), delivering 40+ holes per edge compared to 8–14 for carbide. The higher tool cost is offset by reduced cycle time, fewer tool changes, and better hole quality (Ra <0.25 µm).
What is the primary failure mode when gun drilling Inconel 718?
Notch wear at the depth-of-cut line, caused by the alloy's rapid work-hardening. This is the dominant failure mode distinguishing 718 from 625 — notch wear severity is directly related to the alloy's work-hardening rate.
How do surface finishes compare between 625 and 718 deep hole drilling?
Inconel 625 typically achieves Ra 0.3–0.6 µm with carbide gun drills at optimal parameters. Inconel 718 achieves Ra 0.4–0.8 µm with carbide. With PCBN gun drills, 718 can achieve Ra <0.25 µm — superior to carbide results in either alloy.
Summary
Inconel 625 and Inconel 718 are both nickel-based superalloys with distinct deep hole drilling characteristics:
- Inconel 625 (annealed) — machinability rating ~25% of free-cutting steel. Moderate work-hardening, lower cutting forces, and more forgiving parameters. Recommended starting point: 25 m/min, 0.030 mm/rev, 70–100 bar coolant. Tool life: 20–30 holes per edge with carbide gun drills.
- Inconel 718 (age-hardened) — machinability rating ~12–15%. Severe work-hardening, high cutting forces, abrasive gamma prime precipitates. Recommended starting point: 18 m/min, 0.020 mm/rev, 100–160 bar coolant. Tool life: 6–12 holes per edge with carbide; 30–50 holes with PCBN.
- Key differences — 718 requires 30–50% lower speed, 20–30% lower feed, and 30–50% higher coolant pressure than 625. Notch wear from work hardening is the dominant failure mode in 718 but not in 625.
- Tooling — TiAlN-coated carbide is suitable for both alloys. PCBN is economically justified for production 718 drilling, enabling 50 m/min cutting speed with 5× carbide tool life.
- The aerospace manufacturer in the opening scenario achieved 25 holes per edge in 625 with standard parameters, but needed separate optimisation for 718: reduced speed (18 m/min), higher coolant (140 bar), and peck cycling raised carbide tool life to 14 holes. PCBN gun drills at 50 m/min delivered 40 holes per edge with Ra 0.25 µm finish.