Appearance
A manufacturer of hot work tool steel injection mold cores was drilling Ø6 mm × 360 mm (L/D 60:1) cooling channels in H13 steel (48–52 HRC). The initial process used a TiAlN-coated carbide gun drill at Vc = 25 m/min, f = 0.015 mm/rev, 120 bar coolant, 35° point angle. Tool life was 4–12 m per edge — failure by edge chipping within the first 2–5 m. Cutting forces produced stresses at the cutting edge exceeding the carbide's transverse rupture strength, and cutting edge temperature was 720–850 °C. Corrective actions: reduced speed from 25 to 12 m/min (cutting temperature dropped to 620 °C); switched from TiAlN to thick AlCrN coating (5 µm versus standard 3 µm) for higher hot hardness; increased point angle from 35° to 42° (strengthening the cutting edge); and increased coolant pressure from 120 to 180 bar. After implementation: tool life increased to 18–28 m per edge (4.5× improvement), chipping eliminated, tool cost per bore down 60%.
Material Characteristics
Hardened Steel and Superalloy Classification
| Material | Hardness | UTS (MPa) | Thermal Conductivity (W/mK) | Work-Hardening Rate | Chip Form | Primary Failure Mode for Carbide Tools |
|---|---|---|---|---|---|---|
| H13 (hot work tool steel) | 48–55 HRC | 1,500–1,900 | 28–30 (at 20 °C) | Low | Segmented, saw-tooth | Edge chipping from thermal-mechanical fatigue |
| D2 (cold work tool steel) | 58–62 HRC | 1,800–2,200 | 20–22 | Low | Segmented, brittle | Abrasive wear; edge chipping |
| A2 (air hardening tool steel) | 55–62 HRC | 1,600–2,000 | 24–26 | Low | Segmented | Edge chipping; abrasive wear |
| M2 (high-speed steel) | 60–65 HRC | 2,000–2,500 | 20–25 | Low | Segmented, fine | Abrasive wear; thermal softening of cutting edge |
| 4340 (Q&T, high hardness) | 45–52 HRC | 1,500–1,800 | 38–42 | Low | Segmented | Edge chipping at elevated temperatures |
| 300M (aerospace steel) | 50–55 HRC | 1,900–2,200 | 32–36 | Low | Segmented | Edge chipping; high cutting forces |
| Inconel 718 (aged) | 44–48 HRC | 1,350–1,550 | 11.4 | Very high (n = 0.50) | Serrated, tough | Notch wear at depth of cut; thermal cracking |
| Inconel 625 (annealed) | 25–35 HRC | 850–1,050 | 9.8 | High (n = 0.45) | Serrated | Work hardening; built-up edge |
| Waspaloy (aged) | 40–45 HRC | 1,300–1,500 | 10.2 | Very high | Serrated, tough | Notch wear; surface integrity concerns |
| Hastelloy X (annealed) | 20–30 HRC | 750–900 | 9.0 | High | Serrated | Work hardening; chip breaking |
| Maraging steel (C300) | 50–55 HRC | 1,800–2,100 | 22–25 | Low | Segmented | Edge chipping; high thrust force |
Recommended Cutting Parameters for Hardened Steels and Superalloys
| Material | Condition | Method | Vc (m/min) | f (mm/rev) | Coolant Pressure (bar) | Tool Material / Coating | Expected Tool Life (m) | Expected Ra (µm) |
|---|---|---|---|---|---|---|---|---|
| H13 | 48–52 HRC | Gun drilling | 10–16 | 0.010–0.025 | 120–200 | AlCrN carbide (thick coat) | 15–30 | 0.8–1.5 |
| D2 | 58–62 HRC | Gun drilling | 6–12 | 0.008–0.020 | 120–200 | AlCrN carbide or PCBN | 8–20 | 0.6–1.2 |
| A2 | 55–60 HRC | Gun drilling | 8–14 | 0.010–0.020 | 120–180 | AlCrN carbide | 12–25 | 0.8–1.5 |
| M2 | 60–65 HRC | Gun drilling | 5–10 | 0.005–0.015 | 120–200 | PCBN (tipped) or AlCrN carbide | 5–15 | 0.6–1.2 |
| 4340 | 45–52 HRC | Gun drilling | 12–20 | 0.015–0.030 | 100–180 | AlCrN carbide | 20–50 | 1.0–2.0 |
| 300M | 50–55 HRC | Gun drilling | 10–16 | 0.012–0.025 | 120–200 | AlCrN carbide | 12–30 | 1.0–2.0 |
| Inconel 718 | Aged (44 HRC) | Gun drilling | 6–12 | 0.008–0.020 | 150–250 | AlCrN carbide (thick) | 5–18 | 1.0–2.0 |
| Inconel 718 | Aged (44 HRC) | BTA drilling | 12–20 | 0.06–0.15 | 40–100 | AlCrN inserts | 10–25 | 2.0–4.0 |
| Inconel 625 | Annealed | Gun drilling | 12–20 | 0.015–0.030 | 120–200 | AlCrN carbide | 15–40 | 1.5–3.0 |
| Waspaloy | Aged (44 HRC) | Gun drilling | 5–10 | 0.008–0.018 | 150–250 | AlCrN carbide or PCBN | 4–12 | 1.0–2.0 |
| Hastelloy X | Annealed | Gun drilling | 14–22 | 0.015–0.030 | 120–180 | AlCrN carbide | 15–35 | 1.5–3.0 |
| Maraging C300 | 50–55 HRC | Gun drilling | 10–16 | 0.010–0.022 | 120–180 | AlCrN carbide | 12–25 | 0.8–1.5 |
Tool Material Selection
Tool Material Comparison for Hardened/Superalloy Drilling
| Tool Material | Hardness (HV) | TRS (N/mm²) | Thermal Conductivity (W/mK) | Max Operating Temp (°C) | Relative Cost (vs carbide) | Best Application |
|---|---|---|---|---|---|---|
| Micrograin carbide (0.5–0.8 µm) | 1,600–1,800 | 3,000–4,000 | 80–100 | 800–900 | 1× (baseline) | H13, 4340, 300M at 45–55 HRC; general hardened steel drilling |
| AlCrN-coated carbide (thick, 4–8 µm) | 1,600–1,800 (substrate) | 3,000–4,000 | 80–100 (substrate) | 900 (coating) | 1.2–1.5× | All hardened steels; Inconel 718; Waspaloy — standard choice for most applications |
| PCBN (polycrystalline CBN, 50–90% CBN content) | 3,500–4,500 | 500–1,000 (low TRS — brittle) | 80–130 | 1,000–1,200 | 5–15× | D2, M2 at > 58 HRC; high-volume hardened steel production; tool life 2–5× coated carbide |
| Whisker-reinforced ceramic (Al₂O₃ + SiC whiskers) | 2,000–2,500 | 500–800 | 20–30 | 1,200–1,400 | 3–8× | Inconel 718 at high speeds (> 20 m/min); limited to BTA inserts (not available as gun drill tip) |
| Cermet (TiC/TiN-based) | 1,800–2,200 | 1,500–2,500 | 30–50 | 800–1,000 | 1.5–3× | Limited application in deep hole drilling; used for finish boring of hardened steels |
| PCD (polycrystalline diamond) | 8,000–10,000 | 1,500–2,000 | 2,000 | 600–700 | 3–5× | Not recommended for steel (chemical affinity causes rapid wear); used for non-ferrous hardened materials |
PCBN Gun Drills: When to Use
| Factor | Recommendation |
|---|---|
| Material hardness threshold | PCBN justified for materials > 55 HRC (D2, M2, A2 at maximum hardness) |
| Production volume | Justified for > 100 bores per year in the same material and diameter |
| Tool life advantage | 2–5× coated carbide in D2 at 60 HRC; 3–8× in M2 at 64 HRC |
| Cost premium | PCBN tipped gun drill: €300–800 vs carbide: €80–250 (Ø6–12 mm) |
| Break-even | 15–40 bores (depending on tool cost and change downtime cost) |
| Limitations | Brittle — requires rigid machine setup; no interrupted cuts; sensitive to coolant pressure variation |
| Edge preparation | Requires 20–40 µm edge hone (larger than carbide) to prevent chipping |
Process Reliability
Vibration and Chatter Control
| Technique | Mechanism | Effectiveness | Implementation |
|---|---|---|---|
| Reduced cutting speed | Lowers excitation frequency below the system natural frequency | High — most effective single parameter for chatter reduction | Reduce Vc by 25–50% from the initial trial speed |
| Increased feed rate | Increases chip thickness, which increases process damping (the plowing effect on the clearance face damps vibrations) | Moderate-high | Increase f by 20–40% (monitor tool edge stress) |
| Reduced point angle | Changes the direction of cutting forces; higher point angle (40–45°) directs more force axially, which is stiffer | Moderate | Increase point angle to 40–45° for hardened materials |
| Increased coolant pressure | Provides hydraulic damping at the guide pad/bore wall interface | Moderate | Increase by 30–50 bar from standard |
| Workpiece support (steady rest) | Increases workpiece stiffness, reducing vibration amplitude | High — essential for L/D > 30:1 in hardened materials | Use one or two steady rests on the workpiece OD |
| Tuned mass damper on drill tube | Adds damping to the drill tube at the chatter frequency | High for narrow-band chatter | Requires frequency analysis to tune; expensive |
| Variable spindle speed | Modulates cutting speed to break regenerative chatter | Moderate for wide-band chatter | Requires CNC capability for speed modulation |
Cutting Edge Temperature Management
| Material | Cutting Speed (m/min) | Cutting Edge Temperature (°C) | Limiting Factor | Cooling Strategy |
|---|---|---|---|---|
| H13 (50 HRC) | 12 | 580–650 | AlCrN coating oxidation (900 °C) | 120–180 bar coolant |
| H13 (50 HRC) | 25 | 720–850 | Carbide softening (800 °C) | 120–180 bar insufficient; must reduce speed |
| D2 (60 HRC) | 8 | 550–650 | Edge chipping (mechanical) | 120–200 bar |
| D2 (60 HRC) | 15 | 700–800 | Thermal fatigue of edge | 180+ bar required |
| Inconel 718 (44 HRC) | 8 | 650–750 | AlCrN coating oxidation (900 °C) | 150–250 bar |
| Inconel 718 (44 HRC) | 15 | 800–950 | Work hardening; notch wear | 200+ bar + AlCrN |
| Inconel 718 (44 HRC) | 20 | 900–1,050 | Tool failure within 2 m | Not recommended for carbide |
FAQ
What is the most important parameter for deep hole drilling of hardened steel?
The most important parameter is cutting speed. Hardened steels (45–62 HRC) require cutting speeds of 5–16 m/min — approximately 1/5 to 1/10 of the speed used for the same steel in the annealed condition. The sensitivity is extreme: increasing cutting speed by 5 m/min in D2 at 60 HRC (from 10 to 15 m/min) typically reduces tool life by 60–80% (from 15–20 m to 3–5 m). The reason is the exponential relationship between cutting temperature and tool wear rate. In hardened steel drilling, the cutting edge temperature at 10 m/min is approximately 600 °C; at 15 m/min it is approximately 750 °C; at 20 m/min it exceeds 850 °C. Each 100 °C increase in cutting temperature doubles or triples the tool wear rate through a combination of thermal softening of the tool material, accelerated diffusion wear, and increased thermal fatigue cracking. The recommended approach is to start at a conservative cutting speed (8–10 m/min for > 55 HRC, 12–15 m/min for 45–55 HRC) and increase in increments of 2 m/min while monitoring tool wear patterns. The economic optimum is typically at the speed where tool failure mode transitions from gradual flank wear (predictable, acceptable) to edge chipping (unpredictable, unacceptable).
Can PCBN gun drills be used for deep hole drilling?
PCBN (polycrystalline cubic boron nitride) gun drills can be used for deep hole drilling of hardened steels (> 55 HRC) and offer 2–5× the tool life of coated carbide in these materials. However, PCBN requires specific application conditions: rigid setup — PCBN is brittle (transverse rupture strength of 500–1,000 N/mm² versus 3,000–4,000 N/mm² for carbide) and cannot tolerate interrupted cuts, machine vibration, or coolant pressure fluctuations. The machine must be in good condition with spindle runout < 0.005 mm and stable coolant pressure; larger edge hone — PCBN gun drills require an edge hone of 20–40 µm (versus 10–20 µm for carbide) to prevent edge chipping from the brittle CBN material; higher point angle — 40–45° (versus 30° for carbide) to strengthen the cutting edge; higher cost — PCBN gun drills cost 5–15× more than equivalent carbide drills (€300–800 for a Ø6–12 mm PCBN gun drill versus €80–250 for carbide); and limited geometries — PCBN tips are typically brazed onto a carbide substrate, limiting the complexity of the gun drill tip geometry. The economic justification for PCBN gun drills requires: annual volume of > 50–100 bores in the same material and diameter; tool life improvement sufficient to offset the 5–15× cost premium; and reduced machine downtime from fewer tool changes.
How do I prevent edge chipping when drilling hardened tool steels?
Edge chipping is the most common failure mode in hardened steel deep hole drilling and is caused by the combination of high mechanical stress at the cutting edge and thermal fatigue from cyclic heating and cooling. Prevention requires a multi-faceted approach: cutting speed — stay below 16 m/min for steels > 45 HRC and below 12 m/min for steels > 55 HRC. Exceeding these speeds causes the cutting edge temperature to exceed 700–800 °C, at which the carbide's cobalt binder softens and the edge becomes susceptible to microfracture; edge hone — use a 20–40 µm edge hone (K-land) to strengthen the cutting edge. The hone removes the sharp, stress-concentrating edge and distributes the cutting force over a larger area. The hone size should increase with material hardness; point angle — use a 40–45° point angle for hardened steels, which increases the included angle at the cutting tip and provides a more robust edge geometry; coating — use thick AlCrN coating (4–8 µm) which provides a thermal barrier and reduces the thermal cycling amplitude at the carbide substrate surface; coolant pressure — maintain minimum 120 bar (180 bar preferred) to provide consistent cooling and reduce thermal cycling amplitude. Intermittent coolant delivery (pressure fluctuations) is particularly damaging because it causes the edge to repeatedly heat and quench; and feed rate — maintain 0.010–0.025 mm/rev — too low a feed (< 0.008 mm/rev) causes rubbing rather than cutting, which work-hardens the surface and increases edge loading.
What causes the notch wear at the depth of cut line in Inconel 718?
Notch wear at the depth of cut line is the characteristic failure mode when deep hole drilling Inconel 718 and other nickel-based superalloys. It appears as a localized groove at the point where the cutting edge meets the bore surface (the depth of cut line). The notch is caused by three interacting mechanisms: work-hardened surface layer — the previous cutting pass or the guide pad burnishing action work-hardens the bore surface to 450–550 HV (versus 400–450 HV bulk). This harder surface layer (typically 10–30 µm deep) is 1.5–2× more abrasive than the bulk material, causing accelerated local wear at the cutting edge position that contacts this layer; oxide scale — the high cutting temperatures (800–950 °C for Inconel 718) combined with the nickel and chromium content in the workpiece form hard, abrasive oxide particles at the bore surface that contribute to notch formation; and chip rubbing — the chip being evacuated through the flute or tube rubs against the cutting edge at the depth of cut line, adding an additional mechanical wear component. Prevention strategies include: maintaining a consistent depth of cut from the previous operation (avoid step changes in cut depth that create a hard step at the bore surface); using AlCrN-coated carbide with high hot hardness; applying a chamfer or edge preparation at the depth of cut corner to spread the wear over a larger area; and replacing tools at the first sign of notch development (> 0.20 mm notch depth) before the notch triggers edge chipping.
What coolant pressure is required for superalloy deep hole drilling?
The minimum coolant pressure for production deep hole drilling of nickel-based superalloys (Inconel 718, Waspaloy, Hastelloy) is 150 bar at the tool, with 200–250 bar recommended for optimal tool life and process reliability. This is the highest coolant pressure requirement of any common deep hole drilling material group. The extreme pressure requirement is driven by: cutting zone cooling — superalloys have thermal conductivity of 9–12 W/mK (1/5 of steel), so 75–85% of the cutting heat flows into the tool. Only very high coolant pressure can provide sufficient heat transfer to keep the cutting edge temperature below the tool material's thermal softening threshold; chip breaking — the tough, serrated chips from superalloys require high coolant velocity to break and evacuate. At coolant pressures below 120 bar, chip packing events increase by 3–5×; and guide pad lubrication — the high cutting forces in superalloys (2–3× higher than in steel) generate extreme guide pad pressures that require a robust hydrodynamic coolant film to prevent metal-to-metal contact. At 150 bar, the coolant film between the guide pads and the bore wall provides adequate load support; at 100 bar, guide pad wear rates increase by 2–3×. The practical recommendation is to design the coolant system for a minimum of 200 bar at the pump (to allow for pressure drops through the filter, piping, rotary union, and drill tube) and verify that the pressure at the tool during cutting is at least 150 bar.
Disclaimer: The hardened steel and superalloy drilling parameters, tool material data, and process recommendations presented in this article are based on published technical literature, tool manufacturer testing, and industry-reported experience from aerospace and tool and die applications. Actual results depend on specific material composition, heat treatment, machine condition, and coolant system capability. Extreme caution should be exercised when transferring parameters between different materials or machine tools. Tool failure in hardened/superalloy drilling can cause workpiece damage (scrapped parts) and machine damage (broken drill tube). No guarantee of specific tool life, surface finish, or process reliability is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.