Appearance
A tool and die manufacturer requires deep-drilled coolant holes in D2 tool steel die blocks at 58–62 HRC. Standard gun drilling parameters for pre-hardened steel cause rapid chipping and tool failure within three holes. The process team develops material-specific strategies: for D2 at full hardness, EDM drilling is recommended over gun drilling; for A2 and O1 in the annealed condition, carbide gun drills at 25–40 m/min with 80–120 bar coolant pressure achieve 40–80 holes per regrind. For S7 tool steel at 48–54 HRC, AlCrN-coated micrograin carbide tools with 20–40 m/min cutting speed and reduced feed rates produce reliable results.
Tool Steel Properties for Deep Hole Drilling
| Grade | Type | Annealed (HB) | Hardened (HRC) | Wear Resistance | Toughness | Machinability |
|---|---|---|---|---|---|---|
| A2 | Air-hardening medium alloy | 200–220 | 57–62 | Good | Good | Fair (annealed) |
| D2 | High carbon, high chromium | 220–250 | 58–62 | Excellent | Poor | Poor (annealed) |
| O1 | Oil-hardening | 180–200 | 57–61 | Good | Fair | Fair (annealed) |
| S7 | Shock resisting | 190–220 | 48–56 | Fair | Excellent | Fair (annealed) |
Cutting Parameter Recommendations
| Parameter | A2 (Annealed) | A2 (Hardened 58 HRC) | D2 (Annealed) | D2 (Hardened 60 HRC) | O1 (Annealed) | S7 (48–54 HRC) |
|---|---|---|---|---|---|---|
| Cutting speed — carbide (m/min) | 30–50 | 15–25 | 25–40 | Not recommended* | 30–50 | 20–40 |
| Feed — 6 mm dia (mm/rev) | 0.020–0.040 | 0.010–0.020 | 0.015–0.030 | — | 0.025–0.050 | 0.015–0.030 |
| Feed — 10 mm dia (mm/rev) | 0.030–0.060 | 0.015–0.030 | 0.020–0.045 | — | 0.035–0.065 | 0.020–0.040 |
| Feed — 20 mm dia (mm/rev) | 0.045–0.090 | 0.020–0.040 | 0.030–0.060 | — | 0.050–0.100 | 0.030–0.055 |
| Coolant pressure (bar) | 60–100 | 80–120 | 80–120 | — | 60–90 | 70–110 |
| Recommended coating | TiAlN | AlCrN | AlCrN | — | TiAlN | AlCrN |
*D2 above 55 HRC is not recommended for gun drilling — use EDM or drill in annealed condition before heat treatment.
Tool Geometry by Condition
| Geometry Parameter | Annealed Tool Steel | Hardened Tool Steel (> 50 HRC) |
|---|---|---|
| Point angle | 118–130° | 130–140° |
| Rake angle | 6–10° positive | 4–8° positive (reduced) |
| Relief angle | 8–12° | 10–14° |
| Edge preparation | Sharp with light hone | T-land 0.03–0.08 mm |
| Coating | TiAlN | AlCrN (higher hot hardness) |
| Carbide grade | Micrograin (0.5–1.0 µm) | Fine grain with high Co content |
| Tip displacement | 0.22–0.25 × D | 0.20–0.22 × D |
WARNING
Deep hole drilling of tool steels in the hardened condition (above 50 HRC) is extremely demanding and should be avoided whenever possible. The best practice is to drill cooling channels and holes in the annealed or pre-hardened condition before heat treatment. The exceptions where post-heat-treatment drilling is unavoidable — repair work, design changes, or geometry that would distort during heat treatment — require: AlCrN-coated micrograin carbide gun drills, cutting speed reduced to 15–25 m/min, feed reduced 40–60% from annealed values, coolant pressure increased to 80–120 bar, and tool replacement at VB ≥ 0.10 mm (versus 0.20–0.30 mm in annealed steel). Tool life in hardened tool steel is typically 10–30 holes per regrind compared to 40–100 holes in annealed material.
D2 Tool Steel — Special Considerations
| Aspect | Recommendation |
|---|---|
| Best approach | Drill in annealed condition (220–250 HB) before heat treatment |
| Carbide grade | Micrograin WC with 8–10% Co for toughness |
| Coating | AlCrN (resists abrasive wear from high-volume carbides) |
| Annealed speed (carbide) | 25–40 m/min |
| Annealed feed (10 mm) | 0.020–0.045 mm/rev |
| Coolant pressure | 80–120 bar |
| Expected tool life (annealed) | 40–80 holes per regrind |
| Above 55 HRC | EDM drilling or diamond grinding recommended |
| Chip formation | Fine, segmented chips — good evacuation |
| Primary wear mechanism | Abrasive flank wear from carbide particles in microstructure |
Troubleshooting
| Symptom | Likely Cause | Solution |
|---|---|---|
| Edge chipping in D2 | Excessive carbide particle impact, speed too high | Reduce speed, switch to AlCrN coating, increase Co content |
| Rapid flank wear | Abrasive carbides in tool steel microstructure | Reduce speed, increase coolant pressure, use AlCrN |
| Tool breakage at entry | Hard surface scale, interrupted cut | Pre-drill pilot hole, peck entry, use chamfered entry |
| Poor surface finish | Tool wear, inadequate coolant | Replace tool, verify coolant pressure and EP level |
| Hole oversize | Tool wear, guide pad wear | Replace at VB ≥ 0.20 mm (0.10 mm for hardened) |
| Chatter / vibration | Excessive overhang, feed too low | Reduce overhang, increase feed 15%, check bushing fit |
| Work hardened surface | Dull tool, feed too low | Replace tool, increase feed above 0.015 mm/rev minimum |
FAQ
Can D2 tool steel be gun drilled at 60 HRC?
Gun drilling D2 at 60 HRC is not recommended for production. The high volume of hard carbide particles in the D2 microstructure causes rapid abrasive wear on the carbide cutting edge. Tool life is typically 3–10 holes — uneconomical for production. Alternatives include: (1) drill in the annealed condition (220–250 HB) before heat treatment — this is the standard practice; (2) use EDM drilling for holes in fully hardened D2; (3) use diamond grinding or ultrasonic machining for small-diameter holes. If gun drilling hardened D2 is unavoidable, use AlCrN-coated micrograin carbide with 10% cobalt content, cutting speed 15–20 m/min, and feed 0.010–0.020 mm/rev.
What cutting speed is recommended for A2 tool steel?
For A2 tool steel in the annealed condition (200–220 HB), recommended cutting speed is 30–50 m/min with carbide gun drills. For A2 hardened to 57–62 HRC, reduce to 15–25 m/min. A2 is more machinable than D2 due to lower carbide content, making it the most practical air-hardening grade for deep hole drilling. TiAlN coating is suitable for annealed A2; AlCrN is preferred for hardened A2.
Should tool steels be drilled before or after heat treatment?
Drilling before heat treatment is strongly preferred for all tool steel grades. Drilling in the annealed condition (180–250 HB) provides: 3–5× longer tool life, higher cutting speeds, better surface finish, and lower risk of catastrophic tool failure. The drilled hole may distort slightly during heat treatment (typically 0.02–0.10 mm for D2, less for A2 and O1), so drill undersize and finish after heat treatment if tolerance is critical. If post-heat-treatment drilling is required, limit to through-holes under 20× diameter and use AlCrN-coated carbide with reduced parameters.
What coolant pressure is needed for tool steel deep hole drilling?
Minimum 60 bar coolant pressure for annealed tool steels (A2, O1, S7), increasing to 80–120 bar for D2 and any hardened tool steel. The high coolant pressure requirement is driven by the need to evacuate fine, abrasive chips from the cutting zone and to provide adequate cooling to the tool edge — tool steels generate significant cutting heat even at low speeds. Oil-based coolant with EP additives is required; water-based emulsions are not suitable for tool steel deep hole drilling.
What is the primary wear mechanism when drilling D2?
The primary wear mechanism when drilling D2 tool steel is abrasive flank wear caused by the high volume (12–15%) of hard chromium carbide particles in the D2 microstructure. These carbides (1,200–1,800 HV) act as abrasive particles that wear the carbide tool matrix. Even at low cutting speeds, the abrasive action wears the cobalt binder, causing carbide grain pull-out. AlCrN coating helps but cannot entirely eliminate this wear. This is why D2 is the most difficult tool steel grade for deep hole drilling and why drilling in the annealed condition is strongly recommended.
Summary
Deep hole drilling of tool steels A2, D2, O1, and S7 requires material-specific strategies that depend primarily on the heat treatment condition. Drilling in the annealed condition (180–250 HB) is strongly preferred for all grades, with carbide gun drills at 25–50 m/min, feed rates of 0.015–0.100 mm/rev depending on diameter, and coolant pressure of 60–120 bar. D2 is the most challenging grade due to its high carbide content — avoid drilling above 55 HRC and use EDM as an alternative for fully hardened D2. A2 and O1 are the most practical tool steel grades for deep hole drilling in both annealed and hardened conditions. S7 requires intermediate parameters reflecting its moderate hardness but high toughness. Post-heat-treatment drilling should be limited to applications where pre-heat-treatment drilling is not feasible, using AlCrN-coated micrograin carbide with speeds reduced 40–60% and tool replacement at VB ≥ 0.10 mm.