Tool steels push deep hole drilling to the edge of what conventional carbide can handle. Gundrilling straight cooling lines through a P20 mold block at 28–36 HRC is routine; drilling a hardened D2 die insert at 60 HRC is a battle against carbide wear, work hardening, and vibration. This guide maps the family — D2, H13, P20, O1 and S7 — and gives starting parameters for every condition from annealed to hardened.
Tool steel is the umbrella for a family of high-alloy grades designed for the cutting, forming and molding of other materials. What they share is a high alloy content and the ability to reach hardness far beyond structural steels — and what that costs the driller is wear, work hardening, and demanding chip control. The five grades that dominate deep hole work behave very differently.
| Grade | Class | Annealed | Hardened | Machinability | Typical Deep Hole Job |
|---|---|---|---|---|---|
| D2 | Cold work, 12% Cr | ∼200 HB | 58–62 HRC | Poor when hard | Blanking dies, punches, forming rolls |
| H13 | Hot work, 5% Cr-Mo-V | ∼200 HB | 44–52 HRC | Fair–moderate | Die casting & extrusion dies, forging dies |
| P20 | Pre-hardened plastic mold | Supplied 28–36 HRC | As-supplied | Good–excellent | Injection mold blocks, gundrilled cooling lines |
| O1 | Oil hardening | 183–212 HB | 57–62 HRC | Good (95%) | General-purpose punches, gages, cutting tools |
| S7 | Shock resistant, air hardening | 187–223 HB | 54–60 HRC | Good (95%) | Impact punches, chisels, shear blades |
Machinability in tool steel is driven less by hardness number and more by carbide content and alloy structure. O1 and S7 machine almost like ordinary carbon steel in the annealed state — both are rated about 95% relative to 1% carbon steel — while annealed D2 already feels harder on the edge than its Brinell number suggests.
| Condition | Typical Hardness | Tooling | Expected Tool Life |
|---|---|---|---|
| Annealed | <250 HB | HSS twist or carbide gundrill | Long; resharpening economics only |
| Pre-hardened P20 | 28–36 HRC | Carbide TSC twist, carbide gundrill | Good — standard mold-shop practice |
| Hardened H13 | 48–52 HRC | Coated solid carbide, through-coolant | Up to 210 holes at 30 m/min (test) |
| Hardened D2 | 60–62 HRC | Carbide / CBN, minimal | 6–9 holes — question the design |
The single most important decision in tool steel deep hole drilling is the heat treatment sequence. Almost every cost and quality problem disappears if the hole can be drilled in the annealed billet and the grade hardened afterward — but through-hardening shrinks and distorts bores, so precision holes often must be drilled after hardening anyway. Choose per hole, not per habit.
| Factor | Drill in Annealed | Drill in Hardened |
|---|---|---|
| Cutting speed | 40–70 m/min | 15–30 m/min |
| Tool life | High, resharpening only | Rapid flank wear, edge chipping |
| Vibration risk | Low | High — chip load must be held |
| Straightness | Stable, well-behaved | Drift-prone, needs drill support >40×D |
| Best when | Hole tolerances relaxed by HT distortion | Bore is the datum or finish-machined last |
Values below are starting points compiled from published research and tooling handbooks — confirm with the tool manufacturer and a test coupon before production. Note the wide D2/H13 carbide study ranges (60–108 m/min on 14 mm drills): those tests cover both steels and show coated drills cutting thrust and torque.
| Grade & Condition | Method | Vc (m/min) | Feed (mm/rev) | Notes |
|---|---|---|---|---|
| D2, annealed ∼200 HB | Carbide gundrill | 50–65 | 0.06–0.10 | TiAlN/TiN multilayer lowers thrust force |
| D2, 14 mm drill | Carbide twist (AlCrN) | 60–108 | 0.15–0.25 | Higher feed & speed drop cutting-zone temperature |
| H13, annealed | Carbide gundrill | 50–70 | 0.06–0.16 | Botek guide values, tool steel family |
| H13, 14 mm drill | Carbide twist (AlCrN) | 60–108 | 0.15–0.25 | Coated drills: less thrust, better hole quality |
| H13, hardened 48–52 HRC | Coated carbide, through-drill | ∼30 | ∼0.10 | Test produced up to 210 holes |
| H13 / D2, 45–55 HRC | Solid carbide TSC, 2-flute | 4,000–6,500 rpm | 0.03–0.08 | 70 bar coolant, up to 12×D (TiAlN/AlTiSiN/DLC) |
| P20, 28–36 HRC | Carbide gundrill | 60–90 | 0.08–0.15 | Mold cooling lines, standard practice |
| O1 / S7, annealed | HSS twist or gundrill | 15–25 | 0.04–0.10 | 95% machinability, HSS is enough |
| Tool steel, 200 HB (group 10) | Indexable gun drill (ISCAR TRI-DEEP) | 120–180 | 0.04–0.18 | High-alloyed steel, cast steel, tool steel |
| Tool steel, 325 HB Q&T (group 11) | Indexable gun drill (ISCAR TRI-DEEP) | 120–180 | 0.04–0.18 | Quenched & tempered condition |
Tool steel deep holes live or die on the edge. Carbide grade, point geometry, and coating all interact — and published D2/H13 drilling work consistently shows the coating has the largest single effect on result. For hardened work, one manufacturer's solid-carbide deep hole drill uses an ultra-fine-grain HRC 65 substrate, a 140° point, 30° helix, and a TiAlN/AlTiSiN/DLC multilayer for depth-to-diameter up to 12×D.
| Coating | Strongest In | Why |
|---|---|---|
| AlCrN (aluminum chromium nitride) | H13 / D2 drilling | Lower heat conduction keeps the edge hard; coated drills showed lower thrust, torque, and better hole quality |
| TiCN (titanium carbonitride) | D2 surface finish | Dominant factor (∼95%) on surface roughness in Taguchi drilling tests |
| TiAlN / TiN multilayer | D2 thrust force | Lowest thrust forces and best hole diameters on 5 mm D2 coupons |
| TiAlN / AlTiSiN / DLC multilayer | Hardened 55–65 HRC | Multilayer stack on HRC 65 substrate for deep holes up to 12×D |
Every credible deep hole drilling source on tool steel converges on one coolant rule: neat (100%) drilling oil, not emulsion, not a water-based mix. The Botek/Toolox program for 45 HRC tool steel states it flatly — no mixtures are acceptable. Tool steel chips are abrasive, and the bore surface work-hardens if the cutting zone starves.
| Parameter | Recommendation | Why |
|---|---|---|
| Fluid | 100% high-EP cutting oil | Lubricity + EP additives; emulsions fail the edge in hardened work |
| Pressure (hardened) | ∼70 bar | Forced chip evacuation and edge cooling at 45–55 HRC |
| Filtration | Fine filtration, tool steel chips are abrasive | Recirculating chips accelerate guide-pad and insert wear |
| Temperature control | Stable, ±1–2°C | Straightness and diameter wander with thermal drift |
| Monitoring | Coolant pressure & flow interlocks | Pressure drop = plugged drill tube; stop before tool breakage |
Deep hole drilling of tool steel produces segmented chips, and research on VTM-PLUS tool steel shows something unusual: the chips themselves work harden as cutting speed rises. At 70 m/min the measured chip hardness reached 321 ± 18 HV against a 223 HV annealed starting point — meaning faster cutting throws progressively harder, more abrasive debris through your drill tube.
Feed 0.06–0.16 mm/rev (per grade table). A thin rubbing cut hardens the bore instead of removing it.
Higher speed hardens the chip itself (321 HV at 70 m/min) — which then wears your guide pads faster.
BTA heads and gundrills need clear chip passages; blocked ports pack chips and stall torque.
Deep single-pass is ideal; if pecking, keep retract short so the point never rubs the hardened bore wall.
Drill support (steady rest / bushing) is non-negotiable in tool steel; unsupported bores drift and chatter.
Molds and dies are the heartland. Injection mold bases get straight, gundrilled cooling lines in P20; die casting dies carry conformal and straight channels in H13; blanking and forming tooling carries wear- and shock-related bores in D2 and S7. Gundrilling capability covers Ø8–50 mm lines to 2,000–4,000 mm depth on large dies.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Vibration / chatter at depth | Overhang too long, speed too high | Reduce Vc, add drill support / steady rest, stiffen toolholding |
| Rapid edge wear (D2 especially) | Hard carbides + wrong coating | AlCrN or TiAlN/TiN multilayer; or drill annealed first |
| Tool breakage in the bore | Coolant starvation, packed chips | Monitor pressure/flow, check chip shape, filter coolant |
| Rough bore surface | Edge wear, wrong coating | TiCN coating on D2; resharpen earlier; hold feed |
| Bore drift / banana hole | Unsupported bar, off-square start | Drill support, square entry, spot-face the face first |
| Chipped point | Too-sharp tip angle, work-hardened start | Use the toolmaker's point angle; peck-entry at reduced feed |
| D2 tool life in single digits | Drilling hardened D2 at all | Re-sequence: drill annealed + HT + re-finish, or wire EDM |