Appearance
If you have drilled one tool steel, you have drilled one tool steel. D2 at 27% machinability and O1 at 42% are not in the same class — they share a category name but not a cutting behaviour. The machinist who drills O1 at 90 m/min and expects D2 to survive the same speed will be sorting through broken carbide within the first hole.
Tool Steel Metallurgy and Machinability
Why Tool Steels Are Difficult to Drill
Tool steels are alloyed for wear resistance, hardness at elevated temperature, and dimensional stability during heat treatment — properties that are directly opposed to machinability. The same chromium carbides that give D2 its wear resistance also abrade cutting edges. The same vanadium content that gives H13 its hot hardness also increases cutting forces.
| Alloying Element | Effect on Tool Steel | Effect on Machinability |
|---|---|---|
| Chromium (Cr) | Forms hard carbides, improves wear resistance | Increases abrasive tool wear |
| Vanadium (V) | Refines grain structure, improves hot hardness | Increases cutting forces |
| Molybdenum (Mo) | Improves toughness and hardenability | Moderate reduction |
| Tungsten (W) | Retains hardness at high temperature | Significant reduction |
| Carbon (C) | Determines maximum hardness | Higher C = lower machinability |
| Sulphur (S) | — | Added in some grades to improve chip breakage |
Machinability Ratings
All machinability ratings are relative to AISI 1212 free-machining steel at 100%.
| Tool Steel | Machinability Rating | Key Limiting Factor |
|---|---|---|
| D2 (cold work) | 27% | High-volume chromium carbides — extremely abrasive |
| H13 (hot work) | 46% | Vanadium content increases cutting forces |
| O1 (oil hardening) | 42% | Uniform carbide distribution, moderate abrasion |
| S7 (shock resisting) | 45–70% | Lower carbide volume than D2, tougher matrix |
| W2 (water hardening) | 45% | Low alloy content improves machinability |
Warning: Machinability ratings are a starting guide, not a guarantee. A D2 bar from one supplier can machine differently from another due to variations in annealing practice, carbide distribution, and microstructural uniformity. Always verify with a test cut before committing to production parameters.
Material-by-Material Guide
D2 Tool Steel
D2 is a high-carbon, high-chromium cold work tool steel (1.5% C, 12% Cr). It is the most difficult to drill of the five grades covered here.
| Property | Value |
|---|---|
| Annealed hardness | 15–20 HRC (≈200 HB) |
| Hardened hardness | 58–62 HRC |
| Machinability rating | 27% |
| Primary wear mechanism | Abrasive wear from chromium carbides |
| Chip formation | Segmented, brittle chips in annealed state |
Recommended parameters (carbide drills, annealed):
| Drill Type | Cutting Speed | Feed per Revolution |
|---|---|---|
| Solid carbide twist drill | 60–100 SFM (18–30 m/min) | 0.002–0.005 IPR (0.05–0.13 mm/rev) |
| Carbide gun drill | 100–150 SFM (30–45 m/min) | 0.002–0.004 IPR (0.05–0.10 mm/rev) |
| BTA drill (carbide) | 80–120 SFM (25–35 m/min) | 0.003–0.008 IPR (0.08–0.20 mm/rev) |
Practical tip from experienced machinists: Many users report that running D2 at too low a speed (below 60 SFM with carbide) causes work hardening of the surface, which then breaks the cutting edge on subsequent passes. The recommended approach is to run at the higher end of the speed range with a moderate feed, rather than low speed with high feed.
H13 Tool Steel
H13 is a chromium hot work tool steel (0.4% C, 5% Cr, 1% V, 1.5% Mo). It is significantly more machinable than D2.
| Property | Value |
|---|---|
| Annealed hardness | 15–22 HRC (≈200 HB) |
| Hardened hardness | 48–52 HRC |
| Machinability rating | 46% |
| Primary wear mechanism | Adhesive wear + moderate abrasion |
| Chip formation | Continuous to segmented chips |
Recommended parameters (carbide drills, annealed):
| Drill Type | Cutting Speed | Feed per Revolution |
|---|---|---|
| Solid carbide twist drill | 200–250 SFM (60–75 m/min) | 0.005–0.008 IPR (0.13–0.20 mm/rev) |
| Carbide gun drill | 150–250 SFM (45–75 m/min) | 0.003–0.006 IPR (0.08–0.15 mm/rev) |
| BTA drill (carbide) | 100–200 SFM (30–60 m/min) | 0.004–0.010 IPR (0.10–0.25 mm/rev) |
H13 responds well to coated carbide (TiAlN or AlCrN). Research by Tekaüt et al. (2017) showed that AlCrN-coated drills produced better hole quality with lower cutting forces in H13 compared to uncoated drills at speeds of 60–108 m/min and feeds of 0.15–0.25 mm/rev.
O1 Tool Steel
O1 is an oil-hardening cold work tool steel (0.9% C, 0.5% Cr, 0.5% W). It is one of the most forgiving tool steels for drilling.
| Property | Value |
|---|---|
| Annealed hardness | 15–20 HRC (≈190 HB) |
| Hardened hardness | 58–62 HRC |
| Machinability rating | 42% |
| Primary wear mechanism | Moderate abrasive wear |
| Chip formation | Continuous chips — chipbreaker recommended |
Recommended parameters (carbide drills, annealed):
| Drill Type | Cutting Speed | Feed per Revolution |
|---|---|---|
| Solid carbide twist drill | 200–300 SFM (60–90 m/min) | 0.005–0.008 IPR (0.13–0.20 mm/rev) |
| Carbide gun drill | 200–250 SFM (60–75 m/min) | 0.003–0.006 IPR (0.08–0.15 mm/rev) |
O1 produces more continuous chips than D2 or H13, making chipbreaker geometry important for deep hole drilling. Without a chipbreaker, long ribbon chips can pack in the flute and cause tool jamming.
S7 Tool Steel
S7 is a shock-resisting tool steel (0.5% C, 3.25% Cr, 1.4% Mo) designed for impact applications.
| Property | Value |
|---|---|
| Annealed hardness | 15–20 HRC (≈190 HB) |
| Hardened hardness | 54–58 HRC |
| Machinability rating | 45–70% |
| Primary wear mechanism | Moderate abrasion |
| Chip formation | Segmented chips — breaks readily |
Recommended parameters (carbide drills, annealed):
| Drill Type | Cutting Speed | Feed per Revolution |
|---|---|---|
| Solid carbide twist drill | 200–300 SFM (60–90 m/min) | 0.005–0.008 IPR (0.13–0.20 mm/rev) |
| Carbide gun drill | 200–250 SFM (60–75 m/min) | 0.003–0.006 IPR (0.08–0.15 mm/rev) |
S7's advantage in deep hole drilling is its chip fragmentation behaviour — it produces shorter, more segmented chips than O1 or H13 at equivalent feeds, reducing chip evacuation risk.
W2 Tool Steel
W2 is a water-hardening tool steel (0.6–1.4% C, 0.25% V). It is the simplest composition of the group and the most predictable to machine.
| Property | Value |
|---|---|
| Annealed hardness | 15–20 HRC (≈180 HB) |
| Hardened hardness | 60–65 HRC |
| Machinability rating | 45% |
| Primary wear mechanism | Moderate abrasive wear |
| Chip formation | Continuous chips |
Recommended parameters (carbide drills, annealed):
| Drill Type | Cutting Speed | Feed per Revolution |
|---|---|---|
| Solid carbide twist drill | 200–300 SFM (60–90 m/min) | 0.005–0.008 IPR (0.13–0.20 mm/rev) |
| Carbide gun drill | 200–250 SFM (60–75 m/min) | 0.003–0.006 IPR (0.08–0.15 mm/rev) |
W2's low alloy content makes it more forgiving of coolant interruptions and parameter variation than the other grades. However, its water-hardening characteristic means that heat generated during drilling can cause localised hardening — maintain coolant flow without interruption.
Comparative Parameter Table
Carbide Gun Drilling (Annealed State)
| Material | Machinability | Vc (m/min) | fn (mm/rev) | Coolant Pressure (bar) | Expected Tool Wear |
|---|---|---|---|---|---|
| D2 | 27% | 30–45 | 0.05–0.10 | 70–100 | High — abrasive wear dominant |
| H13 | 46% | 45–75 | 0.08–0.15 | 50–100 | Moderate — adhesive + abrasive |
| O1 | 42% | 60–75 | 0.08–0.15 | 50–100 | Moderate — uniform wear |
| S7 | 45–70% | 60–75 | 0.08–0.15 | 50–100 | Low-moderate |
| W2 | 45% | 60–75 | 0.08–0.15 | 50–100 | Low |
Carbide BTA Drilling (Annealed State)
| Material | Vc (m/min) | fn (mm/rev) | Coolant Pressure (bar) | Typical Insert Grade |
|---|---|---|---|---|
| D2 | 25–35 | 0.08–0.20 | 20–50 | AlTiN-coated carbide |
| H13 | 30–60 | 0.10–0.25 | 20–50 | TiAlN or AlCrN-coated |
| O1 | 35–60 | 0.10–0.20 | 20–50 | TiAlN-coated |
| S7 | 35–60 | 0.10–0.25 | 20–50 | TiAlN-coated |
| W2 | 40–60 | 0.10–0.20 | 20–50 | TiAlN-coated |
Hardened State Drilling
When tool steels are in the hardened state (above 45 HRC), conventional drilling becomes impractical and specialised strategies are required.
| Material | Hardness | Vc (m/min) | fn (mm/rev) | Tool Type |
|---|---|---|---|---|
| D2 | 60–62 HRC | 8–15 | 0.02–0.05 | Carbide, CBN-tipped, or ceramic |
| H13 | 48–52 HRC | 15–30 | 0.05–0.10 | Carbide (AlTiN-coated) |
| O1 | 58–62 HRC | 10–20 | 0.03–0.08 | Carbide or CBN |
| S7 | 54–58 HRC | 12–20 | 0.03–0.08 | Carbide (AlTiN-coated) |
| W2 | 60–65 HRC | 8–12 | 0.02–0.05 | CBN-tipped preferred |
Tip: For hardened tool steels, consider drilling in the annealed state before heat treatment whenever possible. The difference in drilling cost between annealed D2 (30 m/min, 0.10 mm/rev, standard carbide) and hardened D2 (10 m/min, 0.03 mm/rev, CBN tooling) is approximately 5–8× in cycle time alone, without accounting for tooling cost differences.
Deep Hole Drilling Considerations
Chip Evacuation
| Material | Chip Character | Evacuation Strategy |
|---|---|---|
| D2 | Segmented, brittle | Generally good — chips break readily. Monitor for fine abrasive dust in coolant |
| H13 | Continuous to segmented | Moderate pecking recommended at depths > 10× diameter |
| O1 | Continuous, tough | Chipbreaker geometry essential. Increase feed rate if chips are stringy |
| S7 | Segmented, short | Best chip evacuation of the group — least likely to clog |
| W2 | Continuous | Requires chipbreaker or pecking cycles. Can form long ribbon chips |
Coolant Strategy
Through-tool coolant at minimum 50 bar is required for deep hole drilling of all tool steels. Specific considerations:
- D2: High coolant pressure (70–100 bar) helps flush abrasive carbide particles from the cutting zone. Filter coolant to 10 μm or better.
- H13: Standard pressure (50–70 bar) is sufficient. Oil-based coolant reduces adhesive wear tendency.
- O1: Standard pressure. Ensure coolant flow is not interrupted to avoid chip welding.
- S7: Standard pressure. S7 is the least sensitive to coolant variation.
- W2: Critical — any coolant interruption can cause localised hardening from heat buildup.
Guide Bushing Selection
For gun drilling tool steels, the guide bush must be matched to the material:
| Material | Guide Bush Material | Clearance |
|---|---|---|
| D2 | Carbide | Minimum (G6 fit) |
| H13 | Carbide or hardened steel | Standard |
| O1 | Hardened steel | Standard |
| S7 | Carbide | Standard |
| W2 | Hardened steel | Standard |
The abrasive nature of D2 means that carbide guide bushes are essential — a hardened steel bush will wear beyond tolerance within 10–20 holes.
Tool Coating Selection
| Coating | Suitability | Why |
|---|---|---|
| TiAlN (titanium aluminium nitride) | Good for H13, O1, S7, W2 | High oxidation temperature (800°C), good abrasion resistance |
| AlCrN (aluminium chromium nitride) | Best for D2 | Higher hardness than TiAlN, better resistance to abrasive wear |
| AlTiN (aluminium titanium nitride) | Good for all five grades | Highest oxidation temperature (900°C), excellent thermal protection |
| TiN (titanium nitride) | Not recommended for tool steels | Low oxidation temperature (600°C) — breaks down at tool steel cutting temperatures |
| Uncoated carbide | Only for W2 in annealed state | Insufficient wear resistance for alloy-rich grades |
Warning: TiN-coated drills are commonly supplied as "general purpose" and will fail rapidly in tool steels. The coating breaks down above 600°C, which is below the typical cutting edge temperature when drilling D2 or H13 at productive parameters. Specify TiAlN or AlCrN for any tool steel drilling application.
Troubleshooting by Material
| Problem | D2 | H13 | O1 | S7 | W2 |
|---|---|---|---|---|---|
| Rapid flank wear | Reduce speed, check for adequate coolant pressure | Check coating — switch to AlCrN | Reduce speed or check for interrupted cut | Reduce speed | Check for localised hardening from heat |
| Edge chipping | Reduce feed, check runout | Reduce feed — H13 work-hardens at low feed | Reduce feed, use chipbreaker geometry | Check for impact loading | Reduce feed |
| Built-up edge | Increase speed or coolant pressure | Increase speed to > 40 m/min | Increase speed | Less common in S7 | Less common in W2 |
| Oversize hole | Check guide bush wear — D2 wears bushes rapidly | Check runout and guide bush fit | Check spindle alignment | Check for vibration | Check drill point geometry |
| Chip clogging | Unlikely — D2 chips are brittle | Increase peck frequency | Add chipbreaker or increase feed | Unlikely — S7 chips fragment | Add pecking or chipbreaker |
| Poor surface finish | Reduce feed, increase speed if possible | Reduce feed | Reduce feed, check edge condition | Reduce feed | Reduce feed |
FAQ
What is the most difficult tool steel to drill?
D2 is the most difficult of the five grades covered here, with a machinability rating of only 27% (relative to AISI 1212 free-machining steel). Its high chromium carbide content causes rapid abrasive wear on cutting tools. D2 requires lower cutting speeds (30–45 m/min for carbide gun drilling), AlCrN-coated tooling, high coolant pressure, and carbide guide bushes.
Can tool steels be gun drilled in the hardened state?
Yes, but with significantly reduced parameters. Hardened D2 (60–62 HRC) requires cutting speeds of 8–15 m/min and feeds of 0.02–0.05 mm/rev with CBN-tipped or AlTiN-coated carbide tooling. The cycle time is 5–8× longer than drilling in the annealed state. Whenever possible, drill in the annealed state before heat treatment.
What cutting speed should I use for carbide drilling of annealed H13?
200–250 SFM (60–75 m/min) for solid carbide twist drills, or 150–250 SFM (45–75 m/min) for carbide gun drills. Feed rate should be 0.005–0.008 IPR (0.13–0.20 mm/rev) for twist drills or 0.003–0.006 IPR (0.08–0.15 mm/rev) for gun drills.
How does O1 compare to D2 for drilling?
O1 is significantly easier to drill than D2 (42% vs 27% machinability). O1 can be drilled at 2–3× the cutting speed of D2 with carbide tooling. However, O1 produces more continuous chips, requiring chipbreaker geometry or pecking cycles in deep hole applications, while D2's brittle chips evacuate more readily.
What tool coating is best for drilling D2?
AlCrN (aluminium chromium nitride) coating is the best choice for D2. It has higher hardness than TiAlN and better resistance to the abrasive wear mechanism that dominates in D2. For a general-purpose coating that works across all five grades, AlTiN offers the highest oxidation temperature (900°C).
What coolant pressure is required for deep hole drilling tool steels?
Minimum 50 bar (725 PSI) at the cutting edge for gun drilling, with 70–100 bar recommended for D2. BTA drilling typically requires lower pressure (20–50 bar) due to the larger diameter and internal chip evacuation system. Through-tool coolant delivery is essential — flood coolant alone is insufficient for deep hole drilling in tool steels.
How does the chip formation differ between these tool steels?
D2 and S7 produce segmented, brittle chips that break readily — good for chip evacuation. H13 produces a mix of continuous and segmented chips depending on feed rate. O1 and W2 tend toward continuous, tough chips that require chipbreaker geometry or pecking cycles to prevent flute packing in deep hole drilling.
Can I use the same drilling parameters for annealed and hardened tool steel?
No. Hardened tool steel requires cutting speeds 60–80% lower and feeds 50–70% lower than the annealed state. Attempting to drill hardened tool steel at annealed parameters will destroy the tool immediately. Tooling also differs — hardened steels may require CBN or ceramic tooling rather than standard carbide.
Why does D2 wear guide bushes so quickly?
D2 contains approximately 12% chromium, most of which is bound in hard chromium carbides (Cr₇C₃, Cr₂₃C₆). These carbides are harder than hardened steel and act as abrasive particles. When the drill rotates, the carbide particles embedded in the chip stream and on the workpiece surface abrade the guide bush ID. Carbide guide bushes are required for production D2 drilling.
Which tool steel is best for deep hole drilling applications?
S7 offers the best combination of properties for deep hole drilling: good machinability (45–70% rating), segmented chip formation that evacuates reliably, moderate abrasive wear, and sufficient toughness for demanding applications. H13 is a close second, with better thermal resistance but slightly more challenging chip control. D2 is the most difficult and should be avoided for deep hole drilling unless the application specifically requires its wear resistance.
Conclusion
The five tool steel grades covered here — D2, H13, O1, S7, and W2 — span a wide range of drilling difficulty. D2, with its 27% machinability rating and abrasive chromium carbides, demands the most conservative parameters (30–45 m/min for carbide gun drilling), the hardest coating (AlCrN or AlTiN), and the highest coolant pressure (70–100 bar). H13, O1, S7, and W2 are all significantly more forgiving in the annealed state, with recommended cutting speeds in the 45–90 m/min range for carbide gun drilling. The fundamental rule for tool steel drilling is that material condition matters as much as material grade — annealed vs hardened state can change the recommended cutting speed by a factor of three to five, and attempting to drill hardened tool steel at annealed parameters will produce immediate tool failure. For deep hole drilling specifically, chip formation behaviour becomes a primary selection criterion: S7's segmented chips and D2's brittle chips evacuate reliably, while O1 and W2 require chipbreaker geometry or pecking cycles to manage their continuous chip formation.