When I first started deep hole drilling tool steels, I assumed they would behave like the hardened alloy steels I was used to. I was wrong. Tool steels are their own category of misery — high carbide content, through-hardened to the core, and absolutely unforgiving of parameter mistakes. After years of gun-drilling D2, A2, S7, and O1 for mold and die work, here is what I have learned the hard way so you do not have to.
How Tool Steels Differ from Standard Alloy Steels
Standard alloy steels like 4140 or 4340 machine predictably. They work-harden moderately, produce consistent chips, and give you plenty of warning before things go wrong. Tool steels are not that cooperative.
The key difference is carbide volume fraction. Tool steels pack large amounts of chromium, vanadium, molybdenum, or tungsten carbides into the matrix. These carbides are harder than the cutting edge itself. When a gun drill passes through a tool steel workpiece, it is not cutting through homogeneous metal — it is chiseling through a composite of hard ceramic particles embedded in a tough binder. Every carbide particle that breaks loose acts as its own abrasive grit, accelerating flank wear and edge chipping.
Tool steels also arrive at the machine in one of three conditions: annealed (softest, best for drilling), pre-hardened (28-32 HRC), or fully hardened (50-65 HRC). Most of my work falls in the 45-62 HRC range for mold and die applications. At those hardness levels, conventional drilling wisdom goes out the window. You need rigid setups, high coolant pressure, and feeds that generate consistent chip segmentation without exceeding the tool’s edge strength.
D2: High Wear Resistance, Carbide Content
D2 is the most common tool steel I drill, and also the most frustrating. It runs at 58-62 HRC in service, with roughly 12-14 percent chromium and substantial vanadium carbide content. The carbide network in D2 is coarse and unevenly distributed — large primary carbides scattered through the matrix. These carbides act as built-in chip breakers but absolutely destroy cutting edges.
The biggest lesson D2 taught me is that conventional HSS gun drills are a non-starter. I tried them once on a production run of ejector pin holes in a large injection mold. The first hole looked acceptable. By the third hole, the edge was gone and surface finish had climbed from 32 Ra to over 80 Ra. Micro-grain carbide gun drills with a wear-resistant coating are mandatory here.
Chip evacuation is the other challenge. D2 produces short, segmented chips in the best case, but as the tool wears, the chips transition to fine, abrasive dust that packs into the chip flute. I have had multiple gun drill breakages caused by chip packing leading to coolant starvation at the cutting edge. Running higher coolant pressure — 1000 PSI minimum, ideally 1500 PSI — makes a measurable difference in both tool life and hole quality.
For D2, I pre-drill in the annealed state whenever possible, then through-drill after heat treatment. Pre-drilling in the annealed state at 25-35 HRC gives me three to four times the tool life compared to drilling fully hardened D2.
A2: Air-Hardening, More Forgiving
A2 is the tool steel I recommend to anyone getting started with deep hole drilling in hardened materials. At 56-60 HRC, it is only slightly softer than D2, but the carbide distribution is much finer and more uniform. There are fewer large primary carbites to hammer the cutting edge, which translates directly to longer tool life and more predictable chip formation.
In my experience, A2 is about 40 percent more drillable than D2 at the same hardness level. Tool life on micro-grain carbide gun drills running at 80-100 SFM regularly exceeds 200 linear inches before regrind. Chip evacuation is also less problematic — A2 produces tight, consistent 6-9 chips that flow reliably through the flute at 800-1000 PSI coolant pressure.
The air-hardening nature of A2 matters for post-drilling operations. After deep hole drilling, A2 experiences minimal dimensional distortion during heat treat if you follow proper stress-relief cycles. This is critical for mold core holes that need to stay within 0.0005 inches of true position after hardening.
I still use coated carbide for A2, but I have gotten acceptable results from uncoated micro-grain carbide in short runs under 50 holes. The coating is a productivity differentiator, not a requirement, with A2.
S7: Shock-Resistant, Tough
S7 is the oddball in this group. It is designed for impact resistance — punch dies, shear blades, cold work tooling that sees shock loading. At 54-58 HRC, it is on the softer end of the tool steel spectrum, but do not let that fool you. S7 is tough. It has high impact toughness and excellent ductility, which means it does not chip or crack easily during machining.
That toughness, however, creates a different problem: chip control. S7 produces long, stringy chips that are a nightmare in deep hole drilling. The material wants to tear rather than shear, and getting clean chip segmentation requires aggressive feed rates that push the limits of the tool edge. I run S7 at feeds 15-20 percent higher than D2 or A2 to force chip breaking — typically 0.0012 to 0.0016 IPR depending on hole diameter.
Coolant pressure matters more with S7 than any other tool steel I have drilled. The long chips create enormous back-pressure in the flute. If coolant pressure drops below 800 PSI, chip packing is almost guaranteed. I target 1200-1500 PSI and monitor coolant flow continuously during the cycle. A 10 percent drop in flow tells me chips are accumulating and I need to retract before the tool binds.
S7 also responds well to higher surface speeds. I have run S7 at 120-140 SFM with coated carbide gun drills and seen tool life comparable to D2 at more conservative parameters. The toughness of S7 seems to protect the cutting edge from thermal shock better than the more brittle D2.
O1: Oil-Hardening, Good Drillability
O1 is the easiest tool steel I drill, and it is the one I use for process development and training. At 58-60 HRC, O1 has fine carbide distribution, good machinability in the hardened state, and forgiving chip formation. The oil-hardening metallurgy produces a more uniform microstructure than air-hardening or high-carbon grades, which translates to consistent drilling behavior hole after hole.
I have run O1 at 100-120 SFM with CVD diamond-coated carbide gun drills and seen tool life exceed 400 linear inches. Even with standard AlTiN-coated carbide, O1 is predictable and repeatable. The chips are short, well-segmented, and evacuate reliably at 600-800 PSI coolant pressure.
The catch with O1 is dimensional stability. Oil quenching can produce more distortion than air hardening or vacuum hardening. For deep holes requiring tight tolerances after heat treatment, I pre-drill O1 oversize by 0.003-0.005 inches and finish drill after hardening. This two-pass approach adds cycle time but consistently delivers hole tolerances within 0.0005 inches.
Parameter Recommendations by Tool Steel Grade
The table below summarizes the parameters I use as starting points for gun-drilling common tool steels. These assume micro-grain carbide gun drills with a wear-resistant coating, rigid machine setup, and high-pressure coolant through the tool.
| Grade | Hardness (HRC) | Surface Speed (SFM) | Feed (IPR) | Coolant Pressure (PSI) | Expected Tool Life (linear inches) |
|---|---|---|---|---|---|
| D2 | 58-62 | 60-80 | 0.0008-0.0012 | 1000-1500 | 80-150 |
| A2 | 56-60 | 80-100 | 0.0010-0.0014 | 800-1000 | 200-300 |
| S7 | 54-58 | 100-140 | 0.0012-0.0016 | 1200-1500 | 120-200 |
| O1 | 58-60 | 100-120 | 0.0010-0.0014 | 600-800 | 300-400+ |
| Annealed A2/D2 | 25-35 | 150-200 | 0.0015-0.0025 | 500-800 | 400-600+ |
These numbers are starting points, not gospel. Every machine, coolant formulation, and hole geometry shifts the optimal window. I keep a process log for every tool steel grade and adjust parameters based on chip shape, power consumption, and surface finish trends.
Tool Coating Selection for Tool Steels
Coating selection is the single most impactful decision for tool steel deep hole drilling. The coating must handle high compressive stress, abrasive carbides, and elevated temperatures at the cutting edge. Here is what I use and why:
| Tool Steel | Recommended Coating | Why |
|---|---|---|
| D2 | AlCrN or AlTiN | High hot hardness resists abrasive carbides; AlCrN offers better oxidation resistance for higher-speed runs |
| A2 | AlTiN or TiAlN | Balanced wear resistance and toughness; TiAlN works well at moderate speeds |
| S7 | AlCrN or TiCN | AlCrN handles the thermal cycling from interrupted chip formation; TiCN provides good lubricity for chip flow |
| O1 | CVD Diamond or AlTiN | Diamond coating delivers exceptional life in O1’s fine carbide matrix; AlTiN is a cost-effective alternative |
| Annealed tool steels | TiN or uncoated carbide | Lower abrasive wear means standard coatings and even uncoated carbide are sufficient |
I have also experimented with AlCrN/DLC multilayer coatings on D2 and seen a 25 percent improvement in tool life over single-layer AlCrN, though the cost premium makes it viable only for production runs over 500 holes. For short runs, AlTiN is the workhorse choice across most tool steel grades.
For more detail on coating technologies and how they interact with different substrate materials, see my article on gun drill coatings and materials.
Common Problems
Chipping
Chipping is the number one failure mode in tool steel deep hole drilling. It almost always traces back to one of three causes: excessive feed rate that overloads the cutting edge, inadequate coolant pressure that allows chips to recut and impact the edge, or runout in the tool holder that creates uneven chip load per tooth.
I check for chipping by examining the cutting edge under 10x magnification every 25-50 holes. At the first sign of micro-chipping, I back the feed off by 10 percent and verify that coolant pressure is at the target. If chipping persists, the coating is likely the wrong choice for that grade.
Work Hardening
Tool steels work-harden aggressively when the cutting edge dwells in the cut. This happens most often at the start of the hole — if the gun drill rubs instead of shearing on entry, the surface hardens instantly and subsequent passes struggle to penetrate. I address this by using gun drill entry bushings to ensure clean, immediate engagement at the full feed rate.
Work hardening also appears mid-hole when feed rate drops due to chip packing. The material beneath the chip compresses and hardens, creating a localized hard spot that accelerates wear on the next revolution. Maintaining consistent coolant flow is the best prevention.
Micro-Cracking
Micro-cracking is the subtle killer. It does not show up as an immediate failure but as a gradual degradation in surface finish and hole roundness over a production run. Micro-cracks propagate from the cutting edge into the tool substrate, eventually leading to catastrophic edge failure.
The root cause is almost always thermal cycling — rapid heating and cooling at the cutting edge as the tool enters and exits the cut. This is especially problematic in interrupted-cut scenarios like cross-holes or keyways. I combat micro-cracking by selecting coatings with better thermal barrier properties (AlCrN over TiAlN) and ensuring coolant is directed precisely at the cutting edge, not just the flank.
For tool steels above 60 HRC, I also incorporate a 0.0005-inch chamfer on the cutting edge to distribute thermal stress over a larger area. It reduces sharpness slightly but extends tool life by 30-50 percent in the hardest grades.
Internal Links
- Deep Hole Drilling Hardened Steels — general approach for hardened materials including pre-hardened alloy steels
- Gun Drill Coatings and Materials — detailed comparison of coating technologies for deep hole drilling
Key Takeaways
- Tool steels are fundamentally different from standard alloy steels — high carbide content and through-hardened microstructures demand coated carbide gun drills, high coolant pressure, and conservative parameters.
- D2 is the most abrasive tool steel due to its coarse primary carbide network. Run it at 60-80 SFM with AlCrN-coated carbide and 1000+ PSI coolant.
- A2 is the most forgiving hardened tool steel for deep hole drilling, offering 40 percent longer tool life than D2 at similar hardness.
- S7 requires aggressive feed rates and high coolant pressure to control its long, stringy chips. Do not baby it — S7 needs 0.0012-0.0016 IPR to break chips reliably.
- O1 is the easiest to drill and the best choice for process development, with tool life regularly exceeding 300 linear inches.
- Coating selection matters more than any single parameter. AlCrN and AlTiN are the workhorses; CVD diamond is the top performer for O1.
- The three failure modes to watch for are chipping (reduce feed, check coolant), work hardening (ensure immediate edge engagement), and micro-cracking (improve thermal management at the cutting edge).
- Always drill in the annealed state if the process allows — tool life improves three to four times compared to drilling fully hardened.