Deep Hole Drilling Hardened Steels (HRC 40+): Parameters and Tooling That Work

Hardened steels are a different animal. I learned this the hard way — watching a brand-new carbide gun drill snap on the third hole because I treated the material like it was standard 4140. After a lot of scrap parts, broken tools, and late-night spindle-side debugging, I landed on a set of parameters and tooling choices that turn hardened steel deep-hole drilling from a gamble into a repeatable process. Here is what actually works on the floor.

What “Hardened” Means for Deep Hole Drilling

When I talk about hardened steel in the context of deep hole drilling, I am focused on the HRC 40 to HRC 60 range. Below 40 HRC, standard gun-drilling parameters with minor feed adjustments tend to hold up fine. Once you cross 40 HRC, the chip formation changes completely — you get segmented, saw-tooth chips instead of the continuous ribbons you see in softer steels. At 50+ HRC, the material is abrasive enough to erode uncoated carbide in a matter of inches, not feet.

For deep hole drilling, the practical cutoff is around 58-60 HRC. Above that, even CBN-tipped tools struggle with the combination of hardness and the long-chip-evacuation demands of a gun-drill setup. If you are above 60 HRC, you should seriously consider pre-drilling in the soft state (more on that later).

Parameter Adjustments vs. Standard Steel

You cannot run hardened steel with the same speeds and feeds you use for annealed 4140 or 4340. The difference is stark, and the penalty for getting it wrong is a broken drill buried halfway down a bore that may already represent hours of machining.

Speed reduction is the biggest lever. I cut surface speed by roughly 40-60% compared to standard steel at the same hardness level. Going too fast builds heat at the drill tip faster than the coolant can evacuate it, and in hardened steel that leads to immediate edge breakdown.

Feed changes are more nuanced. You actually want to maintain enough feed to get under the work-hardened layer. If the feed is too light, the drill rubs instead of cutting, work-hardens the surface, and the next thing you know you are pushing a dull tool. I reduce feed by roughly 30-50% from standard values, but I never drop below 0.0002 IPR — anything less invites rubbing.

For a deeper breakdown of how these materials behave differently, see my article on deep hole drilling difficult materials.

Tool Coating Requirements

Uncoated carbide is useless above 45 HRC for deep hole drilling. I have tried it, and the edge life measured in single-digit inches of drilled depth. Here is what I use now depending on the hardness range:

• CBN (Cubic Boron Nitride): My go-to for 48-58 HRC. CBN handles the heat and abrasion better than anything else in a gun-drill geometry. The tradeoff is cost — a CBN-tipped gun drill can be 3-4x the price of carbide. But when it lasts 20x longer, the math works out.

• PCBN (Polycrystalline CBN): Technically a subset of CBN, but the binder formulation matters. For impact-resistant drilling with interrupted cuts (keyways or cross-holes already present), PCBN with a ceramic binder holds up better than the metallic-bond CBN variants. I save PCBN for the toughest jobs at 54+ HRC.

• Coated Carbide (AlTiN / TiAlN): Usable in the 40-48 HRC range. AlTiN coatings handle the thermal load reasonably well, but once you cross 50 HRC the coating breaks down and the substrate goes fast. I treat coated carbide as the budget option for light-duty runs only.

The biggest mistake I see is assuming a “hardened steel” coating grade is enough. It is not — the deep hole drilling process adds a continuous rubbing action along the bore wall that standard turning or milling inserts never see. You need the coating thickness and the substrate toughness specifically matched to gun-drill geometries.

I cover the full lineup of coating options in my piece on gun drill coatings and materials.

Coolant Pressure Needs

Standard coolant pressure of 300-500 PSI does not cut it in hardened steel. The chips are shorter and more segmented, which means they pack tighter in the flute and require more hydraulic force to push out. A chip jam in a 40+ HRC bore is not an inconvenience — it is a tool-breakage event.

I run a minimum of 800 PSI at the drill entry, and I prefer 1,000-1,200 PSI for anything above 50 HRC. The coolant volume matters just as much. I target 15-20 GPM for diameters around 0.5-1.0 inches, scaled proportionally. If your machine cannot deliver those pressures, consider an external high-pressure coolant booster — the investment pays for itself in the first dozen jobs.

Coolant filtration is equally critical. Hardened steel chips are abrasive and recirculating them through the system at 1,000 PSI turns your coolant into a lapping compound. I use 20-micron or better filtration and change the coolant more frequently than on standard steel jobs.

Expected Tool Life

Tool life in hardened steel is short compared to standard materials. Here is what I plan for based on actual production data:

These numbers assume proper coolant pressure, good filtration, and no misalignment. Drop any of those, and you lose 50% or more of your tool life immediately. The most reliable indicator I have found is monitoring spindle load — once it creeps up 15% from baseline, I pull the tool regardless of how many inches it has run.

When to Pre-Drill Soft Then Harden

There is a strong case for pre-drilling in the annealed state and then heat-treating afterward. Here is when I make that call:

• Above 58 HRC: Tool life collapses so fast that the cost of CBN drills becomes prohibitive. Drill in the annealed state with standard carbide gun drills, then heat treat.

• Very deep bores (L/D > 50): Chip evacuation in hardened materials becomes exponentially harder as depth increases. Pre-drilling soft avoids the worst of it.

• High production volumes (> 100 parts): The cycle time savings from drilling soft plus the avoidance of CBN tooling costs usually favor pre-drill + harden.

• Tight tolerances after heat treat: If the bore needs to be straight and round to within a few tenths, drill it soft, harden, then finish with a single-point boring or honing operation. Gun drilling hardened steel through a pre-existing soft bore is possible but alignment becomes critical.

The downside is obvious: distortion during heat treatment can shift the bore. You either need to leave stock for a final finishing pass or accept that some positional tolerance will be lost. For parts where the bore location is not critical, pre-drilling soft is almost always the cheaper and more reliable path.

Key Takeaways

• Hardened steel for deep hole drilling starts at 40 HRC. Below that, standard parameters with modest feed reductions work fine. Above 60 HRC, you should seriously question whether gun-drilling in the hardened state makes sense.
• Cut surface speed by 40-60% and feed by 30-50% vs. standard steel. Never drop below 0.0002 IPR to avoid work-hardening the surface.
• CBN tooling is mandatory above 48 HRC. Coated carbide is a temporary solution in the 40-48 range — treat it as such.
• Coolant pressure must be 800 PSI minimum, 1,000-1,200 PSI preferred. Filtration to 20 microns or better is non-negotiable.
• Expected tool life is 60-400 inches depending on hardness and tool material. Monitor spindle load as your primary wear indicator.
• Pre-drill in the soft state if your material exceeds 58 HRC, your L/D is above 50, or you are running more than 100 parts. The cost and reliability math almost always favor drilling before heat treat.