Drilling Hardened Steel: Tools and Techniques

Where HSS Stops Working

High-speed steel drills have a practical hardness limit somewhere between 40 and 45 HRC in the workpiece. Below that range, M42 cobalt HSS handles most applications with appropriate speeds and feeds. Above 45 HRC, you're rapidly into carbide territory — not because HSS is soft, but because the heat generated cutting that hard material degrades HSS faster than the reconditioning cycle can justify.

The failure mode in hard material is thermal rather than mechanical. Hard steels generate more cutting heat per unit of material removed because more energy is required to shear each chip. That heat concentrates at the tool edge. HSS loses hardness at temperatures above 1000 to 1100°F — and in 50 HRC tool steel or D2 at any reasonable cutting speed, you hit those temperatures quickly. The edge softens, deforms, and the drill fails not by chipping but by plastic deformation of the cutting edge.

This means the solution isn't just slowing down — it's changing the tool material. Cobalt grades buy you more thermal margin. Carbide buys you significantly more. Coatings (TiAlN in particular) reduce cutting temperatures by lowering friction. And for the hardest materials, non-cutting alternatives like EDM become the only practical option.

Carbide vs Cobalt: The Real Comparison

For drilling hardened steel in the 40 to 55 HRC range, the comparison between M42 cobalt HSS and solid carbide drills is nuanced. Carbide has higher hot hardness — it maintains cutting ability to 1400°F where HSS would have long since failed. But carbide is significantly more brittle: a carbide drill that catches a hard spot, hits a chip in the hole, or deflects under side load can snap catastrophically where a cobalt drill would survive with visible wear.

The application parameters that favor carbide in hardened steel: rigid machine setup, through-spindle coolant, consistent material hardness, and straight holes without interrupted cuts. CNC machining centers with solid workholding, stable spindles, and reliable coolant are the right environment for carbide drills in hard material.

Cobalt drills remain viable in hardened steel up to about 45 HRC when the setup isn't ideal for carbide — flexible workholding, manual machines, interrupted cuts, or irregular material hardness. The cobalt drill will wear faster than carbide per hole, but it won't snap unexpectedly under conditions that would shatter a carbide drill.

Practical speed guidance for hardened steel: at 40 HRC, M42 cobalt at 20 to 30 SFM; solid carbide at 40 to 60 SFM. At 50 HRC, carbide only at 15 to 25 SFM with through-spindle coolant. At 55+ HRC, carbide at 10 to 15 SFM or reconsider EDM.

EDM as the Alternative for Extreme Hardness

Electrical discharge machining (EDM) removes material through controlled electrical sparks, completely decoupled from workpiece hardness. A material that's 65 HRC conducts electricity just as well as a 30 HRC piece — EDM doesn't care. This makes it the tool of choice for features in through-hardened tool steel, carbide, and other materials that drilling simply cannot handle economically.

EDM hole drilling (sinker EDM with a tube electrode or dedicated EDM drill) can produce holes in any electrically conductive material regardless of hardness. The process is slow — typically 0.5 to 2 inches per hour depending on material and electrode size — but it's consistent and produces zero cutting forces, which matters in thin or fragile hardened parts where drilling thrust would cause cracking or deflection.

The EDM path makes the most sense when: the material is above 55 HRC and hard throughout (not just surface hardened), the required hole diameter is too small for reliable carbide drilling, the part geometry doesn't allow the rigidity needed for carbide, or the quantity is small enough that the setup cost is manageable. For production quantities, the economics usually favor drilling with carbide if the material and geometry allow it.

Practical Setup Checklist for Hard Material Drilling

• Verify actual workpiece hardness with a Rockwell tester before committing to a tool path — nominal hardness specs often have wide ranges.
• Use the shortest possible drill extension — long drills in hard material deflect and snap. If the hole is deep, consider a stub drill for entry and a jobber for the full depth as a second operation.
• Flood coolant at maximum available pressure, or through-spindle if available. Even brief coolant interruption in hard material drilling can cause immediate thermal failure.
• Spot-drill with a carbide spotting drill first — this creates a precise starting location that eliminates walk and reduces entry shock on the main drill.
• Inspect every drill before each use. In hard material, a microchip on the cutting edge that would produce a few more adequate holes in mild steel will cause catastrophic failure in the first pass.
• Never re-use a drill that broke in hard material without thorough inspection. The shock of breakage can create invisible micro-cracks in other drills stored nearby if they were impacted.