Understanding What You Are Up Against
Hardened steel is not just harder steel — it is a fundamentally different cutting challenge. At 40 HRC and below, you are still in territory where carbide-tipped or cobalt HSS tooling can make reasonable progress. Above 55 HRC, you are approaching tool steel hardness and the economics of drilling with conventional tooling break down fast. Knowing where your material sits on the Rockwell scale is the first step in deciding whether drilling is viable, or whether you need a different strategy entirely.
Most hardened components in general machining fall into a few categories: hardened shafts and pins, heat-treated tool steel stock, case-hardened surfaces on otherwise soft cores, and parts that have been inadvertently work-hardened by previous machining. Each category calls for a different approach.
A file test gives you a rough idea before you commit a drill to the work. A new mill file at roughly 60-65 HRC that slides off the surface without biting tells you the material is fully hardened. A file that bites slightly suggests 50-55 HRC range. A file that cuts normally means the surface is below 45 HRC and standard cobalt HSS tooling has a fighting chance.
Annealing: The Underused Option
If you have any flexibility in the process, local annealing before drilling is often the most economical approach. Softening just the area you need to drill allows you to use standard tooling and eliminates the exotic tooling cost entirely.
Spot annealing with an acetylene torch works for most tool steels. Heat the area to a bright cherry red — roughly 1400 to 1500 degrees Fahrenheit — and let it cool slowly. Do not quench. Covering it with vermiculite or firebrick fragments slows the cooling and ensures a full anneal rather than just surface softening. This typically brings the surface down to 20-25 HRC, which is mild-steel territory for drilling purposes.
The catch is that annealing changes the material properties of the part. For a hardened fixture plate where you are adding a hole that does not need to be in the hardened zone, this is fine. For a component where the hardness serves a functional purpose in that area, annealing is not an option. Know which situation you are in before you reach for the torch.
Spot annealing also affects the structure of the surrounding material. On through-hardened parts, the heat-affected zone extends beyond the visible discoloration. Plan your drilling location to give yourself some margin if the hardness of adjacent areas matters for the part's function.
Cobalt HSS vs Standard HSS for Hardened Work
Standard M2 HSS tooling has a red hardness limit of roughly 1000 degrees Fahrenheit. When drilling hardened steel, friction at the cutting edge generates heat quickly, and that heat softens the cutting edge before significant material removal occurs. The drill wears and the situation gets worse with each inch of feed.
Cobalt HSS — M35 at 5% cobalt, M42 at 8% cobalt — extends the red hardness to roughly 1050 to 1100 degrees Fahrenheit. That sounds like a small improvement but it translates to meaningfully longer tool life in heat-generating cuts. For hardened steel in the 40-48 HRC range, cobalt HSS is the minimum viable tooling. Below 40 HRC, the difference between cobalt and standard HSS matters less and standard tooling can be used with careful technique.
The technique adjustments for hardened steel with cobalt HSS: lower speeds by 40 to 60 percent compared to soft steel, higher feed pressure to keep the cutting edge engaged rather than rubbing, and cutting fluid applied generously. Rubbing without cutting generates more heat than cutting does, which is counterintuitive. The goal is to keep the edge in active shear contact with the material, not skating on top of it.
Cutting fluid selection matters more on hardened steel than on soft steel. Sulfurized cutting oil or a heavy-duty soluble oil outperforms water-based coolants because the lubricity component matters as much as heat removal at the low speeds you are running. Keep the fluid flowing continuously — letting it run dry mid-hole is how you lose the drill and potentially work-harden the remaining material.
Carbide Spot Drilling First
One shop floor trick that works well on hardened material: use a carbide spot drill to establish a starting point before committing your cobalt HSS drill to the work. The carbide spot drill can start a clean center on hardened material that would deflect or damage an HSS drill tip. Once you have a clean, well-centered starting divot, the HSS drill has a guide to follow and does not have to survive the lateral forces of starting on a hardened surface.
This is especially useful on case-hardened parts where the surface is hard but the core is softer. The carbide spot drill penetrates the hard case and gives the HSS drill a path into the softer material underneath. The carbide spot drill sees minimal total depth of cut and survives well; the HSS drill never has to deal with the hardest material.
Carbide jobber drills are an option for fully hardened material, but they are expensive and brittle enough that using them on anything but a rigid CNC machine is risky. Hand drilling or drill press work with carbide drills in hard material is a reliable way to snap the drill and embed carbide shards in your workpiece. Reserve carbide drills for rigid setups where you have positive feed control and runout dialed in.
When to Stop Trying with HSS
Above roughly 55 HRC, HSS tooling — cobalt or otherwise — is not the right answer. The economics do not work: drill life is measured in partial holes, cutting fluid consumption climbs, and the quality of the resulting hole is poor. At this hardness level, EDM drilling is the correct process if hole geometry matters, or grinding if you can access the feature that way.
Work-hardened stainless is a related situation where HSS can fail unexpectedly. Stainless steel that has been heavily cold-worked in a previous operation can reach 40 HRC locally even when the base material is rated much lower. If your cobalt drill is losing its edge after just a few holes in what should be standard-grade stainless, work-hardening is a likely cause. The solution is slower speeds, higher feeds, and sharper tools — a dull drill on stainless is a work-hardening machine.
Resharpen Economics on Hardened-Steel Work
Cobalt HSS drills used on hardened material dull faster than in normal use, but the economics of resharpening still favor sending them in rather than replacing them — especially at larger diameters. A 1/2 inch M42 cobalt drill costs several dollars to replace. A proper resharpen restores it to full geometry for less than half that cost.
The wear pattern on drills used in hardened material tends to be concentrated at the outer corners of the cutting lips, where cutting speed is highest and heat generation is greatest. A machine resharpen brings those corners back and restores the full relief geometry, which is difficult to achieve freehand on a worn cobalt drill. The difference between a machine-sharpened cobalt drill and a freehand-sharpened one is more pronounced on hard material because the geometry requirements are tighter — uneven lips or incorrect relief angle means one side of the drill fails while the other is still working.
If you are running hardened steel work regularly, building a rotation of cobalt drills that cycle through MachinistPost's resharpen program keeps your tooling cost predictable and your geometry consistent. You always know what you are putting in the machine, which matters when the margin between a successful hole and a broken drill is narrow.