Drilling Hardened Steel: When It's Possible and When You Should EDM Instead

The most common scenario: a part comes in that needs a hole added after heat treatment. Or a shop gets a job on pre-hardened material that wasn't designed with drilling in mind. The question is the same either way — can we drill this, or does it have to go to EDM?

The answer depends on the hardness level, the tooling available, the hole geometry, and the tolerance required. There's a clear line where conventional drilling stops being practical and EDM (electrical discharge machining) becomes the right process. Understanding where that line is prevents a lot of broken tooling and scrapped parts.

The Hardness Zones

Under 35 HRC: Drill Normally

Material in this range — prehardened tool steels like P20, H13 in softer conditions, and normalized 4140 — drills with quality HSS cobalt at reduced speeds. You need M42 cobalt with split-point geometry, slow SFM (30–50 SFM), and good cutting oil. The drills will wear faster than in annealed material, but it's economically viable for job quantities. Prehardened 4140 at 28–32 HRC is the most common case and handles cobalt drilling well.

35–45 HRC: Carbide Territory

In this range, cobalt HSS degrades too quickly to be practical for more than a few holes. Solid carbide drills with TiAlN or AlTiN coatings are the correct tooling. Speeds drop further — 15–30 SFM for solid carbide in hardened tool steel — and the setup rigidity requirements increase significantly. Any vibration or runout causes premature carbide failure.

This range is doable with carbide but demands attention to every setup detail: minimal overhang, maximum rigidity, through-coolant or directed oil, and tooling that is absolutely sharp. Carbide doesn't telegraph wear the way HSS does — it drills fine until it fails suddenly. Set hole count limits per drill and don't exceed them.

45–55 HRC: Getting Difficult

In the 45–55 HRC range, carbide drilling is possible but the economics become questionable for anything but production volumes. Tool life is short, feeds and speeds must be extremely conservative, and setup requirements are demanding. Many shops send this range to EDM or laser drilling rather than fighting carbide wear in material this hard.

If you need to drill 50 HRC D2 tool steel on a job shop knee mill, you're going to break a lot of carbide. The job should probably be quoted with EDM in the process plan, or the customer should be asked whether the part can be machined before heat treat.

Above 55 HRC: EDM

Above 55 HRC, conventional drilling with any tooling is not practical in a job shop context. The material is effectively harder than the drill. EDM (sinker or wire) is the correct process. EDM removes material by electrical discharge regardless of workpiece hardness — the hardness of the material has minimal effect on the process. This is exactly what EDM was developed for.

When EDM Is the Right Answer Below 55 HRC

Hardness isn't the only criterion. EDM is also the correct choice when:

• Tight tolerance on a blind hole: EDM produces extremely consistent hole geometry without the deflection risk that comes from drilling near the hardness limit.
• Small diameter through significant depth: A 0.060" diameter hole through 1" of hardened material is impractical with conventional drilling at almost any hardness. EDM handles this routinely.
• Complex profile: Shaped holes (non-circular) are EDM work, not drilling.
• No ability to anneal and re-harden: Some parts can't be heat-treated twice. If the part is already finished and hardened, EDM avoids any thermal effect on the workpiece that drilling might introduce.

The "Drill Before Heat Treat" Option

The best answer to hardened material drilling is often to avoid the situation entirely. If you or the customer has input on the process sequence, drill all holes before heat treatment. The material in annealed or normalized condition drills easily with standard HSS. After heat treatment, the holes are already there.

Threading is the usual exception — threads may need to be cut after heat treat in some applications. But through holes, clearance holes, and even precision holes are almost always better drilled in the soft condition and cleaned up with a reamer or jig bore afterward if needed for final accuracy.

Surface Hardening vs. Through Hardening

Case-hardened and nitrided parts have a hard surface layer over a softer core. The rules above apply to the surface layer specifically — once through the case, the drill is cutting much softer material. The entry can be the difficult part. A carbide spot drill or a solid carbide drill to get through the case, then possibly a switch to cobalt for the body of the hole, is a practical approach for shallow case depths (under 0.030").

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