Cobalt Drill Bits for Hardened Steel | M42 Grade Guide

The fix is switching to a cobalt bit — specifically M42 grade — which keeps its edge at temperatures that ruin ordinary drill bits. This guide covers what makes cobalt bits work, which grade to pick, the exact speeds and technique, and the two mistakes that wreck them fastest.

What Makes Cobalt Drill Bits Different from Regular HSS?

Cobalt bits are a subset of High-Speed Steel (HSS), not a separate material. The difference is cobalt added to the alloy — 5% in M35 and 8% in M42 — which boosts red-hardness. That means the bit stays hard even when friction pushes the cutting edge past 1,100°F (593°C), while standard HSS (M2 grade) softens and fails at much lower temperatures.

Regular HSS bits measure about 62 on the Rockwell C scale. M35 cobalt hits roughly 65 HRC, and M42 reaches around 67 HRC. That extra hardness is what lets cobalt bite into hardened steel that eats HSS for breakfast.

M35 or M42 — Which Cobalt Grade Should You Pick?

The choice comes down to the steel you are drilling. M42 with 8% cobalt is the standard for hardened steel up to HRC 50. M35 with 5% cobalt works well for general carbon steel and stainless but lacks the heat tolerance for harder alloys.

For hardened steel at HRC 40–50, reach for M42. For anything above HRC 55, step up to solid carbide — cobalt will struggle and may fracture. For routine jobs on mild steel or stainless, M35 saves money and still outperforms standard HSS. Jinootools’ guide on hardened steel bits walks through the HRC matching process in more detail.

How to Choose and Use Cobalt Bits on Hardened Steel

The procedure starts before the drill spins. Identify the steel’s HRC value first using a hardness chart or the material spec sheet. If it falls between HRC 40 and 50, M42 cobalt is the right call. If it is harder than HRC 55, switch to solid carbide instead.

Look for bits with a 135° split point. That geometry reduces walking when starting a new hole and lets the bit cut faster than the older 118° point, which is designed mainly for following pilot holes. Jobber length is the standard choice for most drilling tasks on hardened steel.

Apply cutting oil before every hole. Cobalt generates serious heat, and cutting oil keeps the edge cool while flushing chips out. Running dry cuts tool life drastically.

The Right RPM and Drilling Technique

Speed is the variable that most often gets wrong. For a 6mm M42 cobalt bit drilling hardened steel, the sweet spot is 500–800 RPM. Running faster than that builds heat faster than the bit can shed it, and the edge breaks down early.

Cobalt is more brittle than standard HSS, so alignment matters. If the bit grabs or the drill wanders, stop and reset. Avoid drilling into intersecting holes or angled surfaces where radial pressure can snap the bit. For handheld drilling or low-rigidity setups, cobalt is the safer choice over carbide, which fractures more easily under vibration.

The a steady stream of small chips, not dust or smoke. Chips mean the bit is cutting cleanly. Dust or discoloration means the speed is too high or the bit has dulled.

Common Mistakes That Kill Cobalt Bits

• Using HSS on hardened steel. It will dull or overheat within seconds. Cobalt or carbide is non-negotiable here.
• Running cobalt at carbide speeds. 800+ RPM on a small M42 bit generates destructive heat. Keep it in the 500–800 range.
• Using a 118° point for new holes. That geometry wanders badly on bare metal. The 135° split point starts clean every time.
• Skipping cutting oil. Dry drilling reduces tool life significantly.
• Using carbide in a hand drill. Carbide is too brittle for handheld work — stick with cobalt for anything that isn’t rigidly mounted.

Top M42 Cobalt Bit Picks for 2026

The market has a handful of reliable options. The table below covers the strongest choices for hardened steel work, from full sets to single standout bits.

If you are deciding between full sets and individual bits, our tested drill bits for hardened steel roundup compares the top options side by side with real shop-floor notes.

For anyone who regularly works with hardened steel, stainless, or high-tensile alloys, that math makes M42 the only sensible choice. Match the grade to the steel, keep the RPM under 800, use cutting oil, and a single bit will last through dozens of tough holes.