Hardened steel — heat-treated tool steel, hardened bolts, springs, bearing races and set screws — defeats standard bits almost instantly. The best drill bits for hardened steel are solid carbide when the metal is very hard, or cobalt for moderately hard work. Getting the choice right depends entirely on how hard the steel actually is.
What to Look For
The deciding factor is the hardness of the workpiece, usually measured on the Rockwell C scale. Below roughly 40 HRC, a quality cobalt bit will still cut with patience and oil. Above that, only carbide stays harder than the steel and can actually shear material away. Solid carbide is extremely hard and holds its edge at high temperature, but it is brittle, so it needs a rigid setup, low speed and zero wobble. Never try to drill fully hardened steel with plain HSS — the bit will simply skate, glaze and burn without ever biting in.
The Best Type of Bit for Hardened Steel
For hard tool steel, choose a solid carbide bit. Carbide is one of the few affordable materials harder than the work itself, so it can cut where cobalt only rubs and polishes. Support the bit well and keep the whole setup rigid to avoid chipping the delicate tip.
- Very hard steel: Solid carbide — harder than the workpiece and heat-stable.
- Moderately hard steel: Cobalt (M42) is tougher and more forgiving than carbide.
- Rigidity: Carbide is brittle — use a press or firm hold and no side load.
- Speed: Low RPM with steady pressure; let the tip shear, not skate.
Technique and Mistakes to Avoid
Clamp the work rock-solid; any flex or chatter shatters a carbide tip in a heartbeat, so a drill press beats a handheld drill every time. Use cutting oil, keep pressure firm and constant, and never peck aggressively or let the bit rattle in the hole. A common mistake is running too fast — high RPM overheats the tip and dulls it. If the steel can be annealed or spot-annealed first with heat, drilling becomes far easier and you may get away with a cobalt bit. Start any large hole with a carbide pilot, then open it up. Our USA-made solid carbide drills are ground for accuracy and edge retention on the hardest jobs.
When to Reach for Carbide-Tipped
Solid carbide is the sharpest, hardest option, but it is also the most fragile and the most costly. If you only face a thin hardened skin over softer steel — a case-hardened part, for example — a carbide-tipped bit can be a smarter buy. The brazed carbide edge punches through the hard surface, while the tougher steel body absorbs shock better than a solid carbide bit would. Match the choice to the depth of hardness, and always keep the setup rigid whichever type you run.