Cobalt Drill Bits for Hardened Steel: Check the Hardness First

Cobalt drill bits can drill some hardened steels, but “cobalt” alone is not a sufficient buying specification. Choose by the workpiece’s hardness, the exact drill’s application data and the equipment needed to run it—not simply by the cobalt percentage.

A useful example is Guhring’s AeroX series 1018: an M42 cobalt-alloyed high-speed steel drill with a 135° split point. Guhring rates hardened steel as a secondary application, rather than optimal. Its linked speed chart gives settings for hardened steels at or below 48 HRC, but leaves the harder, under-66-HRC row blank. That supports a limited application, not permission to drill every hardened part. Nor is 48 HRC a universal limit for all cobalt drills. See the Guhring product specifications and speed/feed chart.

First check the steel—not the bit’s hardness

Ask for the steel grade, heat-treatment condition and workpiece hardness, preferably expressed in HRC: Rockwell C hardness. A listing describing only “hard metal” or “hardened steel” leaves too much unresolved.

Also distinguish a hardness figure for the drill itself from a rating for the steel it can drill. Look for an application statement or cutting-data table explicitly identifying workpiece hardness.

What you know	Buying decision
The steel falls within the exact cobalt drill’s published application range	Cobalt is a candidate; check the required setup and cutting data.
The part is harder than that range	Choose a drill explicitly rated for that hardness, rather than another generic cobalt set.
The hardness or heat treatment is unknown	Identify the material or ask the tool manufacturer before buying a full set.

For comparison, Champion’s CT605 carbide-tipped die drill is explicitly intended for precision drilling of hardened steel and armor plate at 48–65 HRC. Its instructions call for constant coolant flow, frequent chip clearing and light feed with steady pressure. That is a different tool and operating method—not merely a cobalt upgrade. Champion CT605 specifications.

These examples are specification-based comparisons, not a hands-on product ranking. For the harder-material route, see our guide to carbide drills rated for hardened steel.

Does more cobalt make the bit suitable?

Cobalt bits are cobalt-alloyed HSS, not simply ordinary bits with a cobalt surface coating. RUKO describes its 8% cobalt drills as harder and more heat-resistant than its 5% versions. Norseman identifies its Type 150 as M42 HSS containing 8% cobalt, with improved hardness retention at high temperature. RUKO material guide and Norseman Type 150 specifications.

That makes 8% cobalt worth considering for demanding steel work, provided the exact drill is rated for the workpiece. It does not establish a universal hardened-steel capability.

Before ordering, check:

• Application: Does the manufacturer name the steel condition or hardness range?
• Machine requirements: Can your equipment hold alignment and deliver controlled feed? Norseman’s Type 150 specifications require rigidly held, positive-feed drilling equipment; do not assume every cobalt drill is intended for handheld use.
• Dimensions: Verify cutting diameter, shank fit and usable drilling depth. Flute length alone is not necessarily the permitted hole depth; Guhring publishes a separate maximum-depth dimension for series 1018. Guhring dimensional catalog.

For one repair hole, a correctly specified single bit is a more focused purchase than a large set with vague material claims.

Use the exact cutting data—not one “slow” RPM

Speed depends on both the workpiece and bit diameter. Guhring’s series-1018 chart gives 15 surface feet per minute (SFM) for its hardened-steel row at or below 48 HRC. Using the formula RPM = 3.8197 × SFM ÷ diameter in inches, that calculates to approximately:

• 1/4-inch bit: 229 RPM
• 1/2-inch bit: 115 RPM

These are calculated examples from that specific chart, not universal cobalt settings. The formula comes from Norseman’s drilling guidance; use your chosen drill’s own speed and feed recommendations.

Clamp the part securely, use the specified lubricant or coolant and keep the cutting edges sharp. Clear deep holes frequently. If the bit is not advancing, stop and reassess the edge, material match and setup rather than adding excessive pressure. Reduce pressure as the drill breaks through. Wear eye protection. At a drill press, do not wear gloves or loose clothing; stop the machine before clearing chips, and use a brush or vacuum rather than your fingers. CCOHS drill-press guidance.