What Is a Cobalt Drill Bit? | Hard-Metal Power

A cobalt drill bit is a high-speed steel bit with cobalt mixed into the steel itself, giving it the heat resistance needed to cut hard metals like stainless steel.

When stainless steel or titanium laughs at a standard bit, the tool you reach for is probably cobalt. These bits are high-speed steel (HSS) with cobalt alloyed into the metal itself — not coated on top — which lets them hold a cutting edge at temperatures that would soften ordinary HSS. Drill bits made with cobalt are built for tough, heat-generating jobs in hard metals, and they behave differently from the bits you use for wood or soft steel. Knowing what makes them different, and where they shine, is the difference between a clean hole and a snapped bit.

How Cobalt Changes the Steel

Cobalt drill bits start as high-speed steel, then get a dose of cobalt mixed into the alloy. Common grades are M35 with about 5% cobalt and M42 with about 8%. Higher cobalt content generally means better heat resistance, which is the whole point of these bits.

The cobalt raises the steel’s “hot hardness” — its ability to stay hard when friction pushes temperatures up. That property is exactly what you need when you’re cutting stainless steel, cast iron, titanium, or Inconel. A standard HSS bit loses its temper as it heats up; a cobalt bit keeps cutting.

What Cobalt Bits Are Actually For

Cobalt drill bits earn their keep on hard metals where heat buildup is the main enemy. Stainless steel is the classic target. Cast iron, titanium, and the nickel-based superalloys like Inconel also fall squarely in their wheelhouse. The heat resistance keeps the edge alive through sustained cutting, so they’re the right pick for any job that makes a standard bit glow.

That does not make them a universal substitute. For wood, plastic, or soft metals, a cheaper HSS or purpose-made bit does the job. Using a cobalt bit where it’s not needed just spends more money and risks a brittle break on a task that didn’t demand the extra hardness.

The Common Mistakes That Break Cobalt Bits

Three errors account for most ruined cobalt bits. The first is confusing alloy bits with “cobalt-coated” ones. True cobalt bits have the element throughout the steel, so the benefit lasts the life of the bit. A coated version loses its surface layer with wear or resharpening, leaving you with plain HSS underneath.

The second mistake is leaning on the bit. Cobalt bits are more brittle than standard HSS, so forcing them with excessive pressure raises the chance of chipping or snapping. Heat resistance does not mean heat-proof; proper feed, speed, and lubrication still matter.

The third is using cobalt where it’s overkill. If you’re drilling soft material, you’re paying extra for hardness you don’t need and accepting brittleness that can bite back. Match the bit to the workpiece and you get clean holes without the breakage risk. If you’re deciding which set to put in your box, our tested roundup of the best cobalt drill bits for metal compares the top options for hard-metal work.