HSS vs Cobalt: When to Upgrade Your Drill Material

What the M-Numbers Actually Mean

HSS drill materials are classified by AISI/SAE M-series designations. The letter and number describe the alloy composition, and the composition determines the performance envelope. Three grades dominate the drill market: M2, M35, and M42.

M2 is the baseline — the grade most standard black-oxide and bright-finished drills are made from. It contains roughly 6% tungsten, 5% molybdenum, 2% vanadium, and 4% chromium, with carbon around 0.85%. Hardness runs 62 to 65 HRC after proper heat treatment. M2 handles the vast majority of drilling applications in mild and medium-carbon steels, aluminum, brass, and plastics. It's the default for good reason.

M35 adds approximately 5% cobalt to the M2 base. Cobalt is a solid-solution strengthener — it doesn't form carbides the way tungsten and molybdenum do, but it stabilizes the matrix at elevated temperatures. The practical result: M35 holds its hardness better as cutting temperatures rise. Where M2 might start to lose hardness above 1000°F, M35 retains useful cutting ability to 1100°F or beyond. Hardness after heat treat runs 65 to 67 HRC — marginally harder than M2 but much more thermally stable.

M42 pushes cobalt content to 8%, paired with higher carbon and slight adjustments to the carbide-forming elements. Hardness can reach 68 to 70 HRC. M42 is the premium grade for the most demanding applications — nickel superalloys, titanium, hardened steels, and anything else that generates severe cutting temperatures. It's also more brittle than M2 or M35, which matters in interrupted cuts and whenever the setup isn't rock-solid.

Where the Performance Difference Shows Up

In mild steel at normal speeds with adequate coolant, M2, M35, and M42 drills perform nearly identically. The alloy difference doesn't matter when temperatures stay low. This is why many shops run standard M2 for the bulk of their work and never feel the need to upgrade — the application doesn't stress M2 to its limits.

The performance gap opens when heat builds. This happens in several specific situations:

• High-SFM cutting in tough materials. Running stainless steel, tool steel, or alloy steel at aggressive speeds generates cutting temperatures that challenge M2. M35 and M42 hold edge hardness longer under those conditions.
• Interrupted cuts. Drilling through castings with hard spots, through-and-through interrupted cuts, or any application where the drill impacts a hard inclusion repeatedly. Cobalt grades handle thermal shock better.
• Dry or near-dry drilling. Without coolant to carry heat away, cutting temperatures climb fast. Cobalt grades are the practical choice when coolant application is limited.
• Hard materials. Anything above 30 HRC starts to favor cobalt grades. Above 40 HRC, M42 is usually the minimum specification — below that, you're replacing drills too frequently to justify the cost savings on cheaper material.

The upgrade to cobalt makes sense when you're running a material or condition where M2 drills are showing heat-related failure — blue chips, burned margins, rapid dulling without mechanical damage. That's the signal that thermal capacity is the limiting factor, and cobalt addresses exactly that.

Cost-Per-Hole Analysis: When Cobalt Pays Back

M35 cobalt drills typically cost 40 to 60 percent more than equivalent M2 drills. M42 can cost double or more. The payback calculation is about hole count per drill edge.

In a challenging material where M2 produces 150 holes per regrind and M35 produces 300, the cobalt drill is cost-neutral at 2x price and cost-advantageous at anything less. Factor in regrind cost, machine downtime for drill changes, and scrap risk from broken drills, and cobalt's true cost advantage over M2 in appropriate applications is often 30 to 50 percent lower total cost per hole despite the higher purchase price.

The mistake shops make is upgrading to cobalt across the board without doing the analysis. In mild steel at normal speeds, cobalt drills last about as long as M2 and cost more — the application doesn't stress the material to its limits, so you're paying for performance you're not using. Buy cobalt for the applications that need it. Keep M2 for everything else.

One additional consideration: cobalt drills recondition well. A well-maintained M35 drill regrinding on a proper CNC grinder comes back to full performance spec. The cobalt is distributed through the entire drill cross-section — you're not grinding through a coating. Every regrind exposes fresh, fully-alloyed material. This means the total life of a cobalt drill, measured across its full reconditioning cycle, is substantially longer than the first-use cost comparison suggests.