Cobalt vs HSS vs Carbide Drill Bits: What Actually Lasts Longer

Drill 100 holes in 304 stainless and the three materials retire in a fixed order, every time. The plain high speed steel bit at 20 m/min work-hardens the hole wall as it slows, grows its own rub, and dies glazed before the edge wears out. The cobalt bit at 28 to 35 m/min holds its edge through more holes, then still dies of heat long before carbide. The carbide drill bit at 80 to 120 m/min finishes the whole count, if the machine and the clamp hold it rigid.

So the honest answer to "what lasts longer" is not a material name. It is a question about the rigidity you bring and the speed you can feed, because each material only outlasts the one below it when its own speed window is reachable. What follows: the published speed bands, the heat physics behind them, and the cost-per-hole math that flips the shelf-price intuition.

The Snapshot

• Relative life in stainless per grind, from published shop practice: cobalt about 2 to 3 times HSS, carbide 10 times HSS or more, but only on a rigid machine at carbide speeds.
• Speed windows in steel-class work: HSS 20 to 30 m/min, cobalt M35/M42 25 to 40, solid carbide 80 to 150 and up, per supplier drilling handbooks.
• Red hardness is the dividing line: HSS softens around 600 °C, cobalt steels a little higher, carbide stays hard past 800 to 900 °C.
• Toughness runs the other way: HSS bends, cobalt bends nearly as well, carbide does not, so shock and runout decide whether the long-life material ever shows its life.
• "Cobalt" on a hardware tin is a steel, not a metal: M35 or M42 high speed steel with 5 to 8 percent cobalt alloyed in.
• Cost per hole, not cost per bit, settles the argument, and hole volume decides who wins it.

The Hardness-Toughness See-Saw

Every drilling material sits on a see-saw between holding an edge and surviving abuse. Two properties set the position: hot hardness at the tip, and strength in bending.

Hot hardness (red hardness). The tip of a drill in stainless runs hot, and an edge that goes soft is a worn edge. Plain high speed steel loses its temper near 600 °C, the red-heat limit printed in every tool-steel handbook. Cobalt-alloyed grades push that ceiling modestly higher, with hot-hardness retention commonly cited around 610 to 640 °C, though the exact figure is grade-dependent. Cemented carbide is a different class of matter: tungsten carbide grains in a cobalt binder stay hard past 800 to 900 °C. That is why carbide drills sit at three times the surface speed and still hold a flank.

Bending strength. High speed steel carries a transverse rupture strength near 3 GPa and yields before it breaks. Carbide is the inverse: huge compressive strength, but transverse rupture depends on grain size and cobalt binder content, with published grades spanning roughly 1.5 to 3 GPa.

Read that second point as a load statement, not a quality statement. Carbide fails when the load spikes: a chatter pass, a snag on breakthrough, a pull-out from a running spindle. The same shock that dents or bends an HSS web snaps a carbide flute, the same mechanics that break carbide end mills in interrupted cuts, worked through in the material selection guide.

What "Cobalt" Actually Is

Before comparing cobalt vs HSS drill bits, settle the name, because it causes most of the confusion in this category. A cobalt drill bit is a high speed steel. The designation is an AISI M-series tool-steel grade, and the numbers describe alloy content, not a coating or a plating.

• M2 is the baseline general-purpose high speed steel: essentially no cobalt, tough, cheap, the stock HSS bit.
• M35 adds about 5 percent cobalt to an M2-like chemistry. That is what "5% cobalt" on a tin means.
• M42 carries about 8 percent cobalt plus more vanadium and is the premium red-hardness jobber grade. Published M42-to-M35 life gains on stainless land around 20 to 50 percent more holes, grade-dependent.

The cobalt does one job: it raises hot hardness, so the edge keeps its temper at the temperatures stainless drilling produces. The price is a small toughness loss and higher cost, a fair trade in continuous holes and a bad one in a drop-forge job. Nobody makes a drill from solid metallic cobalt, so the buying-lie check is simple. "Cobalt" on a big-box tin means M35 or M42 steel, and anything truly cemented-carbide is labeled carbide.

That answers carbide vs cobalt drill bit questions cleanly. The cobalt option is a better steel, the carbide option is a different material class, and the gap between them is wider than the gap between M2 and M42.

Speed Windows and Heat

The see-saw becomes practical once you read it as speed. Edge heat climbs with surface speed, and each material has a published window where the edge stays hard enough to be worth feeding. Supplier-handbook bands for twist drilling:

• HSS in mild steel: 20 to 30 m/min, the old 60 to 100 SFM rule of thumb.
• Cobalt (M35/M42) in austenitic stainless: 25 to 40 m/min, most shops run 28 to 35.
• Solid carbide in steel-class materials: 80 to 150 m/min and above, per vendor tables for through-coolant geometry.

The dangerous instinct is treating a window as a ceiling. The speed and feed calculator prints the same warning from the other direction: tool life follows a Taylor-type exponent near n = 0.1 to 0.25 in drilling, so doubling the speed takes life down to roughly one quarter to one third. Call it the one-third rule. Run any bit at twice the published band and it earns a third of the holes while spending its life making heat, not chips.

The reverse trap is worse in stainless. Cobalt at HSS speed is not "gentle", it is a rubbing regime, and rubbing builds the work-hardened skin that kills the next hole. This is also why an HSS drill bit for aluminum works fine while the same bit dies young in stainless: aluminum carries heat away with the chip at 10 times the steel removal rate, so the mild-steel window protects the edge there and punishes it in stainless.

Life Under the Same Hole Count

Take the anchor from the opening: a 100-hole duty in 304 stainless, 10 mm through. Published shop experience, not invented test data, plays it out roughly like this.

• HSS (M2): one set of holes at best, cut slower and slower as the flank wears and the wall hardens. To the grinder, and the geometry comes back a little weaker each cycle.
• Cobalt (M35/M42): about 2 to 3 times the HSS holes per grind, holding its edge through the hotter cutting, then dying of heat anyway, usually with the tip discoloring forward.
• Solid carbide: 10 times HSS and more on a rigid setup, often the whole count plus hundreds behind it, and it keeps its geometry between grinds.

That ordering is why drill bit material for stainless steel questions all converge on the same short answer: never plain HSS, cobalt if the machine is modest, carbide if the machine is rigid and the hole count is real. The bands compress in mild steel, where good feed practice lets an HSS bit do respectable duty, and stretch in titanium, where only the top of the ladder survives long enough to matter. The best drill bits for stainless steel are the ones run inside their window, so if you cannot hold 28 m/min or better, a sharper strategy beats a tougher tin.

Rigidity, Geometry and the Machine You Have

The life bands above assume each material gets its own speed, and that assumption breaks on real equipment. On a hobby drill press, a cordless in a two-jaw chuck, or any setup with visible wobble, the carbide option cannot reach its window and dies of the loads it was never meant to take. Runout at the flank is impact at the cutting edge, hundreds of times a revolution. On those machines the shock-tolerant steel wins, and the question becomes what lasts long enough at the speed you can actually hold.

• Hand tools and light drill presses: HSS and cobalt. Cobalt in stainless with slow speed, firm feed and a pilot hole; web-thinning or self-piercing points help any steel bit bite without walking.
• Rigid mill or drilling head with peck capability: solid carbide wins decisively, because the speed advantage shrinks cycle time as much as it stretches tool life.
• Runout threshold: carbide drills want tooling in the 0.01 to 0.02 mm class, hydraulic or shrink-fit holders, per supplier installation notes. That class of bit in a bent chuck produces a chipped margin, not a slow, honest wear land.

Geometry follows the machine too. Split-point and self-piercing grinds cut thrust and walking on manual setups, helix and point angle tune chip flow in stainless, and through-coolant points matter for carbide because it clears chips at feeds flood coolant alone cannot keep up with. Choose the bit for the stiffest system you actually own, not the one the datasheet imagines.

Cost per Hole Math

Shelf price is the wrong denominator. The right one is holes: bit price plus regrinds, divided by total holes retired at that setup.

Worked example at 10 mm jobber size in stainless, illustrative prices, published-experience life bands. An HSS bit at about $4 is good for roughly 100 holes per grind and can take 2 to 4 regrinds. A cobalt M35 at about $8 holds 250 to 300 holes per grind. A solid carbide bit at $45 to $70 is scrap-not-regrind in most shops, since reconditioning carbide only pays at scale, runs 2,000 to 3,000 holes before it is pulled, and cuts them at a fraction of the cycle time.

• HSS: about $0.01 to $0.02 per hole before grinding labor, plus a stoppage every hundred holes.
• Cobalt: roughly the same per hole, half the stoppages.
• Carbide on a rigid machine: $0.015 to $0.035 per hole with zero mid-run changes, and the spindle is cutting, not sitting.

The production math flips the shelf-price intuition. Per hole, all three land in the same few cents, so the decision moves to change-out labor, spindle occupancy and hole-to-hole size consistency, where carbide is structurally ahead. HSS stays the right call only at the bottom of the volume curve, or on the machine that cannot run anything else. The same logic ranks high speed steel end mills and HSS turning tools against their carbide equivalents: the steel version is a low-volume insurance policy, not a productivity choice.

The Side-by-Side Table

Six jobs people actually ask about: the honest pick, the published speed band, what kills the bit, and when to retire it.

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Frequently Asked Questions

Q1. Are cobalt drill bits worth it over plain HSS?

In stainless and any material that runs the edge hot, yes: about 2 to 3 times the holes per grind for a small price step. In mild steel at correct speed, plain HSS keeps up and the cobalt premium buys little. The cobalt vs HSS question is really a "how hot does the hole run" question.

Q2. What drill bit is best for hardened steel above 45 HRC?

Honest answer: none of the three drills it well. Standard geometry shatters or anneals on contact. The working options are specialty carbide pilot-and-bore tooling at very low speed, or designing around the hardness with thread-milling or drilling before heat treatment.

Q3. Do carbide drill bits break easily?

Not if the system is rigid, and predictably if it is not. Carbide tolerates steady load at high speed and fails under shock, runout and pull-out, the mirror image of HSS.

Q4. Can I drill stainless steel with a cordless drill?

Yes, with expectations set: a cobalt bit, a center-punch or pilot start, low steady speed and firm feed, because a slowing bit work-hardens the wall and seals its own fate. Torque sag makes carbide a waste there, and best drill bits for stainless steel threads on the shop forums almost always end with someone describing exactly that mistake.

Q5. So what are the drill bits for stainless steel I should actually buy?

Cobalt on manual equipment, solid carbide on a rigid machine, plain HSS only when you accept grinding often. The machine decides, not the material shelf.

Q6. What are the best drill bits for metal in a general shop kit?

A mixed kit: cobalt jobbers for hand tools and odd stainless jobs, HSS for soft steel and low-volume aluminum, and a short solid-carbide set for the one machine rigid enough to feed them. The full drilling range covers the carbide picks at common sizes.

Q7. Is an HSS drill bit for aluminum a problem?

No. Aluminum is the easy case: sharp point, generous clearance, little heat to fight, and plain HSS cuts it all day.

The Bottom Line

Cobalt beats HSS because it holds hardness a little hotter, and carbide beats both because it holds hardness much hotter and abrades slower doing it. Each win lives inside its own speed window, and each window exists only on a machine rigid enough to reach it. On the weak setup, cobalt in stainless is the longest-lasting bit; on the rigid one, carbide is, by an order of magnitude; on mild steel at low volume, plain HSS still earns its keep.

InsertCore quotes solid-carbide drill geometry against your machine's rigidity, runout and peck capability, with the published speed bands attached to every recommendation, and says so when the honest answer is a cobalt bit.