Hammer mode, which shatters carbide instantly Cobalt bits marketed as suitable — they are for stainless, not for hardened steel Annealing a spring, a fastener or a structural part whose hardness is doing a job

Which drill bit will go through hardened steel?

Solid carbide in a rigid setup, or soften the spot first — HSS and cobalt cannot cut anything much above 50 HRC.

A cutting tool has to be substantially harder than the material it cuts. HSS sits around 62-65 HRC and cobalt HSS a little above, but they both lose hardness as they heat, so against a component hardened to 55-60 HRC they blunt almost instantly. Tungsten carbide stays hard to far higher temperatures and can cut it — at the cost of being brittle, so any wobble, any hand-held wander, and the tip chips. Rigidity therefore matters as much as the bit: clamp the work, use a press or a stable guide, run slowly, and use steady pressure without hammering. If the part's heat treatment does not matter, annealing the spot is far easier than fighting it.

The options, ranked

Ranked best-first for the stated job. The third column is why the others exist.
Option	Good at	Bad at
Solid carbide bit	Hard enough to cut a hardened bolt, a file or a lock shackle where nothing else will.	Brittle to the point of snapping under any side load, so it needs a drill press and a clamped workpiece.
Carbide-tipped masonry bit, reground	A well-known field improvisation: the carbide tip cuts, once ground to a sharper cutting angle.	Crude, slow, leaves a rough hole and needs a bench grinder and patience.
Annealing the spot	Heating the area red and letting it cool slowly softens it so ordinary bits work again.	Destroys the heat treatment of the part, which may be exactly what the part relies on.
Abrasive or spark erosion	A carbide burr, a die grinder or professional EDM will remove hardened material without a drill.	Slow, or requires equipment most people do not have.

Technique that changes the outcome

Clamp the work and use a drill press or a solid guide; hand drilling snaps carbide
Run slow — a few hundred rpm — with firm continuous pressure
Cool with cutting fluid to protect the bit rather than the workpiece
Start with a carbide spotting or centre drill so there is no chance of wandering

What goes wrong

Hammer mode, which shatters carbide instantly
Cobalt bits marketed as suitable — they are for stainless, not for hardened steel
Annealing a spring, a fastener or a structural part whose hardness is doing a job

Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.

Other drill bit questions

For	Use
Tile	A spear-point carbide or diamond bit in rotary mode only — hammer action shatters tile, every time.
Metal	An HSS twist drill, ideally with a 135° split point, run slowly with firm pressure and cutting fluid.
Concrete	An SDS-plus carbide bit in a rotary hammer — a normal hammer drill will get there eventually and blunt several bits doing it.
Masonry	A carbide-tipped masonry bit in hammer mode for brick and block; step up to an SDS rotary hammer above about 10 mm.
Porcelain tile	A diamond core bit, cooled with water, at low speed — carbide spear points do not cut porcelain.
Stainless steel	A cobalt bit at low speed with heavy constant pressure and cutting fluid — and never let it stop cutting.
Wood	A brad point bit for clean holes up to about 13 mm; an auger for depth and a Forstner for a flat bottom.
Plasterboard (drywall)	Almost none — an ordinary HSS or brad point bit in rotary mode, and the real question is which fixing goes in the hole.
Glass	A spear-point carbide or a diamond bit, run slowly under water — and toughened glass cannot be drilled at all.
Plastic	An HSS bit ground to a shallower rake, or a plastic-specific bit, at low speed with a backing board — acrylic especially will crack.
Aluminium	A sharp HSS bit at high speed with a wax or paraffin lubricant — the problem is aluminium sticking to the bit, not hardness.
Large holes in wood	Spade for speed and roughness, auger for depth, Forstner for a clean flat bottom, hole saw for big diameters in thin stock.

Common questions

Which drill bit will go through hardened steel?: Solid carbide in a rigid setup, or soften the spot first — HSS and cobalt cannot cut anything much above 50 HRC.
What should I avoid?: Hammer mode, which shatters carbide instantly Cobalt bits marketed as suitable — they are for stainless, not for hardened steel Annealing a spring, a fastener or a structural part whose hardness is doing a job

Which drill bit should I use for tile? A spear-point carbide or diamond bit in rotary mode only — hammer action shatters tile, every time.
Which drill bit should I use for metal? An HSS twist drill, ideally with a 135° split point, run slowly with firm pressure and cutting fluid.
Which drill bit should I use for concrete? An SDS-plus carbide bit in a rotary hammer — a normal hammer drill will get there eventually and blunt several bits doing it.
Which drill bit should I use for masonry? A carbide-tipped masonry bit in hammer mode for brick and block; step up to an SDS rotary hammer above about 10 mm.
Which drill bit should I use for porcelain tile? A diamond core bit, cooled with water, at low speed — carbide spear points do not cut porcelain.
Which drill bit should I use for stainless steel? A cobalt bit at low speed with heavy constant pressure and cutting fluid — and never let it stop cutting.
Which drill bit should I use for wood? A brad point bit for clean holes up to about 13 mm; an auger for depth and a Forstner for a flat bottom.
Which drill bit should I use for plasterboard? Almost none — an ordinary HSS or brad point bit in rotary mode, and the real question is which fixing goes in the hole.
Which drill bit should I use for glass? A spear-point carbide or a diamond bit, run slowly under water — and toughened glass cannot be drilled at all.
Which drill bit should I use for plastic? An HSS bit ground to a shallower rake, or a plastic-specific bit, at low speed with a backing board — acrylic especially will crack.

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked