Heat Damage in HSS Drills: How to Spot Annealing Before It Ruins the Hole

HSS tools get their hardness from a heat treatment process — they are hardened by heating to around 2200°F and then oil or air quenched. That hardness is not permanent. If the drill reaches roughly 1000–1100°F during cutting or grinding, the temper starts to release and the steel begins to anneal (soften). An annealed HSS drill is softer than the workpiece material it was designed to cut.

The problem is that a lightly annealed drill does not look visibly destroyed. It has discoloration, but it still looks like a drill. It still enters the hole. It just cuts poorly for a few passes and then gives up entirely — and the machinist blames the material or the setup instead of recognizing the drill was compromised before it touched the part.

The Tempering Color Scale

When steel is heated in air, it forms an oxide layer that changes color predictably with temperature. These are the tempering colors for steel, and they are your visual guide to heat damage in HSS drills:

These colors appear on the flute surfaces, near the cutting lips, and around the web. A drill showing blue or gray coloring near the cutting edge should be pulled immediately — it will not recover and will cause quality problems if you continue using it.

Causes of Heat Damage

Speed too high

The most common cause. Running an HSS drill above its recommended surface speed for the material generates heat faster than coolant can remove it. The edge temperature climbs until the temper releases. Always start with speeds at or below the recommended SFM for the material, especially in stainless, titanium, and hard alloys.

Dry cutting in materials that need lubrication

Aluminum, steel, and most alloys need cutting fluid when drilled with HSS. Without lubrication, friction at the cutting lips and along the flute walls generates heat that the chip cannot carry away fast enough. The result is progressively increasing edge temperature with each hole.

Dull drill running past its service point

A dull drill requires more force to cut. More force = more friction = more heat. A drill that is past its service life generates significantly more heat per hole than a sharp one. This is why letting a drill run until it stops cutting is worse than resharpening it at the right interval — by the time it "stops cutting" it has been anneal-damaged for the last several holes.

Grinding damage

Resharpening on a dry bench grinder without sufficient cooling is a major source of heat damage. The grinding wheel removes material rapidly and generates intense localized heat. If the operator does not use coolant or frequent dipping in water, the cutting lips can reach annealing temperature before any visible color change is apparent on the bulk body of the drill. A properly resharpened drill should show no more than light straw color at the cutting lips after grinding.

How to Check for Annealing in the Field

If you suspect a drill is heat-damaged but it does not show obvious color, there is a quick test: run it lightly against a file. An HSS drill at proper hardness (60–65 HRC) will not be scratched by a standard metalworking file — the file skates across the surface. An annealed drill (below ~50 HRC) will show scratches from the file. This is not a precision test, but it reliably catches drills that have lost enough hardness to be useless for production cutting.

Can a Heat-Damaged Drill Be Salvaged?

It depends on where the damage is and how deep it penetrates:

• Brown color at cutting lips only, from cutting (not grinding): Resharpen. Remove the damaged lip material by grinding back past the discoloration. The bulk drill body is fine.
• Blue or gray at cutting lips from grinding: Resharpen past the damaged zone. Test with file after — if the new edge passes the file test, it is usable.
• Blue or gray extending deep into the flute or body: The annealing has progressed too far for surface grinding to fix. The drill should be retired — it will never hold an edge properly.
• File test shows softening throughout: Retire. This drill cannot be resharpened back to usefulness.