Drilling Through Mill Scale and Weld Beads: What Changes and What Doesn't

What Mill Scale Is and Why It Matters for Drilling

Mill scale is the iron oxide layer that forms on hot-rolled steel during the rolling and cooling process. It's that dark blue-gray flaky surface you see on A36, A572, hot-rolled tube, angle iron, and structural sections — anything produced by hot rolling rather than cold rolling or turning.

Mill scale is significantly harder than the base steel underneath. While A36 mild steel runs around 120–150 HBW, mill scale can reach 500–600 HBW in dense areas — harder than some hardened tool steels. It's also brittle, abrasive, and uneven in thickness. The scale layer is typically 0.003" to 0.015" thick, but at welded areas, flame-cut edges, or areas that have been reheated, scale can build up much thicker.

For a drill bit, the first entry through mill scale is always the hardest cut of the entire hole. The scale abrades the cutting edge before the drill is into the softer base material below. If you're drilling dozens of holes in hot-rolled material per day, scale entry damage accumulates and sharpening intervals shorten compared to the same hole count in turned or cold-rolled bar.

What Changes at the Entry Point

The practical effect of mill scale on drill life depends on how much scale surface area the drill contacts at entry, and how the drill transitions from scale to base metal.

A center-punched hole gives the chisel edge a clean start through a specific point in the scale layer. Without a center punch, the drill may wander as it contacts scale (which is hard and abrasive) before engaging the softer base metal, generating lateral forces that work the cutting edge sideways.

For drills entering perpendicular to a flat hot-rolled surface, the scale impact is primarily on the chisel edge and the very tip of the cutting lips. Once through the scale, the drill is in mild steel that cuts easily. The concern is the entry, not the bulk of the hole.

For drills entering at an angle to a hot-rolled surface, the scale exposure is asymmetric — one side of the drill contacts scale while the other is already in base metal. This creates uneven loading that accelerates asymmetric wear and can cause drill wander or oversized holes.

Practical Approaches to Reduce Scale Damage

Center punch every hole. This is non-negotiable on mill-scale surfaces. A center punch breaks through the scale at a specific point and gives the chisel edge a softer starting zone. It also eliminates drill wander at entry, which is the primary cause of asymmetric cutting lip wear on scaled surfaces.

Use a spotting drill first on critical holes. A short, rigid spotting drill (90° or 120° point angle) breaks through the scale at the intended entry point and creates a precisely located conical seat. The production drill then enters the spotting chamfer rather than the scale surface directly. This adds an operation but extends the life of your production drills significantly on high-scale content jobs.

Reduce feed slightly at entry. Don't use your full programmed or manual feed rate for the first 0.020"–0.030" of depth on scaled surfaces. Let the drill establish a seat in the material before loading the chips fully. Once through the scale layer, return to normal feed.

Use cobalt (M42) on high-production structural work. The improved red hardness of M42 doesn't make scale less abrasive, but the edge holds up better to the abrasive wear of scale entry over many holes. If you're drilling hundreds of holes per day in hot-rolled material, the incremental cost of M42 over M2 is returned in longer intervals between resharpening.

Weld Beads: A Different Challenge

Weld material is structurally and chemically different from both the base metal and mill scale. The composition depends on the filler metal (ER70S-6, E7018, 309L stainless filler, etc.), the dilution ratio with the base metal, and the cooling rate after welding. Weld material that cools quickly — thin sections, high interpass temperatures — can have significantly different microstructure and hardness than the base metal.

The surface of a weld bead also has a flux oxide layer (on shielded metal arc), spatter (on MIG), or oxidation layer (on TIG) that behaves similarly to mill scale — hard, brittle, and abrasive. Below the surface oxide is the weld metal itself, which in carbon steel structural welds typically runs 140–200 HBW — softer than scale but harder than the base A36 structure.

The real drilling challenge with welds comes when the drill path crosses a weld bead rather than drilling straight through it. A drill entering hot-rolled plate that has a weld bead crossing the intended hole path will encounter three distinct hardness zones: base metal → weld heat-affected zone → weld bead → weld HAZ → base metal. The HAZ can be harder than either the base metal or the weld bead depending on the preheat and cooling rate.

Drilling Through Weld Crosses

The most common scenario is drilling through a plate that has weld beads on the surface — typically a fillet weld attaching gussets, stiffeners, or bracing to a structural member. The drill enters the base plate, then hits the weld bead at depth.

For this situation:

Mark the hole before welding when possible. If you can plan the hole location to miss the weld bead entirely, you avoid the problem. This isn't always possible in structural fab, but in custom one-off work it's worth a few minutes of planning.

Use higher feed when crossing the weld. This is counterintuitive but correct: when the drill hits weld material, the tendency is to slow down. But slower feed increases rubbing time in hard material and can work-harden the HAZ ahead of the drill. Maintain feed through the transition. If the drill is sharp and the setup is rigid, feeding through weld material produces better results than backing off.

Expect more frequent resharpening on weld-heavy work. Drills that regularly cross weld beads or enter through significant surface contamination wear faster than drills in clean base metal. Your holes-per-resharpen number for weld-crossing jobs will be 30–50% lower than for base metal drilling in the same material at the same size. Track this separately if you do a lot of weld-heavy structural work.

Grind the surface where possible. If the weld bead is on the entry surface and you're drilling through it, a few seconds of grinding with a flap disc to knock down the bead surface before drilling reduces the scale and hard-surface exposure significantly. This is standard practice on high-precision work and worthwhile on any job where drill life matters.

When to Resharpen vs. When to Push Through

After extensive scale or weld work, check edge condition with a loupe before the next job rather than waiting for the scheduled audit. The concentrated abrasive exposure of scale entry can round a cutting edge faster than the same number of holes in clean material. A drill that passes a Friday audit may have been run through 40 scale entries on Monday and need resharpening before Tuesday's job.

The visual sign of scale damage is typically edge rounding concentrated at the outer corner and tip of the cutting lip, with the rest of the edge in reasonable condition. This is different from uniform wear across the cutting lip from normal steel drilling. Scale-damaged drills benefit from a light regrind that focuses on the outer edge while preserving length — a full-depth regrind wastes material on edges that aren't worn.