Abrasion-resistant steels — sold under names like AR400, AR500, Hardox 400, Hardox 500, Creusabro, Bruxite, and others — are quenched and tempered steels designed to resist wear from impact, abrasion, and sliding contact. They're used in dump truck beds, bucket lips, wear liners, chutes, crusher plates, and structural applications where standard mild steel would grind away in service.
The number in the designation refers to Brinell hardness. AR400 runs 370–430 HBW. AR500 runs 470–540 HBW. By comparison, mild steel (A36) runs around 120 HBW and hot-rolled structural plate sits around 150–180 HBW. Hardox grades from SSAB are similar — Hardox 400 and Hardox 500 overlap with their generic AR counterparts in hardness.
For drilling, hardness is the primary challenge. The cutting edge of a standard HSS drill bites into mild steel comfortably at 70–90 SFM. In AR400 at proper speeds, the drill is working against material that work-hardens under the chisel edge and fatigues the cutting lips faster. In AR500, it's harder still — and the window between "cutting" and "rubbing until the drill burns" is narrower.
Solid carbide would theoretically outperform HSS in hard materials. And it does — until the workpiece moves, the fixturing deflects, or the drill catches on a burr. In the field — on structural fab floors, in maintenance shops, at equipment dealers welding up bucket teeth — rigidity is limited. Solid carbide doesn't tolerate flex. One instance of the part shifting or the drill loading up and you have a $25 broken carbide drill.
HSS, especially M42 cobalt grade, is tough. It flexes without fracturing. It can handle a momentary overload that would shatter carbide. For out-of-position work, field repair, and fab shops that can't control every variable in the setup, M42 cobalt HSS is the practical answer for wear plate drilling.
It also resharpens. A cobalt drill that's dull but not damaged comes back to full cutting efficiency with a proper regrind. The geometry that matters — point angle, lip relief, web thickness — can all be restored. You're not throwing away the entire tool, just renewing the cutting edge.
Standard 118° conventional point geometry isn't optimal for hard materials. Here's what works better:
135° point angle. The flatter geometry reduces chisel edge thrust, which matters more in hard material where the chisel is pushing against real resistance rather than scrubbing through soft steel. The shallower attack also distributes cutting force more evenly across both lips, which extends edge life.
Split point or thinned web. In hard materials, anything that reduces chisel edge contact is a win. Split-point geometry nearly eliminates the chisel edge and drops the axial thrust required to start cutting. Less chisel pressure means less work-hardening under the point before the cutting lips engage.
Increased lip relief angle. At low speeds in hard material, the drill needs clearance behind the cutting edge so the flank doesn't rub. Rubbing in hard materials generates heat and work-hardens the surface — a death spiral. Additional relief keeps the flank clear of the freshly machined surface.
No rake reduction for hard materials on HSS. Unlike carbide, which benefits from a negative rake preparation to protect the edge, HSS in hard steels works best with a neutral to slightly positive rake. Negative rake on HSS increases cutting forces and generates more heat, which is already your limiting factor.
When sending AR-plate drills out for resharpening, specify the point angle (135° preferred), mention the material (AR400 or AR500), and ask for split-point geometry if the drill diameter is ¼" or larger. A good resharpening service can optimize the grind for the application.
Speed is the single biggest controllable variable in wear plate drilling life. Going too fast generates heat that softens the HSS cutting edge and accelerates wear. Going too slow generates rubbing, which work-hardens the surface under the chisel and makes subsequent passes harder.
A practical starting point for HSS in AR400:
Surface speed: 25–40 SFM. This is significantly slower than mild steel drilling (70–100 SFM). AR400 wants slow, deliberate cutting.
Feed rate: Maintain consistent chip load. The drill should be feeding, not rubbing. If you can't feel chips forming or see recognizable chip curls, you're rubbing — increase feed or decrease speed.
For AR500, drop another 20–30% from the AR400 numbers. You're in territory where even cobalt HSS is near its thermal limit, and the margin for error is smaller.
Coolant helps, but direct flood coolant at low speeds in hard materials can cause thermal shock to the cutting edge — particularly if coolant application is inconsistent. Consistent flood or cutting oil applied at the entry zone is better than intermittent flooding. Cutting oil (not water-soluble coolant) tends to perform better in hard steel drilling at low speeds because of its film strength.
In hard materials at proper diameters, pilot holes can help or hurt depending on the situation.
When pilots help: Large diameters (above ½") in AR400 benefit from a pilot hole that reduces the area the chisel edge must push through on the final drill. Starting with a ¼" pilot and stepping up to final diameter distributes the aggressive work across two tools and reduces the total thrust required in the first pass.
When pilots don't help: In thin plate (under ½" thick), a pilot hole often reduces the area the final drill needs to remove, but it removes the benefit of the cutting lips working through full-thickness material. In thin plate, going directly to final diameter with the right geometry frequently works just as well and saves the pilot operation.
Diameter steps: Never jump more than 2:1 in diameter between passes in hard material. Going from ¼" to ¾" in one step will shock-load the larger drill entering a pre-drilled hole at an angle. Step through ⅜" or ½" first.
AR plate drills wear fast compared to mild steel work — plan for it. On a production run through AR400, expect roughly 20–40% of the hole count you'd get in structural mild steel before resharpening is needed. Track holes per resharpen in hard material separately from your general drill life data.
Mark your cobalt drills that have been used in wear plate. They're candidates for resharpening sooner than the same drill used on general structural work, and they may have more edge wear than a visual check reveals without magnification.
When resharpening after wear plate work, check web thickness. In hard materials, the web gets thicker relative to drill diameter as the drill is ground shorter with each resharpening. A thicker web means more chisel edge — more chisel edge means more thrust. Web thinning during resharpening is particularly valuable in wear plate applications.
Mail in your worn wear-plate drills. We'll regrind to 135° split-point geometry on our WinsloMatic — optimized for the next run.
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