Duplex Stainless Deep Hole Drilling

Why Duplex Is Hard to Drill

Duplex stainless (~50/50 austenite/ferrite) combines the strength of ferrite with the toughness of austenite. Its thermal conductivity is low, so heat stays in the cutting zone; the austenite work-hardens under any rubbing; and the hard, stringy chips load the cutting edge unevenly. The result is edge notching and rapid flank wear the instant the cut stops being a cut.

Starting Parameters

The Failure Modes to Watch

• Edge notching: the ferrite/austenite bands chip the edge unevenly. Use sharper edge prep and higher rake inserts; regrind gundrills on a tight schedule.
• Work hardening: any rubbing or a dull edge hardens the bore wall — the next pass and the next tool pay for it.
• Stringy chips: ductile austenite makes birds-nest chips. Chip-breaker geometry and adequate feed are non-negotiable.
• Heat: low conductivity means the edge sees the heat. High-pressure oil coolant is the only answer.
• Straightness drift: the alternating microstructure can steer a long drill slightly; counter-rotation helps on critical bores.

Tooling Choices

• Carbide grade: micro-grain K10–K20 with sharp edges; avoid excessive edge hones.
• Coating: TiAlN or TiSiN for heat resistance; some shops prefer uncoated sharp tools for duplex to avoid edge rounding.
• Gundrill geometry: positive rake, adequate guide-pad width, well-matched point angle for the austenite content.
• BTA heads: indexable carbide inserts with positive geometry; check insert seating often — a loose seat notches immediately.

Where Duplex Bores Are Drilled

Drilling Duplex vs Standard Stainless

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