Appearance
An offshore oil and gas manufacturer drills 25 mm diameter holes at L/D 24:1 in super duplex stainless steel (UNS S32750) subsea connector blocks. BTA drilling at 50 m/min and 0.10 mm/rev delivers only 8 holes per edge — adhesion-driven flank wear and flute damage from built-up edge cause rapid failure. Stringy chips cause intermittent jamming. After switching to TiAlN-coated inserts with polished rake faces, reducing speed to 40 m/min, increasing feed to 0.15 mm/rev, and raising coolant pressure from 40 to 70 bar, tool life reaches 30 holes per edge (3.75× improvement) with short comma-shaped chips and no further breakage.
Material Characteristics
Duplex stainless steels have a two-phase microstructure of approximately 50% austenite and 50% ferrite. This microstructure gives them high strength and excellent corrosion resistance but creates significant machining challenges.
Grades and Properties
| Grade | UNS | PRE Value | Tensile Strength | Hardness | Machinability |
|---|---|---|---|---|---|
| Lean duplex | S32101 | 26–28 | 700 MPa | 28 HRC | Moderate |
| Standard duplex (2205) | S32205 / S31803 | 33–36 | 800 MPa | 30 HRC | Moderate–poor |
| Super duplex (2507) | S32750 | 40–42 | 900 MPa | 32 HRC | Poor |
| Hyper duplex | S32707 | 48+ | 950 MPa | 34 HRC | Very poor |
The PRE (Pitting Resistance Equivalent) value correlates inversely with machinability — higher PRE means more alloy content and lower machinability.
Machining Challenges
| Challenge | Cause | Effect on Deep Hole Drilling |
|---|---|---|
| Work hardening | Rapid strain hardening of austenite phase | Accelerates flank wear, makes re-entry difficult |
| Built-up edge | Adhesion of workpiece material to tool | Degrades surface finish, alters cutting geometry |
| Stringy chips | High ductility of austenite phase | Chip jamming, tool breakage risk in deep holes |
| Low thermal conductivity | Alloy content | Heat concentrates at cutting edge |
| High cutting forces | High strength at elevated temperature | Tool deflection, hole straightness issues |
Gun Drilling Parameters
Speed and Feed
| Grade | Vc (m/min) | Feed (mm/rev) | Coolant Pressure | Notes |
|---|---|---|---|---|
| 2205 (standard duplex) | 40–70 | 0.02–0.07 | 70–120 bar | Start at 50 m/min |
| 2507 (super duplex) | 30–55 | 0.015–0.05 | 90–150 bar | Start at 35 m/min |
| Lean duplex | 50–80 | 0.03–0.08 | 60–100 bar | Easier than 2205 |
Feed by Diameter (2205 Duplex)
| Drill Diameter (mm) | Feed (mm/rev) | Speed at 50 m/min (RPM) |
|---|---|---|
| 3–5 | 0.008–0.020 | 3,180–5,310 |
| 6–8 | 0.015–0.035 | 1,990–2,650 |
| 10–12 | 0.025–0.050 | 1,330–1,590 |
| 14–18 | 0.035–0.060 | 880–1,140 |
| 20–25 | 0.040–0.070 | 640–800 |
For super duplex 2507, reduce feed by 20–30% from these values.
Tip: The feed must stay above 0.015 mm/rev for 2205 and 0.012 mm/rev for 2507. Below these thresholds, the cutting edge rubs instead of cutting, accelerating work hardening and causing rapid notch wear. If chip breaking is insufficient at the minimum safe feed, reduce speed rather than reducing feed further.
BTA Drilling Parameters
Speed and Feed by Grade
| Grade | Vc (m/min) | Feed (mm/rev) | Coolant Pressure | Coolant Flow |
|---|---|---|---|---|
| 2205 | 40–65 | 0.08–0.18 | 4–8 MPa | Q = 5–6 × D L/min |
| 2507 | 30–50 | 0.06–0.15 | 6–10 MPa | Q = 5.5–6.5 × D L/min |
| Lean duplex | 50–75 | 0.10–0.20 | 3–6 MPa | Q = 4.5–5.5 × D L/min |
BTA Parameters by Diameter (2205 Duplex)
| Diameter (mm) | Vc (m/min) | Speed (RPM) | Feed (mm/rev) | Coolant Flow (L/min) |
|---|---|---|---|---|
| 18–22 | 45–60 | 650–1,060 | 0.08–0.15 | 90–130 |
| 25–35 | 40–55 | 360–700 | 0.10–0.18 | 130–200 |
| 40–50 | 35–50 | 220–400 | 0.10–0.18 | 200–300 |
| 55–70 | 30–45 | 140–260 | 0.12–0.20 | 300–420 |
Chip Breaker Selection
| Chip Breaker | Application | Feed Range | Chip Shape |
|---|---|---|---|
| GF (general fragmentation) | Duplex 2205, general purpose | 0.08–0.18 mm/rev | C-shaped |
| MF (medium fragmentation) | Super duplex 2507, stringy chip control | 0.06–0.15 mm/rev | Short comma |
| DT (deep trepanning) | Not recommended for duplex | — | Poor chip breaking |
The MF chip breaker is preferred for super duplex grades because it produces a tighter chip curl and more aggressive fragmentation than the standard GF.
Tool Selection
Insert Grades
| Requirement | Recommended Grade | Coating | Edge Preparation |
|---|---|---|---|
| General duplex drilling | IC520, AH725 | TiAlN PVD | Sharp, light T-land |
| Super duplex / high adhesion | IC806, AH8015 | AlTiN or TiAlN multilayer | Polished rake face |
| Maximum tool life | GC2220 (Sandvik) | Inveio® TiAlN | Positive rake, sharp |
Polished rake faces are strongly recommended for all duplex grades. The polished surface reduces the tendency for built-up edge formation, which is the dominant wear mechanism.
Guide Pads
| Material | Recommendation |
|---|---|
| Pad grade | IC928 (WC-Co + TaC) for best wear resistance |
| Coating | Uncoated or TiAlN-coated |
| Clearance | 0.008–0.012 mm per side (tighter than for steel) |
| Number of pads | 2 (standard), with carbide pads only |
Tool Geometry Considerations
| Geometric Feature | Recommendation | Reason |
|---|---|---|
| Rake angle | +5 to +10° (positive) | Reduces cutting forces, lowers BUE tendency |
| Clearance angle | 8–12° | Prevents rubbing on work-hardened surface |
| Corner radius | 0.4–0.8 mm | Balance edge strength with surface finish |
| Edge preparation | Sharp or light hone | Duplex needs sharp edges to cut cleanly |
Coolant Requirements
Why Coolant Is Critical
Duplex stainless steels have approximately 30% lower thermal conductivity than austenitic stainless steels and 60% lower than carbon steel. Heat concentrates at the cutting edge, accelerating diffusion and adhesion wear.
| Cooling Method | Suitability for Duplex Deep Hole Drilling | Notes |
|---|---|---|
| Flood (emulsion) | Required — minimum acceptable | 10–15% oil content |
| High-pressure oil | Recommended | Best chip evacuation |
| Cryogenic LN₂ | Experimental for deep holes | Reduces BUE, research ongoing |
| MQL | Not suitable | Insufficient cooling for duplex |
Pressure and Flow
| Method | Minimum Pressure | Recommended Pressure | Flow Rate |
|---|---|---|---|
| Gun drilling | 70 bar | 90–150 bar | 0.3–0.5 L/min per mm diameter |
| BTA drilling | 4 MPa (40 bar) | 6–10 MPa (60–100 bar) | 5–6.5 × D L/min |
Warning: Duplex stainless steel cannot be deep hole drilled with MQL (minimum quantity lubrication). The cooling capacity of MQL is insufficient for duplex grades, and the reduced chip evacuation force causes chip jamming. For deep holes in duplex, use flood coolant or high-pressure oil. Cryogenic cooling (LN₂/LCO₂) has shown research promise but is not yet production-proven for deep hole drilling of duplex.
Chip Breaking
Duplex steels produce strong, tough chips that resist breaking. Chip control is the primary operational challenge.
Feed vs Chip Shape
| Feed Range (mm/rev) | Chip Shape | Suitability |
|---|---|---|
| Below 0.05 | Continuous stringy chip | Not acceptable — high jam risk |
| 0.05–0.10 | Long curled chips | Marginal — may jam at depth |
| 0.10–0.15 | Short comma-shaped | Good — target for most duplex drilling |
| Above 0.15 | Thick broken chips | Acceptable if chip mouth can handle volume |
Strategies for Better Chip Breaking
- Increase feed first — feed has the strongest influence on chip breaking. A 50% feed increase produces significantly shorter chips.
- Use polished rake inserts — reduced friction helps chips curl more tightly.
- Optimise chip breaker geometry — narrower chip breaker width promotes tighter curl.
- Reduce speed — lower speed reduces chip ductility, helping fracture.
- Apply vibration assistance — low-frequency vibration (LFV) is effective for chip breaking in duplex but adds equipment complexity.
Surface Integrity
Surface Finish
| Grade | Gun Drilling Ra (µm) | BTA Drilling Ra (µm) | Notes |
|---|---|---|---|
| 2205 | 0.4–0.8 | 0.8–1.6 | Good finish at optimal parameters |
| 2507 | 0.5–1.0 | 1.0–2.0 | Can be rougher due to BUE |
Surface Defects
The dominant surface integrity risk in duplex deep hole drilling is built-up edge formation, which:
- Creates grooves and tearing on the bore surface
- Increases roughness by 50–100%
- Can cause localised micro-cracking in the surface layer
- Reduces fatigue life in corrosion-fatigue applications
Prevention: maintain adequate cutting speed (above 30 m/min), use polished coated tools, and ensure coolant reaches the cutting edge.
Tool Life
Expected Tool Life
| Grade | BTA (holes per edge) | Gun Drilling (metres per edge) | Failure Mode |
|---|---|---|---|
| 2205 | 20–40 | 5–15 m | Flank wear, adhesion |
| 2507 | 10–30 | 3–8 m | Flank wear, BUE, chipping |
| Lean duplex | 30–60 | 8–20 m | Gradual flank wear |
Wear Mechanism Analysis
Research by Paro et al. (2001) on drilling duplex stainless steels identified:
- Flank wear — the dominant wear mode, driven by adhesion from BUE
- Flute damage — a major failure mode in deep hole drilling, caused by chip friction in the flute
- Notch wear — present at the depth-of-cut line from work hardening
- Crater wear — limited at recommended speeds, increases above 60 m/min
The 2507 grade exhibits 30–50% faster wear than 2205 at the same cutting parameters due to higher alloy content and work-hardening rate.
Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| Built-up edge on insert | Speed too low or rake face rough | Increase speed 15%, use polished grade inserts |
| Stringy chips, poor breaking | Feed too low | Increase feed to 0.10–0.15 mm/rev |
| Rapid flank wear | Adhesion from BUE | Switch to AlTiN coating, check coolant flow |
| Chip jamming in flute | Chip shape too long | Adjust feed, use MF chip breaker |
| Tool breakage in bore | Chip packing from stringy chips | Increase coolant pressure, reduce peck depth |
| Poor surface finish | BUE or worn insert | Replace insert, verify speed above 30 m/min |
| Oversize bore | Deflection from high cutting forces | Reduce feed, check guide pad condition |
| Excessive burr at exit | Work hardening at breakthrough | Maintain feed through final 2 mm, use sharp edge |
| Coolant pressure drop | Chip blockage in coolant passage | Clean coolant holes, check filtration (5 µm) |
| Notch wear at depth line | Work hardening from low feed | Increase feed above 0.015 mm/rev |
FAQ
Why is duplex stainless steel difficult to deep hole drill?
Its two-phase microstructure (austenite + ferrite) combines high strength, rapid work hardening, low thermal conductivity, and high ductility — all of which challenge the cutting edge and chip evacuation.
What cutting speed should I use for duplex 2205 deep hole drilling?
Gun drilling: 40–70 m/min. BTA drilling: 40–65 m/min. Start at the lower end and increase based on tool wear observation.
What cutting speed should I use for super duplex 2507?
Gun drilling: 30–55 m/min. BTA drilling: 30–50 m/min. Reduce speed by 20–30% compared to 2205.
What feed rate gives the best chip breaking in duplex?
0.10–0.15 mm/rev produces short comma-shaped chips. Below 0.05 mm/rev, chips will be stringy and prone to jamming.
What coating is best for drilling duplex stainless steel?
TiAlN PVD coating with a polished rake face. The polished surface reduces built-up edge formation — the dominant wear mechanism. AlTiN coatings offer better thermal stability for higher-speed applications.
What coolant pressure is needed for duplex deep hole drilling?
Gun drilling: minimum 70 bar, recommended 90–150 bar. BTA drilling: minimum 4 MPa (40 bar), recommended 6–10 MPa (60–100 bar).
Can duplex stainless steel be gun drilled?
Yes — gun drilling of duplex is production-feasible with coated carbide tools, adequate coolant pressure (70–150 bar), and conservative speeds (30–70 m/min depending on grade).
What is the difference in machinability between 2205 and 2507?
2507 has approximately 30–50% lower tool life than 2205 at equivalent parameters due to higher alloy content (PRE 40–42 vs 33–36) and more rapid work hardening.
Is MQL suitable for duplex deep hole drilling?
No. The cooling capacity and chip evacuation force of MQL are insufficient for duplex stainless steel in deep holes. Use flood coolant or high-pressure oil.
What is the best chip breaker geometry for duplex BTA drilling?
The MF (medium fragmentation) chip breaker for super duplex, or GF (general fragmentation) for standard duplex. Avoid the DT (deep trepanning) chip breaker — it does not produce sufficient chip curl for duplex.
Summary
Duplex stainless steel deep hole drilling is challenging but production-feasible with correct parameter selection and tooling:
- Grades — 2205 (standard duplex, PRE 33–36) and 2507 (super duplex, PRE 40–42) are most common; machinability decreases with increasing PRE
- Gun drilling — 30–70 m/min speed, 0.015–0.07 mm/rev feed, 70–150 bar coolant pressure
- BTA drilling — 30–65 m/min speed, 0.06–0.20 mm/rev feed, 4–10 MPa coolant pressure
- Chip breaking — target feed above 0.08 mm/rev for C-shaped chips; use MF chip breaker for super duplex
- Tool coating — TiAlN or AlTiN with polished rake face to minimise built-up edge
- Coolant — high-pressure flood or oil is essential; MQL is not suitable
- The oil and gas manufacturer in the opening scenario increased tool life from 8 to 30 holes per edge (3.75×) by optimising speed, feed, coolant pressure, and insert coating for super duplex 2507