Duplex stainless steels combine the high strength of ferritic stainless steel with the corrosion resistance and toughness of austenitic stainless steel. This dual-phase microstructure — approximately equal proportions of ferrite and austenite — gives duplex grades roughly twice the yield strength of standard 304 or 316 austenitic stainless steels, while providing excellent resistance to chloride stress corrosion cracking.
For deep hole drilling, these properties translate into significantly higher cutting forces, more rapid work hardening, accelerated tool wear, and chip formation behavior that differs from both austenitic and ferritic grades. A gun drill that produces 300 holes in 316L may struggle to produce 50 holes in 2507 super duplex at the same parameters.
This guide covers all common duplex grades used in deep hole drilling applications, with material-specific cutting data from Outokumpu and Sandvik, tool selection guidance, and strategies for managing the specific challenges of these materials.
Duplex Stainless Steel Grades
Duplex stainless steels are classified into four families based on alloy content and corrosion resistance.
Grade Comparison
| Property | LDX 2101 (S32101) | 2304 (S32304) | 2205 (S32205) | 2507 (S32750) |
|---|---|---|---|---|
| Type | Lean duplex | Lean duplex | Standard duplex | Super duplex |
| Yield strength (min) | 450 MPa (65 ksi) | 400 MPa (58 ksi) | 450 MPa (65 ksi) | 550 MPa (80 ksi) |
| Tensile strength (min) | 650 MPa (94 ksi) | 630 MPa (87 ksi) | 640 MPa (95 ksi) | 730 MPa (116 ksi) |
| Elongation (min) | 30% | 25% | 25% | 15–20% |
| Hardness (max) | ~235 HB | ~235 HB | ~250 HB | ~270 HB |
| PREN (pitting resistance) | ~26 | ~26 | ~34–35 | ~42–43 |
| Relative machinability | Best (similar to 304) | Good | Moderate | Most difficult |
PREN = %Cr + 3.3 × %Mo + 16 × %N
Key Characteristics for Deep Hole Drilling
- LDX 2101 — The most machinable duplex grade. Its machining properties are notably better than other duplex grades, making it the preferred choice when deep hole drilling is required in a duplex material. Yield strength is comparable to 2205 but with lower alloy content.
- 2304 — Good balance of strength and cost. Corrosion resistance equivalent to 316L. Moderate machinability — more difficult than 316L but easier than 2205.
- 2205 — The workhorse duplex grade, most commonly specified for pressure vessels, chemical tankers, desalination plants, and oil and gas components. Requires reduced cutting speeds and rigid setups compared to austenitic stainless.
- 2507 — Super duplex with the highest strength and best corrosion resistance. Used in the most demanding environments — offshore seawater systems, high-pressure piping, chemical processing. Most difficult to machine of the duplex family. Cutting speeds must be reduced by 30–40% from 2205 values.
Chemical Composition (Weight %)
| Element | LDX 2101 | 2304 | 2205 | 2507 |
|---|---|---|---|---|
| Carbon (max) | 0.04 | 0.03 | 0.03 | 0.03 |
| Chromium | 21.5 | 23 | 22 | 25 |
| Nickel | 1.5 | 4 | 5.5 | 7 |
| Molybdenum | 0.3 | 0.3 | 3.2 | 4 |
| Nitrogen | 0.22 | 0.10 | 0.17 | 0.27 |
| Manganese | 5.0 | 2.5 | 1.5 | 0.8 |
The molybdenum content is a key differentiator — 2205 contains 3.2% Mo compared to 0.3% in lean duplex grades, contributing to its higher strength and work hardening rate. 2507 contains 4% Mo, the highest of any duplex grade.
Work Hardening Behavior
Duplex stainless steels work-harden at a rate 50–100% higher than austenitic grades like 304 or 316. This is the single most important factor affecting deep hole drilling parameters.
Why Duplex Work-Hardens
During cutting, the dual-phase microstructure undergoes severe plastic deformation. The harder austenite phase is squeezed out of the stagnation zone at the tool tip, leaving only the softer ferrite phase in contact with the tool. This ferrite adheres to the cutting edge, forming a built-up edge (BUE). The BUE periodically breaks off, removing carbide particles from the tool.
The work-hardened surface layer in duplex stainless can reach hardness levels 200–300% above the base material hardness. For 2507 with a base hardness of 270 HB, the work-hardened surface can exceed 800 HV.
Consequences for Deep Hole Drilling
| Effect | Cause |
|---|---|
| Cutting forces 25–35% higher than 316L | Higher yield strength and work hardening rate |
| BUE formation on the cutting edge | Ferrite phase adhesion at moderate temperatures |
| Accelerated flank wear | Abrasive contact with work-hardened chip |
| Notch wear at depth of cut line | Work-hardened surface layer from previous pass |
| Chip packing | High chip toughness — chips resist breaking |
| Tool breakage on re-entry | Work-hardened ring at the bottom of a peck cycle |
Cutting Parameters for Gun Drilling
The following values are compiled from Outokumpu’s official machining guidelines and published research. For duplex stainless, internal through-tool coolant is not optional — it is required for any deep hole drilling above 3×D depth.
2205 (Standard Duplex) — Gun Drilling with Carbide Tooling
| Diameter (mm) | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) |
|---|---|---|---|---|
| 3–6 | 65–130 | 20–40 | 0.002–0.005 | 0.05–0.13 |
| 6–12 | 100–165 | 30–50 | 0.003–0.007 | 0.08–0.18 |
| 12–20 | 100–165 | 30–50 | 0.005–0.009 | 0.13–0.23 |
| 20–35 | 80–130 | 25–40 | 0.006–0.010 | 0.15–0.25 |
Base values from Outokumpu: 2205 drilling with internal coolant — 60 m/min (197 SFM). Adjust by ±15% depending on machine rigidity and coolant system.
2507 (Super Duplex) — Gun Drilling with Carbide Tooling
| Diameter (mm) | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) |
|---|---|---|---|---|
| 3–6 | 50–100 | 15–30 | 0.002–0.004 | 0.05–0.10 |
| 6–12 | 65–115 | 20–35 | 0.003–0.006 | 0.08–0.15 |
| 12–20 | 65–115 | 20–35 | 0.004–0.008 | 0.10–0.20 |
| 20–35 | 50–100 | 15–30 | 0.005–0.009 | 0.13–0.23 |
Base values from Outokumpu: 2507 drilling with internal coolant — 45 m/min (148 SFM). Reduce speeds by 25–30% from 2205 values.
LDX 2101 (Lean Duplex) — Gun Drilling with Carbide Tooling
LDX 2101 has significantly better machinability than 2205 or 2507, with cutting speeds approaching those used for 304 stainless steel.
| Diameter (mm) | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) |
|---|---|---|---|---|
| 3–6 | 130–200 | 40–60 | 0.003–0.006 | 0.08–0.15 |
| 6–12 | 165–260 | 50–80 | 0.004–0.008 | 0.10–0.20 |
| 12–20 | 165–260 | 50–80 | 0.006–0.010 | 0.15–0.25 |
| 20–35 | 130–200 | 40–60 | 0.008–0.012 | 0.20–0.30 |
Cutting Parameters for BTA Drilling
BTA drilling of duplex stainless steel requires reduced parameters compared to gun drilling at equivalent diameters due to the higher feed rates and correspondingly higher cutting forces.
2205 — BTA Drilling with Carbide Inserts
| Diameter (mm) | Speed (SFM) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 12–20 | 115–165 | 0.08–0.15 | 50–80 |
| 20–35 | 100–150 | 0.10–0.20 | 40–70 |
| 35–50 | 85–130 | 0.12–0.22 | 30–60 |
2507 — BTA Drilling with Carbide Inserts
| Diameter (mm) | Speed (SFM) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 12–20 | 80–115 | 0.06–0.12 | 60–100 |
| 20–35 | 65–100 | 0.08–0.15 | 50–80 |
| 35–50 | 50–85 | 0.10–0.18 | 40–70 |
Indexable insert BTA heads can run at higher speeds than brazed carbide heads. Outokumpu’s data specifies 85 m/min for 2205 and 70 m/min for 2507 with indexable insert drills — approximately 40% higher than internal coolant solid carbide drills.
Comparison with Austenitic Stainless Steel
| Parameter | 304/316 | 2205 | 2507 |
|---|---|---|---|
| Gun drilling speed (carbide, internal coolant) | 25–45 m/min | 30–50 m/min | 20–35 m/min |
| BTA speed (carbide, indexable) | 55–80 m/min | 70–85 m/min | 55–70 m/min |
| Relative cutting forces | Baseline | +25–35% | +40–50% |
| Work hardening rate | Moderate | High | Very high |
| BUE tendency | High | Very high | Very high |
| Coolant pressure requirement | ≥ 70 bar | ≥ 70 bar | ≥ 100 bar |
| Expected tool life (relative) | 100% | 50–70% | 30–50% |
Tool Selection
Carbide Grade
| Grade | ISO Classification | Coating | Edge Preparation |
|---|---|---|---|
| 2205 (general) | K20–K30 | TiAlN (PVD) | Honed edge, 0.03–0.05 mm |
| 2205 (production) | P25–P35 | AlTiN (PVD) | Honed edge, 0.03–0.05 mm |
| 2507 (general) | K25–K35 | TiAlN or AlTiN (PVD) | Honed edge, 0.05–0.08 mm |
| 2507 (production) | P30–P40 | AlTiN (PVD) | Honed edge, 0.05–0.08 mm |
| LDX 2101 | K15–K25 | TiAlN (PVD) | Light hone, 0.01–0.03 mm |
TiAlN or AlTiN coatings are essential for duplex stainless steel. Research on drilling of 2205 and 2507 shows that coated tools extend tool life by 150–200% compared to uncoated carbide. The coating provides a thermal barrier at interface temperatures exceeding 800°C and reduces the chemical affinity that drives BUE formation.
Gun Drill Head Profile
- Profile E — First choice for duplex stainless steel. The E profile eliminates tool sticking after the outer corner dulls, which is a common problem in higher-strength stainless grades.
- Profile C — Recommended for 2507 super duplex. Increased back taper and larger coolant gaps accommodate the higher chip load.
- Profile G — Suitable for LDX 2101.
BTA Drill Head
For BTA drilling of duplex stainless:
- Indexable insert heads with tough carbide grade (P25–P35 for 2205, P30–P40 for 2507)
- TiAlN or AlTiN coating
- Chip breakers designed for ductile materials — standard chip breakers for carbon steel may not break duplex chips
- For 2507, consider brazed carbide heads for diameters under 20 mm, with reduced clearance angles
Coolant Requirements
Coolant pressure and delivery method are the most critical process variables for deep hole drilling of duplex stainless steel. Published research comparing external low-pressure cooling (2–4 bar) with internal high-pressure cooling (21 bar) on duplex drilling found:
| Cooling Method | Holes per Tool | Failure Mode |
|---|---|---|
| External (2–4 bar) | ~30 holes | Non-uniform chipping (CH2) |
| Internal (21 bar) | 60+ holes | Gradual flank wear (VB) |
Internal high-pressure coolant extends tool life by a factor of 2 or more and changes the dominant wear mechanism from catastrophic chipping to predictable flank wear.
Recommended Coolant Parameters
| Grade | Gun Drilling | BTA Drilling |
|---|---|---|
| LDX 2101 | 500–800 PSI (35–55 bar) | 400–700 PSI (28–48 bar) |
| 2205 | 700–1,000 PSI (48–70 bar) | 500–800 PSI (35–55 bar) |
| 2507 | 1,000–1,500 PSI (70–100 bar) | 700–1,000 PSI (48–70 bar) |
Coolant Type
| Recommendation | Reason |
|---|---|
| Low-viscosity cutting oil | Best lubricity for BUE prevention |
| High-performance emulsion at 8–12% | Acceptable if oil not available; verify EP additive content |
| Minimum 8% concentration | Below 8%, lubricity is insufficient for duplex |
For 2507 super duplex, oil-based coolant is strongly recommended. The higher film strength of oil compared to water-based emulsion is essential for preventing the metal-to-metal contact that drives BUE formation.
Surface Finish and Dimensional Accuracy
| Parameter | Gun Drilling (2205) | Gun Drilling (2507) | BTA Drilling (2205) |
|---|---|---|---|
| Surface finish (Ra) | 0.4–1.6 µm | 0.4–1.6 µm | 0.8–3.0 µm |
| Diameter tolerance | IT7–IT9 | IT7–IT9 | IT8–IT10 |
| Straightness (per 1,000 mm) | 0.05–0.3 mm | 0.05–0.3 mm | 0.1–0.5 mm |
Duplex stainless steel can achieve the same surface finish as austenitic stainless in gun drilling, provided the tool remains sharp. The key difference is that tool wear in duplex accelerates more rapidly, so the finish deteriorates sooner. For production runs, consider reducing the tool life limit (holes per regrind) by 30–50% compared to 316L.
Common Problems and Solutions
Built-Up Edge (BUE)
Symptom: Rough bore surface, fluctuating cutting forces, visible workpiece material adhered to the cutting edge after retraction.
Cause: Ferrite phase adhesion to the carbide cutting edge. Duplex stainless has a higher tendency to BUE than austenitic grades because the dual-phase microstructure creates more favorable conditions for material transfer.
Solution:
- Increase cutting speed above 150 SFM (45 m/min) for 2205, above 100 SFM (30 m/min) for 2507
- Use TiAlN or AlTiN-coated tooling — essential, not optional
- Ensure coolant pressure meets the minimum values specified above
- Verify coolant concentration (8–12% minimum for emulsion)
- If BUE persists, switch from emulsion to oil-based coolant
Chip Breaking Difficulties
Symptom: Long stringy chips, torque fluctuations, chip packing.
Cause: Duplex stainless steel’s high fracture toughness produces tougher chips than austenitic grades. The chips resist breaking.
Solution:
- Increase feed rate incrementally until chips break into 6–12 mm segments
- For BTA drilling, verify that the chip breaker geometry is designed for stainless steel
- For gun drilling, check that the gun drill’s chip breaker geometry is appropriate
- Increase coolant pressure to improve chip evacuation — duplex chips that pack in the flute are difficult to clear
Rapid Flank Wear
Symptom: Tool life 50–70% shorter than in 316L at equivalent cutting speed.
Cause: The work-hardened chip of duplex stainless is more abrasive than the chip produced by austenitic grades, causing accelerated flank wear.
Solution:
- Reduce cutting speed by 15–25% from the starting value for the first production run, then adjust upward
- Use a tougher carbide grade (P25–P35 for 2205) with adequate edge preparation
- Verify that the edge hone is sufficient — insufficient edge preparation causes rapid flank wear
- Monitor flank wear at shorter intervals than with austenitic stainless
- Set the regrind limit at 0.15 mm flank wear (VB) rather than the 0.25 mm typical for carbon steel
Work Hardening at Tool Re-Entry
Symptom: Tool breaks at the bottom of a peck cycle when feed resumes.
Cause: If the tool dwells at the bottom of the peck, the material work-hardens under the heat and pressure. When feed resumes, the cutting edge encounters this hardened surface and may chip or fracture.
Solution:
- Avoid peck cycles with carbide tools. Use continuous feed for the entire hole depth.
- If peck cycles cannot be avoided (machine limitation), minimize dwell time at the bottom of each peck
- Never allow the spindle to continue rotating while feed is stopped — the rubbing action work-hardens the hole bottom
- Reduce the peck depth to 0.5×D or less to minimize the time between pecks
Deep Hole Drilling Procedure
Pilot Hole
- Depth: 2×D minimum (longer than for carbon steel or 304/316)
- Diameter: 0.013–0.025 mm larger than the deep hole drill
- Point angle: 140–145°, with split point
- Surface prep: Spot face the entry surface to remove any scale or surface irregularities
Entry Sequence
- Drill pilot hole at 100% recommended speed and feed
- Feed the deep hole drill to within 1.5 mm of the pilot hole bottom at maximum 50 RPM with coolant OFF
- Turn coolant ON and allow flow to stabilize (3–5 seconds)
- Begin feed at 50% speed and 75% feed for 2×D past the pilot hole bottom
- Increase to 100% speed and feed for the remainder of the hole
Running the Hole
- Continuous feed required — never dwell while the tool is cutting in duplex stainless
- No peck cycles — the re-entry impact work-hardens the material and chips the cutting edge
- Monitor coolant pressure continuously — a 10% drop indicates chip blockage, which is more critical in duplex than in austenitic grades because duplex chips are tougher and harder to clear
- Monitor chip form — chips should be well-broken segments 6–12 mm in length. Longer chips increase the risk of packing
Breakout and Retract
- Through holes: Reduce speed by 50% and feed by 25% before breakout. The breakout burr in duplex is more pronounced than in carbon steel — plan for a deburring operation.
- Retract: Reduce speed to maximum 50 RPM before retracting, with coolant ON during retraction to flush remaining chips.
Summary
Deep hole drilling of duplex and super duplex stainless steel requires a fundamentally more conservative approach than austenitic grades:
- 2205 — Reduce cutting speeds by 10–15% from 316L values. Use TiAlN-coated carbide with internal coolant at minimum 70 bar.
- 2507 — Reduce cutting speeds by 30–40% from 316L values. Oil-based coolant recommended. Internal coolant at minimum 100 bar.
- LDX 2101 — Can be drilled at speeds approaching 304 values. The most economical duplex grade for deep hole drilling.
- Tool life in 2507 is typically 30–50% of that in 316L at the optimal cutting speed for each material. Plan tool changes accordingly.
- Continuous feed is strongly preferred — avoid peck cycles with carbide tools in duplex stainless.
For related reading, see the Deep Hole Drilling 304 and 316 Stainless Steel Guide, the Deep Hole Drilling Troubleshooting Guide, and the Speeds and Feeds for Deep Hole Drilling Reference Tables.