Appearance
Duplex and super duplex stainless steels do not machine like 316L. Their dual-phase structure — equal parts ferrite and austenite — combines the worst of both worlds for deep hole drilling: the work-hardening tendency of austenite with the notch-wear characteristics of ferrite. When drilling deep holes in these materials, every aspect of the process — tool geometry, coating, coolant pressure, and parameters — must be optimized for the specific duplex grade.
Understanding Duplex and Super Duplex Stainless Steels
Metallurgy and Material Properties
Duplex stainless steels have a balanced microstructure of approximately 50% ferrite and 50% austenite. This dual-phase structure gives them:
| Property | Duplex 2205 (S32205) | Super Duplex 2507 (S32750) | Compared to 316L |
|---|---|---|---|
| Yield strength | ~450 MPa | ~550 MPa | ~200 MPa (2.2–2.8× higher) |
| Tensile strength | ~680 MPa | ~800 MPa | ~485 MPa |
| Hardness | ~30 HRC | ~35 HRC | ~20 HRC |
| Thermal conductivity | ~15 W/m·K | ~14 W/m·K | ~16 W/m·K |
| Work hardening rate | High | Very high | Moderate |
| Machinability rating | 30–40% (of free-cutting steel) | 20–30% | 50–60% |
Common Grades
| Grade | UNS Designation | Applications |
|---|---|---|
| Duplex 2205 | S32205 / S31803 | Chemical processing, oil and gas, marine |
| Super Duplex 2507 | S32750 / S32760 | Offshore platforms, subsea equipment, desalination |
| Lean Duplex 2101 | S32101 | Structural, bridges, storage tanks |
| Duplex 2304 | S32304 | Pressure vessels, heat exchangers |
Why Duplex Grades Are Difficult for Deep Hole Drilling
Challenge 1: Work Hardening
Duplex stainless steels work-harden rapidly during machining. In deep hole drilling, this is particularly problematic because:
- The austenite phase strain-hardens under the cutting action
- The guide pads burnish the bore wall, further work-hardening the surface
- A work-hardened layer of 0.05–0.15 mm depth forms on the bore surface
- Subsequent cutting passes (if reaming or finishing) must penetrate this hardened layer
| Effect on Process | Consequence |
|---|---|
| Increased cutting forces | Higher torque, power consumption |
| Accelerated flank wear | Reduced tool life |
| BUE formation at low speeds | Surface finish degradation, undersize bore |
| Notch wear at depth of cut line | Premature tool failure |
Challenge 2: Chip Breaking and Evacuation
Chips from duplex stainless steels are strong, tough, and tend to form long continuous ribbons:
| Chip Characteristic | Effect on Deep Hole Drilling |
|---|---|
| High ductility and strength | Chips resist breaking, form long tangles |
| Work-hardened chip structure | Chips are abrasive inside the flute |
| Poor curl tendency | Chips straighten instead of coiling |
| Large chip volume | High chip load per hole requires robust evacuation |
A Tungaloy UNIDEX case study on SUS329J3L duplex showed that effective chip breaking at 0.19 mm/rev was critical to maintaining tool life at 14 m per insert — 55% longer than the competitor's 9 m.
Challenge 3: Heat Concentration
The low thermal conductivity of duplex grades (approximately 14–15 W/m·K, compared to 50+ W/m·K for carbon steel) means that cutting heat concentrates at the tool edge:
| Location | Heat Concentration | Effect |
|---|---|---|
| Cutting edge | 70–80% of heat goes into the tool | Thermal softening of carbide binder |
| Guide pads | Heat from burnishing accumulates at pad interface | Galling, material transfer |
| Bore surface | Surface temperature can exceed 300°C | Work hardening, residual stress |
Tool Geometry for Duplex Drilling
Gun Drill Geometry Modifications
| Geometry Feature | Standard (316L) | Duplex Recommendation | Reason |
|---|---|---|---|
| Outer point angle (ϕ) | 30° | 25–30° | Lower angle improves edge strength for higher cutting forces |
| Inner point angle (ψ) | 20° | 18–22° | Balance cutting forces, prevent notch wear |
| Tip offset | d₀/4 | d₀/4 | Maintain standard offset |
| Outer relief | 12° | 8–10° | More edge support for higher loads |
| Inner relief | 18° | 12–15° | Prevent notch wear at the inner edge |
| Edge hone | 0.01–0.02 mm | 0.02–0.04 mm | Strengthen edge against higher forces |
BTA Drill Head Geometry
| Feature | Recommendation for Duplex |
|---|---|
| Carbide grade | Toughness-grade micro-grain (0.5–0.8 μm grain size) |
| Chip breaker | Aggressive chip breaker geometry — short, curved chips |
| Guide pads | DLC-coated or TiAlN-coated pads |
| Pad relief | Increased relief angle to reduce pad contact friction |
| Cutting edge preparation | T-land or chamfered edge for strength |
Chip Breaker Design
Chip control is the single most important geometry consideration for duplex drilling:
| Chip Breaker Type | Best For | Chip Form |
|---|---|---|
| Standard chip breaker | Low feed (< 0.08 mm/rev) | Long, stringy — inadequate |
| Aggressive step breaker | Medium feed (0.08–0.15 mm/rev) | Short 6–9 chips — good |
| Multi-stage breaker | High feed (> 0.15 mm/rev) | Short 3–6 chips — excellent |
| Wavy-edge breaker | Variable conditions | Moderate control |
The goal is consistently broken chips in the 3–9 mm length range — short enough to evacuate through the flute or annular space, long enough to avoid dust.
Cutting Parameters for Deep Hole Drilling
Gun Drilling Parameters
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Coolant Type |
|---|---|---|---|---|
| Duplex 2205 (S32205) | 20–35 | 0.008–0.025 | 100–180 bar | Oil-based, EP 1.5–2% S |
| Super Duplex 2507 (S32750) | 15–25 | 0.008–0.020 | 120–200 bar | Oil-based, EP 1.5–2% S |
| Lean Duplex 2101 (S32101) | 25–40 | 0.010–0.030 | 80–150 bar | Oil-based or high-oil emulsion |
| Duplex 2304 (S32304) | 25–35 | 0.010–0.025 | 100–160 bar | Oil-based preferred |
BTA Drilling Parameters
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Coolant Flow |
|---|---|---|---|---|
| Duplex 2205 | 25–40 | 0.020–0.060 | 30–80 bar | 150–300 L/min |
| Super Duplex 2507 | 18–30 | 0.015–0.050 | 40–100 bar | 150–300 L/min |
Parameter Effects on Tool Life
| Parameter Change | Effect on Tool Life | Recommendation |
|---|---|---|
| Speed reduced 20% | Tool life increases 50–100% | Start at lower end of range |
| Feed reduced 20% | Tool life increases 30–50% | But may worsen chip breaking |
| Feed increased 20% | Tool life decreases 30–50% | Better chips, shorter tool life |
| Coolant pressure doubled | Tool life increases 20–40% | Verify rotary union capability |
TIP
When setting parameters for a new duplex drilling application, start at the lower end of the speed range and the middle of the feed range. Observe chip form first — if chips are long and stringy, increase feed. If tool wear is excessive, reduce speed. Optimize speed and feed in alternating steps.
Coating Selection
Recommended Coatings for Duplex Drilling
| Coating | Max Temp | Hardness | Performance in Duplex | Best For |
|---|---|---|---|---|
| TiAlN | 900°C | ~3,300 HV | Good — standard choice | 2205, general duplex |
| AlCrN | 1,100°C | ~3,200 HV | Excellent — best notch wear resistance | 2507, super duplex |
| TiSiN (S3P) | 1,100°C | ~3,600 HV | Excellent — superior oxidation resistance | High-speed, MQL |
| AlTiN (high Al) | 1,100°C | ~3,500 HV | Very good — high hot hardness | High-production, dry machining |
| TiAlN + TiN | 900°C | ~3,300 + 2,300 HV | Good — TiN top layer for visual wear detection | Production monitoring |
A 2024 study on super duplex 2507 drilling compared TiSiN and AlTiN S3P-coated tools and found that coating selection significantly affected cutting forces. The TiSiN coating provided better performance at higher cutting speeds due to its superior oxidation resistance.
Coating Recommendations by Application
| Application | Recommended Coating | Justification |
|---|---|---|
| Gun drilling 2205 | TiAlN | Good heat resistance, balanced cost |
| Gun drilling 2507 | AlCrN | Best notch wear resistance for super duplex |
| BTA drilling duplex | TiAlN or AlCrN | High temperature resistance for continuous cutting |
| Gun drilling with MQL | TiSiN (S3P) | Superior oxidation resistance in low-lubricity conditions |
| High-production duplex | AlTiN (high Al) | Maximum hot hardness for extended tool life |
Coolant Strategy
Coolant Type
| Coolant Type | Suitability | Recommendation |
|---|---|---|
| Oil-based cutting oil | Excellent | First choice for all duplex deep hole drilling |
| High-oil emulsion (> 15%) | Good | For machines that cannot use straight oil |
| Standard emulsion (5–8%) | Poor — not recommended | Insufficient lubricity, promotes BUE and work hardening |
Oil-based cutting oil with 1.5–2.0% sulfur EP additives provides the best results. The high lubricity reduces friction at the guide pad interface, and the EP additives prevent adhesion and galling.
Coolant Pressure Requirements
| Drill Diameter | Recommended Pressure | Minimum Pressure |
|---|---|---|
| 1–3 mm | 180–250 bar | 150 bar |
| 3–10 mm | 120–200 bar | 100 bar |
| 10–25 mm | 100–180 bar | 80 bar |
| 25+ mm | 80–120 bar | 60 bar |
Super duplex 2507 requires pressure at the upper end of these ranges due to its higher strength and work-hardening tendency.
Case Studies
Case 1: Tungaloy UNIDEX — SUS329J3L Duplex
| Parameter | Value |
|---|---|
| Process | Indexable deep hole drilling (UNIDEX), 60 mm × 440 mm |
| Material | Duplex stainless steel SUS329J3L |
| Cutting speed | 60 m/min |
| Feed | 0.19 mm/rev |
| Insert grade | UC2220 (coated carbide) |
| Tool life | 14 m/insert — 55% longer than competitor (9 m) |
| Key factor | Optimized chip breaking at high feed |
Case 2: Gun Drilling Super Duplex 2507
| Parameter | Value |
|---|---|
| Process | Gun drilling, 8 mm × 400 mm in super duplex 2507 |
| Initial condition | Tool chipping after 8–12 holes; rough surface finish |
| Root cause | Cutting speed too high (30 m/min), insufficient edge hone |
| Correction | Reduced speed to 20 m/min; increased edge hone from 0.015 to 0.030 mm; switched to AlCrN coating |
| Result | Tool life increased to 45+ holes; surface finish Ra improved from 1.2 to 0.6 μm |
Case 3: BTA Drilling of Duplex 2205 Heat Exchanger Tube Sheet
| Parameter | Value |
|---|---|
| Process | BTA drilling, 25 mm × 600 mm in 2205 duplex |
| Failure | Notch wear at outer cutting edge after 30 holes; chips packed in flute |
| Root cause | Chip breaker geometry too mild for duplex — long chips tangled at pad interface |
| Correction | Changed to aggressive chip breaker geometry; increased feed from 0.03 to 0.05 mm/rev; increased coolant pressure from 40 to 70 bar |
| Result | Chips broken into 5–8 mm segments; tool life increased to 120+ holes |
FAQ
Q: What is the most difficult aspect of deep hole drilling duplex stainless steel? Work hardening combined with poor chip breaking is the most challenging aspect. The dual-phase structure work-hardens rapidly under the cutting edge, and the resulting chips are strong and ductile, resisting breakage.
Q: Can 316L parameters be used for drilling duplex 2205? No. Duplex 2205 has approximately 2× the yield strength of 316L and significantly higher work hardening. Cutting speed should be reduced by 30–50% compared to 316L parameters.
Q: What is the best coating for gun drilling super duplex 2507? AlCrN (aluminum chromium nitride) offers the best combination of notch wear resistance and heat resistance for super duplex grades. TiAlN is a good second choice for standard duplex 2205.
Q: What coolant pressure is required for gun drilling duplex stainless steel? 100–180 bar for 2205 duplex, 120–200 bar for 2507 super duplex. Oil-based cutting oil with 1.5–2.0% sulfur EP additives is strongly recommended.
Q: Why do chips from duplex stainless steel break poorly? The high ductility and work-hardening tendency of the duplex microstructure produces chips that resist curling and breaking. Aggressive chip breaker geometries and feed rates above 0.08 mm/rev are typically required.
Q: What tool geometry changes improve duplex drilling performance? Reduced relief angles (8–10° outer, 12–15° inner), increased edge hone (0.02–0.04 mm), and aggressive chip breaker geometry. Sharper edge geometry that works for 316L will chip in duplex.
Q: Can water-miscible coolant be used for drilling duplex? Only high-oil emulsions (> 15% concentration) are acceptable. Standard emulsions (5–8%) do not provide sufficient lubricity and will promote work hardening and BUE.
Q: What is the recommended cutting speed range for gun drilling duplex 2205? 20–35 m/min for gun drilling, depending on drill diameter, depth, and coolant pressure. Start at the lower end (20–25 m/min) and increase based on tool wear observations.
Q: How does the machinability of 2507 compare to 2205? Super duplex 2507 has approximately 30–50% lower machinability than 2205. Cutting speeds should be reduced by 20–30%, and feed rates by 15–25% when switching from 2205 to 2507.
Q: What is the best way to detect notch wear in duplex drilling? Monitor bore surface finish and tool power consumption. Notch wear causes a characteristic deterioration of surface finish at a specific depth corresponding to the depth of cut line. Tool power will increase gradually as notching progresses.