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Super Duplex 2507 Machinability
Super Duplex 2507 (UNS S32750, EN 1.4410) is harder to machine than 304L or 316L austenitic stainless and harder than standard duplex 2205. The combination of high yield strength (550 MPa minimum), high work-hardening rate, and tendency to form built-up edge drives down material-removal rate and shortens tool life. A reasonable benchmark is that super duplex 2507 cuts at approximately 30 percent of the speed of 304L and approximately 60 percent of the speed of 2205, with comparable feed rate. This page covers practical cutting parameters, tool selection, coolant strategy, and the principal failure modes when machining UNS S32750 fasteners and bar.
Super duplex 2507 is supplied to ASTM A479 bar specification with the surface condition (hot-rolled, peeled, ground, or cold-finished) noted on the mill test certificate. Cold-finished bar offers a tighter dimensional tolerance and less surface decarburisation but is otherwise machined identically to hot-rolled bar.
Machinability Rating versus Reference Grades
Machinability is normally expressed as a percentage of free-machining 1212 carbon steel (taken as 100 percent). Comparative ratings for super duplex 2507 against reference stainless grades are tabled below. The numbers are approximate and depend on feature geometry, tool material, and coolant.
| Material | Machinability Rating, percent of 1212 | Relative Speed vs Super Duplex 2507 |
|---|---|---|
| Free-machining 1212 carbon steel | 100 | Approximately 5x faster |
| 304L austenitic stainless | 40 to 45 | Approximately 3x faster |
| 316L austenitic stainless | 30 to 35 | Approximately 2x faster |
| Duplex 2205 (UNS S31803 / S32205) | 20 to 25 | Approximately 1.5x faster |
| Super Duplex 2507 (UNS S32750) | 15 to 18 | 1x (reference) |
| Zeron 100 (UNS S32760) | 15 to 18 | Approximately equal |
| Alloy 625 (UNS N06625) | 10 to 15 | Approximately 0.7x |
Recommended Cutting Speeds and Feed Rates
Cutting parameters below are starting points for production turning, drilling, and milling of super duplex 2507. Heavy first cuts at moderate speed are preferred over light cuts at high speed, because dwelling at low depth-of-cut allows the work-hardened layer at the surface to dominate and accelerate tool wear.
| Operation | Tool Material | Cutting Speed, m per min | Feed Rate, mm per rev or mm per tooth | Depth of Cut, mm |
|---|---|---|---|---|
| Turning, roughing | Coated carbide (TiAlN, Al2O3) | 50 to 80 | 0.30 to 0.50 mm per rev | 2.5 to 6.0 |
| Turning, finishing | Coated carbide | 70 to 110 | 0.10 to 0.25 mm per rev | 0.5 to 1.5 |
| Drilling, solid carbide | Coated carbide | 15 to 25 | 0.05 to 0.20 mm per rev (size dependent) | (diameter dependent) |
| Drilling, indexable | Coated carbide insert | 40 to 70 | 0.04 to 0.10 mm per rev | (diameter dependent) |
| Face milling | Coated carbide | 60 to 90 | 0.10 to 0.20 mm per tooth | 1.0 to 4.0 |
| Tapping (rolled thread preferred) | Cobalt HSS or coated carbide | 3 to 6 | (thread pitch) | (tap geometry) |
| Threading by single point | Coated carbide | 70 to 100 | (thread pitch) | 0.05 to 0.20 per pass |
Tool Selection
Carbide tooling is the default for super duplex 2507. High-speed steel (HSS) is generally avoided except for tapping small threads, because HSS lacks the hot hardness to resist edge plastic deformation under the high cutting forces super duplex generates.
- Carbide grade: medium-grain ISO M-class or P-class carbide is the workhorse. Submicron-grain carbides extend tool life on finishing passes but chip more readily on interrupted cuts.
- Coating: TiAlN or Al2O3 coatings provide thermal barrier and chemical wear resistance. Multi-layer PVD coatings (TiAlN over TiN, with an Al2O3 or oxide top layer) give the best combination of edge strength and crater-wear resistance.
- Edge geometry: positive rake (5 to 10 degrees) reduces cutting force and minimises work hardening of the new surface. A small honed edge (0.02 to 0.05 mm) is preferred over a sharp edge for production runs to suppress chipping.
- Insert chip-breaker: medium-rough geometries with controlled chip flow prevent long stringy chips that would wrap on the workpiece and tool holder.
Coolant Strategy
Flood coolant is mandatory. Through-tool coolant (high-pressure, ideally 70 bar or above) significantly extends tool life on drilling and turning operations because it both clears chips from the cutting zone and breaks up the work-hardened layer ahead of the tool.
- Coolant chemistry: water-soluble synthetic or semi-synthetic emulsion, mixed at 8 to 12 percent. Straight cutting oils are acceptable but generate more heat than emulsions and require strict control of mist for operator health.
- Flow rate: 30 litres per minute minimum on turning, 50 litres per minute on milling, with high-pressure through-tool delivery on drilling.
- Chlorinated extreme-pressure (EP) additives are NOT acceptable on super duplex used in chloride-bearing service (seawater, oil and gas, FGD). Residual chloride from machining coolant has been documented as a stress-corrosion cracking initiator on stainless components in service.
Common Failure Modes
| Failure Mode | Root Cause | Fix |
|---|---|---|
| Rapid flank wear | Cutting speed too high; hot hardness exceeded | Reduce cutting speed by 20 percent; verify coating integrity on insert |
| Built-up edge (BUE) | Cutting speed too low; coolant inadequate | Increase cutting speed; switch to coated carbide; raise coolant flow |
| Notch wear at depth-of-cut line | Work hardening of prior pass surface | Vary depth of cut on each pass to break up notch; use larger nose radius |
| Insert chipping on interrupted cut | Insert too brittle (submicron grain) or geometry too positive | Switch to tougher carbide grade; hone the edge; reduce feed |
| Surface stress-corrosion cracking in service | Chlorinated coolant residue | Switch to chlorine-free coolant; pickle component before service entry |
Thread Manufacturing
For super duplex 2507 fasteners (bolts, studs, screws), thread rolling is preferred over thread cutting wherever the diameter and length permit. Rolled threads work-harden the root zone, raising local strength and improving fatigue performance, and avoid the spiral cutting marks that can serve as crack-initiation sites in fatigue or chloride-stress service. Super duplex stud bolts and hex bolts are normally rolled-thread by default; cut threads are used only on diameters above approximately 50 mm or where the section length exceeds the thread-rolling die capacity.
Related Super Duplex 2507 References
- UNS S32750 reference page
- Mechanical properties
- Heat treatment
- Welding procedure
- Super duplex 2507 fasteners
- Super duplex 2507 pipe
- ASTM A182 F53
Super Duplex 2507 Machinability FAQ
Why is super duplex 2507 hard to machine?
High yield strength (550 MPa minimum), high work-hardening rate, and tendency to form built-up edge all increase the cutting force and shorten tool life relative to austenitic grades. The duplex microstructure also produces uneven cutting forces because the harder ferrite and softer austenite respond differently to the tool, accelerating notch wear.
What cutting speed should I use for turning UNS S32750?
A reasonable starting point is 50 to 80 m per minute for roughing and 70 to 110 m per minute for finishing, both with coated carbide tooling and flood coolant. Adjust downward by 20 percent if flank wear develops within the first few parts; adjust upward by 10 percent if built-up edge appears, which signals the cutting speed is too low.
Are HSS taps acceptable for super duplex 2507?
Cobalt HSS taps (M35 or M42) are acceptable for small thread sizes (M6 to M16) where carbide taps are commercially scarce. Coated carbide taps last longer in production. For diameters above M16, thread rolling is preferred over tapping wherever the geometry permits, both for tool life and for the metallurgical benefit of work-hardened thread roots.
Can chlorinated cutting oil be used on super duplex 2507?
No, not on components destined for chloride-bearing service. Chlorinated extreme-pressure additives leave residual chloride that has been documented as a stress-corrosion cracking initiator on stainless components in seawater, oil-and-gas, and FGD service. Use water-soluble synthetic or semi-synthetic emulsion (8 to 12 percent mix) without chlorinated EP additives, and pickle the component before service entry if chlorinated coolant was used at any point during fabrication.
Is thread rolling or thread cutting preferred for super duplex 2507 fasteners?
Thread rolling is preferred wherever the diameter (typically up to M48 to M64 depending on die capacity) and the section length permit. Rolled threads work-harden the thread root, raising local fatigue strength and avoiding the spiral cutting marks that can serve as crack-initiation sites in chloride-stress or sour-service applications. Cut threads are used only where the geometry exceeds the rolling capability.
What coolant flow rate is recommended?
30 litres per minute minimum on turning operations, 50 litres per minute on milling, with high-pressure through-tool coolant (ideally 70 bar or above) on drilling. The objective is twofold: cool the cutting edge to maintain hot hardness, and break up the work-hardened layer ahead of the tool by mechanical action of the coolant jet.
Does heat treatment affect machinability?
Material in the correct solution-annealed condition machines as described above. Material that has been incorrectly cooled and contains sigma phase will be both harder and more brittle, accelerating tool wear and producing poor surface finish. If machinability seems significantly worse than expected on a particular bar, hardness and metallographic ferrite checks are warranted before the bar is released to production.
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