Duplex stainless steel gives you roughly twice the yield strength of 316L with far better chloride stress-corrosion cracking resistance, which is why it dominates offshore piping, desalination hardware, and chemical processing. It also cuts like a completely different material. Shops that treat duplex stainless steel machining as “316 but harder” burn through inserts, work-harden the surface, and miss tolerance. This guide explains what actually changes and how to get clean, in-spec parts.
What Duplex Steel Actually Is
Duplex grades contain roughly equal parts austenite and ferrite in their microstructure. That balance delivers the corrosion resistance of austenitic stainless with the strength and stress-corrosion resistance of ferritic. Standard duplex 2205 (UNS S32205) is the workhorse. Super duplex 2507 (UNS S32750) pushes chromium, molybdenum, and nitrogen higher for a pitting resistance equivalent number above 40, suited to seawater service. Lean duplex 2304 trades some corrosion resistance for lower cost.
The dual-phase structure is exactly what makes machining hard. The two phases have different hardness and different work-hardening behaviour, so the cutting edge is continually transitioning between them. Add a yield strength near 550 MPa, low thermal conductivity that concentrates heat at the tool tip, and rapid work hardening, and you have a material that punishes light, hesitant cuts more than aggressive ones.
Cutting Parameters That Work
The governing rule is to stay under the work-hardened layer. Depth of cut must always exceed the depth of the layer left by the previous pass, which for duplex means avoiding cuts shallower than about 0.5 mm in finishing and never dwelling or rubbing. Surface speeds run roughly 30 to 40 percent below equivalent 316L values: around 80 to 120 m/min for 2205 turning with coated carbide, and lower still for 2507.
Feed rate should be maintained rather than reduced. A steady, positive feed keeps the edge cutting rather than burnishing. Rigidity is non-negotiable, since vibration multiplies work hardening. High-pressure through-tool coolant directed at the cutting zone removes heat and breaks chips that would otherwise weld to the insert. We apply these parameters across CNC turning, CNC milling, and CNC drilling operations on duplex components.
Tooling and Insert Selection
Sharp, positive-rake geometry with a strong edge preparation gives the best balance between cutting force and edge life. PVD-coated carbide grades formulated for stainless are the default; CVD coatings can work for continuous turning but chip more readily in interrupted cuts. Ceramic and CBN are generally not worth the risk on duplex.
Tool life is shorter than on austenitic stainless and should be managed on a scheduled index rather than run to failure, because a worn edge dramatically increases work hardening and can push a nearly finished part out of tolerance. For features that are impractical to cut mechanically, wire EDM removes the tool-wear variable entirely. Our stainless steel machining guide compares parameters across the austenitic, ferritic, and duplex families.
Preserving Corrosion Resistance
Machining damages duplex corrosion resistance in three ways: it smears the surface, embeds free iron from tooling and fixturing, and can locally overheat the material. Excessive heat above roughly 300°C risks precipitating sigma phase, which is brittle and destroys corrosion performance. Keeping the cut cool is a corrosion issue, not just a tool-life issue.
Post-machining, passivation to ASTM A967 removes embedded iron and restores the chromium oxide film. This step is mandatory, not optional, for any duplex part going into a chloride environment. For maximum performance, electropolishing goes further by removing the deformed surface layer entirely. Our passivation services page explains the nitric versus citric acid choice and when each applies.
Welding and Heat Treatment Considerations
If your part is fabricated rather than machined from solid, heat input control becomes critical. Duplex needs enough heat input to allow austenite to reform in the weld and heat-affected zone, but not so much that intermetallic phases precipitate. Solution annealing followed by rapid quench restores the phase balance where the thermal history is uncertain. Our heat treatment capability covers solution annealing for duplex assemblies, and the custom metal flanges guide covers the facing and sealing standards these parts are inspected against.
Where Duplex Machining Pays Off
Duplex is specified where chloride attack would kill 316L. Offshore and subsea piping, valves, and manifolds are the largest market. Desalination plants use duplex for high-pressure pumps and pipework. Chemical and petrochemical processing uses it for heat exchanger tube sheets and reactor internals. Pulp and paper digesters, marine propulsion hardware, and structural components in coastal infrastructure round out the list. The material premium is repaid in service life, which is why it appears on drawings despite costing more to buy and more to cut.
Achievable Tolerances and Finishes
With rigid setups and controlled tooling, general tolerances of plus or minus 0.05 mm and critical features to plus or minus 0.02 mm are routine on duplex. Surface finish of Ra 1.6 micrometres comes straight off a finish turn, with Ra 0.4 or better achievable through cylindrical grinding on sealing diameters. Expect longer cycle times than an equivalent 316L part and budget accordingly rather than being surprised at quotation.
Frequently Asked Questions
How much slower is duplex than 316L to machine?
Plan on 30 to 40 percent lower surface speed and correspondingly longer cycle times, with shorter tool life.
Why can’t I take light finishing cuts on duplex?
Shallow cuts ride in the work-hardened layer, which accelerates tool wear and degrades the surface. Keep depth of cut above roughly 0.5 mm.
Is passivation required after machining duplex?
Yes. Machining embeds free iron and smears the surface, so ASTM A967 passivation is needed to restore corrosion resistance.
What is sigma phase and why does it matter?
A brittle intermetallic that precipitates with excessive heat, destroying both toughness and corrosion resistance.
Do you machine super duplex 2507?
Yes, with reduced speeds, high-pressure coolant, and scheduled insert changes to protect tolerance.
What tolerances can you hold?
Plus or minus 0.05 mm generally, and plus or minus 0.02 mm on critical features with grinding where required.
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