255 (commonly sold under the trade name Ferralium 255, UNS S32550) is a super duplex stainless steel — the tier above standard duplex grades like 2205 in both alloy content and corrosion resistance. It still relies on the same dual-phase austenite-ferrite microstructure that defines the whole duplex family, but pushes chromium up toward 25-27% and molybdenum up to nearly 4%, well above 2205's roughly 3%. That extra molybdenum, combined with a distinctive copper addition of 1.5-2.5% that most other duplex grades don't carry, gives 255 a pitting resistance equivalent number in the high 30s to around 40 — noticeably ahead of standard duplex and well beyond 316L.

The copper addition is what sets 255 apart chemically from other super duplex grades like F55/Zeron 100: it improves resistance to reducing acids such as sulfuric and phosphoric acid, an environment where straight chromium-molybdenum duplex alloys are comparatively weak. Combined with the strength and stress-corrosion-cracking resistance inherent to any duplex microstructure, that acid resistance makes 255 a common choice for seawater-handling equipment, flue-gas desulfurization systems, chemical processing vessels, and marine and offshore components operating in aggressive chloride and acidic environments where even standard 2205 isn't enough margin.

255 is one of the more demanding materials this site covers, and the reason comes straight from its alloy content. Super duplex grades carry more chromium, molybdenum, and nitrogen than standard duplex, and all three raise both the strength of the ferrite phase and the overall resistance of the material to being sheared. Cutting forces on 255 run meaningfully higher than on 2205, heat generation at the tool edge is more intense, and the copper addition that helps its corrosion resistance does nothing to make the material easier to cut — the abrasive load on the tool comes almost entirely from the chromium and molybdenum content.

Where 255 differs from the plain austenitic problem-child grades like 316, work hardening from a rubbing edge is still a secondary concern compared with the raw mechanical load of shearing through a high-strength ferrite network on every pass. That calls for a genuinely robust insert: strong substrate, wear-resistant coating, and enough edge support to handle sustained high force without chipping or rapid flank wear. Rigidity throughout the machine, toolholder, and workholding chain is non-negotiable at this alloy level — any deflection shows up quickly as accelerated wear or edge failure. Reduced speeds and feeds relative to standard duplex, generous coolant flow, and a grade purpose-built for high-alloy duplex and super duplex stainless are the baseline expectations for working with 255 successfully.