Armco 18, better known in industry literature as 18-18 Plus (UNS S28200), is a chromium-manganese-nitrogen austenitic stainless steel originally developed by Armco Steel as a low-nickel alternative to conventional 18-8 type stainless like 304. The "18-18" name refers to its nominal balance of roughly 17-19% chromium and 17-19% manganese — in place of the nickel that ordinarily stabilizes the austenitic structure in 300-series grades, 18-18 Plus relies on manganese together with a substantial nitrogen addition (0.40-0.60%) to hold a stable, non-magnetic austenitic microstructure at room temperature.

Eliminating most of the nickel content gives this alloy more stable, predictable material cost versus nickel-price swings, while the nitrogen strengthening pushes yield strength well above standard 304/316. It is commonly specified for seawater-handling equipment, pulp and paper bleach-plant components, tall oil distillation columns, heat exchangers, hydrometallurgical process equipment, and other applications where 300-series corrosion resistance is required together with low magnetic permeability and higher mechanical strength than conventional austenitics provide.

Prior data on this page listed a designation field that was disabled in the source (never actually rendered) and had been carried over from an unrelated material page. It has been corrected to the verified UNS S28200 designation for this alloy.

Composition corrected against verified UNS S28200 reference data. Prior data on this page (Fe 66%, Ni 3.7%, Cr 17.2%, Mn 12.5%) did not match this alloy's actual chemistry — 18-18 Plus is essentially nickel-free, which is one of its defining commercial advantages, and nitrogen and molybdenum/copper additions (absent from the prior figures) were added back in.

18-18 Plus shares its core machining challenge with the wider family of nitrogen-strengthened chromium-manganese austenitics (Nitronic-type alloys, 21-6-9, and similar grades): rapid work hardening under the cutting edge. The nitrogen addition that gives this alloy its strength advantage over 304 also means the surface deforms and hardens quickly if the tool rubs instead of shears, so a dull edge or hesitant feed can turn a routine pass into a fight against a work-hardened skin. Unlike high-nickel superaustenitics, 18-18 Plus does not carry the same adhesive, gummy chip behavior driven by nickel content, so built-up edge tendencies are somewhat more manageable — but cutting forces still run noticeably higher than plain 304 because of the elevated strength.

Sharp, positive-rake carbide with adequate edge preparation is the starting point, paired with decisive, constant feed rates that keep the tool cutting below any hardened layer left by the previous pass. Rigid tool and workpiece clamping matter more here than on general-purpose stainless, since chatter or deflection immediately worsens work hardening on the next pass. Coolant helps manage both heat (thermal conductivity is still relatively low, as with any austenitic) and built-up edge at the tool tip.

Values assume favorable cutting conditions: a well-matched insert grade, rigid tool and workpiece clamping, good-quality raw material, and short tool overhang. Nitrogen-strengthened Cr-Mn austenitics run well below general 300-series stainless speeds due to elevated cutting forces.

Turning

Parting / Grooving

Milling