21-6-9 (UNS S21900) is a nitrogen-strengthened austenitic stainless steel in the same general family as Nitronic 50 and Nitronic 60 — the designation itself spells out the core chemistry: roughly 21% chromium, 6% nickel, and 9% manganese, with nitrogen doing much of the work of stabilizing the austenitic structure and, more importantly, substantially raising strength above what a conventional 300-series grade like 304 or 316 can offer. Because manganese partially substitutes for nickel in stabilizing austenite, 21-6-9 also carries a cost and supply advantage over fully nickel-stabilized austenitics while delivering meaningfully higher yield and tensile strength.

That strength-to-corrosion-resistance combination makes 21-6-9 a common choice for aerospace and military hardware, high-strength fasteners, retaining rings, and structural components where 304/316 would be adequate for corrosion but too weak, and where a fully hardenable martensitic or PH grade is not wanted because of its lower corrosion resistance or magnetic behavior. It is typically supplied and used in the annealed or cold-worked (strain-hardened) condition, since it is not age-hardenable.

Chromium, manganese, nickel, and carbon on the prior version of this page were reasonably close to verified UNS S21900 ranges; nitrogen — the element that actually defines this alloy's strength advantage — was missing and has been added back in.

21-6-9 machines noticeably harder than standard 304/316 stainless because nitrogen strengthening raises tensile and yield strength well above conventional austenitics without changing the underlying gummy, work-hardening character of the microstructure. Cutting forces run higher, and the same rapid work-hardening behavior common to all austenitic stainless is more pronounced here — a dull edge, light feed, or any dwell time at the cutting edge quickly burnishes a hardened skin that fights the next pass. Chips are tough and stringy, resisting the clean fracture seen on lower-strength stainless.

Sharp, positive-rake carbide with a coating suited to work-hardening materials (TiAlN or similar) is the starting point, run at moderate-to-higher cutting speeds with a heavier, decisive depth of cut rather than a light finishing pass — counterintuitively, research on this alloy family shows that backing off speed to "go easy" on the material often increases built-up edge and surface roughness rather than reducing tool wear. Rigid setups and constant feed rates matter as much as tool selection, and coolant delivery directly to the cutting zone helps control both heat and adhesion at the tool tip.

Values assume favorable cutting conditions: a well-matched insert grade, rigid tool and workpiece clamping, and short tool overhang. Nitrogen-strengthened austenitics like this one run below general 300-series stainless speeds due to elevated cutting forces.

Turning

Parting / Grooving

Milling