X20CrNiMo22-5-3 is a duplex stainless steel — its roughly 22% chromium, 5% nickel, and 3% molybdenum composition places it in the well-known 2205 family (Wnr. 1.4462), and that Cr-Ni-Mo balance is the giveaway. Duplex grades are deliberately alloyed so nickel content is too low to fully stabilize an austenitic structure, leaving the material with a mixed microstructure of roughly equal parts austenite and ferrite. The previous version of this page classified this alloy as martensitic, which does not match its composition at all — there's no meaningful carbon-hardening mechanism here, and the Cr-Ni-Mo balance is squarely duplex territory.

That two-phase structure is what makes duplex stainless valuable: it combines roughly double the yield strength of a standard austenitic grade like 316 with excellent resistance to chloride-induced pitting and stress-corrosion cracking, thanks to the combined effect of chromium, molybdenum, and (in the full specification) nitrogen. This makes it a common choice for offshore, marine, chemical processing, and pulp-and-paper equipment where 316 isn't strong or corrosion-resistant enough.

The trade-off for that strength and corrosion resistance is machinability. Duplex stainless work-hardens readily like austenitic grades, but its higher strength and the abrasion resistance contributed by the ferrite phase and alloy content make it noticeably harder on tooling than 304 or 316.

Duplex stainless is generally considered more difficult to machine than either straight austenitic or ferritic grades, and this alloy is no exception. The two-phase microstructure means the tool is cutting through two mechanically different constituents in the same pass — the tougher, more ductile austenite phase work-hardens readily like 316, while the harder ferrite phase resists cutting and contributes to abrasive flank wear. The combined effect is higher cutting forces, faster edge wear, and a narrower window between "cutting cleanly" and "rubbing and work-hardening" than on the common 300-series grades.

Because of the higher strength and work-hardening tendency, feed rate discipline matters even more here than on 304 or 316 — light, hesitant cuts let the tool ride on a hardening surface instead of shearing beneath it, which accelerates wear rapidly. A sharp, positive-rake edge with good heat and wear resistance is essential, and insert life is generally shorter than on austenitic stainless of similar hardness, so realistic tool-life expectations and slower speeds are part of machining this grade successfully.

Rigid setups, adequate coolant, and a premium coated carbide grade go further here than on easier stainless grades — marginal tooling or setups that "get by" on 304 often fail outright on duplex.