20Cb-3 is a superaustenitic nickel-iron-chromium alloy developed by Carpenter Technology in the 1930s and registered as UNS N08020 — it is the alloy behind the generic name "Alloy 20" used throughout the corrosion-resistant alloy industry. Unlike standard 300-series austenitics, 20Cb-3 carries roughly 32-38% nickel and 19-21% chromium on an iron balance, plus two additions that define its identity: 2-3% molybdenum and, critically, 3-4% copper. The copper addition is what sets this alloy apart from moly-bearing stainless like 316 — it gives 20Cb-3 outstanding resistance to hot sulfuric acid and other non-oxidizing acids that attack chromium-nickel stainless steels almost immediately. Niobium (columbium) stabilization, denoted by the "Cb" in the name, ties up carbon to prevent sensitization and intergranular corrosion during welding.

Because of this acid resistance, 20Cb-3 is the workhorse alloy for sulfuric acid production and handling, phosphoric acid service, pickling equipment, and other chemical processing environments where 316 stainless would corrode quickly. Typical applications include process piping, valves, pumps, agitators, heat exchangers, and reaction vessels in the chemical, fertilizer, and pharmaceutical industries.

Composition corrected against ASTM B463 / UNS N08020 reference data. The copper addition (3.0-4.0%) is the defining alloying element for sulfuric acid resistance and was missing from prior data for this page.

20Cb-3 machines like a heavier-working relative of the 300-series austenitics, reflecting its superaustenitic chemistry. The combined nickel and copper content make chip flow somewhat gummier than 316, and the material work hardens readily if the cutting edge rubs instead of shearing — a dull edge, light feed, or hesitation in the cut will burnish a hardened skin into the surface that punishes the next pass. Low thermal conductivity, typical of high-nickel alloys, concentrates heat at the tool tip rather than carrying it away in the chip, which accelerates crater wear if speeds run too aggressive.

Positive rake, sharp cutting edges are essential, and heavy, constant feed rates should be maintained to keep the tool cutting below any previously work-hardened layer — dwelling or riding on a dull edge is the fastest way to ruin tool life and surface finish on this alloy. Rigid workholding, short tool overhang, and generous coolant flow all pay off disproportionately compared to machining plain stainless. Because 20Cb-3 is not hardenable by heat treatment, machinability stays consistent regardless of prior thermal history, unlike precipitation-hardening grades.

Values assume favorable cutting conditions: a well-matched insert grade, rigid tool and workpiece clamping, good-quality raw material, and short tool overhang. Adjust down for interrupted cuts or thin-wall parts prone to deflection.

Turning

Parting / Grooving

Milling