Alloy 20 (UNS N08020): Grades, Properties & Supply

“Carpenter 20 / 20Cb-3” and “INCOLOY 020” are producer designations for material to UNS N08020; Sagiant Piping Industries is an independent supplier and stockist, not the alloy producer. Identify the exact specification edition, condition and any producer requirement on enquiry.

Limiting composition from the INCOLOY 020 producer bulletin (a sourced reference, not the declared requirement for every purchase standard). “Columbium (Cb)” is the older name for niobium; the Nb+Ta stabilisation rule is 8×C minimum (e.g. at C=0.05%, that is 0.40%) and does not waive other limits. Some producers publish tighter values — for example Carpenter’s type analysis lists C 0.06 max, P 0.035 max and Ni 32.50–35.00 — so source-specific limits are kept separate and never combined into an invented universal grade. The heat/product certificate and ordered specification edition control acceptance.

The same UNS number does not prove identical producer limits, form, heat treatment, examination or code acceptance. Designations are cross-references and related materials, not substitution approvals or stocked coverage; no arbitrary “Alloy 20L/20H” rows are added without a recognised specification.

Legacy references are retained for existing drawings, replacement parts and historical MTCs, clearly labelled. Withdrawal does not automatically invalidate previously certified stock, and ASTM status does not by itself determine an ASME SB adoption or an existing design code. A-series routes are verification candidates, not an approved substitution table — check exact grade inclusion, class, heat treatment, tests and project acceptance in the ordered edition; do not silently change a purchase specification. Rename “equivalents” as designation cross-references.

Availability is confirmed per enquiry with stocked / sourced / made-to-order status — the ranges below are indicative, not a statement that every size, schedule and form is held.

The introductory ~18" welded and 12" seamless figures are route-dependent, not a contradiction. Class is a flange pressure class, not psi. Keep dimensional standards (B36.10 / B36.19) separate from material qualification; mark undefined size/schedule combinations rather than interpolate. See the schedule chart and weight calculator.

• Alloy 20 was developed for sulphuric acid service; copper and molybdenum give good resistance across a range of concentrations, but performance depends on concentration, temperature, aeration, impurities, velocity and weld condition — not “all concentrations at any temperature”. Confirm with a concentration–temperature corrosion map for the actual fluid.
• Its resistance to chloride stress-corrosion cracking is a recognised advantage over standard austenitic stainless steels; producer statements about 20–40% sulphuric acid specifically concern SCC and should not be read as an unrestricted general-corrosion guarantee.
• Niobium stabilisation reduces susceptibility to intergranular attack by tying up carbon; it does not guarantee zero carbide precipitation or full intergranular resistance after every thermal cycle. Welding, cold work, heat treatment and sensitisation history still matter.
• Also assess phosphoric and mixed acids, chloride pitting/crevices, oxidising vs reducing conditions, deposits and galvanic coupling. Publish corrosion rates only with medium, concentration, temperature, aeration, duration and specimen/weld condition.
• Separate aqueous-corrosion limits from mechanical/code limits and from heat-treatment temperatures; a producer’s elevated-temperature application note is not an acid-service limit, and any sour-service claim needs the applicable standard, condition and environment.