Specifications

Chemical composition and mechanical properties

Limits as given in the ASTM B specifications for N08020.

Pipe, bar and forging requirements are given in ASTM B464, B729, B473 and B462.

Actual values for each heat are reported on the mill test certificate.

Why Alloy 20

Alloy 20 is a niobium-stabilised nickel-iron-chromium alloy with molybdenum and copper, designed for sulphuric acid. It is chosen for sulphuric acid across a wide range of concentrations, phosphoric and nitric acids, pickling liquors and chloride-bearing water. Compared with 316L, Alloy 20 resists sulphuric acid and chloride stress corrosion cracking far better; compared with 904L it has higher nickel; for hot concentrated or heavily contaminated acids, Hastelloy C276 is used instead.

Seamless and welded Alloy 20 pipe

Seamless Alloy 20 pipe and tube to ASTM B729 is made by extrusion or piercing of a billet followed by cold working and annealing, and is used for higher-pressure lines and heat-exchanger tubes. Welded pipe to ASTM B464 and tube to B468 are made from strip or plate and fully annealed, and are economical in thinner walls and larger diameters. Both meet the same alloy requirements.

Acid piping design

In sulphuric acid lines, corrosion is highest where the acid is turbulent, hot or being diluted. Dilution points, where water and acid mix with release of heat, are the most severe and are often made in Alloy 20 or higher alloys even when the rest of the line is carbon steel or lined pipe. Velocity limits should be followed, and long-radius bends used to reduce erosion. Alloy 20 heat-exchanger tubes are used in acid coolers and heaters where stainless steel tubes would fail.

Welding and fabrication

Alloy 20 is welded by TIG, MIG and manual arc processes with matching ER320LR filler or suitable nickel-based fillers, on clean joints free of oil, paint, marking crayon, sulphur and lead, using low heat input and controlled interpass temperatures. Heat tint is removed and welds are pickled where corrosion resistance at the weld matters. Fabrication areas should be kept separate from carbon steel, and dedicated tools used to avoid iron contamination.

Testing and ordering

Common supplementary requirements are an intergranular corrosion test to ASTM A262, positive material identification and third-party inspection by the buyer's nominated agency. Please give the grade and specification, product type, size, schedule or class, quantity and any testing or inspection requirement with your enquiry. Each piece is marked with grade, specification, size and heat number, packed in wooden boxes or on pallets, and traceable to its mill test certificate.

Sizes, supply and packing

Pipes are supplied in random lengths, typically about 6 metres, or cut to length, with plain or bevelled ends and plastic end caps. Tubes are supplied in straight lengths to order, or U-bent for heat exchangers. Every length is marked with grade, specification, size, heat number and seamless or welded construction. For heat-exchanger tubes, please give the outside diameter, minimum or average wall thickness, length and bend schedule with the enquiry.

Certification and traceability

Alloy 20 is often confused with 300-series stainless steels and with other nickel alloys of similar appearance. Every piece carries its grade and heat number, and the mill test certificate reports the full chemistry – including nickel, copper, molybdenum and niobium – and mechanical properties. Positive material identification can be carried out before dispatch on request, and intergranular corrosion testing can be added where the end user specifies it.

Applications and related products

Alloy 20 pipes and tubes are used in sulphuric acid production and handling, steel pickling lines, phosphoric acid plants, chemical and pharmaceutical processing, synthetic rubber and plastics production, and heat exchangers in chloride-bearing water. Matching products include Alloy 20 sheets and plates, Alloy 20 round bars, Alloy 20 flanges and Alloy 20 pipe fittings. Background on the alloy is on the Alloy 20 page. Other materials are on the seamless pipes and tubes page.