DUPLEX ALLOY

摘要

A stainless duplex ferritic-austenitic steel with improved corrosion resistance. The steel consists essentially of about: .03 w/o Max. carbon, 3.0 w/o Max. manganese, 1.0 w/o Max. silicon, 26.0 to 29.0 w/o chromium, 3.5 to 5.2 w/o nickel, 3.5 w/o Max. molybdenum, 0.15 w/o Min. nitrogen and the balance essentially iron. The steel preferably contains about 0.17 to 0.35 w/o nitrogen for improved pitting resistance and for increased austenite content. Welds of the steel preferably contain at least about 17% austenite in the as-welded condition for improved pitting and intergranular corrosion resistance. SPECIFICATION This invention relates to a stainless duplex ferriticaustenitic steel having a unique combination of good mechanical properties and good corrosion resistance properties. Heretofore, a stainless duplex ferritic-austenitic steel, designated AISI Type 329, has been commercially available with: a) good mechanical properties such as high annealed yield strength; and b) good corrosion resistance such as resistance to general corrosion in the presence of strong oxidizing agents (e.g., boiling nitric acid). Typical uses for Type 329 steel have included tube or pipe for heat exchange applications involving severely corrosive, oxidizing environments such as are found in the petroleum refining, petrochemical, chemical, and pulp and paper industries (e.g., in nitric acid cooler-condensers). Type 329 steel has typically had a composition of about 0.08 weight percent (w/o) Max. carbon, 1.0 w/o Max. manganese, 0.75 w/o Max. silicon, 23.0 to 28.0 w/o chromium, 2.5 to 5.0 w/o nickel, 1.0 to 2.0 w/o molybdenum, with the balance essentially iron. Compositions similar to Type 329 steel have been sold, containing down to about .02 w/o carbon, up to about 2 w/o manganese, up to about 6 w/o nickel, up to about 30 w/o chromium, up to about 3.5 w/o molybdenum and/or up to about 0.25 w/o nitrogen. However, the resistance to intergranular corrosion in the presence of strong oxidizing agents and the resistance to pitting in the presence of halides, particularly chlorides, of Type 329 steel has left something to be desired in areas of the steel which have been welded, particularly in areas which have