GB936872A - Improvements in or relating to a process of heat treating austenitic stainless steel and austenitic stainless steels whenever prepared by the aforesaid process - Google Patents

Abstract

Austenitic stainless steels are treated by the process which comprises selecting a steel of the transformation hardening type with or without precipitation hardening superimposed thereon and consisting of 0.01 to 0.4% C, 0.05 to 8.0% Mn, 0.05 to 2.0% Si, 8.0 to 20.0% Cr, 1.0 to 13.0% Ni, trace to 0.6% N, trace to 4.0% of at least one of the metals Al, Mo, V and Cu, the balance being iron and impurities, and containing not more than 5% delta ferrite in the annealed condition at room temperature; hot working, e.g. by rolling, the steel into the form of strip; cleaning the strip; annealing the steel strip at above 1900 DEG F., e.g. for 5-30 minutes, to dissolve the carbides and stabilize the austenite; cooling the steel sufficiently quickly to prevent carbide precipitation; cold working the steel to finish gauge, e.g. to effect at least a 10% reduction in cross-sectional area, so that a portion of the austenite is transformed into martensite and slip bands are provided in the austenite phase; critically annealing the steel at between 1800 DEG and 1875 DEG F., e.g. by heating continuously for 15 seconds-20 minutes, to randomly precipitate a portion of the carbides throughout the microstructure; trigger annealing the steel at between 1650 DEG and 1750 DEG F., e.g. for up to 1 hour, to randomly precipitate additional carbides and unbalance the austenite phase; cooling the steel sufficiently quickly, e.g. by quenching, to prevent further carbide precipitation; sub-zero cooling to below - 80 DEG F. and preferably above -110 DEG F., e.g. for 3 hours, to transform the austenite to martensite; and tempering the steel at between 750 DEG and 1050 DEG F., e.g. for up to 16 hours. The cold working step may include one or more operations such as by rolling to work the steel to within 10 to 40% of the cross-sectional area of the finish gauge and re-annealing of the intermediate gauge, e.g. at between 1900 DEG and 2000 DEG F., prior to the final cold working.