Heat treatment of 2 1/4 Cr--1 Mo steel for breeder reactor steam generators

Commercially, 2 1/4 Cr--1 Mo steel is used in various heat-treated conditions. Present designs for breeder reactor steam generators call for the steel to be used in an annealed or isothermally annealed condition. Future reactor steam generators may offer an incentive to use a normalized-and-tempered steel. The elevated-temperature mechanical property differences between the normalized-and-tempered and annealed or isothermally annealed conditions were assessed to determine the advisability of using the normalized-and-tempered material for breeder reactor steam generators. The mechanical properties of 2 1/4 Cr--1 Mo steel are determined by the microstructure, which can be different for annealed and normalized-and-tempered steel. The extreme in microstructural difference is when the normalized-and-tempered steel is entirely bainite and the annealed steel is 75 to 80% proeutectoid ferrite, the balance bainite. Mechanical property data for annealed and normalized-and-tempered 2 1/4 Cr--1 Mo steel with these microstructures were compared. The results were analyzed in terms of the kinetics of the different metallurgical changes that the various microstructures undergo during an elevated-temperature test or elevated-temperature exposure during service. It was concluded that 2 1/4 Cr--1 Mo steel could be used in all heat treatments that are presently used in commercial practice