A modified 9 Cr-1 Mo ferritic steel has been selected for development by the U.S. Department of Energy as an alternative structural material for breeder reactor applications in which type 304 stainless steel or annealed 2 1/4 Cr-1 Mo steel is currently being used. Different 9 Cr-1 Mo steels are already being used commercially in both Britain and the Lnited States, and a 9 Cr-2 Mo steel (EM12) is being used commercially in France. The 12% Cr steel alloy HT9 is also often recommended for

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... erature service. Cre.ep-rupture data for all six of the above ferricic steels were collected and analyzed to yield rupture life as a function of stress, temperature, and lot-to-lot vari. .xon= in strength. Yield and tensile strength data for the three 9 Cr-1 Mo materials were also examined. All results were compared with the behavior of type 304 stainless steel, and the tensile and creep properties of the modified and British 9 Cr-1 Mo materials we a used to calculate allowable stress values S per Section VIII, Division 1 and S per Code Case N-47 to Section III of the ASME Boiler and Pressure Vessel Code. These values were compared with those currently listed in the code for American com*, ^rcial 9 Cr-1 Mo steel, 2 1/4 Cr-1 Mo steel, and type 304 stainless steel.