Study on evaluation method for decarburization of 2 1/4 Cr-1Mo steel in high temperature sodium

Creep strength of 2 1/4 Cr-1Mo Steel used for steam generator tube of a fast breeder reactor is decreased by decarburization in high temperature sodium utilized for coolant of the reactor. However, proposed evaluation methods for carbon decrease in the steel were not sufficient in the predictability. Then, an evaluation method taking account of the behavior of carbide in the steel was studied in this report. The following results were obtained. (1) Model of the evaluation method for decarburization of 2 1/4 Cr-1Mo steel was set from decarburization test results in 500 to 700 °C of sodium temperature, and the method was developed. (2) Decomposition rate of carbide in the steel to fit test results was calculated with the method. (3) It was clarified that carbon concentration calculated with the method was decreased rapidly at initial stage of decarburization for diffusion of solution carbon, and at next stage the carbon concentration was decreased slowly under influence of decomposition rate for carbide to be dominant rate determining step of decarburization. (4) This method presented the most smallest decarburization rate compared with proposed evaluation methods. And the method presented slight decrease of concentration for steam generator tube for 32 years at 500 °C whose thickness was 0.3 cm.