Decarburization rate of 21/4Cr-1Mo steel in high temperature sodium

Decarburization behavior of 21/4Cr-1Mo steel used for the steam generator tubes of the fast breeder reactor in high temperature sodium was studied by following tow tests. (1) Long term test to grasp outline of decarburization behavior of 21/4Cr-1Mo steel in the temperature range of 380-600degC included operating temperature of steam generator. (2) Accelerated test in high temperature range of 500-700degC to study more detail decarburization behavior of 21/4Cr-1Mo steel. In long term test, it was found that logarithm of diffusion rate constant calculated from decarburization rate of 21/4Cr-1Mo steel was proportional with reciprocal sodium temperature. In accelerated test, however, the relation between them was not the same to the relation of long term test but changed with sodium temperature. From the results, it was clarified that decarburization mechanism was different with temperature and was affected by formation, decomposition and transformation of carbides in 21/4Cr-1Mo steel under the tests. Decrease of carbon concentration of 21/4CR-1Mo steel for 40 years was estimated by the use of diffusion rate constant obtained in the tests. (author)