The Effect of Carbides on the Decarburization Behavior of 2\frac14Cr-1Mo Steel in High Temperature Sodium

2\frac14Cr-1Mo steel is highly promising for construction materials of the LMFBR secondary system. The effect of carbides on the decarburization rate of 2\frac14Cr-1Mo steel in high-temperature sodium was studied.The effect was investigated by immersing specimens with different metallurgical structures and various kinds of carbides in static liquid sodium at temperatures from 550 to 700°C. The changes of carbides in the composition, phase and morphology were examined by the X-ray diffraction method, chemical analysis of carbides, electronmicroscopic observation by extraction replica and the electron diffraction method.The following results were obtained.(1) Decarburization rate of 2\frac14Cr-1Mo steel in sodium can be related to phase, morphology, transformation rate and the coagulation process of carbide precipitates. Carbides dissolve in the order of M3C>lamellar M23C6>granular M7C3\simeqgranular M23C6\simeqM2C>M6C.(2) Carbides dissolve so as to spheroidize themselves. Diffusion of Cr and Mo from carbide to the matrix seems to control the dissolution rate of comparatively stable carbide.(3) Carbides should be stabilized in order to minimize the rate of decarburization. It is most appropriate to make M6C a dominant carbide phase in steel.