| Abstract: | The total driving force for the decomposition of retained austensite and that of martensite, the driving force for nucleation of cementite, the growth rate as well as the incubation period of the formation of cementite that precipitates from martensite and austenite are evaluated. The results reveal that in spite of the fact that the driving force for the decomposition of martensite is one order of magnitude less than that of austenite, the driving force for the nucleation of cementite that precipitates from martensite is far greater than that from austenite and the incubation period of the formation of cementite as it precipitates from martensite is one order of magnitude less than that from austenite. It is obvious that during the short time tempering, cementite firstly precipitates from martensite, while upon tempering with long duration, since the total driving force for the decomposition of the retained austenite is greater and the contraction of martensite resulted from its decomposition would enhance the decomposition of interlath retained austenite, the precipitation of cementite from austenite leads the temper martensite embrittlement-Therefore, as the segregation of the impurities(and cementite) on the grain boundary is not taken into consideration, the mechanism of the TME may be assumed to be the combination of the two related decomposition processes. |
