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Abstract
In this paper the total driving force for the decomposition of retained austenite and martensite are calculated together with the nucleation and growth characteristics of cementite in the two phases. The results demonstrate that the driving force for the decomposition of martensite is an order of magnitude less than that of austenite. However, the driving force for cementite precipitation in martensite is two orders of magnitude greater than in austenite with a much shorter incubation period. On short term tempering cementite precipitates from martensite whereas on longer term tempering decomposition of retained austenite occurs because of the increase in driving force which is enhanced by the contraction of the martensite on decomposition. It is argued that the precipitation of cementite from the austenite results in tempered martensite embrittlement, a mechanism dependent upon the two related decomposition processes. The segregation of trace impurities or the precipitation of cementite at the grain boundaries is not a prerequisite.
MST/240