Separation of overlapping retained austenite decomposition and cementite precipitation reactions during tempering of martensitic steel by means of thermal analysis

During isochronal tempering of martensitic steels, several decomposition reactions take place within to some extent overlapping temperature regions. For the kinetic study of these reactions, differential scanning calorimetry and dilatometry are well established complementary experimental techniques. During tempering stages II and III of martensitic steels with increased carbon contents, the simultaneous transformation of retained austenite and the precipitation of cementite produce strong signals as well in the differential scanning calorimeter as in the dilatometer. For the determination of the individual kinetic parameters and reaction enthalpies, these reactions have to be separated, which is attempted in the present study. This is accomplished by tempering room temperature quenched as well as liquid nitrogen quenched specimens in a differential scanning calorimeter. The investigated material is the SAE 1070, a spring-steel with 10 vol% retained austenite after quenching to room temperature. The data evaluation procedure is explained in detail and the results are compared with literature data.

► Overlapping DSC signals of austenite decomposition and cementite precipitation reaction during tempering of steel have been successfully separated.
► Liquid nitrogen quenching was used to produce fully martensitic specimens in order to eliminate the austenite decomposition reaction.
► Complementary isochronal DSC and dilatometer measurements were used to identify the temperature regions of each reaction.
► This new approach allowed the individual determination of the kinetic parameters as well as the reaction enthalpy of each reaction.