THE ISOTHERMAL AND NONISOTHERMAL KINETICS OF TEMPERING IRON-CARBON AND IRON-NITROGEN MARTENSITES AND AUSTENITES

Straightforward compatible analyses of transformation kinetics have been shown to be possible for both isothermally and nonisothermally conducted experiments, provided an appropriate state variable for the degree of transformation has been adopted. The kinetic parameters can be determined as a function of the degree of transformation; they are constant if a single rate-controlling mechanism operates. The kinetic analyses have been applied to the successive stages of transformation occurring on subjecting iron-carbon and iron-nitrogen martensitic specimens to isothermal and nonisothermal treatments. The degree of transformation was traced by employing dilatometric, calorimetric and magnetometric techniques. Similar kinetic parameters have been derived from the isothermal and nonisothermal experiments; also, the kinetic data obtained from the different techniques agree. Aging of the martensitic specimens starts with a redistribution of the intersitials rate controlled by volume diffusion of interstitials in the martensite matrix: formation of local enrichments, segregation to lattice defects and transfer from "a/b" to "c" octahedral interstices. The developments of the transition carbide and the transition nitride as well as their conversions into "equilibrium" carbide and "equilibrium" nitride have activation energies which are similar for the iron-carbon and iron-nitrogen alloys. Interpretation of the latter data is difficult as the experimental results suggest that more than one mechanism may govern the corresponding transformation rates. Analysis of the decomposition of both iron-carbon and iron-nitrogen retained austenites shows that a single rate-controlling mechanism occurs: volume diffusion of interstitials in the austenite matrix.