Activation Energy of Time-Dependent Martensite Formation in Steel

The kinetics of \( \left\{ {5 \,5 \,7} \right\}_{\upgamma} \) lath martensite formation in (wt%) 17Cr-7Ni-1Al-0.09C and 15Cr-7Ni-2Mo-1Al-0.08C steels was assessed with magnetometry at sub-zero Celsius temperatures. Samples were cooled to 77 K by immersion in boiling nitrogen to suppress martensite formation. Thereafter, thermally activated martensite formation was monitored during: (i) isochronal (re)heating at different heating rates; (ii) isothermal holding at temperatures between 120 and 310 K. The activation energy, \( {\text{E}}_{\text{A}} \), of thermally activated martensite formation was quantified from the results of both isochronal and isothermal tests by applying a Kissinger-like method. In addition, the isothermal data was interpreted applying the approach presented by Borgenstam and Hillert. The results of the independent quantification methods were consistent and indicated an \( {\text{E}}_{\text{A}} \) in the range 9–13 kJ mol⁻¹. Thereafter, the two methods were applied to evaluate the data available in the literature. The overall analysis showed that \( {\text{E}}_{\text{A}} \) varies in the range 2–27 kJ mol⁻¹ and increases logarithmically with the total fraction of interstitials in the steel.