Activation energy of time-dependent martensite formation in steel

The kinetics of {557}γ 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, EA, 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 EA 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 EA varies in the range 2–27 kJ mol−1 and increases logarithmically with the total fraction of interstitials in the steel.