On the stabilization of retained austenite: mechanism and kinetics

Stabilization of retained austenite is now well established and refers primarily to a process where further martensite transformation is hindered. However, the literature shows a wide variety of ways in which this impediment can be brought into play. A short classification of all the types has therefore been attempted at the outset in this paper. Of the various types, the one generally referred to as “thermal stabilization” has been the subject of investigation here. A high carbon steel quenched to room temperature and isothermally aged at various temperatures (below Ms) has then been subjected to further cooling to −112°C. Continuous electrical resistivity changes have been recorded to monitor the onset (and hence degree of stabilization, θ) and course of reappearance of martensite. X-ray diffractometry measurements of retained austenite and macroresidual stress as well as Mössbauer spectroscopic investigations have been conducted to throw some light on the mechanism of stabilization. Most of θ vs. ta (time of aging) have been employed to carry out a kinetic analysis using a Zener-Wert-Avrami-type equation. Activation energies have been obtained corresponding to definite “m” values. Activation energy values of 14 and 28 kcal mol−1 characterize the initial and advanced stages of stabilization respectively. The mechanism appears to be one of C atoms initially diffusing primarily from martensite (m) to the γ-M interface, thus anchoring the normally mobile dislocations, while at later stages the diffusion of C atoms in austenite could also be taking place.