The compiled knowledge in literature regarding the isothermal formation of austenite from different initial microstructures (pure and mixed microstructures), has been used in this work to develop a model for non-isothermal austenite formation in low-carbon steels (C < 0.2 wt%) with a mixed initial microstructure consisting of ferrite and pearlite. Likewise, calculations of relative change in length have been made as a function of temperature, and the differences between theoretical and experimental results have been analysed in 0.1C-0.5Mn low-carbon low-manganese steel. Experimental kinetic transformation, critical temperatures as well as the magnitude of the overall contraction due to austenite formation are in good agreement with calculations.