Mathematical modelling of the onset of transformation from austenite to pearlite under non-continuous cooling conditions

The temperature at which a new phase forms is an important parameter in the genesis of final microstructure. For diffusional transformation processes, prediction of this temperature on cooling, until now, relied upon empirical equations which are based on the cooling rate or degree of undercooling. To develop a method that is capable of predicting the onset of transformation in the steel, applicable to industrial processing conditions, a theoretical study has been conducted to critically examine the consequences and implications of the Scheil additivity rule in relation to the incubation of the austenite decomposition. An "ideal" isothermal transformation curve for the start of transformation from austenite to pearlite has been developed based on the assumption of the consumed fractional incubation time being additive. The curve is characteristic of the chemistry and the austenite grain size of the steel. A mathematical relationship between the experimental time to the start of transformation and the ideal incubation time was quantified, and two methods for deriving an "ideal" T77' curve from the controlled cooling experiments were established in this study. The derived "ideal" T77' curve was used for predicting the start of transformation from austenite to pearlite for three different cooling conditions; namely, continuous, semi-continuous and non-continuous cooling conditions. The predictions were consistent in all cases and compared favourably against other methods which have been employed frequently frequently to estimate the transformation start temperature for non-isothermal conditions.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat