ISIJ International, Vol. 47 (2007), No. 5, pp. 769–771

The decomposition of austenite to ferrite or cementite begins at the equilibrium phase boundary of austenite/ (ferrite austenite) or austenite/(cementite austenite) in binary Fe–C phase diagram. The temperature values on the phase boundaries of austenite/(ferrite austenite) and austenite/(cementite austenite) at a certain carbon content are called Ae3 and Acm temperatures of the steel, respectively. The Ae1 temperature is defined as the eutectoid temperature corresponding to the intersection point of two phase boundaries: austenite/(ferrite austenite) and austenite/(cementite austenite). Many thermodynamic studies have been performed to calculate the phase diagrams, which provide us with various basic information for predicting the quantitative kinetics of phase transformations in low alloy steels. The partitioning effects of alloying elements such as Ni, Cr, and Mo occurring during the austenite decomposition on the phase diagrams of the steels have been investigated with ortho-equilibrium (OE) and paraequilibrium (PE) models in low alloy steels. The schematic drawing of the Fe–Fe3C phase diagram calculated by using the OE and PE models is illustrated in Fig. 1. The phase boundaries calculated with OE model are generally shown at the higher temperatures than those by PE model. Recently, Lee and Lusk have analyzed the experimental Ae1 temperatures of 85 low alloy steels, obtained from the US Steels Atlas of I-T diagrams, using thermodynamic models. They have realized that the measured Ae1 temperatures are located between two theoretical Ae1 temperatures predicted using OE and PE models and suggested a new thermodynamic model for better prediction of the measured Ae1 temperatures. This fact means that two phase boundaries of austenite/(ferrite austenite) and austenite/(cementite austenite) are also located between the calculated phase boundaries by OE and PE models. However, the austenite/(ferrite austenite) boundary calculated by OE model shows a good match as compared with the experimental Ae3 data. 2) This seems reasonable since the ferrite transformation usually occurs at relatively high temperatures at which substitutional and interstitial atoms can quickly diffuse and thus become partitioned. Therefore, the actual phase boundary of austenite/(cementite austenite) should be located between the theoretical phase boundaries around the Ae1 temperature, leading to the actual Ae1 temperatures between two equilibrium phase boundaries. Nevertheless, up to now, the thermodynamic formula for predicting the actual Acm boundaries in low alloy steels has not been proposed yet. We have, therefore, derived a new formula to predict the accurate Acm temperature of the low alloy steels, assuming that the partitioning effect of substitutional elements linearly reduces below Ae1 temperature, resulting in no partitioning of alloy elements at room temperature (T0). 9) The thermodynamic formula for predicting the accurate Acm temperatures of low alloy steels has been newly suggested as follows: