THE PRINCIPLE OF ADDITIVITY AND THE PROEUTECTOID FERRITE TRANSFORMATION

This study critically examines the principle of additivity and the reason that the proeutectoid ferrite transformation is additive. Austenite-to-proeutectoid ferrite transformation kinetics were measured under isothermal and stepped-isothermal conditions for AISI 1010 and 1020 steel grades using a dilatometer and a Gleeble 1500 thermomechanical simulator. The additive nature of the austenite-to-proeutectoid ferrite transformation was experimentally assessed by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results of the tests on the 1010 steel showed that the proeutectoid ferrite transformation with allotriomorphic morphology is additive. Transformation kinetics were measured for the 1020 steel with the ferrite morphology changing from allotriomorphic to predominantly Widmanstatten, and the transformation was additive. However, the stepped-isothermal test in which the ferrite was transformed and equilibrated at the first temperature and then rapidly cooled to the second temperature was not additive. The second part of the study involved developing mathematical models with planar and spherical interface geometries to theoretically assess the additivity of the proeutectoid ferrite transformation. Additivity of the proeutectoid ferrite transformation was tested by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation, providing an explanation for this behavior, although theory would suggest ferrite reaction to be nonadditive.