Volume fractions of proeutectoid ferrite/pearlite and their dependence on prior austenite grain size in hypoeutectoid Fe-Mn-C alloys

It has been generally believed that pearlite transformation in hypoeutectoid steels starts when the average carbon concentration in untransformed austenite reaches the A_cm line after the formation of proeutectoid ferrite. To test this concept experimentally, volume fractions of proeutectoid ferrite/pearlite and carbon contents in the austenite being transformed into pearlite were measured for the Fe-2Mn-0.3C alloy isothermally transformed in the temperature range 848 K to 898 K (575°C to 625°C). It was found that lamellar pearlite can form even when the average carbon content in untransformed austenite is much lower than the A_cm line. This peculiar observation is probably due to the two-dimensional diffusion of carbon, i.e., parallel to and normal to the austenite/pearlite interface, which enables lamellar cementite to grow continuously by supplying carbon atoms to its growth front. This results in proeutectoid ferrite fractions with respect to pearlite being much lower than those predicted by the lever rule. With decreasing prior austenite grain size, proeutectoid ferrite fractions with respect to pearlite were found to increase, but the thickness of proeutectoid ferrite was constant within the range of grain size investigated. This is due to the existence of the critical α/γ interface velocity only below which pearlite (actually cementite) can be nucleated at the migrating α/γ interface. Furthermore, the upper limit temperatures for pearlite formation in the Fe-1Mn-0.33C and Fe-2Mn-0.3C alloys were found to be well between the PLE/NPLE and PE A_e1 temperatures.