Differential Scanning Calorimetry and Microscopy Study of Transformations in Ductile Cast Irons: Part I. Continuous Heating

Abstract

The kinetics of pearlite to austenite and ferrite to austenite transformations in two ductile cast irons are studied using differential scanning calorimetry, optical microscopy, electron microprobe, X-ray diffraction and microhardness techniques. Study of the heating process up to the austenitic range with ferrite-pearlite metal base showed presence of a set of phase transformations taking place in different temperature intervals. These transformations are the Curie transformation, pearlite to austenite transformation, and ferrite to austenite transformation. The change of the heating rate influences in different degrees the shift of the temperature ranges where the different phase transformations take place, depending on the type of transformations, and therefore alters significantly the time-temperature parameters of the phase transformations. It has been found that the peak temperatures of the Curie transformation at heating rates 1 and 50°C/min are 739 and 745°C (for the Fe-3.38C-3.23Si-0.33Mn alloy) and 739 and 746°C (for the Fe-3.75C-2.91Si-0.39Mn alloy), respectively. The increase of the temperatures with the heating rates is more significant for pearlite to austenite and ferrite to austenite transformations. The peak temperature of the ferrite to austenite phase transformation at heating rates 1 and 50°C/min are 837 and 876°C for the Fe-3.38C-3.23Si-0.33Mn alloy and 823 and 853°C for the Fe-3.75C-2.91Si-0.39Mn alloy, respectively. The observed micro-inhomogeneity of the silicon influences significantly the kinetics of the phase transformations. The thermal effect due to the Curie point (Tc) is detected, and the influence of the continuous heating parameters on the Curie transformation is studied.