Kinetic study of the austenite decomposition during continuous cooling in a welding steel

Abstract

The kinetics of austenite decomposition during the continuous cooling of a low-carbon welding steel was determined by dilatometric analysis. Based on the measurements, the critical transformation temperatures of austenite decomposition were determined for both ferrite and pearlite. The cooling conditions were established from the Stelmor® controlled cooling process for the manufacturing of wire rod. It was observed that the critical temperatures decrease when the cooling time was reduced as a result of an increased cooling rate in the range of 600–900 °C. Using the decomposition temperatures, a continuous cooling transformation CCT diagram was created and it may be observed that austenite decomposition occurs in two steps. The kinetic parameters for each step were determined and compared with the Johnson-Mehl-Avrami-Kolmogorov diffusive model. The final microstructure was analyzed using an optical microscope and evaluated by nanoindentation to determine the effect of the cooling rate on the nanohardness of each phase.