Abstract
The austenitization (������) transformation in a 1005 C-Mn steel is monitored in real time at continuous heating rates between 1��C/sec to 10��C/sec using in situ synchrotron x-ray diffraction. Both dilatometry and the in situ x-ray diffraction experiments show that the austenitization transformation proceeds through multiple mechanisms from initiation through completion. Unlike the dilatometry experiments, the in situ x-ray diffraction experiments provide direct evidence for the phases present at specific times during the transformation. Thus, experimental validation is provided for models based on the differing kinetics of the austenitization transformation starting from a heterogeneous microstructure containing pearlite and ferrite. Beginning at temperatures below the A1 transformation temperature, the starting microstructure undergoes a recovery and recrystallization process to relieve stress imparted during the initial thermomechanical treatment of the steel. The austenitization transformation follows, beginning at temperatures above the A1 temperature, with the initial transformation proceeding as the pearlite in the microstructure is dissolved and high carbon concentration austenite is formed. Since the carbon present in the steel is localized near the original pearlite colonies, there is a pronounced heating rate dependant delay before the remaining ferrite grains begin to transform. As temperatures reach 850��C at all heating rates, the remaining ferrite transforms to austenite, since the equilibrium phase diagram indicates that higher temperatures are required to drive the ������ transformation at these lower carbon concentrations. The transformation reaches completion at temperatures above the A3 temperature, and the last ferrite to be transformed is nearly pure iron.