Explaining the abnormal dilatation behavior during the austenite formation in a microstructure of a low-carbon low-alloy steel containing retained austenite

Abstract

During continuous heating of a low-carbon low-alloy steel in a dilatometer, an abnormal macroscopic expansion has been observed in the length change of the sample during the onset of austenite formation. Such abnormal expansion behavior is also detected in the literature data with similar initial microstructures and heating rates on various steels. Here, this work investigates the causes and consequences of this abnormal expansion using in situ high-energy X-ray diffraction analysis. According to the volume investigated by diffraction, a significant amount of austenite forms before the onset of the macroscopic contraction of the sample associated with the bainite-to-austenite transformation. The delay in the macroscopic contraction is due to the retained austenite lattice expansion, arising from the concentration inhomogeneities in this phase produced by carbides dissolution during austenitization. The retained austenite lattice expansion manifests as a macroscopic expansion in the length change of the sample. Such abnormal expansion results in a significant overestimation of the Ac1 temperature, and a systematic shift in the austenite formation kinetics determined by dilatometry in comparison with X-ray diffraction.