Abstract
As-quenched martensite in carbon steels needs to be tempered to restore ductility and toughness for practical applications. During tempering of martensite, microstructural evolutions induced by a series of reactions relevant to carbon diffusion is known to occur. In this study, multi-aspect characterization using advanced techniques such as in-situ neutron diffraction, transmission electron microscopy and three-dimensional atom probe tomography, was performed to investigate the changes in tetragonality, physical properties, microstructure and solute carbon content of high-carbon martensite, with an aim to clarify its low-temperature tempering behaviors. A Fe-0.78 mass%C binary alloy was austenitized and quenched to prepare the as-quenched martensite, followed by tempering via continuous heating at different rates. It was found that various reactions occurred sequentially during tempering, starting from the structure modulation generated by carbon clustering in the 0th stage, then followed by the precipitation of metastable η-carbide preferentially on dislocations in the 1st stage, towards the later decomposition of retained austenite, and precipitation of χ-carbide and cementite in the 2nd and 3rd stages, respectively. After analyzing the experimental results, a compressive residual stress with elastic anisotropy was confirmed in the retained austenite until the temperature range of its decomposition. In addition, the tetragonality and solute carbon content of martensite were found to be continuously decreased especially in the temperature range of the 1st stage. Compared with the tetragonality change of martensite during continuous heating, the lattice volume expansion induced by carbon was found to be more effective to accurately estimate the solute carbon content of tempered martensite.
