Unveiling the effects of Mn, Cr, Al, and Si on the low-temperature tempering behaviors of high-carbon martensite

Low-temperature tempering process of martensitic steels is generally performed to restore toughness and ductility without sacrificing strength for various industrial applications. Several reactions causing changes in both microstructure and physical properties occur sequentially during tempering. In this study, multi-aspect characterization using various advanced characterization techniques was performed to correlate the reaction kinetics and microstructural evolution in a series of Fe-0.8wt.%C-based alloys with 2at.% additions of common substitutional alloying elements (Mn, Cr, Al and Si), with an aim to clarify the alloying effects on the low-temperature tempering behaviors of high-carbon martensitic steels. During continuous heating at a rate of 10 K/min from the as-quenched condition, the reaction kinetics in various alloys was evaluated via calorimetry and dilatometry together with in-situ neutron diffraction, which showed good agreement with the results of microstructural characterization mainly by using three-dimensional atom probe. The reduction in tetragonality and solute carbon content of martensite during the clustering/segregation of carbon and the precipitation of metastable ε/η carbide was retarded by Al and Cr additions, which can be explained by their suppression effects on the carbon diffusivity in BCC-Fe. Moreover, the decomposition of retained austenite occurring in two separate steps was found to be influenced by distinct alloying effects. The retardation effects of Mn and Cr on the early step, and those of Al and Si on the late step, can be explained by their suppression effects on the nucleation of decomposition products, i.e., ferrite and cementite, respectively.