Analysis of the dynamic behavior and local structure of solid-solution carbon in age-hardened low-carbon steels by soft X-ray absorption spectroscopy

To understand the formation mechanism of carbon clusters in low-carbon steel, the dynamic behavior of carbon in solid solutions subjected to aging at low temperatures was investigated by X-ray absorption spectroscopy. While it is hypothesized that the C-Fe bond distance and lattice strain induced by the carbon in solid solution affect the formation and strengthening mechanism of carbon clusters in low-carbon steels, currently, there is no information on the local structure of carbon in ferritic steels. Therefore, in this study, the relationship between the correlation coefficient derived from C K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and mechanical properties, in terms of Vickers hardness, was elucidated. The changes occurring in the spectral shape in the initial stages of aging were found to be clearly associated with the diffusion of carbon in the solid solution. The presence of the carbon-cluster precursor state suggested that the correlation coefficient calculated from the C K NEXAFS spectra experienced variations in spite of a constant Vickers hardness. Furthermore, we proposed a method to analyze the local structure of carbon in solid solution in steels using C K NEXAFS. Finally, our observations illustrate that the local structure of carbon is dependent on age-hardening during low-temperature aging.