Evaluation of Solute Carbon Concentration in BCC Iron by Electrical Resistivity Measurement Considering Carbide Precipitation

Electrical resistivity measurement is a powerful method for evaluating the solute carbon concentration (Csol) in BCC iron. In this study, the effect of carbide precipitation on the accuracy of Csol quantification was examined by applying a model that predicts the electrical resistivity of composite microstructures containing dilute-dispersed spherical carbides. For Fe–C alloys with spheroidized cementite dispersed in ferrite, Csol in ferrite could be estimated with an uncertainty of only several tens of ppm. In Fe–0.2Mn–C alloys, high accuracy comparable to that of Fe–C alloys was achieved when Mn enrichment in ferrite accompanying cementite precipitation was properly taken into account. For Fe–2Mn–0.5Si–C martensitic alloys, Mn partitioning, dislocation annihilation, and grain coarsening do not occur during tempering at low-temperature below 573 K. Therefore, changes in electrical resistivity during tempering directly reflected the decrease in Csol, enabling accurate evaluation when the contribution of carbide precipitation was removed using the spherical dilute-dispersion model. In contrast, high-temperature tempering at 873 K induced pronounced Mn partitioning, and even slight uncertainties in the measured Mn concentration in ferrite led to errors of several hundred ppm in the estimated Csol. Although errors associated with volume fraction and electrical resistivity of carbides also affected the evaluation, their impact was much smaller than that of Mn partitioning. Consequently, except for the high-temperature-tempered Fe–2Mn–0.5Si–C alloy, the final accuracy of Csol measurement can be maintained within several tens of ppm when appropriate corrections are applied.