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Electrical resistivity of low-carbon martensitic steels was measured to estimate the carbon concentration in solid solution. Since electrical resistivity is influenced not only by solute carbon but also by substitutional elements, lattice defects and second phase, the effects of these factors need to be subtracted from total electrical resistivity, in order to obtain the accurate solute carbon concentration by this method. As a result, the effects of dislocations and grain boundaries were much smaller than that of solute elements, being only 1 -2% of the measured electrical resistivity, in martensitic steel. On the other hand, substitutional elements and retained austenite were found to be significantly effective. By subtracting these effects from the measured value, the change in electrical resistivity due to solute carbon, Δρ sol.C , could be formulated as a function of the carbon concentration in solid solution of martensite, C sol , as follows:Δρ sol.C [mΩmm] = 0.25 × C sol [mass%] The estimated solute carbon concentration was confirmed to correspond to the directly measured value by atom probe tomography.

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Age hardening in stable austenitic stainless steel wires with a chemical composition of Fe18%Cr12%Ni and different N contents was investigated to clarify the role of N. Age hardening could be enhanced by increasing the drawing ratio and N content. The highest age-hardening effect was observed at 800 K in the N-bearing specimens. In addition, severe drawing induced low-temperature age hardening at 450600 K. Differential scanning calorimetry (DSC) analysis revealed that age hardening at 800 K may be attributed to particle dispersion strengthening by Cr 2 N. Meanwhile, the exothermic peaks observed at 450600 K were controlled by the pipe diffusion of N. However, nitride precipitates and clusters could not be detected by 3D atom probe analysis, which implies the formation of atomic-scale N products, such as I-S pairs, at the dislocations.