Study of Carbon Steel by Mechanical Spectroscopy beyond the Old Limitations
A new aspect of carbon precipitates in steel is unraveled by mechanical spectroscopy. The method was applied to the study of carbide precipitation in quench-aged Fe-C alloys, mild steel, and pearlitic steel. Measurements carried out at constant temperature below room temperature (RT) down to -55 ºC have revealed a new broad beak (NBP) different from neither the Snoek nor the Snoek-Köster peaks. This peak is characterized by the Debye peak with a distribution in relaxation time. The Arrhenius plot yields a large activation energy and gigantic pre-exponential factor. The intensity of NBP increases with aging at temperatures where ε-carbide is known to precipitates. It starts to decay at aging time too early for the ε-carbide to disappear. NBP is understood by a model of stress-induced reorientation of extra-carbon pairs in ε-Fe3C host lattice. The decay of NBP with aging is accompanied by growth of another peak (NBP’). NBP and NBP’ are ascribed to coherent and incoherent ε-carbides, respectively. Mechanical spectroscopy has a potential to unravel the unknowns about carbon in steel beyond the old limitations of internal friction measurements at constant frequency. Based on the present findings, the directions for future studies are suggested, which include the precise determination of Fe-C phase diagram with respect to ε-carbide, understanding of precipitation hardening in steel and extension of measuring window of mechanical spectroscopy for ε-carbide in steel.