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A new approach for non-isothermal tempering analysis utilizing dilatometry is proposed and was carried out on a medium carbon steel with high silicon and additions of Mo and V for secondary hardening. The method includes a second non-isothermal step performed with the same heating rate (2 °C/min) used for the first step in order to create a baseline for analysis. The results were correlated with several other characterization techniques. Mössbauer spectroscopy confirmed the formation of transition carbides by auto-tempering as well as the presence of retained austenite decomposition (stage II) and cementite precipitation (stage III), which demonstrated significant overlap. Electrical resistivity measurements were correlated with dislocation densities obtained through X-ray diffraction analysis. Transmission electron microscopy dark field images confirmed the secondary hardening assessment from dilatometry.

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High strength heavy gauge plate steels for structural applications are subjected to quenching and tempering treatments. Due to the material thickness, different cooling rates are obtained throughout the thickness of the material and several grades do not fully harden to martensite, resulting in a material containing a mixture of martensite and bainite. Therefore, it is desirable to model the tempering response of martensitic and bainitic microstructures in order to allow tempering optimization for mixed microstructures. The early stages of tempering as well as the high temperature tempering were studied with respect to martensite and bainite with a focus on the influence on the tempering response associated with the presence of the alloying elements molybdenum (Mo), vanadium (V), chromium (Cr), and silicon (Si). Microstructural characterization prior to tempering was performed using light optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Tempering response was assessed through dilatometry, Vickers micro-hardness, and Mössbauer spectroscopy. Finally, tensile and impact properties were studied using tensile and Charpy V-notch testing.