Abstract
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.
Highlights
The current study proposes a different approach for non-isothermal tempering analysis and the technique is applied over the whole range of tempering temperatures
The main difference between the non-isothermal double tempering approach and the conventional non-isothermal tempering utilized by several authors[5,6,7,8,9] is the addition of a second non-isothermal tempering step to be utilized as a baseline. This approach more clearly reveals features associated with the different stages of tempering and reduces the impact on change in length caused by the temperature dependence of thermal expansion coefficient
Ande et al.[30] studied cementite with alloying element enrichment by Mn amongst others based on density functional theory (DFT) and reported a subtle decrease in the cementite unit cell volume associated with the presence of Mn on Fe sites
Summary
A Thermal Analysis (TA) Quenching dilatometer 805 L was utilized for non-isothermal analysis using cylindrical samples measuring 4 mm in diameter and 10 mm in length. P for the non-isothermal procedure, hereafter labeled double tempering. The majority of the tempering reactions take place during the first tempering step, and even though equilibrium is not reached at the different temperatures, significantly less tempering reactions are expected during the second tempering step, and the latter is used as a baseline for data analysis. Subtraction of change in length observed in the second step from the first helps to highlight changes associated with the tempering process. A derivative of the difference in the change in length between first and second tempering steps with respect to temperature plotted versus temperature was utilized to evaluate the tempering response
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