# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Selection of Tempering Conditions  
Dilatometric tests were performed to record the relative change in length (RCL) and its derivative (DRCL) during the continuous heating of the bainitic microstructures from room temperature to the fully austenitic field. As can be seen in Figure 3, RCL vs. temperature dilatometric data show clearly a linear thermal expansion characteristic in the two temperature ranges where no phase transformation occurred, which correspond to the regions of \(\alpha_{\mathrm{b}}+\gamma_{r}\) and single austenitic phase, respectively. Moreover, two main change stages were identified in the DRCL curve (more sensitive to microstructural changes than the RCL curve) at around 550 and 800 \({}^{\circ}\)C, which correspond with decomposition of the bainitic microstructure and the austenitization of the tempered microstructure, respectively. These peaks are negative, which means that a contraction of the matrix occurred during both processes. Finally, it was noteworthy that before starting the second transformation, we observed an inflection in the RCL at about 745 \({}^{\circ}\)C, which is marked by a red arrow in Figure 3a. This change was associated to the Curie temperature, because in Figure 3b, there is an observed a change in the DRCL that corresponds to the spontaneous magnetostriction that occurs below the Curie temperature.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Steel**} & \(f_{\gamma}\) & \(C_{\gamma}\) & \(f_{\alpha}\) & \(C_{ab}\) & \\  & **(\(\pm\)0.03)** & **(\(\pm\)0.06 wt \%)** & **(\(\pm\)0.03)** & **(\(\pm\)0.03 wt \%)** & **HV30** \\ \hline
06 CV & 0.23 & 0.90 & 0.77 & 0.18 & \(596\pm 3\) \\
06C1 MoV & 0.32 & 0.91 & 0.68 & 0.20 & \(601\pm 2\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fraction and carbon content of retained austenite (f\({}_{\gamma}\), C\({}_{\gamma}\)) and bainitic ferrite (f\({}_{\mathrm{i,A}}\), C\({}_{\mathrm{i,A}}\)) obtained from XRD, and hardness results for the initial bainitic microstructures.  
Figure 2: SEM micrographs of bainitic microstructures obtained after isothermal transformation at 250 \({}^{\circ}\)C for 20 h in (**a**) 06 CV and (**b**) 06C1 MoV steels, where \(\alpha_{\mathrm{b}}\) stands for bainitic ferrite and \(\gamma\) stands for retained austenite with film (f) and block (b) morphology.  
As the key factor in determining the tempering resistance of a bainitic microstructure is the austenite thermal stability, it is expected that steel 06C1 MoV containing a higher amount of retained austenite in its microstructure presents a higher deviation from the baseline in the DRCL curve in the peak at lower temperature region. However, Figure 3b shows that the peak corresponding to the austenite decomposition (around 500 \({}^{\circ}\)C) presents a higher area for the 06 CV steel. The peak temperature of this reaction for this steel occurs at a lower temperature than for the 06C1 MoV steel. As the major difference among the studied steels is their Mo content, it was concluded that a high Mo addition has improved the strong effect of V on the temper resistance of steels by slowing down the tempering reactions and the size of carbides that precipitate during tempering [44; 45; 46]. In order to separate the reactions occurring during the retained austenite decomposition, tempering experiments were constrained in the range between 450 and 650 \({}^{\circ}\)C in steps of 50 \({}^{\circ}\)C.