# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Dilatometric Study of the Microstructural Evolution during Tempering  
In this section, the dilatometry results obtained during the isothermal tempering treatments performed are analyzed in detail, and an attempt to establish the sequence by which the microstructure decomposes is made. For this, a first set of generic theoretical calculations based on atomic volumes was carried out to later help in the interpretation of the RCL curves. Considering the previous knowledge about the tempering behavior of martensite and bainite [3; 6; 7; 8; 9; 10; 47; 48; 50; 49; 51; 52; 53; 54; 55; 56; 57], and the fact that the initial microstructure is composed of carbon-rich ferrite and austenite, which both will tend to equilibrium during the tempering process, the following possible scenarios, as schematically presented in Table 5, are studied:  
1. Retained austenite decomposes into a mixture of ferrite and cementie, \(\gamma\ (fcc)\rightarrow\ \alpha\ (bcc)+\theta\). These calculations took into account the fact that the austenite retained in the bainitic microstructure has two distinctive morphologies that are characterized by containing very different levels of C in solid solution [34; 35; 36]. Therefore, \(C_{\gamma}\) was varied between that of the bulk (no C enrichment) and 2.5 wt %, which is a typical C enrichment detected for thin films of retained austenite [34; 35; 36]. 2. Before the decomposition described above, it could occur that part of the austenite goes through an intermediate state where it releases part of its C saturation due to the precipitation of the cementie particles [13; 14], which are mostly rich in Fe but also with a certain amount of Cr and Mo: \(\gamma\ (fcc)\rightarrow\ \gamma^{-}\ (fcc)+\theta\). The calculations for this scenario are made from the XRD results shown in Table 4 for the 06C1 MoV alloy. 3. The excess of C in bainitic ferrite can be released as cementie precipitates, leading to the progressive loss of tetragonality of the bainitic ferrite and finally reaching its equilibrium as a bcc phase, \(\alpha_{b}\ (bct)\rightarrow\ \alpha\ (bcc/bct)\ +\ \theta\). The initial C content of bainitic ferrite, \(C_{ab}\), is assumed to be  
Figure 3: Dilatometry results of (**a**) relative change in length and (**b**) first derivate during a continuous heating up to austenitization temperature for all steels. 0.2 wt %, which is in accordance with both the XRD results presented in Table 4 and previous experiences with similar microstructures [37; 38; 39; 40]. 4. Finally, the possibility is considered that some of the C from the supersaturated bainitic ferrite could be transferred to the retained austenite in a similar way to what happens during Q&P or Q + T processes. The calculations for this scenario are made from the XRD results shown in Table 4 for the 06C1 MoV alloy. In this case, it must be considered that there are no changes in the fractions of the phases involved: \(\gamma+\alpha_{\text{bcf}}^{+}\rightarrow\gamma^{+}+\alpha_{\text{bcf}}^{-}\). On tempering at temperatures higher than 550 \({}^{\circ}\)C, precipitation of VC and M\(\gamma\)C\({}_{3}\), and also M\({}_{6}\)C in the steel containing Mo, carbides may occur simultaneously due to secondary hardening and carbide precipitation from austenite. Although the precipitation of these types of carbides during tempering can have a strong influence on the kinetics and the temperature range of the tempering reactions, its effect on the volume changes that occur during tempering can be neglected due to the relatively small content of these carbides that should be present in the microstructure compared to cementite. Figure 4 shows the equilibrium mass fraction of carbides as a function of the tempering temperature calculated using the MTDATA (National Physical Laboratory, Teddington, UK) together with the SGTE database for the chemical composition of the steel used for this study. As the mass fraction expected