# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Microstructural Characterization of Tempered Microstructures by XRD and SEM  
The results of the above dilatometry tests were correlated with a detailed microstructural characterization to evaluate the influence of the tempering temperature on the thermal stability of the nanostructured microstructures present in both steels. FEG-SEM micrographs of the microstructure developed after isothermal holding for 1 h showed a very similar tendency for both steels. Thus, only representative microstructures are given in Figure 10 for the 06 CV steel after tempering at temperatures ranging from 450 to 650 \({}^{\circ}\)C. Figure 11 gathers the XRD results for the tempered microstructures. As anticipated by dilatometry results, Figure 10a (450 \({}^{\circ}\)C tempering) shows the same microstructure to that found in Figure 2. On the other hand, the XRD parameters deduced from the Rietveld refinement for both as received and tempered materials are basically the same as deduced from Figure 11. Tempering at 450 \({}^{\circ}\)C does not introduce perceptible changes in the microstructure, and retained austenite with both thin-film and blocky island morphologies remains basically unchanged, indicating that this temperature is not high enough for its transformation. The behavior of the steels differs during tempering at 50 \({}^{\circ}\)C. As shown in Figure 11, while 06C1 MoV steel still does not present any appreciable changes in their microstructure, the 06 CV steel already exhibits signs of austenite decomposition, as its fraction decreases by 0.12 as compared to the initial microstructure and the previous treatment at 450 \({}^{\circ}\)C. The decomposition of the austenite is accompanied by the precipitation of carbide particles, but the XRD pattern of this sample did not show any diffraction peak of cementie. The low symmetry of cementie causes a greater number of reflections in the diffraction pattern. The distribution of the scattered X-rays in many directions decreases the intensity of a single peak, and the nanometric sizes, microstrain, and faulting of the carbide particles broadens the reflections. The combination of both effects strongly affects the limit of detection (LoD) of the cementie present in a sample, which decreases to a value ranging from 3 to 5 wt %. As a reference, the LoD for retained austenite for same measurement conditions is about 1 wt % in a low alloyed  
Figure 9: Relative change in length (RCL) as a function of time and temperature for (a) 06 CV and (b) 06C1 MoV steel tempered at 650 \({}^{\circ}\)C.  
steel. Although the bainitic ferrite plates and the retained austenite blocks remain unchanged after tempering for 1 h at 500 \({}^{\circ}\)C, Figure 10b evidences by red arrows the precipitation of fine cementite particles in the location where retained austenite films were originally present. This microstructural evolution during tempering would agree with the decomposition of austenite films described for scenario 1. The remaining austenite is still sufficiently C-enriched to remain stable at room T, and no fresh martensite formation was detected during cooling in the RCL curve of Figure 7.  
Figure 10: SEM micrographs after tempering for 1 h at (**a**) 450 \({}^{\circ}\)C, (**b**) 500 \({}^{\circ}\)C, (**c**) 550 \({}^{\circ}\)C, (**d**) 600 \({}^{\circ}\)C, and (**e**) 650 \({}^{\circ}\)C in the case of 06 CV steel. The higher tempering resistance at 500 \({}^{\circ}\)C of the 06C1 MoV microstructure compared to 06 CV is also evident for the other tempering conditions. As previously shown in the dilatometry results, increasing tempering temperatures leads to greater decomposition of the bainitic microstructures. Although no austenite was detected in their XRD patterns, Figures 7 and 8 show for the 06 CV steel the transformation to martensite of a certain amount of austenite during cooling to room temperature after tempering at 550 and 600 \({}^{\circ}\)C, which indicates that the decomposition of austenite was not complete.