# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Microstructural Characterization of Tempered Microstructures by XRD and SEM
When Figure 10a is compared with Figure 10c,d, it is observed that the number and size of the blocky features decrease as the tempering temperature increases. On the other hand, fresh martensite within the block structures can also be observed in a closer look at these figures as a result of the transformation of the retained austenite during cooling. Although martensitic transformation is also detected during cooling after tempering at 550 and 600 \({}^{\circ}\)C for 06C1 MoV in Figures 7 and 8, Figure 11b shows the presence of 5% and 3% austenite retained at room temperature. Note that the XRD measurements were done after cooling to room temperature, and therefore, the microstructure already contains fresh transformed martensite. For 06C1 MoV steel, Figures 8 and 9 show the formation of martensite during cooling after tempering at 650 \({}^{\circ}\)C and Figure 11b shows the absence of retained austenite in the microstructure, indicating that austenite was depleted in C to levels that make it unstable at room temperature. On the  
Figure 11: Summary of XRD results as a function of tempering temperature for (a) 06 CV and (b) 06C1 MoV steel. other hand, the full decomposition of the retained austenite for the 06 CV steel during tempering at this temperature is concluded by the lack of martensitic transformation in Figures 8 and 9 and austenite in Figure 11b. The decomposition of retained austenite and the precipitation of carbides in this case are evident in the profuse precipitation observed in Figure 10e, so that the presence of cementite was detected in the XRD patterns only in this case, but it was below their limit of quantification (LoQ). Although the XRD reflections of the ferritic phase (\(\alpha\)) may include the contribution of bainitic ferrite (\(\alpha_{b}\)), ferrite (\(\alpha\)), and martensite (\(\alpha^{-}\)), the largest information obtained from these reflections corresponds to bainitic ferrite, since this phase represents around 70% of the initial microstructure. Bainitic ferrite decomposition follows scenario 3, and therefore, increases in tempering temperature should lead to a progressive decrease in the average \(C_{\alpha}\). As shown in Figure 11c, the C content of this phase is very similar and remains almost constant after tempering up to 550 and 600 \({}^{\circ}\)C for the 06 CV and 06C1 MoV steels, respectively. These results agree with those of [PERSON] et al., who, based on the first principles calculations, suggests that ferrite in equilibrium with austenite has much higher carbon solubility if it displays tetragonal crystalline symmetry instead of cubic [40]. As the maximum solubility is achieved at about 400 \({}^{\circ}\)C and extends over a long temperature range, carbon excess will have a high resistance due to being released from bainitic ferrite during tempering in this temperature range. It has been experimentally proved that up to 0.2 wt % can be in solid solution within ferrite and trapped at dislocations sites and bainite lath boundaries [1], since the dislocations density near these boundaries are usually much higher than in the lath interior [63]. The complete decarburization of supersaturated ferrite [57; 61] requires the precipitation of carbide from C trapped in dislocations [62], but for this some dislocations must be removed by recovering during tempering. As carbon is more stable while segregated to dislocations than in transition carbides or cementite [62], the high dislocation density is then stabilized by carbon pinning, causing the recovery of the dislocation network to be strongly inhibited. Thus, the tetragonal structure of bainitic ferrite will be stable up to high tempering temperatures [8; 10], as shown in Figure 11.  
The evolution of the hardness values as a function of the tempering temperature has been correlated with the microstructural evolution described previously.