# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Dilatometric Study of the Microstructural Evolution during Tempering
This, in principle, would allow establishing a distinction in the decomposition behavior of the C-rich thin films (\(\gamma_{f}\)) and C-poor blocks (\(\gamma_{b}\)) of retained austenite present in the initial microstructure; see Figure 2. The fractions of \(\alpha\) and \(\theta\) resulting from the decomposition of retained austenite estimated from these calculations are shown in Figure 5a. Then, it is clear that a maximum fraction of 0.10 of \(\theta\) could be expected from the initial fraction of retained austenite if its complete decomposition is assumed. Figure 4: Calculated equilibrium mass fractions of carbides as a function of tempering temperature. In scenario 2, as shown in Figure 5b, the partial release of saturated austenite C in the form of cementite precipitates leads to an expansion of the system. However, the amount of cementite that is formed in this scenario is always very small. As shown in Figure 5c, it is expected for scenario 3 that tetragonal bainitic ferrite decomposes realizing the C supersaturation as \(\theta\) precipitates: \(\alpha_{b}\left(bct\right)\rightarrow\alpha\left(bcc\right)+\theta\). Since the majority of the carbon atoms in solid solution within bainitic is at dislocation sites, before precipitation can occur, some dislocations must be removed by recovery, but the presence of carbon strongly inhibits recovery [57, 61, 62]. \(\theta\) carbide precipitation from bainitic is very sluggish and occurs gradually as  
Figure 5: Theoretically calculated relative change in length (RCL) and volume fraction of relevant phases involved in the different scenarios described in the main text and detailed in Table 5, (**a**) scenario 1, (**b**) scenario 2, (**c**) scenario 3, and (**d**) scenario 4. The slope of the RCL as function of carbon content is reported as m.  
the tempering temperature is increased. This process is accompanied by a progressive loss of its tetragonal character, and the final structure consists of cementite and a \"defect-free\" bcc ferrite matrix. In Figure 5c, the red dotted line exemplifies the transition between the bct toward the bcc line. Finally, the result of the calculations made for scenario 4 is represented in Figure 5d. This figure shows how a partial enrichment of the retained austenite with C from the supersaturated bainitic ferrite results in system shrinkage, but its order of magnitude is too small compared to the other scenarios. Therefore, it can be assumed that if this mechanism were to occur, it would be overshadowed by any of the other scenarios or that it is not detectable by dilatometry. Figure 6 shows the dilatometry curve obtained during isothermal tempering at 350 \({}^{\circ}\)C. It can be observed a length increase of about 0.01% after 1 h of tempering, but as changes in the microstructure were neither expected nor detected by XRD and HV measurements, it can be attributed to the relaxation of compressive stresses built up during quenching from the austenitizing treatment to the austempering temperature or even to the uncertainty in the RCL measurement. In any case, this scenario has been considered highly unlikely, at least at tempering temperatures up to 450 \({}^{\circ}\)C, since no such enrichment of carbon, from ferrite to austenite, was detected by 3D atom probe tomography [57]. Additional information on the evolution of the microstructure during the entire tempering treatment can be obtained by analyzing in detail the dilatometry curves obtained. Illustrative examples of these curves as a function of the tempering temperature and the treatment time are given in Figure 7 only for the 06C1 MoV steel, since the behavior of both steels is equivalent. Although Figure 3 showed that no changes occur during heating at temperatures below 500 \({}^{\circ}\)C, Figure 7a and b illustrate a contraction in the RCL during isothermal holding at 450 and 500 \({}^{\circ}\)C, respectively.