# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results
### Dilatometric Study of the Microstructural Evolution during Tempering
In the case of the sample held at 450 \({}^{\circ}\)C, the RCL decreased progressively up to \(-0.03\%\), but for the sample held at 500 \({}^{\circ}\)C, the RCL decreased \(-0.06\%\) after 1300 s and then reached a plateau. It is likely that the microstructural changes associated with this shrinkage of the samples are related to the partial decomposition of the C-enriched austenite films according to scenario 1, since its higher content of C gives them a greater driving force for carbide precipitation [6; 7; 8; 10; 13; 14]. Figure 6: Relative change in length (RCL) as a function of time for 06 CV steel tempered at 350 \({}^{\circ}\)C.  
During tempering at 550 and 600 \({}^{\circ}\)C, an RCL decrease occurred, which was followed by a length increase that is more pronounced at 600 \({}^{\circ}\)C, as shown in Figure 7c,d. Austenite films begin to decompose during heating to these holding temperatures, so once isothermal holding begins, the carbide precipitation process triggered in scenario 1 is quickly over. Comparing the RCL curves  
Figure 7: Relative change in length (RCL) as a function of time and temperature for 06C1 MoV steel tempered at (**a**) 450 \({}^{\circ}\)C, (**b**) 500 \({}^{\circ}\)C, (**c**) 550 \({}^{\circ}\)C, and (**d**) 600 \({}^{\circ}\)C.  
of Figure 7c,d, it can be concluded that an increase of 50 \({}^{\circ}\)C in the tempering temperature leads to a significantly lower reduction in length and a decrease in the time required for carbide precipitation. The increase in length observed in the RCL curve after the initial contraction is compatible with the decomposition of the remaining retained austenite depleted in C by both scenarios 2 and 1. However, the RCL curve recorded during cooling to room temperature after the isothermal treatment at 500 and 600 \({}^{\circ}\)C shows that some austenite transformed to fresh martensite. The appearance of a martensitic transformation during cooling means that the amount of carbon in austenite has been reduced continuously during isothermal tempering by the precipitation of carbides (scenario 2). As shown in Figures 7 and 8, the measured Ms temperature decreases as the tempering temperature is increased, indicating that martensite is produced from an austenite with a lower C content. The hypothesis that the austenite decomposition sequence begins with scenario 2 and ends with scenario 1 is reinforced by the dilatometric results shown in Figure 9 for the tempering treatment at 650 \({}^{\circ}\)C. It can be observed that RCL curves increases during isothermal holding at this temperature, revealing that scenario 1 for C-rich films and the possible loss of the tetragonality of the ferrite (scenario 3) have already occurred during heating. For the 06 CV steel, the appearance of a plateau in the RCL curve after 0.08% expansion and the absence of any length change related to the martensitic transformation during cooling to room temperature would indicate that the austenite decomposition has been completed during isothermal tempering. On the contrary, the austenite decomposition of the 06C1 MoV steel is not completed after 1 h at 650 \({}^{\circ}\)C, since the RCL curve does not reach a plateau, and there is some C impoverished austenite available to transform to martensite during cooling. These results would indicate that the addition of Mo delays the kinetics of the reactions that take place during tempering. Figure 8: Measured Ms temperatures on cooling after tempering at the indicated temperature.