# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 2 Materials and Methods  
Two steels containing (in wt %) 0.6% C and 1.7% Si and different amounts of carbide-forming alloying elements such as Mo and V have been designed in the frame of a new development concept of including the secondary hardening reaction under certain tempering conditions in nanostructured bainitic steels [15; 16; 17; 18]. As shown in Table 1, these steels also include 1.3% Mn and 1.7% Cr to ensure low bainite (Bs) and martensite (Ms) start temperatures [19; 20; 21], and the high Si content ensures the absence of cementite precipitation during bainitic transformation [22]. The materials used for this study were manufactured as laboratory cast and heat treated to obtain a nanobainitic microstructure. To achieve this microstructure, both steels were austenitized at 1150 \({}^{\circ}\)C and then isothermally held for 20 h at 250 \({}^{\circ}\)C in a molten salt bath. The starting bainitic structures were subjected to tempering treatments consisting in heating the sample at 5 \({}^{\circ}\)C/s up an annealing temperature ranging between 450 and 650 \({}^{\circ}\)C, holding for 1 h, and then cooling down to room temperature at 5 \({}^{\circ}\)C/s, as shown in Figure 1. Tempering heat treatments were performed in a High-Resolution Dilatometer, Bahr 805D (TA Instruments, Germany), which enables studying phase transformation by monitoring the changes in the sample length. This device uses an induction coil to heat the sample and helium flow to cool it down. Throughout the experiment, the temperature is controlled by a K-type thermocouple welded at the mid length of the sample surface. Dilatometry tests, on cylindrical samples of 4 mm diameter and 10 mm length, were performed using fused silica push-rods, and the longitudinal length changes were recorded using a linear variable differential transducer (LVDT) in contact with the pushrods. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **C** & **Si** & **Mn** & **Cr** & **Ni** & **Mo** & **Cu** & **V** \\ \hline
**06 CV** & 0.6 & 1.7 & 1.3 & 1.7 & 0.2 & 0.2 & 0.2 & 0.5 \\ \hline
**06C1 MoV** & 0.6 & 1.7 & 1.3 & 1.7 & 0.2 & 1.0 & 0.2 & 0.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical analysis (in wt %) of the steels used for this study. Metallographic preparation was performed using a standard procedure that included grinding with SiC papers up to 1200 grit, and polishing in the final step with diamond paste of 3 and 1 \(\upmu\)m. The different microstructures were revealed by etching the polished surfaces with a 2% Nital (2% nitric acid in ethanol) solution and observed in a FEG-SEM Hitachi S48000 microscope operating at an accelerating voltage of 7 kV. In the case of the samples used for X-Ray diffraction, additional polishing and etching cycles were introduced to avoid the presence of martensite transformed form austenite on the surface of the sample during the metallographic preparation. X-ray diffraction (XRD) patterns were recorded using a Bruker AXS D8 diffractometer equipped with a Co X-ray tube working at 40 KV and 30 mA, a Goebel mirror, and a LynxEye Linear Position Sensitive Detector (Bruker AXS, GmbH, Karlsruhe, Germany). Coupled 0-20 scans were carried out over a 20 range of 35-135\({}^{\circ}\) with a step size of 0.01\({}^{\circ}\). The full diffraction patterns obtained were refined using the 4.2 version of Rietveld analysis program TOPAS (Bruker AXS) and the crystallographic information of the different phases (ferrite, austenite, and cementite) for quantitative phase analysis. In addition to global parameters such as background, zero displacement, and scale factors, the refinement protocol also included the unit-cell parameters by the double Voigt approach [23]. For this analysis, a corundum sample was used to remove the instrumental contribution to the width of the diffraction peaks. Theoretical calculations by means of atomic volumes were performed in order to later assist on the interpretation and understanding of the changes detected on the dilatometric curves.