# Retained Austenite Destabilization during Tempering of Low-Temperature Bainite
## 3 Results  
### Initial Microstructure  
Microstructural characterization of the as-received material confirmed the presence of a nanobainitic microstructure in both alloys consisting of a matrix of bainitic ferrite plates (\(\alpha_{\text{b}}\)) and retained austenite. As shown in Figure 2, two austenite morphologies can be differentiated: thin films (\(\gamma_{f}\)) sandwiched between bainite ferrite plates and islands of untransformed residual austenite between the sheaves of bainite, which is blocky in nature (\(\gamma_{b}\)). Although austenite will be enriched in C (\(C_{\gamma}\)) during austempering, experimental observations had shown that austenite films can be more so because of their geometrical isolation between plates of ferrite [34; 35; 36]; the relevance of such will become clear later in this manuscript. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Crystal Structure** & **Unit Cell Atomic Volume** & \(\beta_{\text{i}}\)\((\text{c}^{\text{C}}\text{-}^{\text{1}})\) & **Ref.** \\ \hline \(\gamma\) (fcc) & \(\text{a}_{\text{i}}^{3}/4\) & 2.065\(\cdot 10^{-5}\) & [13] \\ \(\alpha_{\text{b}}\) (bct) & \(\text{a}_{\text{i}}^{2}\text{c}_{\text{i}}/2\) & 1.490\(\cdot 10^{-5}\) & [27] \\ \(\alpha\) (bcc) & \(\text{a}_{\text{i}}^{3}/2\) & 1.244\(\cdot 10^{-5}\) & [13] \\ \(\theta\) (Orthorhombic) & \(\text{a}_{\text{i}}\text{b}_{\text{i}}\text{c}_{\text{i}}/12\) & 6\(\cdot 10^{-6}+3\cdot 10^{-9}\)\(\text{T}+10^{-11}\) T\({}^{2}\) & [28; 29] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Atomic volume and thermal expansion coefficients of the different phases used in this work where \(\text{a}_{\text{i}}\), \(\text{b}_{\text{i}}\), and \(\text{c}_{\text{i}}\) are the lattice parameters and \(\beta_{\text{i}}\) represents the thermal expansion coefficients. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Lattice Parameter (Å) at Room T** & **Ref.** \\ \hline \(\text{a}\)\(=\)\(3.578+0.033\)\(\text{w}_{\text{C}}+0.00095\)\(\text{w}_{\text{Mn}}-0.0002\)\(\text{w}_{\text{Ni}}+0.0006\)\(\text{w}_{\text{Cr}}\) & \\ \(\gamma\) & \(+\)\(0.022\)\(\text{w}_{\text{N}}+0.0056\)\(\text{w}_{\text{Al}}-0.0004\)\(\text{w}_{\text{Co}}+0.0015\)\(\text{w}_{\text{Cu}}\) & [30] \\ (fcc) & \(+\)\(0.0031\)\(\text{w}_{\text{Mo}}+0.0051\)\(\text{w}_{\text{Nb}}+0.0039\)\(\text{w}_{\text{Th}}+0.0018\)\(\text{w}_{\text{V}}\) & [30] \\ & \(+\)\(0.0018\)\(\text{w}_{\text{W}}\) & \\ \hline \(\alpha_{\text{b}}\) or \(\alpha^{\prime}\) & \(\text{a}\)\(=\)\(2.8664-0.01303\)-\(\text{w}_{\text{C}\alpha}\) & [31] \\ (bct) & \(\text{c}\)\(=\)\(2.8664+0.1162\)\(\text{w}_{\text{C}_{\text{n}}}\) & [26] \\ \hline \(\alpha\) (bcc) & \(\text{a}_{\text{Fe}}+\frac{(\text{a}_{\text{Fe}}-0.279\)\(\text{x}_{\text{C}})^{2}(\text{a}_{\text{Fe}}+2.496\)\(\text{x}_{\text{C}})-\text{a}_{\text{Fe}}^{3}}{3\text{a}_{\text{Fe}}^{2}}-0.03\)\(\text{x}_{\text{Si}}+0.06\)\(\text{x}_{\text{Mn}}\) & [26] \\ \(+0.07\)\(\text{x}_{\text{Ni}}+0.31\)\(\text{x}_{\text{Mo}}+0.05\)\(\text{x}_{\text{Cr}}+0.096\)\(\text{x}_{\text{V}}\) & \\ \hline \(\theta\) (Orthorhombic) & \(\text{a}\)\(=\)\(4.5246\) & \\ \(\theta\) (Orthorhombic) & \(\text{b}\)\(=\)\(5.0885\) & [28; 29] \\ \(\text{c}\)\(=\)\(6.7423\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Theoretical lattice parameters of the different phases at room temperature, where w is the concentration in weight%, x in mole fraction and \(a_{Fe}\)\(=\)\(2.88664\) is the lattice parameter of pure ferrite. The results of the Rietveld refinement performed on XRD patterns obtained for as-received conditions and their Vickers hardness values are summarized in Table 4. This table shows that the greatest difference observed between both materials lies in a greater volume fraction of retained austenite (f\({}_{\gamma}\)) for the 06C1 MoV steel, while the carbon content in bainitic ferrite (\(C_{ab}\)) and austenite (\(C_{\gamma}\)) and the Vickers hardness (HV) are almost identical for both steels.