## 6 Conclusions  
The suitability of metastable austenitic stainless steel for the calibration of LUS measurements for in situ twinned grain size measurements in HSLA steels was investigated. The LUS measurements were validated using a novel approach based on parent austenite orientation maps reconstructed from the orientation maps of martensite measured at room temperature. The present findings can be summarized as follows:  
1. Parent austenite reconstruction using martensite orientation maps was used to verify that stainless steel with metastable austenite at room temperature can be used for the calibration of laser ultrasonic measurements of twinned grain size in HSLA steels.
2. There are significant differences in the twinning density between HSLA and austenitic stainless steels. At smaller grain sizes, differences were also observed between the HSLA steels.
3. A twinning-sensitive method is necessary to validate the LUS data. The parent austenite grain reconstruction of martensitic orientation maps measured using EBSD was found to be suitable for this purpose.  
Conceptualization, M.M., A.J., B.H., D.L. and T.N.; methodology, M.M. and T.N.; software, M.M., A.J. and T.N.; validation, M.M., A.J. and T.N.; formal analysis, M.M. and T.N.; investigation, J.L., M.M., A.J. and T.N.; resources, L.B.; data curation, M.M.; writing--original draft preparation, T.N.; writing--review and editing, T.N., M.M., B.H. and D.L.; visualization, T.N.; supervision, B.H.; project administration, M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.  
This work was partially financed by the strategic innovation program for metallic materials of Vinnova, the Swedish Energy Agency, under Formas grant no. 2018-03916, as well as the European Union's Research Fund for Coal and Steel (RFCS) research program under grant agreement no. RFCS-2018-847296.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Alloy** & \(\begin{array}{c}D_{r}\\ [\mu\text{m}]\end{array}\) & \(\begin{array}{c}D_{nr}\\ [\mu\text{m}]\end{array}\) & \(\begin{array}{c}p\end{array}\) & \(\begin{array}{c}fit_{r}\\ [^{\circ}]\end{array}\) & \(\begin{array}{c}D_{LUS,}[\mu\text{m}]\end{array}\) \\ \hline HSLA 1 & 6.4 & 7.1 & 0.015 & 2.23 & 6.8 \\  & 13.4 & 17.8 & 0.019 & 2.30 & 18.7 \\ \hline HSLA 2 & 5.3 & 6.9 & 0.045 & 1.84 & 6.5 \\  & 11.3 & 13.9 & 0.017 & 1.85 & 12.9 \\ \hline HSLA 3 & 4.4 & 5.0 & 0.027 & 1.79 & 3.7 \\  & 7.5 & 8.9 & 0.021 & 1.66 & 6.3 \\  & 14.3 & 20.8 & 0.022 & 1.74 & 16.6 \\  & 14.3 & 20.9 & 0.022 & 1.77 & 23.3 \\ \hline Tool steel & 5.2 & 7.7 & 0.063 & 1.30 & 4.1 \\  & 9.8 & 13.1 & 0.026 & 1.43 & 12.0 \\  & 12.5 & 17.1 & 0.021 & 1.43 & 12.9 \\  & 16.6 & 24.0 & 0.019 & 1.47 & 17.2 \\ \hline QP & 9.3 & 12.0 & 0.024 & 1.67 & 7.7 \\  & 14.7 & 23.0 & 0.025 & 1.58 & 18.1 \\  & 22.1 & 41.3 & 0.021 & 1.67 & 28.2 \\ \hline Experimental compositions & 7.4 & 9.2 & 0.027 & 1.65 & 11.8 \\  & 8.4 & 9.6 & 0.015 & 1.91 & 7.0 \\ \hline \end{tabular}
\end{table}
Table 1: The grain sizes \(D\), \(D_{n}\), and \(D_{LUS}\), twinning density \(p\), and the reconstruction fit for the low-alloy steels.