# Investigation of Impact Behavior of STS304L Steel Plate Under Cryogenic Temperature
## 3 Results and Discussion
### EBSD Analysis: Volume Fractions of Strain-Induced Martensite  
The EBSD analysis region is depicted in Figure 12, and the martensite fraction results at various temperatures are presented in Figure 13 and Table 4. Strain-induced martensitic transformation under impact loading affects both energy absorption capacity and stiffness. Additionally, strain-induced martensite formation accelerates at lower temperatures [29]. Therefore, when evaluating the temperature sensitivity of STS304L, both strain-induced martensitic transformation and mechanical properties must be considered. At 20 \({}^{\circ}\)C, the martensite fraction remained relatively constant across all analyzed regions. However, in the cryogenic environment (\(-100\)\({}^{\circ}\)C and \(-193\)\({}^{\circ}\)C), the martensite fraction increased significantly with deformation owing to the acceleration of strain-induced martensitic transformation. Region 2, located at the end of the hemispherical radius, exhibited the highest martensite fraction, as follows: 77.85% at \(-100\)\({}^{\circ}\)C and 96.87% at \(-193\)\({}^{\circ}\)C. This region experienced both axial deformation and significant thickness deformation because it was subjected to localized shear stress. Figure 11: (**a**) Energy absorption ability and (**b**) plate deflection versus temperature. Figure 12: Applied condition and EBSD analysis regions. Except for Region 4, all regions exhibited the same trend--strain-induced martensitic transformation increased as temperature decreased. This aligns with previous studies, which showed that lower temperatures accelerate strain-induced martensitic transformation when deformation exceeds a critical threshold [29]. The higher dislocation density at lower temperatures plays a key role in this accelerated transformation. Because strain-induced martensite is harder and stronger than austenite, it resists deformation when impact energy is transferred to STS304L, ultimately increasing stiffness and impact resistance. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Temperature** & **Fraction** & \begin{tabular}{c} **Region 1** \\ **[\%]** \\ \end{tabular} & \begin{tabular}{c} **Region 2** \\ **[\%]** \\ \end{tabular} & \
\begin{tabular}{c} **Region 3** \\ **[\%]** \\ \end{tabular} &
\begin{tabular}{c} **Region 4** \\ **[\%]** \\ \end{tabular} \\ \hline
20 &
\begin{tabular}{c} Martensite \\ Austenite \\ Martensite \\ \end{tabular} & 13.94 & 19.26 & 16.99 & 23.78 \\
20 &
\begin{tabular}{c} Austenite \\ Martensite \\ \end{tabular} & 86.06 & 80.74 & 83.01 & 76.22 \\
20 &
\begin{tabular}{c} Martensite \\ Austenite \\ \end{tabular} & 76.29 & 77.85 & 46.54 & 24.61 \\
\begin{tabular}{c} Austenite \\ Martensite \\ \end{tabular} & 23.71 & 22.15 & 53.46 & 75.39 \\
24 &
\begin{tabular}{c} Martensite \\ Austenite \\ \end{tabular} & 89.56 & 96.87 & 60.65 & 26.97 \\
25 &
\begin{tabular}{c} Austenite \\ \end{tabular} & 10.44 & 3.13 & 39.35 & 73.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of EBSD analysis. Figure 13: EBSD phase maps of STS304L specimens showing strain-induced martensite (Fe-BCC, red) and retained austenite (Fe-FCC, blue) at different temperatures: (**a**–**d**) 20 \({}^{\circ}\)C, (**e**–**h**) \(-\)100 \({}^{\circ}\)C, and (**i**–**l**) \(-\)193 \({}^{\circ}\)C.  
Strain-induced martensitic transformation is also influenced by strain rate. In the drop-weight impact tests, the strain rate was not constant, because the impactor velocity decreased after contact. The average strain rate can be defined as follows:  
\[\dot{\epsilon}=V/L \tag{3}\]  
where \(\dot{\epsilon}\) is the average strain rate, \(V\) is the axial velocity upon impact, and \(L\) is the axial deformation. The calculated average strain rates were 225/s at 20 \({}^{\circ}\)C, 256/s at \(-\)100 \({}^{\circ}\)C, and 262/s at \(-\)193 \({}^{\circ}\)C, all of which fell within the high strain rate range. At high strain rates, adiabatic heating (localized temperature increase) is a critical factor affecting mechanical behavior [30].