# Study on L-Bending Springback of 45 Steel Leather Cutting Tool Coupled with Local Induction Heating
## 2 Materials and Methods  
### Experimental Scheme  
The sheet material employed in this investigation is the Chinese standard 45 steel, which is equivalent to 1045 steel (AISI). The chemical composition of the 45 steel is shown in Table 1, and the dimension of each sheet is 190 \(\times\) 38 \(\times\) 3 mm. A high-temperature tensile experiment [23] was used to produce the stress-strain curve of 45 steel at room temperature and 800 \({}^{\circ}\)C under the condition of a strain rate of 1 s\({}^{-1}\). Table 2 displays the thermophysical performance parameters of 45 steel at various temperatures [24]. The trend in the thermophysical performance parameters with temperature is shown in Figure 1a-c. Moreover, the interfacial heat transfer coefficient (IHTC, in W/m\({}^{2}\) K) is required in the thermodynamic coupling simulation because of the transfer of energy from the heated sheet to the mold and air, as shown in Figure 1d. The IHTC is a value that varies with contact pressure because the pressure between the sheet and the mold affects the transfer of thermal energy.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Composition** & C & Si & Mn & Cr & Cu & Ni & Fe \\
**Proportion** & 0.43 & 0.27 & 0.60 & 0.15 & 0.20 & 0.22 & bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of 45 steel (wt.%).  
Figure 1: Thermophysical performance parameters of 45 steel: (a) specific heat capacity curve with temperature, (b) thermal conductivity curve with temperature, (c) the modulus of elasticity curve with temperature, and (d) IHTC change curve with contact pressure.  
Figure 2 shows the relevant information on the L-bending process of the sheet. The sheets were machined into a geometric shape with serrations at one end, as shown in Figure 2a, before bending. The bending of the sheets occurs in the local red region of Figure 2a, and the deformation of the sheets is mainly concentrated in this region. The detailed structure of the bending mechanism and the induction heating equipment is shown in Figure 2b. The apparatus is primarily made up of two parts: the outer and the inner molds. The sheet was fixed by the inner mold and bent by the rotation of the outer mold, and different bend angles were achieved by varying the rotation angle of the outer mold. The entire process of the sheet bending during local heating is depicted in Figure 2c.  
Through the usage of two infrared thermometers, the retractable induction heating device continuously measured the sheet's temperature during the induction heating process and the subsequent sheet's bending process. According to [PERSON] et al. (2018), L-bending needed a greater temperature than V-bending in order to considerably reduce springback. This was a result of the differing thermodynamic boundary conditions and forming properties of L-bending and V-bending, causing the higher cooling rate of L-bending. The springback of the sheet under L-bending was only fully decreased when the local heating temperature was not lower than 800 \({}^{\circ}\)C ([PERSON] et al., 2018). Therefore, 800 \({}^{\circ}\)C was chosen as the high-temperature bending temperature in this study, and the room temperature group was set for comparison. The sheet required 7.5 s to heat up, and the width of the heating area was 5 mm. Figure 3 depicts the heating process curve determined using the infrared thermometer. Because the bending process took place only 1.5 s after the sheet had finished heating, the temperature change in the sheet during processing was ignored. The experiment used various bending angles of 15, 30, 45, 60, 75, and 90 degrees with a rotational speed of 60 rad/s for the outer mold.  
Figure 2: The relevant information on the L-bending process of the sheet: (**a**) the geometry of the sheet, (**b**) bending equipment, and (**c**) bending and springback process of the sheet.