# Study on L-Bending Springback of 45 Steel Leather Cutting Tool Coupled with Local Induction Heating
## 3 Results
### Springback and Stress Analysis under Local Heating at 800 \({}^{\circ}\)C  
As shown in Figure 8, the amount of springback under local heating at 800 \({}^{\circ}\)C increases with the increase in the bending angle, which is due to the increase in the bending angle causing a greater amount of deformation around the bending area of 45 steel sheets, leading the two sides of the bending area to produce greater compressive and tensile stresses, respectively. This causes a greater amount of unbalanced stress and springback. Additionally, when the sheet is heated locally to 800 \({}^{\circ}\)C, the forming performance of the bending area is improved, and when the 15\({}^{\circ}\) and 30\({}^{\circ}\) bending is carried out, as shown in Figure 9, the plastic deformation area still takes up a sizable percentage of the sheet. That is to say, after the local heating of the sheet at 800 \({}^{\circ}\)C, when the bending angle of the sheet is not lower than 15\({}^{\circ}\), the main influencing factor of springback is always the unbalanced compressive and tensile stresses in the bending area. The springback at various angles at 800 \({}^{\circ}\)C decreased by 75.2%, 68%, 58.4%, 58.1%, 53.1%, and 49%, respectively, compared to room temperature, which supports the feasibility of local heating and demonstrates that local heating effectively enhances the forming performance of bent sheets. The main reason for the reduction in springback is that local heating reduces strain hardening around the bending zone and thus the equivalent stress around the bending area.  
Figure 9: Stress field in the thickness of the bending area of the sheet before unloading at different bending angles at local heating under induction local heating.  
Figure 9 shows the sheet after bending at 90\({}^{\circ}\) under local heating at 800 \({}^{\circ}\)C and that only the bending area has plastic deformation. There is no reverse stress distribution and'springforward' phenomenon caused by reverse bending outside the bending area after unloading under global heating, as reported by [PERSON] et al. (2015). The'springforward' phenomenon makes it difficult to ensure the straightness accuracy of the sheet, which is more likely to occur when the sheet is heated globally at high temperatures. This also illustrates why local heating has greater forming accuracy than global heating. Because local heating only affects a tiny portion of the heat-affected zone and heats up quickly, it has less of an impact on how the structure and hardness of the heat-affected zone change. Figure 11 shows that local heating significantly lowers the sheet's stress level relative to room temperature, which reduces the hardness of the material and increases the likelihood of plastic deformation, which, in turn, enhances the forming performance of the 45 steel material. In addition, during induction local heating, a large amount of energy is transferred from the heat-affected area to the non-heated zone due to heat conduction, which causes the cooling rate during the forming process to be much faster than that of global heating. This aids in the work hardening process in the formed area, where, following induction local heating, a hardness that had been somewhat lowered can be restored to the original hardness or even higher ([PERSON] et al., 2015).  
Figures 10 and 11 depict the equivalent stress of the sheets before unloading, as obtained by the finite element simulation. As can be seen, the sheet's bending area experiences more stress at room temperature than it does at 800 \({}^{\circ}\)C. This difference in stress levels results in a less uneven distribution of stress in the bending area, which, in turn, lowers the springback angle.