November/December AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 2 6 directions. Residual stress is distributed from the bottom to straight section of the workpiece. Stress gradually increas- es with the radian from the bottom out- er wall, begins to decrease during the transition period, and increases quick- ly upon entering the straight section reaching a maximum value. At a deformation temperature of 150 ° C, strain-induced martensite transformation due to residual stress disappears. Also, residual stress chang- es with increasing temperature as part temperature becomes more uniform. Figure 4 shows that residual stress at the straight section decreases at the higher forming temperature. MARTENSITE PHASE CONTENT Specimen phase composition and variation were analyzed qualitative- ly and quantitatively during plastic deformation. Figure 5 shows the x-ray diffraction curve of 304 stainless steel in different positions. The diffraction peaks are γ (111), γ (200), γ (220), α ʹ(110), α ʹ(200), and α ʹ(211). There is a notice- able α ʹ martensite peak, and a marked peak strength of the outer wall straight section position, which confirms that α ʹ-martensite peak strength increases with increasing deformation. The direct comparison method was used to quan- titatively calculate diffraction peak. The martensite volume fraction increased in the straight section (Fig. 5). This also suggests that γ austenite transforms to α ʹ-martensite during the forming pro- cess, and martensite content reached a maximum in the straight section. EFFECTS OF RESIDUAL STRESS Residual stress stems from the in- homogeneity of microstructure in the formed material. Thus, stress distribu- tion in the specimen outer wall is due to the stress state and deformation vari- ables during the forming process. Dif- ferences in the stress state in the outer wall, transition section, and straight section of the specimen directly cause differences in residual stress. Stress does not change with the change in ra- dian from the beginning of the punch contacting the sheet to the end of forming; it is always subjected to bi- axial tensile stress. However, it is af- fected by biaxial compressive stress in the arc segment. In the transition sec- tion, the stress state remains the same during the entire forming process, be- ing one-directional in both tension and compression. However, in the straight section, the stress state changes from biaxial tensile stress and one-directional com- pressive stress to one-directional com- pressive stress and one-directional tensile stress. Therefore, the transition section of the specimen is a transition zone and it is the location of the chang- ing stress state. The complex stress state of the straight section leads to maximum residual stress. During the forming process, deformation is high- est in the straight section. Thus, cracks often appear in this area. In addition, the complex stress state and larger Fig. 2 — Diagram of x-ray diffraction test principle. Fig. 3 — Distribution of residual stress measurement points. Fig. 4 — Residual stress values at measurement points on outer wall of specimens formed at room temperature and 150°C.