3.1 Mechanical properties
The mechanical properties of the experimental bend are shown in Table 3. The results revealed that there is difference in mechanical properties between X90 parent pipe and the bend. The yield strength and tensile strength of the parent pipe is 748 MPa and 847 MPa, respectively. The strength of X90 bend decreased 30 ~ 80 MPa compared with the parent pipe, but the plasticity and toughness improved. According to the data in Table 3, different positions in the X90 bend reflected the different mechanical properties. The yield and tensile strength in the outer arc side is 750 MPa and 848 MPa, respectively. The elongation is 21%. The yield strength, tensile strength in the inner arc side and the neutral axis is lower about 50 MPa than that of outer arc side. However, the impact toughness at -10 ℃ of outer arc side is the weakest in X90 bend, the impact absorbed energy is 153 J, and the energy of other positions is more than 200 J that is larger than that of the parent pipe. During hot induction bending, the outer arc side was subjected the effect of tensile stress and heat treatment. Therefore, the position with a high strength and low toughness. Moreover, there is no effect of deformation on the position of neutral axis, and with the same mechanical properties of the straight part.
Table 3 Tensile and Charpy impact properties of the bend.