Analysis of tensile fracture mechanism of nodular cast iron

After normalizing, the tensile strength and elongation of nodular cast iron are improved at 930 ℃. In nodular cast iron, carbon exists in the form of graphite balls. Although there are few graphite balls, dislocation accumulation is easy to form at the interface between graphite balls and matrix, and microcracks at the graphite matrix interface are easy to occur at the dislocation accumulation during stretching. With further stretching, the graphite matrix interface microcracks will elongate along the interface between graphite ball and matrix until graphite and matrix are separated. This phenomenon is called graphite ball and matrix debonding. After the graphite debonding, pits are formed on the matrix in contact with the graphite ball, which is easy to produce local stress concentration, resulting in the crack source of the matrix. Moreover, because the graphite matrix in front also produces graphite debonding, with the increase of tension, the crack will expand to the front square graphite ball, thus forming a main crack. When the main crack propagates so repeatedly, the ductile iron is finally broken.

The SEM diagram of the micro crack in the longitudinal section of the fracture under the normalizing condition of 930 ℃ is shown in the figure. It can be seen from the figure that there is graphite debonding at the interface, but the microcracks in the matrix do not germinate. This may be because most of the matrix is pearlite, which is composed of ferrite and cementite. According to the pearlite shear fracture model proposed by Miller et al., a large number of cracks in pearlite and pearlite lamella expand at 45 °. In the drawing process, the ferrite lamella breaks first, but the cementite lamella does not break immediately, but becomes slender, so that the pearlite matrix is pulled by the cementite lamella, which increases the overall plastic deformation capacity of the casting. However, cementite in matrix is interstitial compound, which will reduce plastic deformation of nodular cast iron.

In conclusion, compared with as cast, the tensile strength increases from 688mpa to 759mpa and the elongation decreases at the normalizing temperature of 870 ℃. This is because the graphite ball is surrounded by ferrite in the matrix of as cast nodular cast iron, and the crystal structure of ferrite is body centered cubic and has good plasticity. During the tensile process of as cast nodular cast iron, after the graphite ball is separated from the matrix, when the microcrack extends into the graphite ball hole, the graphite ball hole plays the role of crack passivation. Compared with nodular cast iron normalized at 870 ℃, the tensile strength of nodular cast iron normalized at 930 ℃ has little change, and the elongation increases from 5.4% to 9.5%. This is due to the decomposition of cementite in the matrix of nodular cast iron, which improves its elongation.