Analysis of tensile fracture mechanism of nodular cast iron

After normalizing, the tensile strength and elongation of nodular cast iron are improved from 930 ℃. In nodular cast iron, carbon exists in the form of graphite balls. Although these graphite balls are few, 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 microcrack at the graphite matrix interface will extend along the interface between the graphite ball and the matrix until the graphite and the matrix are separated. This phenomenon is called the debonding between the graphite ball and the matrix. 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.

SEM image of longitudinal section micro crack at fracture surface of 930 ℃ normalized sample

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 process of drawing, 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 nodular cast iron. However, the cementite in the matrix is a interstitial compound, which will reduce the plastic deformation of nodular cast iron.

To sum up, compared with as cast, under the normalizing temperature of 870 ℃, the tensile strength increases from 688mpa to 759mpa, while the elongation decreases. This is because the ferrite in the matrix of as cast nodular cast iron surrounds the graphite ball, while 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, while 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.