The low magnification morphology of tensile fracture of three grades of gray cast iron is shown in Figure 1.
It can be seen from Figure 1 that the tensile fracture of the three grades of gray cast iron samples at room temperature is rough and uneven, and the fracture surface is granular crystal. Because the three grades of gray cast iron are brittle materials, the fracture occurs almost instantaneously, and the fracture surface is perpendicular to the stress direction. The tensile fracture type of the three grades of gray cast iron samples belongs to obvious brittle fracture.
The high magnification morphology of tensile fracture of three grades of gray cast iron is shown in Figure 2.
As can be seen from Fig. 2, the tensile fracture surfaces of the three grades of gray cast iron samples are composed of uneven cleavage surfaces, and contain some curved tearing edges, depressions, flake graphite and broken matrix structure. It shows that the tensile fracture mechanism of three grades of gray cast iron samples at room temperature is cleavage fracture, which is mainly cleavage fracture in both matrix and flake graphite. Compared with the ordinary HT250, the low alloy HT250 added with trace alloying elements can be seen from the tensile fracture photos that the fragmentation of its matrix and flake graphite is more serious. The small displacement of the fragmented matrix changes the original stress concentration position, so that the destructive force acts on the new matrix again, resulting in secondary cracks and prolonging the progress of fracture, At the same time, trace alloying elements enhance the matrix strength of low alloy HT250 through solid solution strengthening. As a result, the tensile strength of low alloy HT250 is higher than that of ordinary HT250.