For gray cast iron, its mechanical properties mainly depend on the matrix structure and flake graphite in it. The shape and distribution of graphite play a decisive role in its properties. Because the elastic modulus, yield strength and tensile strength of pearlite matrix and graphite are different, the shape, size and distribution of graphite in gray cast iron are different from those of elastic modulus Tensile strength and fatigue strength have a certain influence. In this test, the three grades of gray cast iron are pearlite matrix, and the flake graphite is type a graphite, but the tensile strength and fatigue strength of the three kinds of gray cast iron are very different. Compared with ordinary HT250, the tensile strength and fatigue strength of low alloy HT250 added with trace alloying elements are increased by 17.3% and 13.1%, which are generally considered to be determined by the morphology and distribution of flake graphite . Figure 4-5 shows the morphology and layout of ordinary HT250 and low alloy HT250 graphite.
It can be seen from the figure that in ordinary HT250 and low alloy HT250, the flake graphite is generally distributed in an undirected manner. The shape and size of ordinary HT250 flake graphite are different, and some parts have certain directionality. The distribution shape of low alloy HT250 flake graphite added with trace alloy elements is uniform; The grade of ordinary HT250 graphite is mixed distribution of grade I and grade II, and the grade of low alloy HT250 graphite is grade II. Due to the great brittleness of graphite and almost no molding, when stress concentration occurs, its deformation is extremely inconsistent with the matrix, resulting in splitting effect on the matrix. When stress concentration occurs on sheet graphite with certain directionality, it is easier to split the matrix. Due to certain directionality, after cracking the matrix, a large number of cracks expand in the same direction, With the further development of stress concentration, the cracks at the tip of flake graphite are connected with each other and finally cause fracture. The flake graphite of low alloy HT250 added with trace alloy elements is evenly distributed, non directionally distributed and has good resistance to stress concentration. When the matrix is split, the small cracks at the tip of the non directionally distributed flake graphite are not easy to connect and form large cracks. Therefore, at room temperature, the tensile strength and fatigue strength of low alloy HT250 are higher than that of ordinary HT250.