Fracture morphology and fracture mechanism of gray cast iron with scrap

The microstructure is the main factor affecting the fracture morphology and fracture mechanism of metal materials. The notch effect at the tip of flake graphite makes gray iron castings prone to stress concentration during loading, which is the preferred position for crack generation and propagation. In addition, the amount, size and distribution of pearlite and hard inclusions in gray cast iron will also have a great impact on it.

As shown in the figure, the tensile fracture morphology of grey cast iron samples with different scrap content is shown. First of all, on the tensile section of 1-2 # sample shown in figure (a) and (b), there are too many large flake graphite, and there are small plastic deformation in the cast iron matrix, which corresponds to the ferrite block in the coarse flake pearlite or free large ferrite in the gray cast iron structure. However, the size and quantity of graphite sheets on the section of sample 3 are small (Fig. (c)). As the graphite in the gray iron castings with 100% scrap steel is obviously refined and the bending degree increases, the graphite flake in sample 3 has much less splitting effect on the cast iron matrix than that in sample 1-2. Secondly, as the spacing of pearlite sheets in sample 3 was greatly refined, the sorbite structure was formed (Fig. (f)), and the fracture was fine lamellae, which was perpendicular to the main fracture surface. The refinement of sorbite structure also has obvious strengthening effect on 3 sample. Finally, MNS particles with large size are distributed on the tensile section of sample 1-2, preferentially distributed at the pearlite cluster boundary and pearlite / graphite interface, and a small amount are distributed in pearlite clusters (as shown by the arrows in figures (d) and (E)). Because tin nucleates and grows with MNS as heterogeneous nucleus, its distribution in gray iron is roughly equivalent to that of MNS. The interface between these larger hard inclusions and cast iron matrix is poor. It is obvious that the hard phase on the tensile section has become the crack source or crack propagation path in the fracture process of gray cast iron.