The defect phenomenon and cause of the fast broken graphite in ductile iron casting

Fast graphite fragmentation

The broken fast graphite is a kind of deformed graphite which often appears in the large end face (wall thickness ≥ 100mm) ductile iron or in the hot spot. On the macro fracture surface, it can be seen that the black spots with the size of 1-3mm are densely distributed in the central area of slow cooling casting. The part with broken graphite has loose texture and deteriorated mechanical properties, especially the plasticity index.

It was found that the fragmentary graphite had its own eutectic group by deep corrosion and SEM. In a eutectic group, the fragmentary graphite is connected with each other, and because it is formed during slow solidification, the eutectic group can develop and grow up, so it is much larger than the spherical graphite eutectic group, and its geometry is generally spherical. Because these graphite are very small and often cut and branched, the metal matrix in the granular graphite eutectic group is usually ferrite.

Causes

The mechanism of the formation of fragmentary graphite is still unclear. The results of SEM show that the molten iron has an erosive effect on the fragmentary graphite. The first one is the fragmentary graphite eutectic. Later, due to the slow solidification process, the size of the eutectic clusters formed is large, and because of the frequent and small branching of the fragmentary graphite, the connection between the ends is loose. Under the action of the hot convection of the molten iron, it is possible to make the graphite close to the boundary of the eutectic clusters be eroded into free fragments. In addition, under the action of thermal convection, the larger size pieces of graphite split into smaller size pieces of graphite. Therefore, they are free from the eutectic group and float at the boundary of the eutectic group.

Secondly, due to the slow solidification, the precipitated graphite ball is much larger than the ordinary primary graphite ball. When the size exceeds a certain value, the iron content in these graphite spheres increases. With the further growth of these graphite balls in the molten iron, internal stress will be formed due to the size change. Because of the increasing internal stress in the process of expansion, when the stress exceeds a certain value, the graphite ball begins to break and form fragments. In the solidification process, the molten iron convection can make these fragments smaller, and they are rushed into the dendrite by the hot turbulence of the molten iron, forming the crystal core of the fragmentary graphite.

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