For cast iron, there will always be some unavoidable casting defects, such as shrinkage cavity and porosity. Casting defects have a great impact on the fatigue performance of materials. Some casting holes will induce fatigue cracks to nucleate and initiate in advance and become a new fatigue source. There will be casting holes on the surface and inside of the material, although the gray cast iron material is less sensitive to notch effect, However, the casting holes in the material become the main location of stress concentration, and the cracks are easy to nucleate at the casting holes. With the further development of stress load cracks, these casting holes not only reduce the time required for fatigue crack nucleation of gray cast iron, but also shorten the fatigue crack propagation period and greatly reduce the fatigue life of gray cast iron. The more casting holes, the larger the size, the more significant the effect on the fatigue life of gray cast iron. As shown in the figure, the metallographic photos of low alloy HT250 casting holes and the morphology of fatigue crack origin area of fatigue fracture are shown.
As shown by the solid line arrow in figure (a), there are a small number of casting holes in the metallographic structure of low alloy HT250, which makes the gray cast iron easy to cause stress concentration when subjected to stress load. Figure (b) shows the morphology of the fatigue crack origin area of the fatigue fracture of the same sample, and the solid line circle in the figure is the casting holes in the material. It can be seen from the photos that the appearance of casting holes has become the origin of fatigue cracks. Due to the stress concentration caused by casting holes, the areas around the holes can become the development area of fatigue cracks. As shown by the solid line arrow in figure (b), new cracks have appeared around the casting holes because of the differences in mechanical properties between tissues, The crack development does not propagate in a single direction, but develops radially in multiple directions. At the same time, the fatigue strip is also observed around the casting hole, as shown by the dotted arrow in figure (b). The fatigue strip generally appears with the fatigue source, which is the characteristic of fatigue crack development. The appearance of fatigue crack and fatigue strip proves that the casting hole has become the fatigue source of fatigue fracture of low alloy HT250. The existence of casting hole greatly reduces the fatigue life of gray cast iron.