Reasons Analysis on Cracking of Ductile Iron Sleeve of a Ship Anchor Machine

The physical and chemical testing and failure reason analysis were done to the fractured ductile iron sleeve of a ship anchor machine. The results show that both the coarse grain of the casting because of the high casting temperature of the liquid iron and the long sealing time, and the more porosity and pore defects because of the liquid iron not feeding in time when solidification, significantly reduced the strength and increased the brittleness of the casting. The strong impact of shot peening treatment make micro cracks produce on the workpiece surface, which brought great cracking possibility to the workpiece. The brittleness of the ferrite at subsurface layer of the workpiece was high due to the high silicon content, which further increased the cracking tendency of the workpiece. Under the high pressure of inner cavity of the sleeve, the multiple step crack sources were generated at the weak positions of the upper cover edge, and the crack continued to expand to the whole upper cover.