See Table 1 for the chemical composition of metal mold centrifugal casting vanadium titanium alloy gray cast iron cylinder liner.

Through the physical and chemical analysis of the black spot defect part of the cylinder liner, it is found that the graphite in this part is mainly coarse type A, and the hardness value is lower than that of the normal part hb10 ~ 30. It is confirmed that the black spot defect is essentially caused by uneven and segregation in the structure. Due to the different properties of different chemical elements in the cooling solidification process of cast iron, segregation is common in casting. For example, manganese can delay ferrite nucleation at the graphite / metal interface, and may slow down the diffusion of carbon to austenite, resulting in segregation.

More importantly, due to the solidification characteristics of centrifugal casting, segregation may be formed in the radial and vertical sections of the casting; Under the action of centrifugal force, the phase with different specific gravity in molten iron will also cause specific gravity segregation.

Factors that may lead to black spot defects include incomplete melting of inoculant, too slow pouring speed, too large difference in microstructure hardness, difference in cooling and solidification conditions, change of machining process, abnormal vibration of centrifugal casting machine, etc.