Crack is one of the most dangerous casting defects of engine gray cast iron. Undetected cracks will expand and extend with the operation and vibration of the engine. At least, they will cause water leakage and oil leakage of components, and at worst, they will lead to the damage and collapse of the whole engine, resulting in irreparable losses.
According to the temperature range of cracks, cracks can be divided into hot cracks and cold cracks. At the later stage of solidification, the metal liquid has formed a solid skeleton. If it is stressed at this time, the dendrites crack and cannot be completely supplemented by the nearby metal liquid, it will form a hot crack. Cold crack refers to the crack caused by the local stress exceeding the strength of gray cast iron after the liquid metal is completely solidified. The hot crack is formed in the liquid-solid mixing period. The crack is wide and tear like. The surface of the crack is seriously oxidized and shows oxidation color without metallic luster. When the cold crack is formed after complete solidification, the cold crack generated at high temperature will also be black brown due to oxidation, and the oxidation color on the crack surface will gradually fade with the decrease of formation temperature, with metallic luster at low temperature. No matter how severe the cold crack is, it is also one of the characteristics of the cold crack and the hot crack.
(1) The hot crack on the top is related to the bottom injection process. After pouring, the top temperature is lower than the bottom temperature, which is easy to produce large stress during solidification. Especially for gray cast iron with pouring weight mainly concentrated in the lower part of V-type engine block, the tensile stress on the top in the later stage of solidification process will be greater, which is more likely to produce hot cracks.
(2) Install cold iron at the parts prone to hot cracks to promote its early solidification and ensure that it has sufficient rigid strength before being stressed, which can prevent the occurrence of hot crack casting defects.
(3) The fundamental reason for the formation of various cracks and casting defects is that the stress formed in the solidification process of gray cast iron is too large and too concentrated. According to the structural characteristics of the blank, selecting a reasonable pouring process design can effectively reduce the formation of stress in gray cast iron or eliminate the bad transmission of stress in the solidification process.
A V-shaped engine block blank produced by ZHY casting, 12 cylinders, weighing 1172kg, made of ht280 material, the sand core adopts the cold core box process, the sand mold adopts the alkaline phenolic resin self hardening sand process, and the oil bottom joint surface faces up. In actual production, cracks and casting defects often appear at the connecting opening between the gray cast iron oil bottom joint surface cylinder and the cylinder. Figure 1 and 2 show the location and shape of the cracks respectively. It is analyzed that the crack appears in the thick part of gray cast iron blank, and the shape conforms to the characteristics of hot crack. It is confirmed that this crack is a hot crack. The thermal crack can be divided into internal crack and external crack. The crack opening of V-type engine body is outward, which belongs to external crack.