The cylinder heads of high-speed and heavy-duty marine diesel engines are mostly high toughness(see Figure). The cooling solidification mode of nodular cast iron is paste solidification, which is easy to produce defects such as small number of graphite balls, large ball diameter, low mechanical properties, spheroidization recession and inoculation recession. In addition, the cylinder head is traditionally produced by vermicular cast iron, but now it is produced by nodular iron. Due to its complex structure and difficult to control the cooling and solidification rate of each part, it is particularly difficult to cast, and it is easy to produce casting defects such as leakage.
From trial production to mass production of high toughness ductile iron cylinder head, the product quality fluctuates greatly, and the yield is less than 80%. The main defects leading to low yield are: shrinkage in the valve guide hole, shrinkage in the bolt hole, air holes around the oil injection hole, shrinkage at the edge of the gate in the exhaust pipe, air holes, etc.
For castings with complex structure and high internal quality requirements, when the structure is limited and direct feeding measures cannot be taken, special sand combined with cold iron process can accelerate the cooling speed of local hot joints, eliminate shrinkage cavities and porosity, and effectively solve the defects of shrinkage cavities and porosity at hot joints.