Factors Influencing Solidification Structure and Mechanical Properties of P-B Cast Iron for Marine Engine Cylinder Liner

摘要

P-B cast iron s have been widely applied to the cylinder liner for marine diesel engines, since they are composed of flaky graphite, hard phases(γ-Fe_3C-Fe_3P eutectic) and pearlite matrix which offer a higher wear resistance. Recently, the strengthening of cylinder liner is required to raise the engine's output. However, large graphite flakes develop in thick castings and reduce the strength. Therefore, the control of graphite structure is essential to attain a higher tensile strength. Application of chill blocks to the sand mold, for raising the cooling rate of casting, should refine the solidification structure and enhance the mechanical properties. However, the experiments on the effects of 20 to 90 mm thick steel chill blocks on the 180mm thick rectangular casting showed that the chill blocks rather lowered the tensile strength. Thermal and metallographic analysis of castings revealed that the chill blocks raised both the cooling rate and the temperature gradient near the eutectic freezing front. Though a higher cooling rate resulted in finer graphite flakes, a higher temperature gradient made the graphite structure thinner and distribute d along heat flow direction in interdendrite region. These densely distributed thinner graphite flakes lowered the tensile strength. Decrease in C content of melt or a potent inoculation with longer fading time is required to improve the properties.