Effects of Casting Condition on Solidication Structure and Mechanical Properties of P-B Cast Iron

Abstract

  P-B cast irons are widely applied to the cylinder liner for marine diesel engines, since they are composed of flaky graphite, hard phases (γ-Fe₃C-Fe₃P eutectic) and pearlite matrix which offer higher wear resistance. Recently, the strengthening of the cylinder liner is required to raise engine output. However, large graphite flakes develop in thick castings and reduce the strength. Therefore, the control of graphite structure is essential to attain higher tensile strength. Application of chill blocks to the sand mold, for raising the cooling rate of casting, should refine the solidification structure and provide higher strength. However, experiments on the effects of 20 to 90 mm thick steel chill blocks on the 180mm thick rectangular casting showed that the chill blocks lowered tensile strength on the contrary. Thermal and metallographic analysis of castings revealed that the chill blocks raised both the cooling rate and 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 distributed along heat flow direction in the interdendrite region. These densely distributed thinner graphite flakes lowered the tensile strength.