Effects of Si Content on the Microstructure and Properties of Spherical Graphite Cast Irons Manufactured by Continuous Cast Process

摘要

The effects of Si content on the microstructures, mechanical properties, oxidation resistance and thermal expansion of ductile cast irons have been studied using optical microscopy, scanning electron microscopy, hardness test, oxidation test, dilatometry, and tensile test. The spherical graphite cast irons are manufactured by horizontal continuous cast process. The volume fraction of pearlite structure in as-cast alloys continuously decreased down to approx 3 percent Si and reached nearly zero at the above Si content. The hardness and tensile strength of ductile cast iron showed a minimum at approx 3 percent Si. The result was due to an overlapping effect of matrix strengthening of pearlite structure and solid solution hardening by Si element. The tensile ductility increased continuously as Si content increases from 2.4 percent to 3.85 percent. the oxidation rate has a constant value regardless of Si content in the temperature range of 600-700 deg C. On the other hand, the oxidation rate decreased with the increase of Si content in the temperature range of 800-1000 deg C. Specially, in this temperature range, the oxidation resistance more enhanced at Si content above approx 3.4 percent. This result directly related to the diminution of Fe_3O_4 type oxide layer. The coefficient of thermal expansion has more larger values at annealed state than as-cast state.