Abstract
Primary and eutectic structures of hypoeutectic cast iron were studied using a static directional solidification apparatus, which is able to control both the freezing rate R and temperature gradient G. Also, dynamic directional solidification method which can not control the freezing conditions were used to find out the critical cooling rate necessary to obtain the graphite eutectic structure. Cast iron employed was Fe-3.1%C-2.4%Si and Fe-3.1%C-2.8%Si. Freezing range and temperature gradient were selected within the scope of transformation of graphite structure to ledebulite structure.
As the relation between the dendrite arm spacing λp of primary austenite and cooling rate RG was shown by λp=k1⋅(RG)−0.5, λp should be in proportion to G−0.5 at constant R. Silicon had little effect on both λp and the dendrite arm spacing λs of secondary austenite. But λp would probably be affected by the degree of discrepancy of chemical composition from the eutectic. λp becomes larger near the eutectic composition than the hypoeutectic. λs would presumably be affected by the freezing time of primary austenite dendrite. λs becomes larger with longer freezing time. Two types of secondary austenite dendrite were observed. One grows naturally and another grows by connecting with neighboring arms. The value of n of ledebulite eutectic in the equation of λs=k2⋅(RG)−n was larger than that of graphite eutectic. Critical cooling rate was 1.7 and 2.7°C/sec., respectively, at 2.4 and 2.8%Si under the conditions of the static method. These values were smaller than the values obtained in the dynamic method. This tendency was more remarkable at low silicon content.
