| Many studies have been done to explain the effect of grain boundaries in austenite shell on the growth of spheroidal graphite . But, its effect on the melting procedure of spheroidal graphite cast iron is unknown. Therefore, in this work, melting procedure of spheroidal graphite cast iron and the role of grain boundaries in austenite shell on melting of spheroidal graphite have been studied, by changing the condition of melting and the content of phosphor. The main results are summarized as follows. 1. Steadite melts under the eutectic temperature. Therefore, the melting phenomena of spheroidal graphite cast iron is changed by the existence of steadite. 2. In spheroidal graphite cast iron containing little phospher, melt appears first at the outer region of austenite shell in which Si is condensed, regardless of heating rate. In this case, grain boundaries in austenite shell give little effect on the melting procedure of spheroidal graphite. 3. In spheroidal graphite cast iron containing phosphor above 0.3%, melt appears first along the interface between spheroidal graphite and austenite shell, at low heating rate. It seems to be caused by moving of liquid phase to spheroidal graphite along grain boundaries in austenite shell. But, at high heating rate, melting proceeds with remaining austenite shell around spheroidal graphite. |
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