Study on the Initiation of Intergranular Fracture of Ferritic SG Cast Iron Caused by Magnesium Containing Inclusions Under Cyclic Heating to 750 deg C

摘要

Distinct ductility deterioration occurs after ferritic spheroidal graphite cast irons suffer from a certain number of cyclic heating up to 750C. This study investigated the effect of magnesium containing inclusions on the ductility deterioration behavior of heat resistant 4 wt percent high-silicon spheroidal graphite cast irons with increased residual magnesium content. A small microstructural difference was found in the vicinity of the eutectic cell boundaries, where the magnesium containing inclusion particles tended to agglomerate and the amount of inclusion particles with increasing residual magnesium content. the thermal cracks initiated mainly from the surface by according to the following four steps: (1) stress induced crack initiation, (2) intergranular crack linking, (3) major crack formation, and (4) major crack inward extension. Cracks were found to be the dominant factor in ductility deterioration pertains to the magnesium containing inclusion. Experimental results show that the oxide inclusions were composed mainly of magnesium, phosphorus and cerium. From tensile fractography of the specimens alter cyclic heating, it was found that tensile ductility deterioration was always accompanied by distinct intergranular fracture. The abovementioned cell boundary inclusions also contributed to this fracture.