Abstract
The effects of the strain rate and contents of phosphorus and magnesium on the intergranular embrittlement (IE) at about 673K in ferritic spheroidal graphite cast iron were investigated by tensile and bending tests. Bending test, which is very useful for evaluating the occurrence of IE, confirmed that IE occurred when the Mg/P ratio exceeded more than about 1.5, even if the P content was as high as 0.03 %. From the electron probe microanalysis on the intergranular micro-cracks observed in the specimen loaded just prior to bending fracture, it was found that IE was caused by the embrittlement of ferrite grain-boundaries consistent with the austenite grain-boundaries formed in the interdendritic region of the eutectic-cell, and that in such grain-boundaries, the magnesium segregated. The spheroidal graphite cast iron with high manganese content, which was also embrittled at 673K when Mg/P was more than 1.5, fractured in intergranular mode even at room temperature. Auger analysis on its intergranular surface fractured at room-temperature in AES chamber revealed that there exists numerous fine metallic magnesium particles, indicating that the intergranular embrittlemen must have been caused by the segregation of the metallic magnesium to the austenite grain-boundary during solidification.
