Abstract
Pig iron was melted in a CaO crucible in a vacuum atmosphere to achieve low S and O contents. The melt was spheroidized with a Fe-Si-Mg alloy to obtain spheroidal graphite cast iron with lower Mg contents, 0.006∼0.029 mass %, than usual. Tensile tests were carried out on these materials at room temperature and 673K to study 673K intergranular fracture embrittlement. Some of these materials showed 673K embrittlement in spite of a Mg/P compositional ratio lower than 1.5. Also despite lower intergranular area fraction less than 10 %, which was lower than previously reported, tensile elongation was very low. Concentration of both Mg and P at the same place, suggesting the formation of Mg3P2, and concentration of only Mg were observed on the intergranular fracture surface. The remaining unreacted Mg after the formation of Mg3P2 during solidification concentrated at the ferrite grain boundaries after ferritizing heat treatment. It is suggested that degradation of the ferrite grain boundaries strength due to the segregation of Mg results in 673K embrittlement.
