Effect of Phosphorus on Intermediate Temperature Embrittlement of High Silicon Spheroidal Graphite Cast Irons

Abstract Ferritic spheroidal graphite cast irons can be susceptible to intermediate temperature embrittlement between 350 °C and 500 °C. It has been reported that the corresponding loss of ductility increases with the sulfur and magnesium content of the alloy but can be eliminated if the phosphorus content is adapted to ensure a sufficiently low ratio of magnesium to phosphorus contents. Using a low-sulfur, low-phosphorus and high-silicon alloy, previous work clarified the role of magnesium and suggested that small MgO precipitates, developing at grain boundaries due to diffusion of oxygen from the atmosphere during tensile tests, contributed to the generation of brittleness. The present work aimed to clarify how the addition of phosphorus could effect this process and make spheroidal graphite cast irons immune to brittleness at intermediate temperatures. The findings suggest that Mg–P precipitates appeared at high temperature during the cooling process after casting, and that they catch excess magnesium at grain boundaries. During mechanical testing, these Mg–P precipitates tighten the oxygen that penetrates along the grain boundaries to form large Mg–P–O precipitates that are not detrimental to ductility, while preventing the massive formation of small Mg–O precipitates.