The mechanism of intermediate temperature embrittlement of cast irons by magnesium

Metallic alloys such as steels and nickel-based alloys are potentially vulnerable to high-temperature brittleness in the range of 500 °C to 1000 °C that results in a loss of ductility. Ferritic spheroidal graphite cast irons show similar loss of ductility at temperatures between 350 °C and 500 °C, suggesting the term intermediate temperature brittleness. Previous works have shown that sulfur and magnesium are detrimental while phosphorus added at a reasonable level is beneficial in preventing loss of ductility in ferritic spheroidal graphite castings. In order to clarify the role of magnesium, a ferritic spheroidal graphite cast iron with high silicon content and low sulfur and phosphorus contents was subjected to tensile tests between room temperature and 650 °C. It was found to be susceptible to embrittlement at intermediate temperatures around 400 °C. MgO precipitates were observed on the fracture surface, which increased in size with test temperature, confirming the role of the environment that was reported by previous researchers. This finding is cross-checked with the results of the literature and allows us to propose a scheme for intermediate temperature embrittlement of ferritic spheroidal graphite iron.