A Discussion on the Effect of Segregation on Intermediate Temperature Embrittlement of Ferritic Spheroidal-graphite Cast Iron

Abstract

Two ferritic spheroidal-graphite cast irons with different silicon contents were used to investigate the effect of impurity segregation on the intermediate temperature intergranular fracture which occurs in tension at about 300°C to 500°C. The specimens were quenched from different temperatures between 820°C and 400°C during furnace cooling in ferritisation heat-treatment. After heat-treatment, the specimens were deformed in tension at various temperatures between 20°C and 520°C with constant crosshead speed of 0.01 mm/sec. The results show that intermediate temperature intergranular fracture can occur only with a higher silicon content. Apart from intergranular fracture, ferritic spheroidal-graphite cast iron may fracture in the cleavage mode when deformed in tension in the intermediate temperature range. The cleavage fracture propagates radially from a central region. Located in the central region are microvoids with magnesium-enriched particles. These particles are also found on some intergranular facets. The fracture mode of intermediate temperature embrittlement is a function of heat-treatment prior to tension. Fracture of the specimens quenched from a higher temperature between 820°C and 400°C was of cleavage type. It changed to intergranular fracture as the quenching temperature was lowered. According to this transition, and based on current understanding of impurity segregation in ferro-alloys, the authors suggest that grain boundary segregation of phosphorus influences the fracture mode of intermediate temperature embrittlement.