In Situ fracture observation and fracture toughness analysis of pearlitic graphite cast irons with different nodularity

Effects of microstructural modification and microfracture mechanisms on fracture toughness of pearlitic graphite cast irons with different nodularity were investigated by in situ observation of microfracture process. Six pearlitic graphite cast irons were fabricated by adding a small amount of Mg as a nodularizing element for graphite, and their microstructures including pearlite, ferrite, graphite, and eutectic carbide were analyzed. Most of ferrites were observed in a layer shape around graphites because of carbon-depleted zones formed near graphites. As the nodularity and nodule count increased, fracture toughness linearly increased in the cast irons except the iron containing many fine graphites. According to in situ observation of microfracture process, cracks initiated at nodular graphites and carbides even at a small load, and then propagated readily through the adjacent graphites or carbides, thereby resulting in the lowest fracture toughness. The cast iron having widely spaced graphites and ferrite layers thickly formed around graphites showed the highest fracture toughness because of the blocking of crack propagation by ductile ferrite layers and the crack blunting and deflection by graphites, which was also confirmed by the R-curve analysis.