Effect of residual magnesium content on thermal fatigue cracking behavior of high-silicon spheroidal graphite cast iron

This study investigates the thermal fatigue cracking behavior of high-silicon spheroidal graphite (SG) cast iron. Irons with different residual magnesium contents ranging from 0.038 to 0.066 wt pet are obtained by controlling the amount of spheroidizer. The repeated heating/cooling test is performed under cyclic heating in various temperatures ranging from 650 degrees C to 800 degrees C. Experimental results indicate that the thermal fatigue cracking resistance of high-silicon SG cast iron decreases with increasing residual magnesium content. The shortest period for crack initiation and the largest crack propagation rate of the specimens containing 0.054 and 0.060 wt pet residual magnesium contents are associated with heating temperatures of 700 degrees C and 750 degrees C. Heating temperatures outside this range can enhance the resistance to thermal fatigue crack initiation and propagation. When thermal fatigue cracking occurs, the cracks always initiate at the surface of the specimen. The major path of crack propagation is generally along the eutectic cell-wall region among the ferrite grain boundaries, which is the location of MgO inclusions agglomerating together. On the other hand, dynamic recrystallization of ferrite grains occurs when the thermal cycle exceeds a certain number after testing at 800 degrees C. Besides, dynamic recrystallization of the ferrite matrix suppresses the initiation and propagation of thermal fatigue cracking.