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-silic on spheroidal graphite (SG) cast iron. Irons with different residual magnes ium 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 crac king resistance of high-silicon SG cast iron decreases with increasing resi dual magnesium content. The shortest period for crack initiation and the la rgest crack propagation rate of the specimens containing 0.054 and 0.060 wt pet residual magnesium contents are associated with heating temperatures o f 700 degrees C and 750 degrees C. Heating temperatures outside this range can enhance the resistance to thermal fatigue crack initiation and propagat ion. When thermal fatigue cracking occurs, the cracks always initiate at th e surface of the specimen. The major path of crack propagation is generally along the eutectic cell-wall region among the ferrite grain boundaries, wh ich is the location of MgO inclusions agglomerating together. On the other hand, dynamic recrystallization of ferrite grains occurs when the thermal c ycle exceeds a certain number after testing at 800 degrees C. Besides, dyna mic recrystallization of the ferrite matrix suppresses the initiation and p ropagation of thermal fatigue cracking.