Effect of Micro-structural Dispersity of SiMo Ductile Iron on Thermal Cycling Performance

High alloyed by silicon and molybdenum (SiMo) ductile iron is a common material used for car exhaust systems, and its micro-structural dispersity depends on intrinsic parameters, which include alloy composition and inoculation efficiency, as well as extrinsic factors, such as casting wall thickness and molding material, which define the cooling rate during solidification. Micro-structural dispersity refers to sizes of structural constituencies and space distribution within the micro-structure. A variation in the micro-structural dispersity can significantly affect high-temperature performance of SiMo ductile iron during static oxidation and transient thermo-mechanical loading conditions. In the first published part of this study, high-temperature static oxidation tests were performed on SiMo ductile iron solidified in a casting with varying wall thicknesses from 5 to 100 mm. In addition, the faster solidified specimens with extremely high micro-structural dispersity were taken from near the chilled casting surface. It was shown that above the critical temperature diapason, increasing micro-structural dispersity intensified the surface degradation due to intensive decarburization (de C ). In this second part of the study, the specimens with different micro-structural dispersity were subjected to constrained thermal cycling by applying different cycle schedules to quantify interactions between thermal fatigue and oxidation. It was shown that the performance of SiMo ductile iron could be improved by optimizing the micro-structural dispersity for different transient thermo-mechanical conditions.