Thermo-Cycling Fatigue of SiMo Ductile Iron using a Modified Thermo-Mechanical Test

Cast exhaust manifolds are made from silicon and molybdenum (SiMo) alloyed ductile iron. These complex-shaped, thin-walled cast components are subjected to heating and cooling cycles during service under mechanically constrained conditions. This type of load condition can induce multiaxial stress that can promote thermo-mechanical fatigue (TMF). In addition, high temperature oxidation in combination with thermo-mechanical load can accelerate degradation of the thin wall cast component during extended service. In this article, a modified mechanically constrained thermal cycling test, which generates transient multi-axial stress, is presented. Finite Element Modeling (FEM) was performed to design a specimen geometry and simulate the stresses and deformation during sequential thermal cycles. The test system uses a heating unit that passes direct current through the sample (Joule heating) and monitors the loads generated by thermal expansion of the specimen. The measured loading profile during thermal cycling was similar to the profiles obtained from FEM simulations. Several tests were performed using SiMo ductile iron with multiple thermal cycle profiles to verify the effect of temperature and oxidation time on TMF fatigue. In addition, thermal diffusivity, differential scanning calorimetry, dilatometry, and oxidation tests were performed to analyze high temperature properties of SiMo cast iron in quasi-static condition. High resolution microscopy was used to analyze the thermo-cycled specimens and it was found that the oxidation and decarburization can significantly affect the resistance of SiMo ductile iron crack formation during thermal cycling. The suggested test method permits the use of a small test specimen directly extracted from a thin-walled casting with the original cast structure and can be used for verification of critical service temperature, alloy design, and ductile iron process optimization