Embrittlement of SiMo Ductile Cast Iron Exhaust Manifold and Evaluation using Sehitoglu Damage Model

Sehitoglu damage model is often applied to evaluate thermomechanical fatigue (TMF) performance of the components in the environment of high temperature in finite element analysis (FEA). SiMo ductile cast irons have been widely used for exhaust manifolds in propulsion systems. The manifold experiences TMF due to the limitation of thermal expansion in the assembled condition. Mechanical strain and damage are therefore introduced by the constraints. On the other hand, it is known that ductile cast iron exhibits embrittlement at the temperature around 400°C due to the addition of magnesium (Mg) in order to obtain graphite nodules. This mechanical behavior at 400°C, which has to be considered in design, makes the ductile cast irons only partially satisfy the assumptions of the Sehitoglu damage model.

In the present work, a two-step approach is presented to evaluate the sensitivity of the manifold geometry to the 400°C embrittlement using the Sehitoglu model. A strain-life (E-N) curve of the ductile iron at 400°C is needed. According to the study by Kobayashi et al, the degree of the 400°C embrittlement varies with Mg and phosphorus (Mg/P) weight percentage ratio, and the embrittlement temperature ranges from about 350°C to 450°C depending on strain rate. In practice, this creates some uncertainties in the correlation study. A chemical composition with a stable Mg/P ratio is preferred in order to obtain a stable mechanical property of the manifold at 400°C.