Low Cycle Fatigue Behavior of Heat‐Resistant Austenitic Cast Steels at 950 °C

Exhaust manifolds are frequently subjected to severe thermal cycling due to startup and shutdown of automobile engines; consequently, high‐temperature fatigue is the predominant failure mode for these components. The research about the relationship between high‐temperature fatigue properties and as‐cast microstructure in austenitic cast steels, which are the most popular materials for exhaust manifolds, is limited. In this study, strain‐controlled low cycle fatigue tests are conducted at 950 °C on two lab‐cast and one commercial‐cast austenitic cast steels with different morphology of primary Nb(C,N). The results indicate that decreasing the lamellar spacing of primary Nb(C,N) is an effective way to increase the LCF lifetime at the high strain range, hence the “Chinese‐script” primary Nb(C,N) is a more desirable morphology. The combination of cracking and prior oxidation of the dispersed plate‐like and blocky primary Nb(C,N) can result in the reduction of LCF lifetime. The influence of the primary Nb(C,N) morphology on the LCF lifetime is negligible at a low strain range, hence lowering the secondary dendrite arm spacing (SDAS) to less than 30 μm significantly extends the LCF lifetime of austenitic cast steels. Additionally, the plastic strain energy density is an effective parameter to predict the LCF lifetime at 950 °C in austenitic cast steels. This paper investigates the low cycle fatigue behavior of austenitic cast steels at 950 °C. The results reveal that the morphology of primary Nb(C,N) has a great influence on LCF properties at the high strain amplitude, while lowering SDAS to less than 30 µm can extend LCF lifetime at the low strain amplitude. Plastic strain energy density is effective for LCF lifetime prediction at this condition.