Failure analysis of a natural gas engine exhaust manifold

The causes of exhaust manifold failure of a natural gas engine were investigated in terms of both the material and structure. First, the material analysis, including chemical component and mechanical properties, were carried out by material tests and data analysis. Then, the temperature and thermal stress of exhaust manifold under thermal shocks were calculated by finite element method. Finally, the initiation and propagation of cracks and improvement design of exhaust manifold were discussed. The results showed that the highest temperature of exhaust manifold was 715 C-degrees and the maximum thermal stress was 455 MPa. The yield deformation occurred during thermal shocks. And the sharp structural change at the critical induced thermal stress area raised the possibility of shrinkage pores. Therefore, the low cycle fatigue failure was the main reason of the exhaust manifold failure. And structure optimization and alternation of material properties were two most basic and effective technological approaches.