The Effect of Metal-Carbide Morphology on the Thermomechanical Fatigue (TMF) Behavior of Cast Austenitic Alloys for Exhaust Manifolds

Abstract Out-of-phase thermomechanical fatigue (OP-TMF) tests between 600 °C and 950 °C have been conducted for three cast austenitic alloys with different metal-carbide (MC) morphologies: dense skeleton, sparse skeleton and blocky carbides. The alloy with dense skeleton-like MC exhibited longer TMF life than the other two, even though their chemical composition and casting process were similar. Fractography analysis indicated that the fatigue cracks initiated from the specimen surface for all the alloys in this study. The morphology of Nb(C,N) has an obvious effect on inelastic deformation. Alloys with skeleton-like Nb(C,N) precipitates have better ductility as compared to alloys with isolated blocky precipitates. Dense skeleton-like Nb(C,N) is found to delay OP-TMF crack initiation and propagation, resulting in longer TMF lives.