Creep behaviour, creep damage and precipitation in the austenitic ductile cast iron D5S at 750 °C

The austenitic ductile cast iron D5S, used for exhaust manifolds in heavy-duty truck engines, was creep tested at 750 °C in air under constant load with initial stresses of 18.5–50 MPa. Four of the specimens (20–50 MPa) were tested to rupture (22–5476 h) and the remaining one (18.5 MPa) was interrupted in the secondary creep regime after 6494 h. Secondary creep was generally predominant at lower stresses whereas primary and tertiary creep were predominant at higher stresses. The damage was characterized by various types of voids/creep cavities around the graphite nodules which were interlinked by shear cracks upon fracture. The results show that significant amounts of finely dispersed Cr7C3 carbides, larger Fe2Ni2Si particles, and an unknown carbide precipitated during creep exposure. Dislocation substructures which had developed during creep were revealed through EBSD.