The effects of Ni on the microstructure and fracture behaviour of ferritic-pearlitic ductile iron under fatigue

This study investigated the effects of nickel (Ni) on the microstructure and fracture behaviour of ferritic-pearlitic ductile iron (DI) under fatigue. It was found that the presence of Ni had effects on both the matrix and graphite nodules. In the matrix, it increased the volume fraction of pearlite. On the graphite nodules, it decreased their number and increased their size by lowering the eutectic temperature, which enabled extended growth time. When a ferritisation heat treatment was carried out, the presence of Ni produced a finer ferrite grain size, attributed to its impact on nucleation temperature. When a normalisation treatment was carried out, the presence of Ni reduced the pearlite interlamellar spacing, leading to increased hardness, yield and tensile strength, and decreased ductility. Fatigue testing indicated that failure occurred at lower cycle counts in the presence of Ni, due to promoting brittleness and defect agglomeration in pearlitic regions. Fractography was carried out on the fracture surfaces of the failed specimens and highlighted the presence of macro shrinkage porosity and carbide clusters as dominant cause for crack initiation. By segregating within the pearlitic regions, Ni was found to have a strengthening effect but also to increase the susceptibility to defect formation, thus reducing the fatigue performance.