Affects of graphite morphology and matrix structure on mechanical properties of cast ions

A comparative study of the affects of graphite morphology and matrix structure on mechanical properties was carried out on spheroidal, compacted and flake graphite irons by a short austenitizing plus austempering treatment. Transformation kinetic data showed that compacted graphite iron had the fastest, spheroidal graphite iron the second and flake graphite iron the slowest austenitizing rate. In spheroidal and compacted graphite irons the strength increased, while ductility decreased, with increasing the amount of bainite in matrix due to prolonging austenitizing time. On the other hand, the increase of bainite structure in the matrix had no significant affect on the mechanical properties of flake graphite iron. Fractographic examinations showed that the fracture surface of spheroidal graphite iron changed from a ductile mode to a brittle mode when its matrix changed from ferrite to bainite dominant. The flake graphite iron ruptured with brittle mode no matter what matrix it had. Compacted graphite iron exhibited an intermediate type of fracture surface. With a short austenitizing plus austempering treatment, the mechanical properties of spheroidal and compacted graphite irons could be improved and extended to a very wide range.