Chiang Mai Journal of Science

     Cast irons continue to be first choice materials for cast components in a wide range of engineering applications. They solidify by non-faceted/faceted eutectic reactions where, via a grey iron reaction, the non-faceted metal phase is normally austenite and the non-metal faceted phase is graphite, or via a white iron reaction to form austenite and faceted iron carbide fe3C. The eutectic reaction which occurs depends on several factors such as charge materials, melting and pouring conditions, composition, liquid metal treatments, casting design, mould materials, and cooling conditions. In some cases, the two reactions can occur within the same cast section. For most applications, production conditions are controlled to ensure that only the graphitic eutectic reaction occurs to provide a range of flake, compacted or spheroidal graphite irons. These are often referred to as the engineering cast irons. Eutectic carbide reactions are only required when producing malleable irons or when resistance to wear is needed as in the special cast irons such as Ni-Cr and high Cr white Irons. In all cast irons the final structures can be controlled by alloying and heat treatment to provide ferritic, pearlitic, austenitic, acicular, or mixed matrix structures.      The faceted nature of graphite growth controls the as-cast microstructure such that in flake graphite irons the graphite forms a continuously branched network within eutectic cells, thus limiting mechanical properties. In flake irons properties can be moderately improved by inoculation treatment to reduce eutectic cell size. The faceted nature of graphite allows its growth behaviour during solidification to be modified by the addition of small amounts of elements such as Mg and Ce which can change the graphite morphology from flake to spheroidal significantly improving mechanical properties. However, the sensitivity of graphite morphology to quite small amounts of certain elements also means that the structure and properties of graphitic irons can be seriously deteriorated by the presence of impurities such as S, Pb, Sb, Ti, etc., and by the dissolved gases H, O, and N. This paper reviews how optical microscopy and scanning electron microscopy has proved to be invaluable in developing understanding and control of the microstructures and properties of engineering cast irons.