Nucleation Mechanisms in Ductile Iron

The present paper reviews different mechanisms for graphite nucleation in ductile iron (DI) and how these are affected by the inoculation process. Theories describing the fundamentals of graphite formation are given and the strengths and weaknesses of each theory discussed. Effects of key elements in the nucleation process, such as silicon (Si), calcium (Ca), strontium (Sr), barium (Ba), aluminum (Al), magnesium (Mg), cerium (Ce), sulfur (S), oxygen (O) and nitrogen (N) are described and discussed, and the importance of non-metallic heterogeneous compounds in the iron such as sulphides, oxides, nitrides and silicates are considered. Studies of nucleation and growth of graphite are shown, and the complex interaction between the Mg treatment and the inoculation process is described. The importance of the crystal structure and the stability of the nuclei to become a potent site for graphite formation are reviewed, and examples of potent and non-potent nucleation sites are shown. The paper arrives with a more comprehensive understanding of graphite nucleation in DI, and explains the key difference between Mg and the other three elements Ca, Sr and Ba in this respect.