29-01-2013, 04:38 PM
MECHANISM OF SPHEROIDAL GRAPHITE FORMATION
MECHANISM OF SPHEROIDAL.pptx (Size: 96.64 KB / Downloads: 23)
Introduction
Traditionally, cast iron inoculants are based on ferro silicon, graphite or calcium silicide, the former being the most common (Patterson, 1 978; Hughes, 1980). Since pure Si and ferrosilicon are found to b e ineffective as inoculants, their nucleation potency depends on the presence of minor elements such as Ca, Al, Zr, Ba, Sr, Ti, etc. in the alloys (Dawson, 1961; Dawson, 1966; Kanetkar, 1984; Lownie, 1963; McClure, 1957; Mickelson, 1967).
At present, the role of these minor elements are partly understood, but still complex matters related to formation of different types of nucleation sites in DI remains to be understood completely.
Theories for graphite nucleation mechanism
Several theories that exist in the literature explain the phenomena of heterogeneous nucleation of graphite in solidifying cast iron. In the following, some of the most established theories are described and discussed.
THE GAS BUBBLE THEORY
THE GRAPHITE THEORY
THE SILICON CARBIDE THEORY
THE SALT-LIKE CARBIDE THEORY
THE SULPHIDE/OXIDE THEORY
THE SILICATE THEORY
THE GAS BUBBLED THEORY
According to Karsay, graphite tends to crystallize onto any given surface or imperfections such as cracks, pinholes, inclusions, etc. (Karsay, 1976).
The gas bubble theory states that graphite can form only if its crystallization is protected by the presence of some sort of phase boundary. The needed phase boundaries are provided by the presence of carbon monoxide bubbles in the melt. The carbon monoxide bubbles are very finely dispersed in the melt and their size is less than 10 µm. Karsay presented the gas bubble theory as illustrated in Fig. 1,