UHVEM observation for the dual structure in spheroidal graphite of cast irons

Abstract

The phenomenon that spheroidal graphite cast iron was formed by adding magnesium (Mg) was discovered in 1949 [1][2]. Since then, a number of researches have been made on this subject. Due partly to increase in research on the graphite‐spheroidizing, there are various theories such as interfacial energy theory, nuclear theory, bubble theory,,, etc. Among them interfacial energy theory is becoming dominant as of now [3]. However the role of Mg as the graphite‐spheroidizing agent has not been yet identified. And Mg particle is often observed inside of spheroidal graphite experimentally. In order to get the information on the internal structure of spheroidal graphite in cast iron, Ultra‐High Voltage Electron Microscopy (UHVEM) observation was carried out. The composition of the cast iron in this experiment was FCD450 (Fe:Bal., C:3.645, Si:2.533, Mg:0.0464, Mn:0.368, P:0.018, S:0.0114, Cu:0.176 (wt%), Yodoshi Co.). The position of Mg in spheroidal graphite was predicted from a surface observation result by EPMA. Focused ion beam (FIB) cut out into approximately rectangular parallelepiped shape carbon sample from a spheroidal graphite of FCD450. This sample was observed by UHVEM in Osaka Univ. at 2.0MeV. Figure 1 shows a bright field image (BFI) of the internal structure of spheroidal graphite. The electron beam in acceleration voltage of 2MeV was transmitted approximately 8μm thickness of graphite (inclination of the incident beam: 30°). Figure 1 indicated that whole area was graphite including the internal structure of spheroidal graphite roughly composed of a core region surrounded by annual rings of a layered intermediate region (central domain) and a layered outer region with some radial factors (covering domain). Only the covering domain has the radial and superimposed contrast. This radial factor originates from the metal inclusion that grew radially from the central domain. The shape of the external region of the central domain (without superimposed contrast) could be regarded as the almost truth sphere, which might become core for growing graphite crystal spherically. Primary crystal of graphite might be spheroidized by the solidification in liquid state, and crystal growth of graphite reaches the eutectic point at the external region of the central domain. After that, the covering domain formed by the cooperation growth between graphite and austenite. The spheroidal graphite in cast iron has the dual structure consisting of a spherical central domain and a covering domain. This obtained data using by UHVEM has provided new knowledge of the internal structure of spheroidal graphite in cast iron. And this study might provide important visual data to identify a role of the Mg in the graphite‐spheroidizing of cast iron.