It has also been confirmed that spheroidal graphite is formed by addition of elements which significantly inhibits spheroidization of graphite in Mg treated spheroidal graphite cast iron. The inhibitory action is mainly caused by segregation as a solid or liquid in molten iron, and it seems that it will result in the formation of spheroidal graphite only when they exist as gas, bubbles.

  In this work, in order to clarify its possibility, behavior of gas forming elements accompanying precipitation of graphite into gas bubbles was investigated. Electron Probe Micro Analyzer used for the analysis is not good for a light element such as Mg, since its characteristic X-ray has low energy and long wavelength, the detection sensitivity is low because the spectrum width is widened.

  In addition, considering that element is trace amount from bubbles, spheroidal graphite produced by gas bubbles of heavy elements Bi and Pb was investigated.

  Consequently, the following result was obtained.

(1) Formation of spherical graphite by elements inhibiting spheroidization in Mg-treated spheroidal graphite cast iron is due to these gas bubbles.

(2) Spheroidal graphite is formed by addition of Bi and Pb

(3) From the observation of the internal structure of the spheroidal graphite by the scanning electron microscope, it was revealed that the spherical graphite made of Bi and Pb is also an annulus-like cross-sectional structure like Mg, Ce and Ca.

(4) From the results of EPMA observation, it became clear that Bi, Pb tended to be concentrated to spherodal graphite rather than matrix. It was also shown that the distribution of these elements in the spheroidal graphite does not concentrate at the central part but is distributed uniformly along the basal plane concentrically.

(5) The results of this work as described above clarify that it can not be explained by conventional graphite spheroidizing theory except for the bubble theory.

  Heavy section ferritic spheroidal graphite iron samples were produced according to an industrial procedure by magnesium (Mg) treatment. Test pieces were obtained from the samples and the distribution of Mg in the test pieces was analyzed by Wavelength Dispersive X-ray Spectrometry using Computer-aided Micro Analyzer. As a result, Mg-like hale formation was detected around almost all graphite nodules. The halo was concluded to be the trace of Mg gas bubbles which provided the site for the nucleation and growth of spheroidal graphite and contributed indirectly to graphite spheroidization. This Mg halo was found to exist between the primary and secondary graphites.

  Many scientific interpretations about the formation theory of spheroidal graphite have been attempted, but there is no unified one yet. To understand the crystal growth of spheroidal graphite correctly, there is a need to visualize the internal structure of spheroidal graphite three-dimensionally. Using ultra-high voltage electron microscopy, the transmission images can be taken from graphite samples with 15 micro meter thickness. Three-dimensional images could be constructed from tilt-series transmission images by using UHVEM. These visual images with diffraction contrast clarify real crystal growth nondestructively.

  Nucleation position, floating and morphological change of graphite nodules during solidification of hypereutectic ductile cast iron with Mg addition was observed in-situ using synchrotron radiation X-ray imaging. Graphite nucleated on pre-existing inclusions (i. e. MgS and Mg-containing oxysulfide) in the melt around austenite dendrites and most of the graphite nodules floated due to the buoyancy force. The floating graphite nodules were engulfed or entrapped austenite dendrites within several seconds. Floating distance of graphite nodules was shorter than dendrite arm spacing. The nucleation events near the dendrites and solute exchange between austenite dendrites and graphite nodules resulted in the short floating distance. The engulfment / entrapment of graphite nodules into austenite dendrites promoted the secondary nucleation of graphite in the melt and consequently influenced size and number of graphite nodules. In addition, two different phenomena also contributed to the morphological transition from spheroidal shape. One is the coalescence of graphite nodules and the other is the growth of graphite nodules in the austenite.