Abstract
The eutectic cell count in cast iron is regarded as a measure of nucleation at eutectic solidification, and so this is one of the important factors determining the soundness and mechanical properties of casting. There are various methods proposed to delineate eutectic cells. However it is not always easy to reveal them, depending on the contents of sulfur and phosphorus or structure of matrix etc.
The authors studied to etch eutectic cell structure in cast iron by means of potentiostatic technique. As a preliminary investigation, samples of Fe-Si and Fe-P alloys were prepared to determine the etching condition electrolytically. Each samples were polarized in 10 N-NaOH aq. solution and taken current density-potential curves by potential-sweep method and current density-time curves at constant potentials. From these results, it is known to be able to detect phosphorus and silicon segregation in ferritic cast iron by etching potentiostatically at +100mV and +600mV vs. Hg-HgO reference electrode respectively.
Then the authors applied this technique to cast iron samples of different graphite structures and nearly eutectic composition.
They observed;
(1) Silicon in grey cast iron is concentrated in the core of eutectic cells. On the other hand, phosphorus is enriched at the cell boundaries.
(2) It is shown that there is a tendency increasing cell count and reducing its size with increase of phosphorus contents.
(3) It is able to reveal the cell structre of inoculated cast iron substaintially free from sulfur and phosphorus by etehing silicon segregation at +600mV.
(4) Especially in inoculated sample, it is observed that some cells are interconnected and seen as only one cell at low magnification.
(5) There are two types of fine graphitic structures classified with respect to silicon segregation. One is very remarkably uneven and other is rather uniformly distributed.
(6) In spheroidal graphite cast iron, it is known that graphite nodules are randomly aligned. They are present not only in the dendritic but also in the interdendritic regions.
From these observations the authors have opinion that eutectic cells in flaky or spheroidal graphite iron can not be really estimated by counting them at low magnification (in the case of flaky graphite iron) nor of graphite nodules, and hence eutectic cell models being proposed are not generally valid.
