Examination of the Size and Morphology of Austenite Grains in Lamellar Graphite Cast Iron

Abstract

There is a current discussion concerning the austenite grains of lamellar graphite irons and its growth at different carbon equivalent contents. The term “grain” is often employed to refer not only to the primary austenite, but also to the colonies or cells formed during the eutectic solidification. This investigation applies advanced experimental methods to study the solidification of lamellar graphite iron. A better understanding of the solidification process should lead to the design of better casting procedures. Samples of lamellar graphite iron were cast and directly austempered after solidification in order to reveal their solidification macrostructure. A typical ingot structure was found, with a narrow columnar-to-equiaxed transition. Electron backscatter diffraction technique was applied to confirm these macroscopic observations. Color etching was employed to determine the size of eutectic colonies at the columnar and equiaxed regions. The EBSD results confirm previous proposals, revealing that the solidification grains result from the growth of austenite dendrites. These grains include several eutectic colonies or cells inside, all these colonies or cells having the same crystalline orientation as the primary austenite dendrite. An analysis of the critical gradient for columnar-to-equiaxed transition could be developed. The size of the eutectic colonies at the columnar zone was found to be smaller than the size of the eutectic colonies at the equiaxed zone. Besides, the size of equiaxed grains strongly decreased with the increase in cooling rate, while no evident differences were obtained when the size of the columnar grains was measured.