Study of macro- and micro-structure and morphological evaluation of graphite in a nodular cast iron obtained by casting in the form of an ingot (Atena Editora)

Abstract

Cast irons are Fe-C-Si alloys with carbon content above 2%, and nodular or ductile cast iron is characterized by having free carbon in the form of spheroidal graphite nodules. The objective of the current work is the systematic study of the macro- and micro-structure of a nodular cast iron, obtained by casting in the form of an ingot in a sand mold, correlating the structural and morphological aspects of the graphite nodules with solidification. The material studied was provided by the company Pam Saint-Gobain Canalização, located in Barra Mansa-RJ, Brazil. For the analysis, the central part of the ingot was selected and a 1 cm thick profile was removed through a total cross-section. The entire profile was metagraphically prepared. Structural analysis was performed by optical microscopy, applying qualitative and quantitative evaluation techniques. As a result, it was determined that the size and quantity of nodules per area vary within the same ingot, according to the solidification profile. Regions closer to the surface of the ingot, where the cooling speed is higher, the average size of the graphite nodules was smaller and with a higher concentration per area, than in the central regions, where cooling is slower and the size of the nodules was larger and smaller number of nodules per area. Also, in the central region of the ingot, macrodefects in the form of pores and voids were observed. It is concluded that the variation in the speed at which cooling occurs, resulting from heat extraction rates and thermal gradients at each moment of solidification, is a determining factor for the alteration of the microstructure of the nodular cast iron ingot.