On the solidification of compacted and spheroidal graphite irons

Abstract: A good understanding of the solidification process of a cast material is essential to understand how the combination of alloy composition and the casting process variables combines into the solid cast component and its performance. The wrong combination may result in poor performance or casting defects. Spheroidal graphite has been well researched in ductile irons (SGI) where it is predominant. Spheroidal graphite is also present in smaller amounts in compacted graphite irons (CGI), but its nature and role in this material is less understood. Recent associations of spheroidal graphite in CGI with shrinkage porosity problems highlights the need for better understanding in this area. The importance of the dendritic austenite structure to the properties and solidification behaviour of cast irons has been highlighted in recent research. However, progress is to a degree limited by lack of practical means to characterize the structure.In the present work, the transition of a cast iron from SGI to CGI though remelting was studied. As the fraction of SG dropped, the tips of the compacted graphite tended to lose contact with the melt at a later stage. After this occurred, solidification appeared to be dominated by spheroidal graphite. Compacted and spheroidal graphite was found to solidify mostly segregated, and the increased recallescence induced by a higher fraction of compacted graphite displayed small influence on the size distribution of spheroidal graphite apart from the total number and fraction. The partitioning of Si, Mn and Cu in SGI and CGI was found to agree well with each other, as well as with theoretical predictions under the assumptions of zero diffusion of the elements in the solid. This shows that the proportions of spheroidal and compacted graphite has small or no influence on the evolution of these elements in the melt during solidification. A method for characterization of the dendritic austenite in quenched cast irons was introduced and evaluated. The method includes a technique for producing a visual contrast between the ledeburite matrix and the dendritic austenite, and a scheme for producing binary images from the resulting micrographs which are suitable for automatic image analysis. Measurements of the volume fraction and surface area per unit volume of the dendritic austenite structure using the introduced method was found to agree reasonably with traditional point counting and line intercept techniques. The difficulty in finding the exact boundary was proposed to be the major source of systematic disagreement.