Influence of Graphite Morphology and Matrix Structure on Chip Formation During Machining of Gray Irons

Abstract

Gray cast irons are frequently characterized in the literature as easily machined materials. However, the machinability of gray irons is influenced by a complex interplay of the graphite and its surrounding matrix structure, which is not well understood. As a result, the causes of variability in the machinability of gray irons remain elusive. In this paper, the role of both graphite morphology and matrix structure in discontinuous chip forma­ tion during machining of gray irons is investigated. Compari­ sons with chip formation process in ductile irons and leaded­ steel are made. Gray irons with both type A and D graphite produced by the continuous casting process are investigated. Machining charac­ teristics are observedfor ferrite,ferritelpearlite and full pearl­ ite matrix structures. Both slow-speed machining studies and high-speed turning with a quick-stop-device (QSD) at typical cutting conditions are investigated. During machining, early fracture of cast iron microstructure occurs along the flakes, well ahead of the cutting tool. The fracture distances ahead of and beneath the cutting tool in­ crease as the graphite becomes coarser. A machining-affected zone (MAZ) is observed ahead of and beneath the cutting tool, and consists of three regions: 1) a decohesion zone; 2) afracture zone; and 3) a shattered zone . Measurements of the MAZ show that the deformation of gray irons during machining is influ­ enced not only by the matrix structure but also by the type and size of the graphite flakes. The machining characteristics of ductile irons are shown to be considerably different from those of gray irons. The MAZ in ductile irons is more plastic, with fewer fracture events. The chips form in a semi-continuous fashion. Furthermore, the deformation is more pronounced for ductile irons with high ferrite content. The graphite nodules in the plastic zone undergo severe deformation and become preferentially elongated in the shear plane direction, ahead of the cutting tool.