Influence of graphite morphology and matrix structure on chip formation during machining of continuously cast ductile irons

摘要

The machinability of ductile irons is influenced by a complex interaction between graphite and the matrix structure that is not well understood. As a result, the causes of variability in the machinability of ductile irons continue to be perplexing. The role of both graphite and matrix structure in chip formation during machining of ductile irons is investigated. Comparisons with chip formation process in gray irons and leaded-steel are made. Ductile irons with various matrix structures produced by the continuous casting process are investigated. Machining characteristics for ferritic, ferritic/pearlitic, and fully pearlitic matrix structures are observed. Two methods are used in the study: slow-speed machining studies and high-speed turning with a quick-stop-device (QSD) at typical cutting conditions. In the slow-speed studies, a high magnification, low speed video camera system attached to the machine is used to characterize chip formation. Frame-by-frame analysis is used to study the deformation and fracture events around the cutting tool. The QSD disengages the tool and brings the cutting process to an abrupt stop leaving the machining chip in place for analysis. A scanning electron microscope is then used to carefully examine the sub-surface damage ahead of and below the cutting tool. During machining, microfracturing is observed to occur close to the cutting tool. Severe plastic deformation is shown to occur with the formation of semi-continuous chips. The graphite nodules ahead of and beneath the cutting undergo severe deformation and are preferentially elongated in the shearing direction. The higher the ferrite content in the matrix structure, the more the plastic deformation and elongation of the graphite nodules. The fully ferritic matrix compares fairly with the machining characteristics of leaded-steel. But gray iron machining characteristics are fundamentally different from those of ductile irons. The gray irons exhibit a "machining affected zone" (MAZ) ahead of and beneath the cutting tool that consists of three regions: a decohesion zone, a fracture zone and a shattered zone. Very little plastic deformation is observed and fracture events are predominant.