Cast Iron Graphite Shape, Fine Structure Are Keys To Cheaper, Faster Machining

Dr. Robert C. Voigt, professor of industrial engineering, says, "If you can increase the life of a cutting tool or reduce product breakage during manufacturing by using more consistently machinable cast iron, costs will go down. We think we have the first strong evidence that connects microstructure and graphite particle shape with machinability."

Penn State researchers detailed their study in a paper, "Influence of Graphite Morphology and Matrix Structure on Chip Formation During Machining of Gray Cast Irons," presented at the American Foundryman's Society meeting in St. Louis, Mo. in March.

The authors are Dr. Ruben Marwanga, research assistant and Fulbright Fellow; Voigt, and Dr. Paul H. Cohen, professor of industrial and manufacturing engineering.

The group looked at commercial grades of gray and ductile cast iron, using slow speed machining as well as a high-speed quick-stop-device. They used a high magnification video camera system to record the process and then examined the machined samples with optical and scanning electron microscopes.

Magnified 50 times in the research videotapes, the surface of the cutting tools resembles a plow turning soil as it machines the surface of the cast iron sample. The tapes clearly demonstrate that cracking occurs along the graphite flakes and that severe deformation occurs in the surrounding matrix structure. The longer the graphite flakes, the longer the fracture distance ahead of and below the cutting tool.

The research also revealed that fine free graphite exists at the tool/chip interface of all gray irons and performs an important lubrication role. The graphite at the interface forms a thin solid film that separates the tool from the work and reduces strain and friction. The result is lower tool/sample interface temperatures and enhanced machinability.

The researchers also found that the type of matrix or fine microstructure in which the graphite was embedded played a role in machinability. They examined typical cast iron microstructures, including ferrite, ferrite/pearlite, and fully pearlite, and found that the higher the amount of ferrite, the easier it is to machine.

Gray irons have different machining characteristics than ductile irons and leaded steels. Ductile irons are more plastic and form longer chips than gray irons. Leaded steels show a much higher ductility than ductile irons, longer deformation distances and the formation of continuous chips during machining.

Voigt notes that the researchers are currently forming a consortium of companies to further examine the role of graphite and structure on cast iron machinability. Their goal is a consistently machinable cast iron that will provide better performance when machined rapidly by automated machining cells. The research was supported, in part, by grants from Dura Bar Division of Wells Manufacturing Co., Woodstock, Ill.