Machinability of gray iron—Mechanics of chip formation

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 graphite morphology and matrix structure that is not well understood. As a result, the causes of variability in the machinability of gray irons remain illusive. The role of both graphite morphology and matrix structure in discontinuous chip formation during machining of gray irons is investigated. Comparisons 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. The results show that early fracture of cast iron microstructures occur along the graphite flakes well ahead of the cutting tool. The results also show a region labeled the “machining affected zone” (MAZ) region ahead and beneath the cutting tool. The area consists of, a decohesion zone, a fracture zone and a shattered zone. Examination of the MAZ showed that the machining of gray irons is influenced by the matrix structure and the graphite type and size. The machining characteristics of ductile irons are shown to be considerably different from those of gray irons.

Keywords:

• cast irons
• gray iron
• ductile iron
• machinability
• machining
• microstructure