The anisotropic cutting behavior of free-machining steels

A determination of the anisotropy in the shear strength during machining of a group of free-machining steels including 1117, 11L17, and 1215 and a plain steel, 1018, is presented. The three free-machining steels are anisotropic in the as-received condition, but the nonfree-machining steel is not anisotropic. In terms of the extremes, 1018 steel has a shear strength of about 34,000 psi when cut in both the longitudinal and transverse directions, whereas 1215 has a shear strength of about 40,000 psi when cut in the longitudinal direction but only 24,000 psi when cut in the transverse direction. By fully annealing the free-machining steels, the anisotropy is reduced in 11L17 and 1215 and is eliminated in 1117. Examinations of the shear zones in stopped-chip specimens show that the freemachining additives form inclusions and that, in the shear zone during machining, long sharp cracks emanate from the tips of the deformed inclusions or there is a separation of material at the matrix-inclusion interface. No such voids or microcracks form in the plain steel containing only impurity levels of sulfur (1018). Since tensile ductility depends on inclusion content and the orientation of inclusions relative to the specimen axis, and the same is true for the shear strength during machining, then, as expected, there is a relationship between the two for all the steels at the same strength level of 50,000 psi; the effective shear strength during machining decreases with decreasing tensile ductility.