Abstract
In this paper it is attempted to estimate quantitatively how the microscopic embrittlement action of free-machining additives, such as void formation and void coalescence around MnS particles, brings about the macroscopic free-machining effects. This consists of determination of stress-strain curve of resulfurized steel based on the behaviour around particles and evaluation of free-machining effects by FEM cutting simulations of both of resulfurized and plain carbon steels according to the stress-strain curves obtained above. In ordes to obtain the stressstrain curve, loaded-unloaded stress fields around the single particle or the coalesced two particles, which were calculated separately with FEM, are inserted into the matrix sheared and stressed up to a given strain with an assumption that the calculated stress fields around the particles closely resemble to those in real resulfurized steel. The bulk shear stress was then obtained as the mean value of the inserted stress fields over the specimen. The stress-strain curve obtained is in good agreement with experimental result. In spite of the same friction characteristic on the rake face used in the FEM cutting simulation for both plain carbon and resulfurized steels, a higher shear angle, a lower cutting force, and a smaller tool-chip contact length were obtained for the resulfurized steel. It is thus confirmed that the embrittlement action of free-machining additives is one of significant factors in the improvement of machinability.
