Effects of Tuning the Microstructure on the Mechanical Properties and Machinability of Free-Cutting Steels

Abstract

The effect of microstructure on the mechanical properties of steels has been extensively investigated over the past 20 years. This work reports on the effects of microstructural features, such as grains, interfaces, precipitates, and the number and morphology of inclusions, on the mechanical properties and machinability of free-cutting steels. The results show that the element S is aggregated toward the element Mn to produce MnS inclusions, and the MnS inclusions obtained are distributed uniformly in the matrix. The MnS inclusions in the steel impeded the deformation of crystalline grains during the compression tests, increasing the fracture strength. In addition, the cracking-induced effect of the inclusions improves the machinability of the free-cutting steel. The minor additions of elements (e.g., Te) change the distribution of inclusions in the steel. The enhancement of mechanical strength and machinability of free-cutting steel is found to be related to the structural defects, where the grain boundaries and spherical and/or spindle-like inclusions are uniformly distributed in the matrix.