Effects of inclusion morphology on growth of built-up edge in low carbon free cutting steels

In order to experimentally assess effects of MnS morphology on growth of built-up edge (BUE) in low carbon free cutting steel, quick stop tests were conducted for a leaded steel and two non-leaded steels which contained different type of MnS inclusions. Deformation structure in chip shear zone of tested specimens was microscopically observed. In the leaded steel, micro-cracks grew from the deformed globular shaped MnS in the vicinity of BUE, which seemed to contribute to form a small BUE. It was clarified from TEM observation that this small BUE was composed of elongated fine ferrite subgrains microstructure, in which a lot of fine cementite particles dispersed. In the non-leaded steels, the BUE originating from the non-leaded steel containing the globular shaped MnS was small similar to the leaded steel, but one from the steel containing the slender shaped MnS developed bigger. The difference in size of BUE was affected the surface roughness in the non-leaded steels. It is considered that difference in deformation characteristics of different type of MnS affects the size of BUE through the generation of micro-cracks in the shear zone in the vicinity of BUE.