Mirror Surface Properties of Steel Containing Free-Cutting Element in Electric Discharge Machining : Compositions and Microstructure of the Steel Containing Free-Cutting Element

In this paper we describe the effect of die-sinking EDM under mirror finishing conditions on mirror surface properties of four-different types steel, SKD11, NAK55, PX5 and SKD61, which contain the free cutting element, sulfur. In particular, the influences of the quantity of sulfur on the machined steel surface, the composition and microstructure with die-sinking EDM. The following conclusions were obtained. 1) A mirror surface of steel containing more than 0.04% sulfur could not be realized. 2) In the case of steel with 0.1% sulfur, the surface property is dependent on the machining direction. 3) Pits are generated on the machined steel surface because the aggregated part of the MnS fiber preferentially melts with the circumferential base metal. In addition, the quantity and shape of pits change according to those of MnS. In the longitudinal section, the shape of the pit is a series of channels, whereas in the transverse section it is a cirele.