Electro-discharge machining (EDM) versus hard turning and grinding—Comparison of residual stresses and surface integrity generated in AISI O1 tool steel

Electro-discharge machining (EDM) appears as an alternative to grinding and hard turning for the machining of tool steels because EDM allows the machining of any type of conducting material, regardless of its hardness. Nevertheless, other factors must be taken into account in the selection of machining processes, especially in the case of responsibility parts. These factors are related with surface integrity: residual stresses, hardness and structural changes generated by the machining processes. In this work, the surface integrity generated in AISI O1 tool steel by wire electro-discharge machining, hard turning and production grinding is studied and compared. Production grinding generates compressive stresses at the surface, and a slight tensile peak, accompanied by a decrease in hardness beneath it. No structural changes are noticeable. Hard turning generates slight tensile stresses in the surface, accompanied by an increase in hardness and in the amount of retained austenite. Just below the surface compressive stresses are obtained, as well as a decrease in hardness and in the volume fraction of retained austenite. Wire electro-discharge machining (WEDM) generates tensile stresses at the surface and well below it, accompanied by the formation of a superficial “white layer” where there is a noticeable increase of the volume fraction of retained austenite and of the hardness. Consequently, among the three machining processes studied, WEDM is the most detrimental to surface integrity and, consequently, to the service life of the machined parts, because it promotes crack formation and propagation.