The electro-discharge machining surfaces of high-chromium white irons

Abstract

Electro-discharge machining (EDM) is widely used with hard and brittle materials, which are difficult to machine using conventional techniques. In the EDM process, spark discharges between the electrically conductive workpiece and a tool electrode increase the temperature so that localized melting occurs. Approximately 15% of this molten metal is removed by flushing, while the remainder resolidifies on the workpiece surface to produce a re-cast layer [1]. This re-cast layer has a rapidly quenched strucmre. A heat-affected layer may also be formed in the workpiece material due to the high temperatures involved in the process. The total depth of the recast layer and the heat affected regions may be between 2 and 130 ~m, and increases with the pulse energy used in the EDM process [2]. The extreme thermal and transformation stresses generated in the re-cast layer may also produce extensive surface cracks, which generally do not propagate deeper than the re-cast layer. The presence of these surface cracks and tensile residual stresses in the re-cast layer can deleteriously affect mechanical properties, such as fatigue resistance [3].