Electron microscopic characterization of surface zones thermo-chemically modified by electrical discharge machining

Machining processes with main thermal impact like electro discharge machining (EDM) result in a change of temperature in the workpiece with temporal and spatial temperature gradients which have a large influence on the thermo-chemically modified surface microstructure and therefore a high impact on the functional properties of the workpiece. However, as most state variables (like the temperature) cannot be measured directly during processing, further insight can be gained only by simulations. This problem can be addressed by using micro structural features like the secondary dendrite arm spacing for calculating the cooling rate and by validating the simulated temperature profiles with these values. This study analyzes the subsurface microstructure of EDM machined 42CrMo4 steel with a ferrite-perlite and a quenched-tempered matrix by a range of microstructure analysis techniques. Results of electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA) and analysis of secondary dendrite arm spacing of secondary electron images are presented and their dependence on the main processing parameters is discussed qualitatively and quantitatively.