Towards a better understanding of the surface features of electro-discharge machined tool steels

Abstract

The paper reports on an extensive experimentation with various etching reagents to reveal the types of micro-structure found on different tool steels under different machining conditions. The recast layer can broadly be classified into multi-layer, single layer with features and single-layer featureless types. Of greatest interest is the single-layer type. It was observed that for this type of recast, three sub-layers can be discerned. The top-most sub-layer is most prominent for AISI A2, D2 and D6 tool steels and consists largely of columnar grains projecting inwards from the surface-dielectric interface. The inner-most sub-layer consists mainly of cellular (for AISI 01 steel) or spike-like dendrites (for AISI A2, D6 and D2 steels). The intermediate sub-layer is most conspicuous in AISI 01 steel and is made up of complex interlocking dendrites. Preliminary results obtained from examining the effects of reducing the steel temperature gradient at the work-dielectric interface is also presented. The objective of this is to minimise the crack susceptibility of EDM surfaces.