Effect of the heat treatment on the microstructure of carbide tool electrodes for electrical discharge drilling

Carbide tool electrodes for electrical discharge drilling were subjected to heat treatment at various temperatures and in different atmospheres for the same duration, respectively, in order to reduce the electrical conductivity of the lateral surface. Hardness tests show that the heat treatment does not induce a loss of strength in the electrode body. Microscopic analyses revealed that the lateral surface, characterized by a two-layer structure comprising a surface and a diffusion layer of varying characteristics and morphology, such as surface roughness, degree of coverage, delaminations, or a columnar microstructure, changes. It is revealed that the electrical conductivity is a function of the total layer thickness made up of the thickness of the surface layer and a diffusion layer. The electrical conductivity drops with increasing total layer thickness. In the context of electric discharge drilling tests, it was found that the reduced electrical conductivity leads to a decreased material removal rate and a simultaneously reduced linear electrode wear. Heat treatments ensuring consistent erosion times resulted in a wear rate reduction of the tool electrodes of up to 10 %.