Study on Stress Distribution in Electric Discharge Machining (5th Report)

Reduction in Residual Stress and Improvement in Degenerated Layers by Heat Treatment

Abstract

As reported previously, on surface of a workpiece by EDM, serious working defects arise such as residual stress and unusual structurs caused mainly by severe thermal actions and the existence of working fluid, due to EDM.
These degenerated layers are a harmful influence on the mechanical and physical properties; for example, a reduction in the fatigue limit, impact property of materials, thermal fatigue characteristics and various other machine elements. Hence, the quality of products (workpiece) largely depend on the treatment methods of products after EDM.
It is difficult to perfectly remove the working defect in EDM, because the working mechnism of EDM is mainly based on thermal action. Accordingly, it is important to control the generation of working defects by selecting more adequate working conditions, and decrease or remove unusual structures.
However, there are few reports on the reduction and improvement of working defects that ganerate on the surface of workpieces by EDM.
This paper deals with the improvement for degenarated layers on surface of a workpiece. Exactly, behaviors or changes in the residual stress and other unusual structures are investigated by tempering the workpieces after EDM, using X-ray methods.
Consequently, it was proven that the effect of a reduction for residual stress occurred during two-cycle tempering in the case of the low temprature(250°C) condition, and a great reduction occurred when tempering in high temperature (550°C) condition.
Furthermore, it became clear that the stresses due to structure transformation and precipitation of carbides such as Fe₃C, M₇C₃ (M; Fe or Cr) take part in the generation of residual stresses as well as thermal stress which have been suggested as being normal.