Isao Ogata

Study on Stress Distribution in Electric Discharge Machining(5th Report). Reduction in Residual Stress and Improvement in Degenerated Layers by Heat Treatment.

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.

Study on die steel for die casting. (4th Report). Thermal fatigue characteristics of hot die steel after CVD treatment.

The thermal fatigue characteristics of hot die steel samples coated by the film of TiN, TiC, TiCN, TiC-TiN, TiC-TiCN-TiN, W2C or TD(VC) process have been studied by means of thermal fatigue test, erosion test, X-ray probe microanalysis and scannning electron microscopy. The samples were prepared using the CVD or TD(VC) processes. In the erosion test using a molten bath of aluminium, the TiC and TiC-TiN coated samples showed more effective erosion resistance than the other coated samples. The number of total cracks per area for the TiN and TiC-TiCN-TiN coated samples was smaller than the other coated samples during the thermal fatigue test. The growth of TiN was found in the matrix of the TiN coated sample after the thermal fatigue test.

RESIDUAL STRESS OF HOT DIE STEELS AFTER ELECTRIC DISCHARGE MACHINING

Electric discharge machining (EDM) is a very effective working process in die making. The surfaces formed by EDM were presented with an eroded layer which cause a large influences on the quality or accuracy for the hot die steel. The residual stress at the surface of hot die steel after EDM has been measured, using X-ray diffraction method. The results obtained can be summarized as follows; (1) The residual tensile stress decreased gradually with increasing in depth below the surface for the annealed SKD61 sample. (2) Stress distribution curves of the quenched and tempered SKD61 sample were different from those of the annealed SKD61 sample. The compressive stress was often found at the surface of the quenched and tempered SKD61 sample. The peaks of the tensile stress were found for below the surface. (3) It was shown that the stress affecting range increased with increasing in the energy supplying duration (τon) from 50 to 1200μs.