Surface integrity and fatigue resistance of spark-machined parts

In this paper, the effects of the surface integrity on the fatigue behaviour of EN X155CrMoV12 tool steel machined by Electrical-Discharge (ED) process are reported. Surface roughness, metallurgical transformations, work hardening, surface residual stresses and thermal residual crack density were selected to characterize the surface integrity. Three points fatigue tests on notched specimens (Kt=1.6) were used to evaluate the fatigue life and the fatigue behaviour of specimens. The results of the surface integrity and fatigue behaviour characterization of ED machined specimens were compared to results obtained for milled specimens of the same steels. The milling process was selected for comparison as it is usually used as a preliminary machining operation before finishing by the ED. It is shown that the thermal crack density and the tensile surface residual stresses are the main factors controlling the fatigue behaviour of the ED machined specimens. A reduction of 35% in the fatigue life at 106 cycles was measured for the ED machined specimens comparatively with the milled ones. indeed, for the milled specimens crack free surfaces with only compressive residual stresses were generated.