Evaluation of very high cycle fatigue zones in 42CrMo4 steel with plate-like alumina inclusions

Quenched and nitrided 42CrMo4 specimens, treated by different reactive filters were tested in very high cycle fatigue regime using ultrasonic method. Despite total alumina reduction fatigue limit showed no improvement due to large alumina plates whose parameters were studied. Areal roughness of fatigue crack zones was measured by confocal laser scanning microscopy. Periodic arrest marks were studied and used for crack growth rate calculation. Smooth area was experimentally defined as a zone between fine granular area and fisheye by imaging, roughness minima, fixed Delta K = 6.4 and crack growth rate of similar to 10(-10) m/cycle. Classification of internal crack zones is proposed.