Effects of Case Depth and Notch Shape on the Rotating Bending Fatigue Properties of Carburized Steel

Abstract

For various kinds of carburized case depths in SCM-21, both smooth specimens and notched specimens with various notch shapes were tested under the rotating bending fatigue to evaluate the fatigue strength properties of carburized steel. The main results obtained are as follows : (1) The maximum value of the crack initiating stress σ_W1 and of the crack propagating stress σ_W2, namely, the maximum fatigue strength at 10⁷ cycles, were obtained for notched specimen as well as the maximum fatigue limit σ_W0 for smooth specimen in the case of the specimen with effective carburized case depth of about 0.91.2 mm, corresponding to 1824% of the half the diameter of the specimen. (2) The existence of the non-propagating crack at 10⁷ cycles was observed in the case of the notched specimens with the notch depth of t≥0.24 mm and the notch root radius of ρ=0.08 mm. (3) The following equation was found to hold between the nominal crack propagating stress at the crack tip σ'_2W and the effective crack length l_eff·σ'ⁿ_W2·l_eff=Const.
The exponent n determined was 7.2 for specimen A, 14 for specimen B and 22 for specimen C. (4) For smooth specimen, the appearance of the fish-eye pattern was observed on the fractured surfaces in the case of the specimen with the effective carburized case about 1.0 mm or less deep, but could not be certainly observed in the case of the craburized case depth exceeding 1.2 mm.