The long life fatigue failure of a case hardened unnotched steel specimen starts just under the hard surface layer and “Fish eye” is found on the fractured surfaces. In the previous experiment, it was found that the rotating bending fatigue strength of carbonitrided unnotched specimens was fairly higher than the value estimated based on the strength for fatigue crack initiation and propagation at the critical portion in the specimen cross section.<br>To examine the reason for the difference between the experimental and estimated fatigue strengths, fully reversed bending fatigue tests were made on two kinds of Cr-Mo steel specimens of rectangular test section with surface layers hardened by low temperature gas carbonitriding. Specimen A was provided with hard cases only on the upper and lower surfaces of the test section, and specimen B was carbonitrided on the whole surface.<br>By using specimens A, crack initiation and propagation on the side surfaces was watched during the fatigue tests. Cracks were initiated just below the hard layer (0.4∼0.6mm distant from upper or lower surface) and propagated to fracture.<br>At a considerable wide range of stress amplitude (about 10kgf/mm²) below the fatigue fracture limit, cracks were initiated but soon arrested due to compressive residual stress of the hard layer. The minimum stress amplitude where non-propagating cracks were observed, coincided mostly with the estimated fatigue limit based on the strength of the critical portion.<br>In the fatigue tests of specimen B, “Fish eye” was found on the fracture surfaces of all specimens failed beyond 3×10⁵ stress cycles. The mean depth of the fish eye center from upper or lower surface of test section was 0.47mm. Comparison of the S-N relation of specimen B with that of specimen A, suggested the slow propagation of internal crack in specimen B, but the difference in fatigue limit between B and A was comparatively little.<br>Three specimens of kind B which endured to 10⁷ cycles at the stress amplitude levels less than 4kgf/mm² below the fatigue limit, were fractured under rather high stress levels to show the internal failure. A non-propagated internal crack was found on one of these forced fracturd surfaces.<br>The higher fatigue strength of case hardened unnotched specimen than the estimated value is supposed to depend mainly on the following factors.<br>(1) Arrest of crack initiated in the critical portion due to compressive residual stress in the hard layer.<br>(2) Slower propagation of the internal crack than the external one.