It has been understood that the fatigue strength of machined materials increases by the hardening work in surface layers. In these hardened surface layers, microstructures, such as dislocation structure, might be variable with the extent of working. And there might be variation in surface structure during the fatigue.
Therefore, in order to clarify the surface structural changes during the fatigue of the annealed low carbon steels and the surface-rolled ones under three different working conditions, in this paper, the observations on the notched part of the rotating bending fatigue specimen are performed by light microscope and electron microscope with replicas, and the properties of slip line and fatigue crack initiation have been made clear.
The summarized results are as follows:
(1) The morphology of slip lines are variable from the annealed specimens to the surface-rolled specimens. In the former the fine slip lines are formed near the grain boundaries, and in the latter the slip bands are formed locally in the ferrite grains. These slip bands increase in length and width as the number of cycles increases.
(2) The higher the rolling pressure rises, the more preferential slip bands are formed in the ferrite grains. The microcracks in the highly rolled specimens tend to initiate within the slip bands before these slip bands interfere each other.
(3) Under the same cyclic stress condition, the time of emergence of the slip lines is earlier in the surface-rolled specimens, but that of microcracks is slower than that in the annealed specimens.
(4) The direction of the slip lines in the surface-rolled specimens is perpendicular to the maximum tensile stress axis.
(5) The slip ratio during fatigue increase more slowly in the surface-rolled specimens than in the annealed specimens.
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