Fatigue Crack Initiation and Propagation Behavior and Fracture Surface of Eutectoid Steels.

To clarify the mechanism of fatigue failure in eutectoid steel, fatigue crack initiation and propagation behaviors have been studied in detail under rotating bending fatigue tests using rail steels. The fatigue failure surface was also examined by using SEM.The main results obtained are as follows;(1) Fatigue cracks initiated at the interface between ferrite and pearlite in case of rail steel with pro-eutectoid ferrite. In rail steel without pro-eutectoid ferrite, fatigue cracks occurred at the inter-lamellar of pearlite due to the slip.(2) The fatigue crack preferentially propagated along the inter-lamellar of pearlite in the early stage. As it grew, however, it propagated perpendicularly to the stress axis by changing its direction at every pearlite block.(3) On the fatigue failure surface in eutectoid steel, the pearlite lamellar was observed, which looks apparently alike striation. But the typical striation pattern of fatigue was not observed.