Fatigue Properties of Carbon Steels (Containing 0.13-0.41%C) with Various Heat Treated Structures

Abstract

The relationships between microstructures and cyclic properties such as the Bauschinger effect, fatigue damage, crack initiation, and crack propagation in carbon steels were investigated. For each heat treatment the cyclic stress or strain response was measured until the crack initiation. Optical metallography and transmission microscopy were used to characterize the microstructural changes during testing. Fatigue crack propagation properties were reported using the empirical equation da/dN=C (ΔK) ^m.
It was found that Bauschinger behaviour was well reflected in low cycle fatigue properties, and the higher was Bauschinger stress of the structure, the more remarkable was the softening. During high cycle fatigue process the dispersion of the fine pearlite and cementite is found to facilitate the cross slip and to result in high plastic strain, but the existence of subboundaries tends to restrict the increase of the plastic strain. Fatigue crack initiation sites depended on the heat treated structures for one steel. It was found that at the low ΔK level fatigue crack propagation properties were not affected by various strengthening structural factors, but generally controlled only by the mechanical properties of matrix.