Low-cycle fatigue behaviour and microstructural evolution of pearlitic and bainitic steels

Microstructural analysis of new bainitic steel subject to heat treatment was performed and directly compared with those achieved from the microstructure of R-260 pearlitic rail steel. Mechanical properties, fracture behaviour, and low-cycle fatigue (LCF) tests of the bainitic steel were also evaluated and collated with those achieved from pearlitic rail steel. The LCF tests were performed at room temperature at varying strain amplitudes under total strain control. The basic characteristics of the relationship between the low - cycle fatigue parameters of steels were evaluated based on the Coffin - Manson formula. The obtained test results demonstrate that the two microstructures exhibit different cyclic stress responses, i.e. the cyclic strengthening of pearlitic steel and a cyclic softening in bainitic steel. The variations in the amount of retained austenite in the bainitic structure, before and after LCF, were evaluated by X-ray diffraction and electron transmission microscopy (TEM). The results indicate that during deformation, a partial transformation of the residual austenite into the deformed martensite alpha' has taken place. Moreover, the quasi-cleavage character of the fracture surface was observed for bainitic steel with a higher deformation degree during the LCF test.