Deformation Behaviour of Near Fully Pearlitic Railway Steels During Monotonic and Cyclic Loading

Licentiatavhandling, 2006

The main conclusions that could be drawn from the present work are that the deformation behaviour of fully and near fully pearlitic steels is very similar and that a combination of low temperature, prestraining and high deformation rates is critical for the performance. This was shown from microscopy studies, tensile, impact and low cycle fatigue (LCF) testing. Near fully pearlitic steels consist of a dominating volume fraction of pearlite and a minor fraction of proeutectoid ferrite or cementite. The interlamellar spacing was shown to be the dominating microstructural parameter which determines the yield stress for all pearlitic structures. Furthermore, the yield stress is influenced by strain rate and temperature with evidence of dynamic strain ageing (DSA) above 100ºC. To study the effect of these parameters, a normalising technique was used to account for minor changes in microstructure caused by heterogeneous cooling. The work hardening was found to be nearly unaffected by changes in strain rate, temperature and interlamellar spacing and was also very similar for fully and near fully pearlitic steels. Ductility was increased by the presence of proeutectoid ferrite and also by a decreased interlamellar spacing. However, changes in neither strain rate nor temperature had any significant effect below the DSA regime. Impact testing showed that the ductile-brittle transition temperature was about 100ºC. The low cycle fatigue performance of the material was shown to be similar for fully and near fully pearlitic steels at room temperature. During the dominating fraction of total life, the stress amplitude in strain control testing increases linearly with increasing cycle number and depends on the material and the actual testing conditions. At a low climatic temperature (-60ºC), the material fails in a brittle manner, with no signs of shearing as in the case of room temperature testing. Tensile prestraining generally decreases the number of cycles to fracture.