Influence of Microstructure on Mechanical Properties and Damage Initiation of Bainitic Steels in Railway Applications

In this PhD thesis, we investigated possible steel candidates for use in railway crossings in order to reduce the damage in them. Pearlitic R350HT together with Bainitic grades including CrB, B1400 and carbide free B360 were investigated for their mechanical properties such as ultimate strength, yield strength, ductility and hardness. The influence of their microstructure on these mechanical properties was studied using microscopy techniques such as light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The effect of an isothermal heat treatment was also investigated on the bainitic steels which were mostly manufactured using continuous cooling. Carbide free bainitic steel B360 was found to have the highest strength, ductility and toughness among all the steels. These properties became even better after the isothermal heat treatment. It was decided to investigate this grade further in detail regarding its damage initiation properties. Micromechanical modelling and in-situ experiment with micro Digital Image Correlation (μDIC) was used to measure local strain maps during tensile loading. Microscopic strain partitioning was used to investigate the damage initiation behavior of this steel before and after the isothermal heat treatment. The deformation localization in the Continuously Cooled Carbide Free Bainitic Steels (CC-CFBS) (B360) was modelled using elastic plastic and crystal plasticity material models. Both models were validated using the in-situ tensile experiment. A 2D real geometry was used as the micromechanical Representative Volume Element. The blocky retained austenite (BRA) was considered as martensite from the beginning of the loading since during the experiments, it was confirmed that large portion of the BRA transform into martensite in a strain-induced transformation mechanism. The main damage mechanism in this steel was observed to be the strain localization in narrow bainitic channels between martensitic islands and the large BRA (which turn into martensite) and in the interfaces of bainite with martensite. The initiated micro cracks can later fracture the martensitic islands. xii Other factors such as the interface of martensite/bainitic ferrite, the orientation of this interface and the phase morphology also influence the damage initiation in the continuously cooled B360 steel. An isothermal heat treatment was performed on this steel in order to remove/reduce the main damage initiating factors such as martensitic islands and the large BRA which was proved to improve the mechanical properties and damage characteristics . The deformation localization in isothermally heat treated CFBS (B360-HT) was modelled and the modelling results were validated using the in-situ experimental tensile tests. The effect of the isothermal heat treatment on B360 was to remove martensite, form finer bainitic microstructure and remove the unstable large BRA. As a result, small and homogeneously distributed BRA was observed in the B360-HT. The combination of numerical simulation and in-situ test revealed that the new proposed microstructure of carbide free bainitic steel has less strain localization compared to the continuously cooled B360 steel. The maximum local strain was reduced from 35% to 25% using the isothermal heat treatment. In the B360-HT, the strain bands usually form in 45 to the tensile axis. This new proposed microstructure of carbide free bainitic steel could be a good candidate to be used in the crossing nose.