Microstructure and Fatigue Properties of Railway Steels for Switches and Crossings

Switches and crossings (S&Cs) are an integral part of any rail network, allowing for necessary flexibility of directing trains from one track to another at junctions. They are also the most vulnerable part of the network due to the complex shape and the severity of the loading on them. During service, they undergo rolling contact fatigue, normal and shear stresses as well as impact loads leading to damage and severe deterioration of the mechanical properties of the steel. Therefore the choice of railway steel is very important to control material degradation and damage for safety and reliability. Understanding the damage and deformation mechanisms of the steels used for crossings, including plastic deformation, crack formation and propagation, are important in order to prevent failure. The main focus of this thesis is to study the mechanical and microstructural properties of manganese steel used in railway crossings. Worn S&C components extracted from the track were studied through extensive metallographic examinations, including optical microscopy, electron microscopy and micro-hardness profiles. X-ray tomography was used for three-dimensional mapping of fatigue crack networks within the S&Cs. The gradient in the residual stress profile from the rail wheel contact surface to different depths from the surface was analyzed using synchrotron as well as laboratory X-ray diffraction. Uniaxial, pure torsion and biaxial (in phase and out of phase) low cycle fatigue tests were performed to study the behavior of manganese steel, as well as head hardened pearlitic steel at different strain amplitudes to do a comparative analysis of their properties and deformation mechanisms under well controlled laboratory conditions. The deformed microstructures after fatigue tests were studied by transmission electron microscopy (TEM). The hardening and the deformation of the manganese steel are quite different from that of commonly used pearlitic rail steels, but the crack morphologies were found to be quite similar. The investigations from the worn crossings revealed that the crack propagation in manganese steel crossing nose is predominately transgranular with the cracks mostly following a path free from twins through relatively soft grains. The residual stress measurements revealed significant compressive stresses on the running surface of the nose of a manganese steel crossing, but the presence of cracks reduces the residual stress significantly to around one-fifth of the original value. Large residual strains were also obtained at depths of 15 mm from the running surface. Mechanical tests of pearlitic and manganese steel showed significant differences in the cyclic response of the two materials, where hardening was prominent in case of manganese steel, the pearlitic steel showed cyclic softening. The mechanical test data correlate well with the deformation micrographs obtained by TEM. For manganese steel, under uniaxial loading dislocation cell structure formation is observed but when the shear mode is introduced in the biaxial fatigue test, formation and growth of stacking faults along with dislocation controls the mechanical properties. For pearlitic steel, the dislocation morphologies are similar under all conditions; the change in mechanical properties is thus due to differences in density of dislocations.