Crack formation within a Hadfield manganese steel crossing nose

Abstract

Switches and crossings in rail networks suffer from complex loading which may induce severe damage and defects, including formation of cracks that can result in rail breakage. This paper focuses on the microstructure and crack network in a damaged Hadfield manganese steel crossing nose. The extent of deformation has been quantified by hardness measurements, optical microscopy and scanning electron microscopy (SEM) including electron back scattering diffraction (EBSD). It is found that the wheel contact causes high deformation hardness of over 600 HV, around three times that of the base material, and the strain hardening extends up to a depth of about 10 mm from the running surface. Microscopy indicates the deformation microstructure is composed of bands of both deformation twins and deformation induced dislocation boundaries. The complex crack network within the nose of the crossing has been investigated using 3D X-ray tomography, where both surface and subsurface cracks are detected with the majority of the cracks originating from the surface. The crack network has been related to the observed deformation microstructure and it has been found that although the hardening and the deformation of the Hadfield manganese steel is quite different from that of commonly used pearlitic rail steels, the crack morphologies are found to be quite similar for the two materials.

2D and 3D characterization of rolling contact fatigue cracks in manganese steel wing rails from a crossing

“…Figure 7 shows an optical micrograph of a subsurface crack in the wing rail of the manganese steel crossing investigated in this work having the same wavy appearance intersecting several grains with several crack branches and a seemingly random crack propagation. This is in correspondence to the cracks found in a manganese steel crossing nose studied previously in [9], where a more detailed study showed the cracks avoided the twinned regions (harder areas of the matrix) and were mostly transgranular giving the wavy appearance by following the weakest link in the microstructure. Therefore, although the plastic deformation is caused by slip in pearlitic grade without twinning, and for manganese steel deformation is from both dislocation motion as well as twinning, the crack networks have a similar appearance for pearlitic steel straight tracks and manganese steel crossings irrespective of the very different loading conditions in the two cases.…”

“…The work hardened layer is observed to extend to 10 mm below the surface. This depth of the deformation in the investigated wing rails is found to be nearly the same as found in case of the manganese nose rail studied previously [9]. Although the previously studied crossing was fairly a new one (5 years in service) compared to the crossing in the present study (13 years in service), the yearly traffic density through both the switches was approximately the same (14MGT).…”

“…The damage and deformation in the manganese wing rail is found to be quite similar to that in the manganese nose rail studied previously [9]. Although the geometry of a wing rail is different from the nose, the microstructural characteristics are similar.…”

“…Not much work has been published in the literature on the damage and degradation behaviour of manganese steel crossings, in contrast to the use of manganese steel in abrasive wear applications such as rock crushing equipment. In a previous work [9] we investigated the nose of a manganese steel crossing which was in service in the Danish rail network for five years. The direction of traffic was mostly towards the nose and hence the nose was severely damaged with cracking and visual spallation.…”

“…The experimental techniques used for the present investigation are the same as those used in the previous study for the crossing nose, namely characterization with optical microscopy and hardness measurements, as well as X-ray tomography for 3D mapping of the crack network. With X-ray tomography cracks can be mapped non-destructively and with good resolution compared to the traditional method of repeated grinding, polishing and imaging of the crack cross section by optical microscopy [9].…”