TY - BOOK

T1 - Influence of material properties on the in-service behavior of rails

AU - Kammerhofer, Christoph Nikolaus

N1 - no embargo

PY - 2014

Y1 - 2014

N2 - The continuously increasing demands on rails resulting from enhanced requests in terms of axle loading, train speed and traffic face the rail producer with the challenge of providing more damage resistant rails with respect to rolling contact fatigue (RCF) and wear, while simultaneously guaranteeing additional requirements like sufficient ductility, weldability and low residual stresses. Among the many forms of RCF, head checking is particularly dangerous as those periodic cracks which emanate from the gauge corner of the high rail in curves of a certain radius range may cause complete failure of the whole rail and in further consequence may lead to derailment of the train. Huge efforts have been undertaken by rail producing companies in providing head check resistant rails. In particular, it is of prime importance that by optimizing the microstructure in terms of RCF and wear, other relevant properties, like ductility for instance, are not deteriorated. As the actual cause for head check initiation is not precisely known at the moment it is not straight forward which mechanical properties have to be optimized. During service the initial base microstructure which is affected by the wheel-rail contact is constantly modified in the way that a nanostructured and aligned surface layer is formed, caused presumably by high tangential loads in combination with micro-slippage. It seems therefore that the severely shear deformed surface layer with its inherent mechanical properties is responsible for head check initiation. The morphology of the base material and its response to the loading in terms of hardening determine however the properties of the surface layer to a great extent. In this thesis microstructure-property relationships of fully pearlitic and novel bainitic rail steels, both in the undeformed as well as severely plastically deformed state, were investigated in detail. Based on these results four criteria were formulated which seem to be a prerequisite for head checking. Finally, recommendations are given how the microstructure of a head check resistant rail steel should be composed of.

AB - The continuously increasing demands on rails resulting from enhanced requests in terms of axle loading, train speed and traffic face the rail producer with the challenge of providing more damage resistant rails with respect to rolling contact fatigue (RCF) and wear, while simultaneously guaranteeing additional requirements like sufficient ductility, weldability and low residual stresses. Among the many forms of RCF, head checking is particularly dangerous as those periodic cracks which emanate from the gauge corner of the high rail in curves of a certain radius range may cause complete failure of the whole rail and in further consequence may lead to derailment of the train. Huge efforts have been undertaken by rail producing companies in providing head check resistant rails. In particular, it is of prime importance that by optimizing the microstructure in terms of RCF and wear, other relevant properties, like ductility for instance, are not deteriorated. As the actual cause for head check initiation is not precisely known at the moment it is not straight forward which mechanical properties have to be optimized. During service the initial base microstructure which is affected by the wheel-rail contact is constantly modified in the way that a nanostructured and aligned surface layer is formed, caused presumably by high tangential loads in combination with micro-slippage. It seems therefore that the severely shear deformed surface layer with its inherent mechanical properties is responsible for head check initiation. The morphology of the base material and its response to the loading in terms of hardening determine however the properties of the surface layer to a great extent. In this thesis microstructure-property relationships of fully pearlitic and novel bainitic rail steels, both in the undeformed as well as severely plastically deformed state, were investigated in detail. Based on these results four criteria were formulated which seem to be a prerequisite for head checking. Finally, recommendations are given how the microstructure of a head check resistant rail steel should be composed of.

KW - rail steel

KW - rail damage

KW - rolling contact fatigue

KW - wear

KW - headcheck

KW - high pressure torsion

KW - nanostructure

M3 - Doctoral Thesis

ER -