Defect Growth Characterization in Modern Rail Steel

This thesis investigates the fatigue characterization of modern head-hardened rails, with a specific focus on detail (i.e. transverse) fracture. This study provides necessary information to determine a safe and economically viable rail inspection interval. Safe inspection interval has previously been established for legacy, i.e. non-head-hardened rails. The head hardening process, which evolved over the past several decades, has been designed to improve rail wear resistance by increasing hardness. Unfortunately, increasing hardness, which is related to strength, typically results in reduction of toughness and fatigue life. This means that while improved wear resistance can extend the wear life of the rail, its fatigue life can simultaneously be reduced. Consequently, the safe inspection interval for legacy rails is not necessarily valid for modern rails. Thorough material characterization of modern rails in reference to legacy rails is necessary to establish the applicability of the legacy rail inspection interval to head hardened rails. Three modern rails (i.e. ArcelorMittal's AHH, HH, and SS - standard, control-cooled rail) and two unused legacy rails produced in 1977 and 1984, were investigated here. The SS rail, the legacy rails and existing data were used as a reference. As expected, the two modern head hardened rails (i.e. AHH and HH) are significantly harder and stronger than the control rail (i.e. SS) and the two legacy rails. Uniform pearlitic microstructure was observed in all rails, with hardness and strength variation caused primarily by pearlite spacing, which is controlled by the cooling rate and the alloy content. Despite the strength variation, toughness test results are fairly uniform across all rails, with some spatial variation inside the rail heads. Similarly, no significant differences in fatigue crack growth rates between modern and legacy rails have been observed (especially between AHH and legacy rails). These results indicate that the head hardening process designed to improve wear resistance, does not have a significant negative impact on fatigue life of rails. It is important to note that improving wear resistance of modern rails, without sacrificing fatigue properties in reference to legacy rails is a significant enhancement in rail manufacturing technology. However, it can result in fatigue becoming the limiting factor for the overall life of the rail, which places higher emphasis on rail inspection and characterization of fatigue and fracture properties. Residual stresses due to heat treatment and roller straightening were also investigated in the AHH and HH rails by means of neutron diffraction measurements supplemented by advanced numerical analysis. The results show that the largest stress component ( 350MPa) is the longitudinal stress, which is also the most consequential for fatigue growth of transverse defects. Given long beam time required to penetrate the rail material, full 3D distribution of residual stresses is difficult to obtain. Additionally, interpretation of the residual stress state measured with smaller specimens, such as plane stress slices and half-rail samples cut along the longitudinal, vertical symmetry plane, is very challenging due to significant level of interaction between different stress components. This means that extracting a rail specimen by cutting, not only relives the stress component normal to the cut plane, but it also affects the remaining stress components. Considering the importance of the longitudinal residual stresses for transverse crack growth rates, their magnitude and distribution, as well as the effect of rail-wheel contact, require further investigation.