Master of Philosophy
School of Mechanical, Materials and Mechatronics Engineering
Rathod, Chandrahas Ramappa, Advanced microstructural characterisation of insulated rail joints, Master of Philosophy thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2015. http://ro.uow.edu.au/theses/4609
Insulated Rail Joints (IRJs) experience a much shorter fatigue life than continuous rail sections, especially IRJs located in heavy haul rail corridors. IRJs are one of the most critical components of the rail system since their failure can lead to multiple catastrophic consequences, including; malfunctioning of the track signalling system due to metal contact across the insulating rail gap and, in cases where joints are damaged at the fishplate or rail bolt holes, derailment of railway vehicles.
A literature review on the topic revealed that these joints suffer from a wide range of damage, predominately resulting from either the gradual accumulation of metal under plastic flow across the railhead in the vicinity of the rail end post, or rail end spalling and/or rail end post insulation damage. Other failure modes relate to problems with fishplates/joint bar and rail bolt holes. These include: fishplate cracking, delamination of fishplates, bolt looseness and bolt hole cracking. Therefore, IRJs are regarded as high-risk component and are maintained via expensive maintenance programs requiring high standards. Every year rail operators around Australia and the world install tens of thousands of rail joints, due to their early failure, or the risk of failure of either the signalling system and/or the structural track integrity. For the Australian heavy haul rail industry, the economic costs of IRJ maintenance are very high.
In this study, approaches to both advanced characterisation of IRJ damage and to improving IRJs service lifetimes were investigated. Damage accumulation in the vicinity of the joint was characterised by advanced metallographic (optical microscopy, SEM and TEM) and neutron diffraction techniques. In the early stage of the project, surface hard facing was selected as a potential approach to improving IRJ lifetimes, and surface clad rail samples were produced and studied via simulation testing and testing of ex-service rails.
The initial hard facing experiments involved comparison of the performance of ex-service laser clad IRJ samples (431 stainless steel cladding produced by Jarvie Engineering Private Limited) with that of uncoated IRJ samples manufactured from normal head hardened 60kg grade rail. The results indicated IRJ lifetimes can be improved by a hard facing of rail ends. Follow-up investigations were performed using both robots TIG MMAW hard facing deposition approach, and experimentation with a choice of cladding metal and geometry of the hard-face rail surface. Experimentation included advanced metallography and electron microscopy, and limited wear testing of cladding materials. Full-scale wheel on track rig testing was also performed on both coated and uncoated samples, which involves cyclic rolling contact loading on railhead surface was the main objective to investigate the railhead damage.
It was concluded that ferritic type commercial hard facing alloys, and maraging steels, appear good candidate materials for hard facing by weld deposition in the vicinity of IRJs. They also appear more suitable than the initial 431 stainless steel used in the laser cladding experiments as deleterious effects of thermal contraction during cooling are less in the ferritic alloys. It was also concluded and that geometry of the hard facing profile should include a steep angle of hard facing with the rail surface where it intersects the top surface of the rail head.
Neutron diffraction analysis of ex-service IRJs revealed significant stress evolution in the vicinity of the top surfaces of the rail ends abutting the insulating gap. This distribution is characterised by a compressive layer of approximately 5mm deep and a counterbalanced tensile layer located 5-15mm at the sub-surface region. It exceeded the distribution along the continuous rail. Residual stresses analysis for the laboratory rig tested rail ends revealed similar characteristic to those found in ex-service rail ends. However, in contrast to the ex-service rail ends, the stress distributions in the test rig samples were slightly different and this difference was attributed to the localization of wheel load under the particular test conditions. A separate neutron diffraction examination of residual stresses near bolt-holes across ex-service IRJ fishplates revealed a stress evolution characterised by both compressive and tensile stresses, extended in a 900 and 450 angles to the longitudinal plane. The later results were consistent with a common observation of fishplate fracture originating at bolt holes at 450 to the running rail surface.