Use of high intensity X-ray analysis as tool to create new, fundamental models for phase transformations and residual stress in ductile cast iron

Recent advances in high resolution X-Ray methods that involve use of synchrotron facilities have made it possible to do high resolution, in-situ experimental studies of phase transformations in engineering materials thus providing detailed and accurate information on the processes that take place during such phase transformations. This paper describes how such facilities can be applied to study solidification of cast iron and formation of residual stress after eutectoid transformation by resolving the processes in 3D and time

Damage and defects in railway materials: influence of mechanical and thermal damage on crack initiation and propagation

With rising societal concern for the environment, railways offer travelers a sustainable alternative to other forms of fossil-fueled transportation. In order to be competitive with airlines, for example, the railway industry must provide safe, efficient, and affordable service. In the current state, frequent delays on many networks and some rare accidents lead travelers to question the reliability and safety of the rail transport system. This problem is a complex mishmash of different matters which must be addressed from various perspectives. This study focuses on the materials in wheels and rails themselves.Railway wheel and rail materials are subjected to extreme stress in the field; increasing speeds, loads, traffic, at times in harsh weather conditions aggravate this. The combination of loadings affects the materials in a multitude of ways, for example through mechanical and thermal damage. One of the most common types of mechanical damage in the railway industry is rolling contact fatigue (RCF), the effect of which is frequently manifested in the surface of railway components, as cracks. Thermal damage, on the other hand, can affect both the surface and bulk of the material.The aim of this project is to properly characterize mechanical and thermal defects in railway components, and evaluate their effects on crack initiation and propagation, and on mechanical properties. This has been approached through extensive characterization of field samples with a certain type of cracks called squat cracks, which in some cases may lead to rail break. The squat crack networks were examined through a variety of methods, followed by the recreation of similar defects in the laboratory. Finally the effect of such defects on the microstructure, crack initiation and propagation in laboratory experiments was evaluated.Squat crack networks were characterized using several methods; detection limits of each technique have been clarified, and it was concluded that using a combination of methods, the network can be accurately described on many scales. In a second part, well-defined thermal damage on rail surfaces called white etching layers (WELs) similar to those found in field were produced using laser welding equipment, and the effect of these WEL spots on crack initiation and fatigue life has been shown. The WELs reduce fatigue life by providing a crack initiation site; both by stress and strain concentration and by decreasing ductility. The effect of thermal damage on bulk properties was also investigated using microscopy techniques including electron backscatter diffraction (EBSD) and differential-aperture X-ray microscopy (DAXM). It was found that the variation in local misorientation and residual strains decrease with increasing annealing temperature. Additionally, a method to examine crack face friction has been identified, and using this method, similar crack face features to those observed in cracks from field are created in the lab. The results from the friction experiments can be used as input towards crack propagation experiments

Validation of physical properties of hypoeutectic HCWCI alloys towards comprehensive process simulation capabilities

Abstract: High chromium white cast iron (HCWCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in mining and engineering industry. The wear resistance and mechanical properties of HCWCI mainly depend on the type, size, morphology of carbides and the matrix structure, i.e. austenite or martensite. Cast components of HCWCI are relatively cheap to produce which makes them highly demanded in the mineral processing, power plant, cement industries, etc. However, residual stresses in HCWCI alloys can compromise their application and service-life. Currently there is not enough literature to properly model and predict the residual stresses produced when casting HCWCI components. To meet the need of practical production for the HCWCI manufacturing processes, simulation work on different manufacturing process is a feasible method for HCWCI castings. A hypoeutectic composition of HCWCI material was designed and manufactured to simulate the real manufacturing processes as closely as possible. Hole-drilling method (HDM) and MAGMASOFT simulation program were used in this present study to measure residual stresses of the castings at different positions and/or depth. The stress measurements were determined on the produced alloy or casting in the as-cast conditions, full annealing and machined conditions From the preliminary investigation, i.e. HDM, it was observed that the surface of a casting had excessively high residual stress compared to the layers beneath and the stresses were only equi-biaxial for only up to a few millimetre fractions below the surface. Furthermore, it was observed that the residual stresses were generally tensile and/or compressive in nature at all positions on both gross and net weight on the produced alloy in as-cast conditions. MAGMASOFT method could only predict the, surface residual stresses and the predicted residual stress results are under estimated and/or under predicted. This resulted inconclusive conclusions to be made. Since, only 2 castings were tested for residual stress measurement using HDM. In addition, the magnitude of the residual stresses would depend on the casting section thickness and the effect of unwanted material from net-shape casting component induces either tensile stresses or compressive stresses, depending on the proximity of region being investigated. It is suggested that further work be carried out to reach conclusive results.M.Tech. (Metallurgy