The railway industry constantly seeks advancements in train speed, axle load capacity, reliability, and rail longevity. Rails undergo complex and severe loading during operation due to wheel/rail contact, resulting in two main damage mechanisms: rolling contact fatigue (RCF) andwear. Furthermore, frictional heating during wheel/rail contact causes local temperature rise, leading to microstructural processes on the rail surface, known as white etching layer (WEL) and brown etching layer (BEL). This project aims to gain insight into the microstructural changes in rail steels, with a primary focus on understanding the origins of detrimental surface layers like WEL and BEL. By achieving this understanding, the lifespan of the rails can be extended and the maintenance frequency can be reduced, which has significant effects on the sustainability of the railway network as well as overall life cycle costs. Additionally, the project explores the microstructural characteristics of recently developed steel grades with enhanced resistance to rolling contact fatigue....@en

R350HT is a standard premium heat-treated rail steel and the reference for new rail steel development. The present study discusses an experimental characterization of fatigue crack growth rate and fracture toughness for this refined pearlitic rail steel in mode-I-loading. The tests are carried out on compact tension specimens extracted from the rail head with the straight notch pointing to the rail foot. As a result, the crack path orientation approximates deep rolling contact fatigue cracks. The fracture surfaces obtained under cyclic and monotonic loading are compared by means of scanning electron microscopy. The results are analyzed and discussed with reference to the morphology of the fracture surfaces for the crack initiation sites, fatigue crack growth region, and the final fracture region, evidencing the role of the microstructure, and inclusions on the fracture behavior. From the analysis of the crack path and fracture surface, it is concluded that the refined microstructure and ferrite ductility play an important role in fracture behavior.

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The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.

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Invar alloys exhibit low thermal expansion and are useful in applications requiring dimensional stability when subject to temperature changes. Conventional production of Invar faces certain challenges that can be offset by exploiting additive manufacturing processes. This study employed pulsed gas tungsten arc welding (GTAW) to deposit Invar 36 alloy blocks at five heat inputs (HI) ranging from 200 to 550 J mm⁻¹. The results show that the microstructure comprised of columnar grains and remained in the austenitic phase regardless of the HI. Ductility dip cracking was found to prevail in all the blocks except the block deposited at the lowest HI. The decreased susceptibility to cracking with a reduction in the HI was due to the preservation of the grain boundary area, consequently leading to an improved partitioning of strain among the grain boundaries. On lowering the HI from 550 to 200 J mm⁻¹ the average yield strength, tensile strength and elongation improved by 16%, 23% and 38%, respectively. The HI had a negligible effect on the mean linear coefficient of thermal expansion (CTE) in different temperature ranges as the CTE values were nearly identical between the blocks deposited at 200 and 550 J mm⁻¹. In general, the CTE in the building direction was slightly higher than the travel direction, with a maximum difference between the CTE of the two directions being 15%. In summary, this work demonstrates the application of the cold wire GTAW process as an alternative to conventional/laser based methods for realizing the functional properties of Invar.

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The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

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The influence of soaking and cooling rates on the final microstructure of a R260Mn pearlitic railway steel subjected to fast heating is investigated. Fast-heating experiments followed or not by soaking and cooling at different rates were performed using quenching dilatometry on railway steel specimens obtained from the rail head. Additional cyclic heating and quenching experiments were done to investigate the evolution of the microstructure during thermal cycling, which is relevant for railway applications. The final microstructure is characterized via microhardness measurements, optical microscopy and scanning electron microscopy. The microstructural features are distinguished and the influence of each condition is detailed. The study allows the construction of transformation diagrams during cooling after fast heating. Furthermore, comparison between the final microstructures obtained in controlled laboratory conditions and field White Etching Layers is presented. The obtained results can serve as a guideline for future reproduction of White Etching Layers in laboratory conditions and interpretation of field conditions.

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