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In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.

In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.

In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.

In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.

In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.

Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.

Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads occur together with the high contact stresses, which combined can affect the mechanical properties of the material. In this study, the effects of annealing on local microstructure and mechanical properties in pearlitic railway wheel steel were investigated using electron microscopy and micro-hardness. It is found that after annealing at 650 degrees C, the room temperature hardness reduces about 25%, accompanied by significant spheroidization of cementite in the pearlitic colonies, though the size and the orientation gradients of the pearlitic colonies have not changed much. The relationship between the microstructural changes and the mechanical properties are discussed.

Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

Block braked railway wheels are subjected to thermal and rolling contact loading. The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep, relaxation and static recovery of the wheel material. At the same time, the rolling contact loading implies a very fast mechanical load application. This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures. The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening. The Delobelle overstress function is employed to capture strain rate dependent response of the material. The model also includes static recovery of the hardening to capture slower viscous (diffusion dominated) behaviour of the material. Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time, uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model. These experiments were performed under isothermal conditions at different temperatures. In the ratchetting tests, higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model. The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.

One of the most important aspects in railway operation is the interaction between rail and wheel. Railway wheels are commonly made from medium carbon steels (similar to 0.55 wt.% C), heat treated to a near pearlitic microstructure with some 5-10% pro-eutectoid ferrite. During the operation of freight trains, where block brakes are used, high thermal loads are evolved because of recurring braking and occasional slippage. Thus the combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of the current investigation is to evaluate the mechanical behaviour of wheel material (UIC ER7T) subjected to non-proportional biaxial fatigue loading, as this simulates the actual working conditions in a better way than uniaxial loading. Axial-torsional low cycle fatigue tests were performed at room temperature and elevated temperatures using thin walled specimens to study the cyclic stress-strain properties of this material. The results showed large influence of temperature on the ratcheting behaviour of the material. Biaxial non-proportional loading gave much higher strain hardening as compared to uniaxial loading. Hardening due to dynamic strain ageing can be seen in the biaxial tests at temperatures around 300 degrees C.

The large deformations occurring in the surface layer of rail heads strongly influence the material behavior. Cracks typically develop from this critical region. However, conventional material testing of this layer is not possible due to the large gradients. A method for obtaining a similar material through predeformation is therefore investigated. Subsequent analyses of the obtained material can improve our understanding of the material behavior close to the running band. Hardness, shear lines and cementite lamella orientation distributions are compared in order to determine whether the obtained material is similar to that of the field samples extracted from rails that have been in service for several years. The predeformation method produces a consistent material, which is representative of the material in the rail field samples that have accumulated large shear strains.

Pearlitic steels are commonly used for railway rails because they combine good strength and wear properties. During service, the passage of trains results in large accumulation of shear strains in the surface layer of the rail, sometimes leading to crack initiation. Knowledge of the material properties versus the shear strain in this layer is therefore important for fatigue life predictions. In this study, fully pearlitic R260 rail steel was deformed using a bi-axial torsion-compression machine to reach different shear strains. Microstructural parameters including interlamellar spacing, thickness of ferrite and cementite lamellae and dislocation density in the ferrite lamellae, as well as hardness were quantitatively characterized at different shear strain levels. Based on the microstructural observations and the quantification of the microstructural parameters, the local flow stresses were estimated based on boundary strengthening and dislocation strengthening models. A good agreement was found between the estimated flow stresses and the flow stresses determined from microhardness measurements. d

One of the most important aspects in railway operation is the interaction between rail and wheel. The contact patch between these two components is around the size of a small coin, and since high loads act on this small area, stresses will give rise to wear and damage in both components. Frictional forces on the surface of wheels and rails caused by recurring train acceleration, braking, curving and occasional slippage can cause cyclic plastic deformation and heating, which in turn causes an aligned, anisotropic microstructure with altered mechanical behaviour. Control of material property degradation is an important topic for guiding maintenance, as well as ensuring safety of railways, since it will allow for a more accurate prediction of material wear and lifetime. The thesis focuses on the mechanical properties of railway wheel and rail steels after exposure to elevated temperatures and plastic deformation. Specifically examined are the carbon wheel steels, UIC ER7T and ER8T (~0.55 wt. %C) and rail steel R260 (~0.72 wt. %C). During their service life, the surface layers of rails and wheels are subjected to very high rolling contact loads. These lead to accumulation of large shear strains close to the running surface. Moreover the high thermal loads that wheels experience when block brakes are used can cause severe degradation of the material microstructure, more specifically spheroidisation of the pearlite, which combined with plastic deformation (that makes the material more prone to spheroidisation) can lead to severe deterioration of the material’s mechanical properties. Both un-deformed and pre-strained wheel materials were heat treated at various temperatures from 250°C to 600°C for various durations, and the change in room temperature hardness was analysed. Additionally, Electron Backscatter Diffraction Analysis (EBSD) was used to evaluate if orientation gradients in the pearlitic colonies affect the spheroidisation of the pearlitic microstructure, that is

One of the most important aspects in railway operation is the interaction between rail and wheel. The contact conditions give rise to wear and damage in both components. Medium carbon steels are used in these components due to their combination of high strength and good wear properties in relation to cost. In service, high surface temperatures develop because of frictional heating on traction, braking, curving and occasional full slippage. Furthermore, long-term block braking may heat the wheel rim to over 500°C. It is thus relevant to examine the high temperature performance of wheel material as well as the decrease in strength after thermal exposure. In the current thesis, two railway wheel steels are examined. These are the medium carbon steels UIC ER7T and ER8T (~0.55 wt.% C), heat-treated to a near pearlitic microstructure with some 5–10% pro-eutectoid ferrite in the wheel tread surface. Specimens were extracted from virgin wheels and pre-strained either monotonically or cyclically, to imitate plastic deformation developing in the wheel tread surface in service. Both un-deformed and pre-strained materials were heat treated at various temperatures from 250°C to 650°C for various time durations, and the change in room temperature hardness as measured before and after heat treatment was analysed. Samples were analysed using Scanning Electron Microscopy (SEM), to examine microstructure degradation. Additionally, Electron Backscatter Diffraction Analysis (EBSD) was used to evaluate if orientation gradients in the pearlitic colonies affect the spheroidisation of the pearlitic microstructure that is observed at higher temperatures.Analyses after the room temperature hardness measurements showed that hardening due to strain ageing takes place at around 300°C while microstructural degradation caused softening at higher temperatures. Spheroidisation of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained. T

In the current work the deterioration of mechanical properties of railway wheel steels (UIC ER7T and ER8T) is in focus. These are medium carbon steels (∼0.55 wt.% C) heat treated to a near pearlitic microstructure with some 5-10% pro-eutectoid ferrite. During operation of trains, high thermal loads are evolved because of recurring acceleration, braking, curving and occasional slippage. It is thus relevant to examine the high temperature performance of wheel material and evaluate the decrease in strength after thermal exposure as well as the degradation of fatigue properties. Samples were extracted from virgin wheels and pre-strained to around 6.5% strain as well as cyclically deformed, to also account for the change in properties that is induced by plastic deformation inherent in the wheel tread surface. Both un-deformed and pre-strained material was heat treated for different times in the temperature range of interest, from 250°C to 600-700°C. Hardening was observed in both conditions around 300°C followed by softening at higher temperatures. Spheroidization of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained.

In the current work the deterioration of mechanical properties of railway wheel steels (UIC ER7T and ER8T) is in focus. These are medium carbon steels (∼0.55 wt.% C) heat treated to a near pearlitic microstructure with some 5-10% pro-eutectoid ferrite. During operation of trains, high thermal loads are evolved because of recurring acceleration, braking, curving and occasional slippage. It is thus relevant to examine the high temperature performance of wheel material and evaluate the decrease in strength after thermal exposure as well as the degradation of fatigue properties. Samples were extracted from virgin wheels and pre-strained to around 6.5% strain as well as cyclically deformed, to also account for the change in properties that is induced by plastic deformation inherent in the wheel tread surface. Both un-deformed and pre-strained material was heat treated for different times in the temperature range of interest, from 250°C to 600-700°C. Hardening was observed in both conditions around 300°C followed by softening at higher temperatures. Spheroidization of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained.

In this contribution, the mechanical behaviour of a near pearlitic wheel steel of type ER7 is studied. Isothermal experiments of cyclic loading combined with a hold time are performed at several temperatures, ranging from room temperature up to elevated (below austenitization) temperatures. The experiments show a viscous behaviour at temperatures above 300°C. To capture the cyclic hardening/softening and viscous behaviour of the material, a Chaboche model of viscoplastic type is presented and calibrated against the experimental data. The robustness and uniqueness of the obtained material parameters are then ensured by employing sensitivity and correlation analyses. The main goal of this study is to improve the modelling of wheel materials subjected to thermal loading due to tread braking and also to highlight the importance of viscoplastic material modelling. In this regard, finite element analyses of generic heavy haul wheels, subjected to high power drag braking loads, are carried out and comparisons between analyses with plastic and viscoplastic material models are shown. Results are presented for simulated global wheel behaviour, i.e. axial rim displacements during and after braking, and also residual stresses after braking. A conclusion is that the obtained results for a generic wheel with an S-shaped web, which builds substantial stresses in the wheel rim during braking, is rather sensitive to the choice of material model. Substantial differences are found already at 400°C. Moreover, the results indicate that a generic Low-stress wheel, which builds less stresses in the rim during braking, is less affected by the choice of material model. However, at temperatures higher than about 500°C also the results for this wheel are significantly affected by the choice of material model.

The focus of this study is the degradation of a near pearlitic microstructure under combined mechanical and thermal loadings leading to changes in mechanical properties. More specifically, it is examined how the orientation gradients inside the pearlite colonies, affect the spheroidisation. Samples were extracted from virgin near pearlitic railway wheels and prestrained, thereafter heat treated at different temperatures. Microstructural characterization by scanning electron microscopy (SEM) and Electron Backscatter Diffraction Analysis (EB SD) was performed and evaluated. Results showed that spheroidised areas appear to have lost their initial orientation gradients after spheroidisation and obtain a more uniform orientation. More sub-grain boundaries are present after exposure to higher temperatures.

The present study describes the development of a theoretical model for estimating the thickness of homogeneous surface layers on spherical and irregular shaped powder particles using the XPS depth profiling. As opposed to flat surfaces, such an approach for substrates of specific geometry is not straightforward. One needs to consider those geometrical factors associated with the experimental setup and sample's roughness, which impose an angle dependence on the photoelectrons peak intensity, X-ray flux and ion etch rate over the surface in question in order to evaluate it. The novelty of the current model lies in the introduction of geometrical freedom in connection to the experimental arrangement, which can be tailored to match the needs of contemporary instruments. The model was evaluated experimentally by analyzing the surface oxide layers on metal powder grades of different morphology. Complementary analytical techniques such as high resolution (HR) SEM and focused ion beam (FIB) were used in combination to further characterize the surface layers prior to the XPS investigations. The results reveal that the estimation of the oxide/metal interface from the measured relative metal intensity signal as function of etch depth using the model is in extremely good agreement with the corresponding values from direct measurement of HR SEM on FIB cross-sectioned powder samples. The model deviates slightly only for the irregular shaped powder, which can be regarded as means for quantification of surface roughness of the material. The model is used as a basis for a computer software that estimates the thickness of surface layers for powdered materials.

In the current work the deterioration of mechanical properties of railway wheel steels is in focus. These are commonly made from medium carbon steels (~0.55 wt.% C) heat treated to a near pearlitic microstructure with some 5–10% pro-eutectoid ferrite. The two steel grades studied here are very common on trains in Europe: the R7T grade is mainly used for freight trains and the R8T grade is mostly used for motorized passenger trains. During operation of trains, high thermal loads are evolved because of recurring acceleration, braking, curving and occasional slippage. It is thus relevant to examine the high temperature performance of wheel material and evaluate the decrease in strength after thermal exposure as well as the degradation of fatigue properties. Samples were extracted from virgin wheels and pre-strained to around 6.5% strain, to also account for the change in properties that is induced by plastic deformation inherent in the wheel tread surface. Both un-deformed and pre-strained material was heat treated for different times in the temperature range of interest, from 250°C to 700°C. Hardening was observed in both conditions around 300°C followed by softening at higher temperatures. Spheroidization of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained.