Influence of combined thermal and mechanical loadings on pearlitic steel microstructure in railway wheels and rails
Doktorsavhandling, 2018

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 observed at higher temperatures. Uniaxial (tension-compression) and biaxial (including torsion) low cycle fatigue tests were performed to study the behaviour of R7T and R8T material at different temperatures. The influence of hold times as well as the ratchetting behaviour with mean stress effects were also studied. Virgin rail material was twisted using a biaxial machine to various shear strain levels to create a microstructure representative for the surface layer observed in field samples. The microstructure was characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness measurements.

The results showed that wheel material 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. The spheroidised areas appear to have lost their initial orientation gradients after spheroidisation and obtain a more uniform orientation. Cyclic tests at elevated temperature revealed cyclic hardening at around 300°C, as an effect of dynamic strain ageing. At higher temperatures, cyclic softening followed due to a combination of increasing thermal activation and spheroidisation. Biaxial testing showed a more severe effect of strain hardening and shorter fatigue life. For the rail material, the dislocation density was found to increase with increasing shear strain. The flow stresses calculated using microstructural parameters such as dislocation density and interlamellar spacing of the pearlite seem to agree well with those evaluated from hardness measurements.

Multiaxial fatigue

Low cycle fatigue (LCF)

Pearlite

Hardness

TEM

Rail steels

Thermal effects

EBSD

Wheel steels

Spheroidisation

Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg, Sweden
Opponent: Professor (Univ. Prof. Dr.) Reinhard Pippan, ESI-Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences, Austria

Författare

Dimitrios Nikas

Chalmers, Industri- och materialvetenskap

Materialteknologi

Nikas, D. , Zhang, X ,Ahlström, J Evaluation of local strength via microstructural quantification in a pearlitic rail steel deformed by simultaneous compression and torsion

Nikas, D., Ahlström, J High temperature uniaxial ratcheting and bi-axial low cycle fatigue behaviour of railway wheel steel

Did you know that the contact patch under a railway wheel is around the size of a small coin? As if this wasn’t enough, the weight applied on this small surface can reach the equivalent of 10 average sized cars. Combined with frictional forces in the surface of wheels and rails caused by train acceleration, braking, and curving we get wear, frictional heating and damage in both components. So it is important to control and understand the material behaviour that will allow for a more accurate prediction of wheel and rail wear and life. This leads to improvement in maintenance practices as well as ensures safety, reliability and sustainability of railways.

In this thesis the mechanical properties of railway wheel and rail steels after exposure to elevated temperatures and plastic deformation are in focus. We examined some of the most commonly used materials that are used these days. The loads and temperatures mentioned above cause damage in the outer surface of rails and wheels as well as deterioration of the microstructure of the material on a microscopic level. This damage usually appears in the form of cracks and/or destruction of microscopic features that are responsible for the strength of these materials.

To examine the materials in different loading conditions we used mechanical testing machines and to see the microstructure and how it changes we used microscopes capable of very high resolution and magnification. With these, the materials’ properties and how their microstructure responds to different loading scenarios and temperatures can be observed and captured. Both undeformed material and material taken after being used in the field were examined. The wheel materials were examined after exposure and loading at various temperatures from 250°C to 600°C and the rail material was loaded in various ways in the laboratory to imitate the loading in field.

The results indicate that wheel material gets stronger around 300°C while for temperatures above this, softening occurs. As mentioned above this softening is due to the microstructure being damaged due to the high temperature and loads. Some loading conditions show more severe effects than others. As for the rail we managed to produce a microstructure, similar to the one that we can find on the surface of used rails. Proper implementation of these results will lead to improved lifetime prediction capabilities of these materials and improved railway operation.

In2Track

Europeiska kommissionen (Horisont 2020), 2016-12-01 -- 2020-12-31.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Tribologi

Bearbetnings-, yt- och fogningsteknik

Annan materialteknik

Styrkeområden

Transport

Materialvetenskap

ISBN

978-91-7597-801-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4482

Utgivare

Chalmers tekniska högskola

Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg, Sweden

Opponent: Professor (Univ. Prof. Dr.) Reinhard Pippan, ESI-Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences, Austria

Mer information

Senast uppdaterat

2018-09-19