| 1. |
- Ahlström, Johan, 1969, et al.
(författare)
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Short-time tempering kinetics of quench hardened pearlitic steels
- 2010
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Ingår i: Conference proceedings ICTPMCS-2010, 31 May – 2 June 2010, Shanghai, China. ; , s. 6 pp-
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Konferensbidrag (refereegranskat)abstract
- In many industrial processes such as welding, and operating environments like railway wheel/rail contact, certain material volumes are exposed to very short temperature pulses that endure for times of the order of 1 s or shorter. When pearlitic steel is exposed to temperatures above the effective austenitisation temperature, martensite forms upon rapid cooling. In the current work tempering of a martensitic carbon steel was performed, with a laser heat source to create short time top-hat temperature pulses, and also with conventional salt bath experiments for longer tempering times. Temperatures were varied in the range of 500 to 700°C, and times between 0.05 to 3000 s. It was found that the martensitic test samples showed a very rapid initial softening upon tempering. During the first 0.1 second the hardness decrease was measured to 35-55% in the tested temperature range. Thereafter the additional hardness decrease was limited to ~10-15% of the original hardness, even after 30 min of tempering. Thus the use of time-temperature models for predicting tempering properties, that relies on constant or linear dependence of the activation energy cannot be extrapolated to very short time processes.
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| 2. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Analysis of wear debris in rolling contact fatigue cracks of pearlitic railway wheels
- 2014
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 314:1-2, s. 51-56
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Tidskriftsartikel (refereegranskat)abstract
- In the current study a severe subsurface crack network in a railway wheel has been studied and compared to typical rolling contact fatigue cracks found in the wheel tread surface. Microstructural characteristics, chemical composition and microhardness within the cracks and around crack faces were examined. While the two damage types are principally different, both showed similar crack characteristics, with short cracks branching along the main crack paths and a discontinuous sheared layer of wear debris and metallic flakes within them. Analyses of the wear debris showed that it does not originate from external contamination or being the result of corrosion primarily. Instead it has most likely been produced by shear deformation and wear mechanisms within crack faces caused by mixed-mode crack growth. Although microstructural appearance at lower magnification seemed to differ from the bulk material, at high magnification a lamellar structure was observed consisting of layers of deformed metallic flakes and particles of the base metal. Auger electron spectroscopy was used to analyze these sheared layers; higher concentration of oxygen was measured in between flakes, indicating the presence of oxides and flakes being of similar chemical composition as the base material. A possible explanation is that these layers are created due to high shear forces and friction between crack faces in the service of the wheel. With continued rolling the material being sheared by the cyclic relative motion of the crack faces disintegrates into smaller wear debris particles with concurrent oxidation.
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| 3. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Characterisation of plastic deformation and thermal softening of the surface layer of railway passenger wheel treads
- 2013
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 300:1-2, s. 200-204
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Tidskriftsartikel (refereegranskat)abstract
- Microstructure, plastic deformation and annealing properties of the material in the outermost tread layer of used passenger railway wheels were characterised. Large deformation existed up to 5 mm depth below the surface and material flow, i.e. shear strain, was present both in tangential- and transverse directions of the wheel. Close to the field side and flange root moderate rolling contact fatigue was observed, where also the largest work hardening and shear deformation were measured. Annealing trials at temperatures around 500 degrees C showed softening of the deformed surface layer at lower temperatures and at higher rate than for non-deformed material.
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| 4. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Influence of short heat pulses on properties of martensite in medium carbon steels
- 2013
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 561, s. 321-328
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Tidskriftsartikel (refereegranskat)abstract
- The process of tempering a martensitic medium carbon steel was investigated with the aim to study resulting material properties. The experimental results were used to model residual stresses caused by local heating, with the finite element method. Tempering was followed for times from 0.1 s up to 1 h by using laser heating and conventional salt bath furnace treatments within the temperature interval 500–700 °C. In addition, the thermal expansion was evaluated using dilatometry. Experiments showed that the initial stages of martensite decomposition, associated with loss of crystal tetragonality, proceed almost instantly. An initial large decrease of hardness within the first tenth of a second of the tempering process was measured, followed by only limited further softening with increased tempering time. Thus for the current material the tempering time had limited influence on hardness, governed primarily by the peak temperature during the heating process. Finite element modelling of rapid local heating and cooling showed that the tempering behaviour and associated dilatation effects yield a peak temperature dependent residual stress field with a broad tensile stress distribution for the case of un-tempered martensite. However, for tempered martensite the residual stress field depends primarily on the heating rate and peak temperature and shows large gradients with tensile stresses in the surface and compressive below. Thereby, for both cases, residual stresses were obtained but with completely different residual stress gradients.
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| 5. |
- Cvetkovski, Krste, 1983
(författare)
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Influence of thermal loading on mechanical properties of railway wheel steels
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Material integrity and properties of wheels are critical in railway traffic, as wheels fulfil the important function of transferring load and traction from the vehicle to the rail track. Steels with a pearlitic microstructure are commonly used for wheels due to their height strength, low cost and good wear properties. However, the pearlitic microstructure and behaviour can be altered by thermal and mechanical load being present in the contact between wheel and rail. As very high power is available, a few milliseconds of time where the slip between wheel and rail becomes large can cause small material volumes in the contact to be heated several hundred degrees Celsius.The present work was initiated with the main purpose to investigate the effects of rapid thermal heating and cooling on wheel material. Cyclic and monotonic mechanical testing was performed to study the effects of thermal softening on virgin wheel material and on used wheels taken out from service. Furthermore, material in both pearlitic and martensitic state was investigated during rapid heating and cooling cycles by methods as laser and resistive heating for different loading conditions.It was shown that alloying composition of different wheel steels could decrease sensitivity to thermal loads, while plastic deformation had the opposite effect when the material was subjected to long time thermal loading. For the typically very short heating times present in the wheel rail interface no significant permanent effects on mechanical properties were measured for pearlitic material. However, for martensitic material, substantial permanent hardness decrease progressed within fractions of a second at elevated temperature. This rapid tempering behaviour was observed to progress even faster in the presence of an external load. Moreover, the inherent different behaviours of pearlite and martensite can result in different residual stress fields for the case of local heating on the tread surface, affected by heating rate, peak temperature and duration.Some additional effects of frictional heating and the influence of wear debris within cracks for rolling contact fatigue cracks were also investigated by use of image and chemical analysis.
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| 6. |
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| 7. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Monotonic and cyclic deformation of a high silicon pearlitic wheel steel
- 2011
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 271:1-2, s. 382-387
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Tidskriftsartikel (refereegranskat)abstract
- The development of railway wheel steel grades has gone towards higher strength to enhance rolling contact fatigue resistance and ability to withstand thermally induced damage throughout the service lifetime. In this study a recently developed wheel steel material for passenger trains containing high levels of manganese and silicon, approximately 1 wt% of each, was tested in low cycle fatigue at different total strain amplitudes. In addition, monotonic tensile and Charpy V-notch impact testings in the temperature range −60 °C to 180 °C were performed. The results were compared to another steel grade commonly used in Europe for railway wheels. The low cycle fatigue results show similar cyclic lifetime Nf, and fatigue stress amplitude development for both materials, but the highly alloyed steel exhibits reduced cyclic softening and hardening during the fatigue life and is in this respect more stable in its behaviour. As is typical for pearlitic–ferritic steels both materials show a pronounced monotonic strain hardening. It was concluded that the increased levels of manganese and silicon have minor effects on monotonic behaviour but substantial influence on cyclic behaviour at ambient temperatures. At increased temperatures, though, there are indications of additional effects on monotonic behaviour. However, the high temperature properties have to be investigated further to gain full understanding of mechanisms and effects.
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| 8. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Rapid thermomechanical tempering of iron–carbon martensite
- 2014
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Ingår i: Materials Science and Technology. - 1743-2847 .- 0267-0836. ; 30:14, s. 1832-1834
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Tidskriftsartikel (refereegranskat)abstract
- Tempering of martensite under simultaneous compressive stress has been studied within thetemperature range of 20–400 deg C. Resistive heating was utilised to obtain rapid heating and coolingcycles of a few seconds. Material was obtained from a medium carbon pearlitic railway wheelsteel, quench hardened to obtain martensitic structure. Above approx 150 deg C dilatation effectswere observed below the global yielding point of the material. Microstraining around dislocationsin the body centred tetragonal crystallographic structure or viscous flow at higher temperatureswas a probable explanation to this material behaviour. Hence, external stress may have animportant influence on the tempering progression of martensitic steel. The trials also showed thattempering of martensite progresses fast, is near instantaneous and is independent of thepresence of external stress or not.
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| 9. |
- Cvetkovski, Krste, 1983, et al.
(författare)
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Subsurface crack networks and RCF surface cracks in pearlitic railway wheels
- 2012
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Ingår i: 9th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems, CM 2012; Chengdu; China; 27 August 2012 through 30 August 2012. ; , s. 425-431
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Konferensbidrag (refereegranskat)abstract
- Subsurface cracks developed by rolling contact fatigue (RCF) are both a lifetime limiting factor for railway wheels and a safety issue. In the current study a severe subsurface crack network has been studied and compared to RCF cracks on the surface. Microstructural characteristics, determination of local chemical compositions with Auger electron spectroscopy and microhardness within cracks and around crack faces were examined. Both damage types showed similar crack growth characteristics, with small cracks branching from the main cracks. Continuous third body material resembling wear particles were identified in many cracks. The analyses showed that this material does not originate from external contamination or oxide inclusions but is produced by shear deformation and wear within crack faces by mode II and III crack growth. The third body material showed a higher hardness than the surrounding base material indicating a high shear deformation and work hardening. While microstructural appearance at lower magnification appeared different to the bulk material, high magnification revealed that the third body material mainly consisted of deformed flakes of base material, wear and oxide particles. Auger electron spectroscopy confirmed the theory that the third body consists of several layers of deformed metallic flakes covered with thin oxide layers indicated by the variation in oxygen content.
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| 10. |
- Cvetkovski, Krste, 1983
(författare)
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Temperature Stability of Railway Wheel Steels - Influence on Microstructure and Mechanical Properties
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Being the single component transferring load and traction from the vehicle to the rail, the selection of materials and structural properties of railway wheels are critical. Carbon steel with a pearlitic microstructure is the most commonly used material due to its high strength, low cost and good wear properties. However, due to the two‐phase microstructure, pearlite is susceptible to softening at higher temperatures and degradation of mechanical properties. A new type of wheel steels with increased alloying content of silicon and manganese has shown improved resistance to rolling contact fatigue. However, a clear correlation between increased alloying content and strength and how the softening behaviour of the material affects the fatigue properties have not been studied so far. To foster further knowledge on these issues, two wheel steels were used in this work: a high Si‐Mn alloyed steel and a widely used lower alloyed steel.In virgin states the two selected steels were found to have fairly similar mechanical characteristics in a wide range of temperatures and different strain rates. Upon annealing, however, both materials begin to soften at temperatures above 500 °C. Higher temperatures approaching the ferrite‐austenite transformation temperature (about 750 °C) accelerates the microstructural degeneration. However the high Si‐Mn‐alloyed steel showed to have a better resistance against thermal softening, resulting in better mechanical properties and increased fatigue life compared to the low alloyed steel. Investigation of the microstructure revealed that the softening was caused by spheroidisation of cemenite lamellas in pearlite. Heavy pre‐deformation before annealing emphasised this degeneration of pearlite even more. Both silicon and manganese are considered to retard the softening towards higher annealing times and temperatures, by a decrease in carbon flux, which thereby limit the spheroidisation rate. The cementite spheroidisation leads to lowering of hardness, monotonic and cyclic strength and fatigue life times. As the high alloyed steel resists microstructural degeneration better at elevated temperatures it retains the mechanical properties better after high temperature exposure. This is particularly evident for the fatigue performance. As an example it can be mentioned that a certain high temperature exposure leads to hardness decrease of about 20 % for the high alloyed steel, while the low alloyed steel suffers a hardness reduction of about 25 %. For fatigue life times, however, the difference between the two materials is as much as a factor three in favour for the higher alloyed steel.
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