| 1. |
- Bakshi, S. Das, et al.
(author)
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Dry rolling/sliding wear of nanostructured bainite
- 2014
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 316:1-2, s. 70-78
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Journal article (peer-reviewed)abstract
- The abrasive wear of carbide-free bainitic steel under dry rolling/sliding conditions has been studied. It is demonstrated that this nanostructure, generated by isothermal transformation at 200 °C, has a resistance to wear that supersedes that of other carbide-free bainitic steels transformed at higher temperatures. The experimental results, in combination with a theoretical analysis of rolling/sliding indicates that under the conditions studied, the role of sliding is minimal, so that the maximum shear stresses during contact are generated below the contact surface. Thus, the hardness following testing is found to reach a maximum below the contact surface. The fine scale and associated strength of the structure combats wear during the running-in period, but the volume fraction, stability and morphology of retained austenite plays a significant role during wear, by work-hardening the surface through phase transformation into very hard martensite
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| 2. |
- Bakshi, S. Das, et al.
(author)
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Dry rolling/sliding wear of nanostructured pearlite
- 2015
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In: Materials Science and Technology. - 0267-0836 .- 1743-2847. ; 31:14, s. 1735-1744
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Journal article (peer-reviewed)abstract
- The dry rolling-sliding wear behaviour of pearlite that has an interlamellar spacing of just 85 nm has been characterised. Its wear resistance is found to be comparable to that of much harder bainitic steels. Microstructural observations indicate that there is substantial plastic deformation of both ferrite and cementite components of pearlite in the vicinity of the wear surface. Plasticity is not expected from Hertzian analysis that assumes a smooth contact surface. Instead, it is likely to be a consequence of exaggerated stresses due to surface roughness. The material remains ductile to shear strains in the order of 4. Diffraction data indicate that the coherent domain size is reduced to about half the interlamellar spacing and that some of the cementite may dissolve and contribute to the expansion of the lattice parameter of ferrite
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| 3. |
- Garcia-Mateo, Carlos, et al.
(author)
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Nanostructured steel industrialisation : Plausible reality
- 2014
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In: Materials Science and Technology. - 0267-0836 .- 1743-2847. ; 30:9, s. 1071-1078
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Journal article (peer-reviewed)abstract
- It is not the first time that a consortium of steel makers, end users and scientists end up with unique approaches and developments in the physical metallurgy of steels. The present paper reveals the scientific and technological developments of a consortium sharing a common intrigue and interest for a unique microstructure, nanostructured bainite. Also known as low temperature bainite, its unique properties rely solely on the scale of the miscrostructure obtained by heat treatment at low temperature (150-350°C). Careful design based on phase transformation theory, some well known metallurgy facts and the necessary industrial experience were the ingredients for a further step towards the industrialisation of these microstructures.
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| 4. |
- Hernandez, Sinuhe, et al.
(author)
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High temperature three-body abrasive wear of 0.25C 1.42Si steel with carbide free bainitic (CFB) and martensitic microstructures
- 2016
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 360-361, s. 21-28
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Journal article (peer-reviewed)abstract
- In the present work, the wear behaviour of different steels has been investigated under a three body abrasive environment at room and elevated temperatures. High-silicon steel (0.25C-1.42Si) was austempered at 300 and 320 ˚C in order to obtain two carbide-free bainitic steels with different mechanical properties. The same steel subjected to two different quench and temper heat treatments was used as a reference material for mechanical and wear testing. The steels were subjected to three-body abrasive wear by means of a high temperature continuous abrasion tester (HT-CAT). The tests were done at 25, 300 and 500 °C respectively. All samples showed similar wear rates at room temperature. At 500 °C, the material austempered at 320 ˚C showed the highest toughness and the lowest wear rate. High temperature hardness and impact toughness tests showed that abrasive wear is not only influenced by hardness but also by the toughness of the material. Owing to their good strength/toughness combination CFB steels could prove to be an important material for abrasive wear applications
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| 5. |
- Kankanala, Anusha, et al.
(author)
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The tribological behaviour under rolling/sliding conditions of carbide-free bainitic steel austempered at different temperatures
- 2010
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In: 14th Nordic Symposium on Tribology. - Luleå : Luleå tekniska universitet. - 9789174391244
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Conference paper (peer-reviewed)abstract
- The dry rolling/sliding wear behaviour of Si alloyed carbide free bainitic steel austempered at different temperatures and sliding distances has been evaluated. Three different batches of samples were austempered in a salt bath maintained at 250, 300 and 350 °C respectively for 1h. Rolling with 5% sliding wear tests were performed with each batch of samples for different test cycles, namely 6000, 18000 and 30000 cycles respectively, in order to study the wear performance of these specific steels. An in-depth microstructural characterization has been carried out before and after the wear tests in order to link the wear behaviour to the microstructure of each sample. The wear resistance has been expressed by means of the specific wear calculated from the mass loss after the tests. The worn surfaces were analyzed by the scanning electron microscopy and X-ray diffraction techniques. Micro-hardness profiles were also obtained to analyze strainhardening effects beneath the contact surfaces. The results indicate that the harder material exhibited overall lower wear rates for all test cycles studied i.e., the one austempered at 250°C showed superior rolling/sliding wear resistance than the rest. It is also interesting to note that the hardness increment and thickness of the hardened layer increases with increasing the austempering temperature and number of test cycles.Furthermore the results appear to indicate that the initial roughness of the samples has no major effect on the outcome of the results. The higher wear performance of the sample austempered at 250°C has been attributed to its superior mechanical properties provided by its finer microstructure. It has been evidenced that all samples suffer the TRIP phenomenon since, after wear, no retained austenite could be detected by XRD.
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| 6. |
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| 7. |
- Leiro, Alejandro, et al.
(author)
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Fatigue of 0.55C-1.72Si Steel with Tempered Martensitic and Carbide-Free Bainitic Microstructures
- 2014
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In: Acta Metallurgica Sinica (English Letters). - : Springer Science and Business Media LLC. - 1006-7191 .- 2194-1289. ; 27:1, s. 55-62
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Journal article (peer-reviewed)abstract
- High-Si spring steel was heat treated in three different ways: Quenching and tempering at 460 °C to obtain a tempered martensite microstructure, and austempering at 300 and 350 °C, respectively, to obtain two different carbide-free bainitic microstructures. In the steel austempered at 350 °C, both the bainite lath thickness and retained austenite content were higher than those of the steel austempered at 300 °C. Rotating-bending fatigue tests were done in order to evaluate the effect of each heat treatment on the high-cycle fatigue behavior of the steel. When the austempering temperature was 300 °C, the endurance limit was increased by 25% despite a 5% reduction in tensile strength when compared with that of the quenched and tempered steel. The relationship between endurance limit [Rfat (50%)] and ultimate tensile strength (Rm) was higher for the austempered samples in comparison with that of the quenched and tempered material. Therefore, it is believed that the presence of retained austenite affects the relationship between endurance limit and tensile strength.
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| 8. |
- Leiro, Alejandro
(author)
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Microstructure analysis of wear and fatigue in austempered high-Si steels
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Research on austempering of high-Si steels has increased over the last years due to their attractive combination of strength and toughness. The presence of over 1.5% of silicon prevents the precipitation of cementite during austempering and the resulting microstructure consists of bainitic ferrite and retained austenite. Currently, research has not dealt widely with wear and fatigue properties of carbide free bainitic (CFB) steels. Their attractive combination of strength and toughness is a result of their refined microstructure and the transformation of retained austenite to martensite under deformation (also known as the TRIP effect), making them interesting from an industrial standpoint. In this work, the effects of austempering temperature and microstructure parameters on the wear and fatigue properties of CFB steels have been investigated. Additionally their wear and fatigue behavior was compared with that of quenched and tempered (QT) steels. Initial studies focused on the effect of the austempering temperature on wear properties of spring steel. Bulk hardness decreased when the austempering temperature increased due to a higher retained austenite content and coarseness of the microstructure. This caused an increase of the wear rate. Later investigations showed that if two CFB steels have similar hardness, then the one with higher retained austenite content, tends to have a lower wear, rate suggesting that transformation induced plasticity (TRIP) aids in reducing wear. Further studies pertained to 3-body abrasion and reciprocating sliding wear characteristics of austempered steels with lower carbon content. In austempered condition these steels had lower strength and higher toughness than in quenched and tempered condition. In reciprocating sliding, it was found that wear was proportional to hardness during run-in. However, once steady state wear was reached, the wear rate was similar for all materials. In abrasion, all steels showed equivalent wear at room temperature, despite differences in hardness. At higher temperatures all materials had the same hardness and the differences seen in wear were attributed to the changes in material toughness. Fatigue studies were conducted on spring steel in austempered and quenched and tempered conditions. Despite having lower tensile strength, the austempered steels showed similar or higher endurance limit than the quenched and tempered material. This was attributed to the presence of retained austenite in austempered steels which can delay crack nucleation and propagation. Carbide-free bainitic steels provide improved wear resistance thanks to their improved toughness and deformation hardening. The effect of the amount of retained austenite on wear is still unclear as some results show ambiguous behavior. In fatigue, the preliminary results have shown an improvement in fatigue life which can be attributed to the presence of the TRIP effect but more research is needed before clear conclusions can be drawn. CFB steels are a very interesting material that is worth investigating further in more specific engineering applications where wear and fatigue are an issue
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| 9. |
- Leiro, Alejandro, et al.
(author)
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Tribological behaviour of carbide-free bainitic steel under dry rolling/sliding conditions
- 2011
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 273:1, s. 2-8
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Journal article (peer-reviewed)abstract
- The dry rolling/sliding wear behaviour of Si alloyed carbide free bainitic steel austempered at different temperatures and sliding distances has been evaluated. 60SiCr7 spring steel samples were austempered in a salt bath maintained at 250, 300 and 350 °C respectively for 1 h. Rolling with 5% sliding wear tests were performed using self mated discs for three different test cycles, namely 6000, 18000 and 30000 cycles. The aim was to study the wear performance of the 60SiCr7 steel with a carbide-free microstructure containing different amounts of retained austenite. An in-depth microstructural characterization has been carried out before and after the wear tests in order to link the wear behaviour to the microstructure of each sample. The wear resistance has been expressed by means of the specific wear calculated from the mass loss after the tests. The worn surfaces were analyzed by scanning electron microscopy and X-ray diffraction. Micro-hardness profiles were also obtained in order to analyze strain-hardening effects beneath the contact surfaces. The results indicate that the material with highest hardness—the one austempered at 250 °C—exhibited the lowest wear rate in every case. It was also observed that the hardness increment and thickness of the hardened layer increases with increasing the austempering temperature and number of test cycles. Finally, the results appear to indicate that the initial roughness of the samples has no major effect in the wear rate of the samples above 2500 cycles. The higher wear performance of the sample austempered at 250 °C has been attributed to its superior mechanical properties provided by its finer microstructure. It has been evidenced that all samples undergo the TRIP phenomenon since, after wear; no retained austenite could be detected by XRD.
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| 10. |
- Leiro, Alejandro
(author)
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Wear and fatigue properties of isothermally treated high-Si steels
- 2012
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Licentiate thesis (other academic/artistic)abstract
- In recent years steels containing Si contents of 1,5% or more have been isothermally transformed in order to obtain a microstructure consisting of bainitic ferrite laths and retained austenite. The refined microstructure and presence of retained austenite has allowed the manufacture of steels with high tensile strength and considerable elongation. The work in this thesis is motivated by the need for more knowledge, especially regarding wear and fatigue properties before they can be used in engineering applications.In this work the rolling-sliding wear resistance and fatigue strength of ausferritic (carbide free bainitic) steels have been investigated. The dry rolling-sliding tribological behavior of 60SiCr7 steel, with 1.65% of Si was investigated in austempered conditions. The obtained ausferritic microstructure contained laths in the sub-micron range. It was found that the retained austenite content decreased and the hardness increased with lower austempering temperatures, and these changes resulted in decreasing the wear rate.The wear behavior of nano-structured ausferritic steels was investigated using the same rolling-sliding conditions as the previous study. If steels of the same hardness are compared, the wear rate was reduced by half in nano-structured steels in relation with the results obtained previously for the 60SiCr7 steel. Initial hardness was an important property in reducing rolling-sliding wear. Surface hardness after wear was inversely proportional to the wear rate. It was found that the increased plasticity obtained from the transformation of retained austenite into martensite (TRIP effect), present in these steels can be beneficial for the wear resistance.The effect of austempering 55Si7 spring steel on its fatigue strength was also investigated. Three heat treatments were done, isothermal transformation at 300 and 350°C respectively and quenching and tempering at 460°C. The samples were tested in rotating-bending fatigue. It was found that by austempering at 300°C the endurance limit was improved by approximately 25% in comparison with the other two heat treatments. The improved fatigue life was mainly due to the carbide-free microstructure. The transformation of austenite into martensite improves fatigue strength, but this depends on the stability of the retained austenite. With the current work some insight has been gained on the behavior of carbide-free bainitic steels in wear and fatigue. The future work will include high resolution techniques in order to further investigate in-depth both the wear and fatigue mechanisms. In addition, other wear modes will be investigated, particularly pure sliding.
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