| 1. |
- Banijamali, S. M., et al.
(författare)
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Effect of Ce Addition on the Tribological Behavior of ZK60 Mg-Alloy
- 2021
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Ingår i: Metals and Materials International. - : Springer. - 1598-9623 .- 2005-4149. ; 27:8, s. 2732-2742
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Tidskriftsartikel (refereegranskat)abstract
- The present work aims to study the tribological behavior of an extruded ZK60 alloy in the presence of Ce; in a previous study, among ZK60 alloys with different Ce addition rates, an alloy with 3 wt% of Ce was found to exhibit the most promising mechanical (e.g., hardness and strengths) properties, while its wear behavior remained unknown. The results of microstructural examinations by optical and electron microscopes show that Ce addition reduces the grain size from 6.1 to 2.0 μm. Besides, in addition to the precipitates already distributed in the base alloy (Mg7Zn3), Ce could promote the formation of a new precipitate (MgZn2Ce), increasing the total fraction of the precipitates. These microstructural evolutions enhance the strengths of the studied ZK60 alloy, as the yield and tensile strengths increase from 212 to 308 MPa and from 297 to 354 MPa, respectively. A pin on disc tribometer was employed to study the wear behavior of the developed alloy under different normal loads (5, 20, 40, and 60 N). The results show that the base and Ce-added alloys exhibit almost a similar frictional behavior, while the wear resistance of the Ce-added alloy is higher within the load ranges applied: (i) in low load conditions (5 and 20 N), where the abrasive wear is the active mechanism, the precipitates in the Ce-added alloy could enhance the wear resistance. (ii) Under the load of 40 N, oxidative wear is also an operative wear mechanism, leading to a sharp increase in the wear rate of the alloys. In this condition, Ce could provide a protective oxide layer, which could improve the wear resistance of the alloy. (iii) At a load of 60 N, both studied alloys exhibit a similar wear rate due to a severe oxidation condition. Therefore, beyond this loading condition, the microstructural evolutions (e.g., change in precipitation behavior) caused by Ce addition can no longer contribute to the enhancement of wear resistance.
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| 2. |
- Caballero, Francisca G., et al.
(författare)
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Advanced Heat Treatments and Complex Ferritic Structures for Bearing Steels
- 2019
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Ingår i: Metals. - : MDPI. - 2075-4701. ; 9:11
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructured bainitic steels exhibit an optimum strength/toughness combination as a consequence of their extremely fine structure. They have also demonstrated potential for wear-resistance applications. The aim of this work was to develop bearing steels by the multi-scale control of complex ferritic structures, designed using atomic transformation theory and processed by novel heat treatments. Based on the results, the new ball bearings outperformed conventional grades, approaching more expensive material options.
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| 3. |
- Moghaddam, Pouria Valizadeh, et al.
(författare)
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Dry sliding wear of nanostructured carbide-free bainitic steels : Effect of oxidation-dominated wear
- 2020
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 454-455
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure has a profound impact on sliding wear behaviour. This paper aims to understand the effect of carbide-free bainitic microstructure on oxidation-dominated wear in a self-mated dry sliding contact and compare the results with quenched and tempered martensitic microstructure. The results show improved wear resistance of the carbide-free bainitic steel austempered at low temperature. Hence, the retained austenite content of the carbide free bainitic microstructure is not the only indicator of excellent wear resistance. Compared to tempered martensitic microstructure, the carbide-free bainite offers a higher resistance against the formation of brittle white etching layer during the sliding wear. In summary, the formation of a thin and mechanically stable compositional mixed layer on top of the surface together with a hard underlying substrate are the main reasons behind the improved wear performance of carbide-free bainitic steel austempered at low temperature.
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| 4. |
- Moghaddam, Pouria Valizadeh, et al.
(författare)
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High temperature tribology of TiAlN PVD coating sliding against 316L stainless steel and carbide-free bainitic steel
- 2021
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Ingår i: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 159
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Tidskriftsartikel (refereegranskat)abstract
- Reciprocating sliding wear tests were performed on TiAlN coating against 316L stainless steel and carbide-free bainitic steel at temperatures of 40, 400 and 800 oC. The results indicate that material transfer is more pronounced for the softer stainless steel at lower temperatures but at 800 oC, carbide-free bainite exhibits relatively more material transfer. Friction coefficient of stainless steel increases when temperature increases. However, for carbide-free bainite, there is a reduction in friction coefficient at elevated temperatures. This can be attributed to formation of an easily sheared iron oxide layer at elevated temperatures. In case of stainless steel, generation of a thin tribofilm containing aluminium oxide and oxidised transferred material can protect the TiAlN coating against wear at 800 oC.
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| 5. |
- Moghaddam, Pouria Valizadeh
(författare)
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On the role of microstructure in wear of nanostructured carbide-free bainitic steels
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The significance of steel production for the development of sustainable society and economy is immense. Today’s fast growing global economy poses an increasing demand for improving the properties of steel. The development of nanostructured carbide-free bainitic steel with an excellent combination of strength and toughness is an attempt to satisfy this global demand. During austempering, the precipitation of cementite can be suppressed by addition of approximately 1.5 wt% silicon and a duplex microstructure comprising of extremely fine aggregates of retained austenite and bainitic ferrite can be obtained. Owing to their excellent mechanical properties, these novel steels exhibit considerable potential to replace quenched and tempered bearing steel or pearlitic rail steel. In these applications, wear play a crucial role in determining the performance of the components. However, the majority of studies have been limited to mechanical properties of these steels but not much attention has been paid to their tribological behaviour. Notably, the role of retained austenite and bainitic ferrite on wear performance has not yet fully understood. Furthermore, machining process is an inevitable step in the manufacturing of metal products. During the machining, contact temperature can rise to several hundred degrees. However, high temperature tribological behaviour of these steels in conjunction with coated cutting tool has not yet been addressed. Therefore, the aim of the present research work is to gain a deeper understanding of the correlation between microstructure and tribological performance of carbide-free bainitic steels in various conditions.To achieve this aim, tribological behaviour of nanostructured carbide-free bainitic steels has been investigated under dry rolling/sliding, sliding and two-body abrasive wear conditions. A number of steel grades were austempered under a wide range of temperatures and durations to obtain different carbide-free bainitic microstructures. The results have been compared with that of quenched and tempered bearing steel. Moreover, high temperature tribological behaviour of carbide-free bainitic and 316L stainless steels during interaction with TiAlN PVD coating has also been studied under dry reciprocating sliding condition.The results show that a relatively higher retained austenite content and its stability enhance wear resistance under rolling/sliding condition. Moreover, wear performance of carbide-free bainitic steel has been found to be superior to that of the quenched and tempered bearing steel. However, under sliding condition, the effect of retained austenite on wear heavily depends on sliding speed. At low sliding speed and under adhesive-dominated wear condition, higher retained austenite content results in improved wear performance due to the higher work hardenability. In contrast, at high sliding speed where oxidative wear is dominant, a microstructure with the lowest content of retained austenite exhibits the highest wear resistance. The higher amount of bainitic ferrite provides a hard underlying substrate for a thin and mechanically stable compositional mixed layer and thereby enhances the wear resistance. The formation and microcracking of a brittle white etching layer of quenched and tempered steel is responsible for its inferior wear resistance. Under two-body abrasive wear conditions, higher retained austenite content leads to improved wear resistance. A microstructure providing an optimum combination of hardness and toughness shows the best abrasive wear resistance. During reciprocating sliding, the tribological response of carbide-free bainitic steel is altered with increasing temperature. Higher temperature results in severe material transfer from carbide-free bainitic steel to TiAlN coated cemented carbide. Furthermore, at elevatedtemperature, a porous oxide layer grows on the transferred materials and reduce friction coefficient.In summary, there is no simple and general relationship between microstructure and wear resistance. Depending upon the dominant wear mechanisms and operating conditions, retained austenite and bainitic ferrite affect the wear behaviour in radically different manners.
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| 6. |
- Moghaddam, Pouria Valizadeh, et al.
(författare)
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The role of retained austenite in dry rolling/sliding wear of nanostructured carbide-free bainitic steels
- 2019
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 428-429, s. 193-204
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Tidskriftsartikel (refereegranskat)abstract
- The dry rolling/sliding wear of nanostructured bainite has been investigated and compared with that of a conventional quenched and tempered bearing steel. In order to elucidate the role of retained austenite on the wear performance, high silicon hypereutectoid bearing steel with an identical alloy composition was heat treated to obtain different microstructures with similar hardness and different amounts of retained austenite. The results indicate that the nanostructured bainite can meet the minimum hardness requirements for bearing applications. Moreover, the nanostructured bainite outperformed the tempered martensitic steel in terms of wear resistance. The work hardening capacity and thus wear resistance increases due to the transformation of retained austenite into martensite. The results of XRD analyses show that the higher stability of retained austenite and strength of bainitic ferrite leads to better wear performance. It is demonstrated that the stability of retained austenite outweigh the influence of retained austenite content on wear resistance. Adhesion and oxidation were identified as the main wear mechanisms. In addition to microstructure, surface oxidation also plays a prominent role in determining the wear resistance.
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