| 1. |
- Xiang, Shengmei, 1991-
(författare)
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Oxidation, Creep and Fatigue Synergies in Cast Materials for Exhaust Manifolds
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The future development of engines of heavy-duty vehicles towards reduced CO2 emission will increase the exhaust gas temperature and render the exhaust atmosphere more corrosive. The current service material of exhaust gas components - a ferritic ductile cast iron called SiMo51 - will soon meet its upper-temperature limit. Three alternative materials were suggested in a previous study: SiMo1000 (ferritic, nodular cast iron), D5S (austenitic, nodular cast iron), and HK30 (austenitic, cast steel). Together with SiMo51 (reference) the alternative materials are investigated in the present thesis with respect to performance and degradation mechanisms, under the individual or collective influence of high-temperature fatigue, corrosion, and creep.Firstly, fatigue, corrosion and corrosion-fatigue at 800oC were studied to establish their degradation mechanisms and relative performance. The individual influence of fatigue and corrosion was studied using low-cycle fatigue (LCF) tests in argon, and oxidation tests in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2(bal.)), respectively. The collective influence of fatigue and corrosion was studied using LCF test in the synthetic exhaust gas. The degradation mechanisms were analyzed through extensive characterization of the tested specimens. Different crack-initiation mechanisms were found for the various combinations of materials and atmospheres. In argon, crack initiation was generally caused by secondary phases at the surface (graphite in SiMo51/SiMo1000, graphite and intermetallics in D5S) and near-surface casting defects (in all materials). In the exhaust atmosphere, crack initiation was generally influenced by oxide intrusions (formed from oxidized graphite in SiMo51 and expressed as dendrite boundary corrosion in HK30), internal fracture of intermetallics (in D5S), decarburization creating microcracks/stress concentrations (in SiMo1000), and near-surface casting defects (in all materials). The relative performance was analyzed using fatigue and oxidation curves.Secondly, two improvements were attempted for SiMo1000, a modified casting geometry for improved graphite morphology and a surface treatment method, nitrocarburizing. The first attempt resulted in significantly reduced decarburization, decreased initial crack size formed by graphite/matrix debonding and an improved corrosion-fatigue life of 8 to 16 times. The second attempt resulted in two types of microcracks after the process and a self-sustained growth of the diffusion layer, when subjected to high-temperature corrosion. A strong corrosion-fatigue synergy was found, reducing the fatigue lifetime by 84-89%.Thirdly, the collective influence of fatigue and creep was studied for D5S using regular LCF tests (reference) and creep-fatigue tests, with either tension or compression dwell. Both dwell directions reduce fatigue life but promote different creep-fatigue-corrosion interactions. Tension dwell produces a clear creep-fatigue synergy, generating creep pinholes near graphite nodules. Typically, such damage is observed in regular creep tests of several months. Compression dwell decreases lifetime more than tension dwell due to increased peak tensile stress and a more pronounced surface crack initiation by an oxide wedging mechanism.The investigation in the present study gives a better understanding of the correlation between microstructure and corrosion-fatigue/creep-fatigue properties in materials used for exhaust gas components. Moreover, the combination of fatigue tests in argon/exhaust atmosphere, oxidation tests in the exhaust atmosphere, creep-fatigue tests, and creep tests from a previous study shows how corrosion, fatigue, and creep individually and synergistically affect the material performance at elevated temperature.
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| 2. |
- Freitas de Abreu, Marcio
(författare)
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Cavitation Erosion Mechanisms in Cast Irons
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research presented in this thesis investigated the mechanisms by which cavitation erosion damage develops in lamellar graphite iron (LGI) and austempered ductile irons (ADIs). This has been achieved by image sequences of surface erosion on test samples in tandem with weight change measurements. Cavitation erosion is caused by the appearance and collapse of bubbles in a liquid which undergoes rapid pressure oscillations. Imploding bubbles release heat, shockwaves and high-speed microjets which may strike nearby solid walls and damage them.The heavy-duty automotive industry encounters this problem in the engine cooling system. The combustion chamber requires precise temperature control for optimal operation and excess heat must be removed by a liquid coolant. In trucks, the coolant liquid achieves this by circulating around the cylinder liner, a hollow cylindrical part that encloses the combustion chamber and prevents its gases from escaping. However, the engine’s intense vibrations create repeated pressure variations in the coolant, and bubbling ensues. With prolonged operation, the cylinder liner’s wet outer wall may be severely worn, resulting in surface roughening, eroded patches and pits. Cavitation is responsible for great losses due to vehicle downtime and maintenance costs. The present work aims, therefore, at analyzing the behavior under cavitation exposure of cast irons that are currently used, or being considered for use, in the cooling system.Cylinder liners are currently made of lamellar graphite iron with a matrix structure consisting of pearlite and a network of steadite, and the analysis for this material has been presented in Paper 1. Austempered ductile irons are candidate materials for pumps and other components of the cooling system due to their very good mechanical properties; three ADIs of increasing hardness, obtained from different heat treatments of a spheroidal graphite iron, have been analyzed in Paper 2. Experiments consisted of an ultrasonic vibratory probe to which material samples were attached and subsequently immersed in a beaker containing engine coolant. The samples were weighed and photographed in an SEM after several predetermined time intervals. This produced a detailed sequence of images which, in combination with mass loss data, can explain the mechanisms by which cavitation damage initiates and develops in these materials. The text of this thesis summarizes the findings presented in the appended articles and compares the behavior of LGI and ADI.
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| 3. |
- Öberg, Christian, 1989-
(författare)
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Creep Behavior of High Temperature Cast Materials for Exhaust Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on creep of four cast materials intended for exhaust manifolds in heavy-duty truck engines. Two of the materials are ferritic ductile cast irons, SiMo51 and SiMo1000, one is an austenitic ductile cast iron, D5S, and another one is an austenitic cast steel, HK30. The ductile cast irons, rich in carbon, have a microstructure with graphite nodules and precipitates, mainly carbides and intermetallics. The cast steel, on the other hand, being meagre in carbon, has precipitates but lacks graphite nodules. During service, the exhaust components are thermally cycled up to 800 °C in a locked stated, bolted to an engine block. This gives rise to creep deformation, fatigue, oxidation and microstructural changes. Driven by the development of environmental friendly engines of lower emissions, the exhaust gas temperature is increasing, continuously leading to higher demands on the materials.The main aim was to investigate the creep behavior and related phenomena of the included materials. A secondary aim was to compare results from three types of tests, i) SRTC (stress relaxations with thermal cycling), provoking stress relaxations in a locked specimen subjected to thermal cycling, ii) STT (sequential tensile test), changing the strain rate at selected strain levels during a tensile test at a selected temperature, iii) CL (constant-load creep test), i.e. traditional creep testing, applying a constant load at a given temperature. SRTC and STT are intended as quick and cheap methods while CL is generally considered slow and associated with high costs. Results of the three methods were regularly compared in Norton plots, i.e. double logarithmic plots of stress and strain rate.Results of i) SRTC (in compression) and ii) STT (in tension) were generally in very close agreement which indicates that creep of the included materials is independent of loading direction. In addition, the creep rates obtained by SRTC were also constant with number of cycles. Both findings facilitate modeling of cyclic creep, although this was not in the scope of the present thesis. There were discrepancies between data sets of CL and SRTC/STT which could not be explained, although several reasons were discussed. In addition, the time-dependent creep damage which develops during a slow CL test is always missed in quick stress relaxation tests or tensile tests.The microstructural events taking place during creep were documented using LOM, SEM and EBSD microscopy techniques, with various etching and sample preparation procedures.When CL tested at 700 °C, SiMo51 showed primary creep, more or less directly followed by tertiary creep. The tertiary creep regime was in turn divided into two stages of which the first was associated with the formation of typical creep cavities around the graphite nodules and at the grain boundaries, and the second associated with larger cracks between the graphite nodules. Oxidation was significant but not enough to be held responsible for the tertiary creep stages. The oxidation on the surface and around the graphite nodules was explicitly studied. Layered oxides were identified by combining EDX data with thermodynamic calculations.Both D5S and HK30 were CL tested at 750 °C, reflecting a higher service temperature of these materials compared with SiMo51. After prolonged creep exposure, HK30 exhibited typical creep cavitation at the grain boundaries, precipitation of sigma phase and G phase, oxide intrusions and recrystallization in a thin layer at the specimen surface. D5S exhibited various types of cavities/voids around the graphite nodules (like SiMo51 at 700 °C) and fracture occurred by shear cracks growing nodule-to-nodule. Various precipitates developed during creep.
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| 4. |
- Arora-Jonsson, Stefan, Professor, 1969-, et al.
(författare)
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A new understanding of competition
- 2021
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Ingår i: Competition. - Oxford : Oxford University Press. - 9780191924460 - 9780192898012 ; , s. 1-25
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Competition is currently found in many, if not most, social domains, such as the economy, politics, public services, sports, culture, higher education, and science. But competition is not endemic to any of these fields. Rather, it has been constructed by those involved or by observers. We ask what competition is and how it can be introduced into a new context. Critically reflecting on insights from economics, management studies and sociology, we define competition as a combined social construction of four factors: actorhood, relationships among actors, scarcity, and desire. We shed light on asymmetric constructions of competition among different actors. Our definition leaves open the effect of competition on behaviour: competition may lead competitors to avoid interaction with each other or to cooperate; it may motivate people to try harder or it may demotivate them. Finally, we discuss issues for further research that follow from our understanding of competition.
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| 5. |
- Arora-Jonsson, Stefan, Professor, 1969-, et al.
(författare)
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Competition unbundled : taking stock and looking forward
- 2021
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Ingår i: Competition. - Oxford : Oxford University Press. - 9780192898012 - 9780191924460 ; , s. 221-236
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In this final chapter, we discuss how the preceding chapters illuminate some fundamental questions about competition. How is it constructed? How are the behaviours resulting from competition managed? What are the consequences of competition? How can competition be removed? And, how do these factors vary with the good people compete for? Our aim is to provide an outline of a social science research programme on competition that is long overdue. A central message of the book is that competition seems ubiquitous but that it should not be taken for granted or be naturalized as an inevitable aspect of human existence. Its emergence, maintenance, and change are often the result of intention and purposive efforts, and a central challenge for social science is to learn more about these developments.
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| 6. |
- Arora-Jonsson, Stefan, Professor, 1969-, et al.
(författare)
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The Origins of Competition : institution and organization
- 2021
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Ingår i: Competition. - Oxford : Oxford University Press. - 9780192898012 - 9780191924460 ; , s. 61-76
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Institutional conditions and organizational efforts are key for understanding how and why the elements constituting competition come about. Several contemporary institutions favour the construction of competition. States, organizations, and individuals are increasingly being seen as actors that are able to compete with each other, and globalization and other recent institutional developments have increased the number of relationships that they can construct. An impressive economic development during the last century has not prevented a continued sense of scarcity and new desires have emerged, not least the desire for status positions. Fourth parties organize social life in ways that create competition by intention or unintentionally. Organizers of contests in politics, markets, sports and many other sectors construct actors, relationships, and scarcity for something desired. Similar efforts have been an essential part of many recent reforms of organizations and markets. The increasing number of prizes and rankings in almost any area of life produces scarcity and is likely to create new desires for goods that per definition are scarce.
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| 7. |
- Arora-Jonsson, Stefan, Professor, 1969-, et al.
(författare)
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Where Does Competition Come From? : The role of organization
- 2020
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Ingår i: Organization Theory. - : Sage Publications. - 2631-7877. ; 1:1, s. 1-24
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Tidskriftsartikel (refereegranskat)abstract
- Although an ever-increasing number and types of organizations are expected to compete, the origins of competition have been a neglected topic. By assuming that competition simply emerges, organization theory currently lacks an understanding of when and why organizations compete. In this article we critically review and extend existing literatures on competition to offer an organizational theorization of the origins of competition. We argue that competition is the social construction of its four constitutive elements: actors, relationships, scarcity and desire. Furthermore, we show that three types of actors – those who compete, those who adjudicate the competition, and those who have an interest in creating competition – can construct competition independently or in concert. We also discuss different types of organized competition; the role of rankers, prize givers and other actors interested in creating competition; and competition as an unintended consequence of organization. Finally, we outline future research on competition and organization that follows from our conceptualization, along with some normative implications.
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| 8. |
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| 9. |
- Bomark, Niklas, 1984-, et al.
(författare)
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Convincing Others That They are Competing : The Case of Schools
- 2021
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Ingår i: Competition. - Oxford : Oxford University Press. - 9780192898012 - 9780191924460
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Bokkapitel (refereegranskat)abstract
- The past decades have seen numerous attempts to introduce competition into new sectors of society, but we still know little about the processes by which competition is realized in a new setting. We study three decades of organizational efforts of a Swedish municipality that sought to introduce competition for students among its upper secondary schools following a national reform in the early 1990s. Our study shows that declaring competition was far from sufficient for its realization; the path to competition was lined with hesitation, uncertainty, and a rich variety of organizational challenges to be overcome. One particularly vexing challenge was to convince the principals of the schools that they should view each other as competitors for students. Our findings contribute to previous literature by demonstrating that competition need not be a prerequisite for choice; that several organizers of competition may operate at once; and, more generally, that competition is introduced through stepwise, piecemeal processes.
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| 10. |
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Competition : What it is and Why it Happens
- 2021
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Samlingsverk (redaktörskap) (övrigt vetenskapligt/konstnärligt)abstract
- The spread of competition into all areas of society is one of the master trends of modern society. Yet, social scientists have played a surprisingly modest role in the analysis of its implications as the discussion of competition has largely been confined to the narrow context of economic markets. This book opens up competition for the study of social scientists. The central message of the book is that competition seems ubiquitous but it should not be taken for granted or be naturalized as an inevitable aspect of human existence. Its emergence, maintenance, and change are based on institutions and organizational efforts, and a central challenge for social science is to learn more about these processes and their outcomes. With the use of a novel definition of competition, more fundamental questions can be addressed than merely whether or not competition works. How is competition constructed—and by whom? Which institutional and organizational foundations need to be considered? Which behaviours result from competition? What are its consequences? Can competition be removed? And, how do these factors vary with the object of competition—be it money, attention, status, or other scarce and desired objects? The chapters in the book investigate these and more questions in studies of competition among and within schools, universities, multinational corporations, auditors, waste-disposal firms, and fashion designers and users. The chapters are written by scholars from several social science fields: management, organization studies, sociology, anthropology, and education.
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