| 1. |
- Xiang, Shengmei, et al.
(författare)
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Corrosion fatigue of austenitic cast iron Ni-Resist D5S and austenitic cast steel HK30 in argon and synthetic diesel exhaust at 800 degrees C
- 2020
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Ingår i: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 132
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Tidskriftsartikel (refereegranskat)abstract
- To increase the understanding of the corrosion-fatigue mechanisms in two cast iron-alloys used in exhaust manifolds, low-cycle fatigue tests at 800 degrees C in argon and synthetic diesel exhaust as well as isothermal oxidation tests in the exhaust atmosphere are conducted. The corrosion impacts on the fatigue life of the materials are quantitatively evaluated from comparing the G-N curves, and examined from characterization through SEM, EDX, EBSD and EPMA. The materials show very different behaviors to the synergistic effect of corrosion and fatigue. Different theories have been suggested based on the findings.
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| 2. |
- Xiang, Shengmei, et al.
(författare)
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Influence of ferritic nitrocarburizing on the high-temperature corrosion-fatigue properties of the Si-Mo-Al cast iron SiMo1000
- 2021
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123 .- 1879-3452. ; 143
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Tidskriftsartikel (refereegranskat)abstract
- The influence of ferritic nitrocarburizing on the high-temperature corrosion-fatigue properties of the Si-Mo-Al cast iron SiMo1000 is investigated in the present study. It was found that nitrocarburizing effectively increases the surface hardness, but dramatically decreases the fatigue and oxidation resistance of SiMo1000. The fatigue resistance is reduced due to two types of microcracks formed after nitrocarburizing. The oxidation resistance is dramatically diminished due to the formation of microcracks, and the depletion of aluminum in the matrix from nitride precipitation during the exposure at 800 degrees C. The corrosion-fatigue synergy is found to cause severe decarburization (i.e. graphite depletion) in nitrocarburized SiMo1000.
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| 3. |
- Xiang, Shengmei, et al.
(författare)
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Influence of graphite morphology on the corrosion-fatigue properties of the ferritic Si-Mo-Al cast iron SiMo1000
- 2020
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123 .- 1879-3452. ; 140
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Tidskriftsartikel (refereegranskat)abstract
- The influence of graphite morphology on the corrosion-fatigue mechanism in the cast compacted graphite iron SiMo1000 is investigated. Two batches of SiMo1000 with the same chemical composition but different nodularity are tested using low-cycle-fatigue tests in argon and a synthetic exhaust atmosphere at 800 degrees C. Decreased graphite nodularity is found to significantly reduce the corrosion resistance of SiMo1000, causing severe decarburization on the sample surface. Besides, the fatigue life is also dramatically reduced with decreased graphite nodularity. The synergy of decarburization and fatigue is found to reduce fatigue life in one of the batches.
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| 4. |
- Xiang, Shengmei, et al.
(författare)
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Influence of tension and compression dwell on the creep-fatigue properties of the austenitic cast iron Ni-resist D5S
- 2021
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 814
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Tidskriftsartikel (refereegranskat)abstract
- Creep-fatigue tests with either tension or compression dwell and reference low-cycle fatigue test without dwell were conducted on the austenitic cast iron D5S in atmospheric air at 800oC to investigate the influence of dwell time on lifetime and the corresponding failure mechanisms. The addition of tension or compression dwell reduces the lifetime, by up to 50% and 80% in the tested range, respectively. Compared with tension dwell, compression dwell is found to be more detrimental and could further reduce the lifetime by up to 60%. Tension and compression dwell are seen to cause a thicker and thinner gauge length, respectively. Tension dwell causes the formation of creep pinholes at the graphite/matrix interface and subgrain boundaries, leading to the formation of internal microcracks. Compression dwell results in forming large cavities at the graphite/matrix interface. The intermetallics in an untested sample are found to contain G phase, Ni31Si12, ?-Fe, and M7C3. The intermetallics in an LCF-tested sample are found to contain G phase, Ni31Si12, M7C3, and ? phase. Cracks inside intermetallics are found to form at the interfaces between Ni31Si12/? phase, M7C3/? phase, and Ni31Si12/M7C3.
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| 5. |
- 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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| 6. |
- 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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| 7. |
- Öberg, Christian, et al.
(författare)
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Creep behaviour, creep damage and precipitation in the austenitic cast steel HK30 at 750 degrees C
- 2020
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 797, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- The austenitic cast steel HK30, used in exhaust manifolds of heavy-duty truck engines, was studied with respect to creep, creep damage, oxidation and precipitation at 750 ◦C in air. Even though creep clearly affects the lifetime of the manifolds, there are very few studies available on the topic. The results of the present study provide a basis for materials selection as well as creep data input to component lifetime simulations. In the present study, five specimens were subjected to constant loads resulting in creep rupture times between 1 and 94 days. The minimum creep rate follows Norton’s creep law with a Norton parameter (n) of 8.9 and the logarithmic creep rupture time increases linearly with reduced stress. At low stresses secondary creep is predominant observing oxide intrusions from the specimen surface, precipitation of sigma and G-phases, creep cavitation at grain boundaries and weak dislocation networks. At high stresses direct transitions from primary to tertiary creep are observed as well as stronger dislocation networks, but no oxide intrusions, no precipitation, or cavitation are observed. The material has grains of cm size and intergranular fracture is predominant at all stress levels. The segregation of elements during solidification was studied by Scheil/Gulliver simulations and precipitations of sigma and G-phases in the last solidified regions were predicted using Thermo-Calc.
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| 8. |
- Öberg, Christian, 1989-, et al.
(författare)
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Plastic deformation and creep of two ductile cast irons, SiMo51 and SiMo1000, during thermal cycling with large strains
- 2024
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Ingår i: Materials Science Forum. - Switzerland : Trans Tech Publications Inc.. - 0255-5476 .- 1662-9752.
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Tidskriftsartikel (refereegranskat)abstract
- Cyclic deformations of two ferritic, ductile cast irons, SiMo51 and SiMo1000, were studied in air and Ar using a new method, SRTC (stress relaxation with thermal cycling). Locked specimens were thermally cycled up to 800 °C with isothermal holds, varying temperature interval, heating/cooling rates and hold times. A description of the mechanical response to thermal cycling of a locked specimen is given.
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