| 1. |
- Fischer, Tim, et al.
(författare)
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Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell : A dislocation density-based crystal plasticity study
- 2022
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 860, s. 144212-
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Tidskriftsartikel (refereegranskat)abstract
- To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800 degrees C, a physics-based crystal plasticity model that describes the complex rate-and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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