| 1. |
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| 2. |
- Kasedde, Hillary, 1984-
(författare)
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Characterization of Raw Materials for Salt Extraction from Lake Katwe, Uganda
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Uganda is well endowed with economic quantities of salt evident in the interstitial brines and evaporite deposits of Lake Katwe, a closed saline lake located in the western branch of the great East African rift valley. Currently, rudimentally methods of salt mining based on solar evaporation of brine continue to be used for salt extraction at the lake. These have proved to be hazardous and unsustainable to the salt miners and the environment. In this work, literature concerning the occurrence of salt and the most common available technologies for salt extraction is documented. Field studies were undertaken to characterize the salt lake deposit and to devise strategies of improving salt mining and extraction from the salt lake raw materials. The mineral salt raw materials (brines and evaporites) were characterized to determine their physical, chemical, mineralogical, and morphological composition through field and laboratory analyses. In addition, laboratory extraction techniques were undertaken to evaluate possibilities of future sustainable salt extraction from the lake deposit. Also, PHREEQC simulations using Pitzer models were carried out to determine the present saturation state of the lake brine and to estimate which salts and the order in which they precipitate from the brine upon concentration by evaporation.Results reveal that the raw materials from the salt lake contain substantial amounts of salt which can be commercialized for optimum production. The brines are highly alkaline and rich in Na+, K+, Cl-, SO42-, CO32-, and HCO3-. Moreover, they contain trace amounts of Mg2+, Ca2+, Br-, and F-. The lake is hydro-chemically of a carbonate type with the brines showing an intermediate transition between Na-Cl and Na-HCO3 water types. The evaporites are composed of halite mixed with other salts such as hanksite, burkeite, trona etc, with their composition varying considerably within the same grades. The laboratory extraction experiments indicate that various types of economic salts such as thenardite, anhydrite, mirabilite, burkeite, hanksite, gypsum, trona, halite, nahcolite, soda ash, and thermonatrite precipitate from the brine of Lake Katwe. The salts crystallize in the order following the sequence starting with sulfates, followed by chlorides and carbonates, respectively. Moreover, thermodynamic modeling in PHREEQC accurately predicted the solubility and sequence of the salt precipitation from the lake brine. Understanding the sequence of salt precipitation from the brine helps to control its evolution during concentration and hence, will lead to an improved operating design scheme of the current extraction processes. The work providesinformation towards future mineral salt exploitation from the salt lake.
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| 3. |
- Xiang, Shengmei, 1991-, et al.
(författare)
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Comparison of the high-temperature corrosion-fatigue resistance of cast alloys for exhaust manifolds
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The corrosion fatigue behavior, fatigue resistance and corrosion resistance of four materials used in exhaust manifolds have been evaluated through conducting low-cycle fatigue tests at 800oC in synthetic exhaust and argon atmospheres as well as oxidation tests in the exhaust atmosphere. The four materials are two ferritic ductile cast irons SiMo51 and SiMo1000, one austenitic ductile cast iron Ni-resist D5S and one austenitic cast stainless steel HK30. Quantitative material rankings were given for the four materials in the two atmospheres through comparing the corresponding e-N and S-N curves. The crack growth mechanisms of the austenitic materials were studied in detail in the paper. Brittle intermetallic phases have been found in both materials but observed to have different levels of impact on their corrosion fatigue properties. The Ni-, Mn-, Si-rich intermetallic phase found in D5S greatly affects both fatigue and corrosion behavior of this material. However, the Fe-,Cr-rich sigma phase shows an insignicant impact as the crack growth path only occasionally follow the distribution pattern. In addition, severe oxide spallation was found in D5S leading to a rapid weight loss during the oxidation test.
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| 4. |
- 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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| 5. |
- Xiang, Shengmei, 1991-, et al.
(författare)
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High-Temperature Corrosion-Fatigue Behavior of Ductile Cast Irons for Exhaust Manifolds Applications
- 2018
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Ingår i: Materials Science Forum. - Switzerland : Trans Tech Publications Inc.. - 0255-5476 .- 1662-9752. ; 925, s. 369-376
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, low-cycle fatigue (LCF) tests and oxidation tests in controlled atmospheres are carried out at 800ºC on two ductile cast irons SiMo51 and SiMo1000. The LCF tests are conducted in argon and synthetic exhaust gas, whereas oxidation tests are carried out in the latter atmosphere. S-N curves and weight-gain curves are presented. The crack growth mechanisms and oxidation mechanisms are investigated, as well as the synergistic effects. A surprising finding of increased fatigue resistance in the oxidizing atmosphere is partly explained.
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| 6. |
- Xiang, Shengmei, 1991-
(författare)
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High-Temperature Corrosion-Fatigue of Cast Alloys for Exhaust Manifolds
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The introduction of gas-driven Otto engine and the corresponding usage of bio-fuels in heavy-duty engines will render the exhaust atmosphere more corrosive and bring a higher working temperature to exhaust manifolds. The current service material, a ferritic ductile cast iron called SiMo51, will soon meet its upper temperature limit set by the ferrite-austenite transformation at 860ºC. Three alternative materials, as well as SiMo51 serving as reference, are investigated in the present thesis emphasizing on high-temperature corrosion fatigue. The first aim of this study is to obtain material data and give a quantitative ranking of the materials’ performance. Low-cycle fatigue (LCF) tests at 800ºC in a synthetic exhaust gas (5%O2-10%CO2-5%H2O-1ppmSO2-N2 bal.) are conducted to evaluate the materials’ performance in simulated real working scenarios, where high-temperature, corrosive atmosphere and fatigue conditions during testings are similar to the conditions experienced by the exhaust manifolds. To evaluate the individual effect from high-temperature fatigue and isolate the impact from corrosion, the materials are tested under the same settings but in an argon atmosphere. To evaluate the individual effect from high-temperature corrosion and isolate the impact from mechanical deformation, oxidation tests are carried out at 800ºC in the same synthetic exhaust gas. The second aim is to identify and understand different oxidation behavior and failure mechanisms in the materials, realized by considerable characterizations of the tested specimens.From the fatigue tests, it is found that the austenitic stainless steel HK30 has the highest fatigue resistance, followed by the austenitic cast iron Ni-resist D5S, and the ferritic ductile cast irons SiMo1000 and SiMo51, a ranking valid in both atmospheres. In the exhaust atmosphere, for instance, the improvement in fatigue strength at 15,000 cycles relative to SiMo51 are 260%, 194% and 26%, respectively. Different crack initiation and propagation mechanisms are found for the various combinations of materials and atmospheres. In the exhaust atmosphere, for instance, crack initiation is assisted by oxide intrusion in SiMo51 and crack propagation is affected by crack branching in HK30, mechanisms not observed in argon. By comparing the S-N fatigue curves in the two atmospheres, the influence of oxidation on fatigue life is evaluated. The fatigue life of the cast irons are surprisingly found to be higher in the exhaust atmosphere. Several explanations are suggested for this, considering their very different oxidation behaviors. This study provides accurate test data that can be used to help industry avoid over-dimensioned design. The investigation of the failure mechanisms promotes better understanding of the correlation between microstructure and mechanical properties. Moreover, the combination of fatigue tests in argon, fatigue tests in exhaust and oxidation tests in exhaust, shows how corrosion and fatigue individually and synergistically affect the materials’ performance at high temperature.
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| 7. |
- 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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| 8. |
- 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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| 9. |
- 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
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Low-cycle fatigue tests and creep-fatigue tests with either tension or compression dwell were conducted on the austenitic cast iron Ni-resist 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. Distinct creep-oxidation-fatigue interactions are observed from the microstructural characterization of the creep-fatigue tested specimens. The intermetallics in the untested sample contain G phase, Ni31Si12, α-Fe, and M7C3. χ phase was found after LCF tests.
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| 10. |
- 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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| 11. |
- 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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| 12. |
- Zhu, Baohua, et al.
(författare)
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Comparison of electrochemical and surface characterisation methods for investigation of corrosion of bipolar plate materials in molten carbonate fuel cell Part I. Electrochemical study
- 1999
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Ingår i: Corrosion Science. ; 41:8, s. 1497-1513
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Tidskriftsartikel (refereegranskat)abstract
- The corrosion of bipolar plate and current collector materials in molten carbonate fuel cells (MCFC) was investigated. Tafel extrapolation, linear polarisation resistance, chronopotentiometry and electrochemical impedance spectroscopy (EIS) were used to determine corrosion rates of iron and iron-based alloys in molten carbonate melts for both cathode and anode MCFC environments. During the initial stages, agreement among the methods was good but could have differed after a corrosion layer formed on the surface. The corrosion rate for a fully immersed sample at the open circuit potential seemed to be controlled by external mass transfer of water. Iron-chromium alloys exhibit reasonable corrosion resistance on the cathode side, provided that the chromium concentration exceeds 25%. The corrosion of bipolar plate and current collector materials in molten carbonate fuel cells (MCFC) was investigated. Tafel extrapolation, linear polarisation resistance, chronopotentiometry and electrochemical impedance spectroscopy (EIS) were used to determine corrosion rates of iron and iron-based alloys in molten carbonate melts for both cathode and anode MCFC environments. During the initial stages, agreement among the methods was good but could have differed after a corrosion layer formed on the surface. The corrosion rate for a fully immersed sample at the open circuit potential seemed to be controlled by external mass transfer of water. Iron-chromium alloys exhibit reasonable corrosion resistance on the cathode side, provided that the chromium concentration exceeds 25%.
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| 13. |
- Zhu, Baohua, et al.
(författare)
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Comparison of electrochemical and surface characterisation methods for investigation of corrosion of bipolar plate materials in molten carbonate fuel cell Part II. Surface analysis
- 1999
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Ingår i: Corrosion Science. ; 41:8, s. 1515-1528
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Tidskriftsartikel (refereegranskat)abstract
- The corrosion of bipolar plate materials in molten carbonate fuel cells (MCFC) was studied by electrochemical techniques. These methods were complemented by thermogravimetric analysis (TGA) and surface analyses, such as SEM, EDX and XRD. The concentration profiles of oxygen, lithium, iron, chromium and nickel in the oxide scales were determined by means of glow discharge optical emission spectroscopy (GDOES). It has been shown that corrosion layers, formed on the iron-chromium and stainless steels surfaces, consist of two layers. The outer layer is porous and has large pores; the inner layer is more compact and may contain small pores. The corrosion of bipolar plate materials in molten carbonate fuel cells (MCFC) was studied by electrochemical techniques. These methods were complemented by thermogravimetric analysis (TGA) and surface analyses, such as SEM, EDX and XRD. The concentration profiles of oxygen, lithium, iron, chromium and nickel in the oxide scales were determined by means of glow discharge optical emission spectroscopy (GDOES). It has been shown that corrosion layers, formed on the iron-chromium and stainless steels surfaces, consist of two layers. The outer layer is porous and has large pores; the inner layer is more compact and may contain small pores.
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| 14. |
- Zhu, Baohua
(författare)
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Corrosion of current cullector materials in the molten carbonate fuel cell
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The corrosion of current collector materials in MoltenCarbonate Fuel Cells (MCFC) is investigated. The essential aimsof this investigation were to study the corrosion behaviour ofdifferent materials, in varying cathode and anode MCFCenvironments, and to study the contact corrosion resistancesbetween the MCFC current collector and electrodes. For thesepurposes, pure iron, iron-chromium binary alloys and severalcommercial steels were investigated in molten carbonate meltswithin the pot-cell laboratory set-up. In addition, the contactcorrosion resistances, between an AISI 310 current collectorand two cathodes (NiO and LiCoO2), were studied in a laboratory fuel cell.Post-tests were done to study the corrosion products formed atthe surfaces.In cathode environments, corrosion potential increased overtime as a protective corrosion layer slowly formed. Eventually,the potential reached a stable value close to the cathodeoperating potential. The main cathode reaction, as corrosionpotential increased, changed from water reduction to oxygenreduction. Corrosion rate under the operating cathode conditiondepended on the chromium content; the higher the concentrationof chromium, the lower the corrosion rate. The corrosion ratesof ferritic steels, with high chromium content, and AISI 310were higher at the so-called outlet operating condition incomparison to the standard and so-called inlet conditions. Thecorrosion rate was higher at the beginning of the exposure,which resulted in a relatively fast corrosion layer growth thatslowed as the protective layer was formed. It was shown thatthe corrosion layers, formed on iron-chromium alloys, AISI 310and ferritic high chromium-containing steels, consisted of twolayers. The outer layer was porous and iron rich, while theinner layer was quite compact and rich in chromium and/oraluminiumTherefore, the corrosion behaviour was dependent onthe corrosion layer structure at the metal surface.In anode environments, the beneficial behaviour of aluminiumin ferritic alloys, with high aluminium contents, was due tothe formation of aluminium oxide and/or lithium aluminium oxideat the surface. The corrosion rates at the standard and outletconditions were of the same order of magnitude, while thecorrosion rates at the inlet conditions were considerablyhigher. The lower temperatures and higher carbon dioxideconcentrations in the inlet conditions appeared to result in asurface layer deficient in aluminium. A modified theoreticalmodel was developed to evaluate the corrosion current densitiesfrom experimental polarisation curves or linear polarisationresistance measurements in anode environments. The fittingswere found to be very good.An experimental method was developed forin-situmeasurements of the contributions to the totalohmic losses at the cathode in a laboratory scale MCFC. Thecontact resistance between the cathode and current collectorcontributed quite a large value to the total cathodepolarization. The corrosion layer, formed between the LiCoO2cathode and AISI 310 current collector, wasiron-rich and more porous, and contained a small amount ofcobalt. This was deemed to consist of a two-phase oxide, whichresulted in a lower conductivity. The corrosion layer, formedbetween the NiO cathode and AISI 310 current collector, wasrich in nickel. The corrosion layers on the AISI 310, incontact with the cathode, had a different composition comparedto samples immersed in carbonate melts.Key words: molten carbonate fuel cell (MCFC), corrosion,current collector, contact corrosion resistance.
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| 15. |
- 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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| 16. |
- 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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| 17. |
- Öberg, Christian, et al.
(författare)
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Creep behaviour, creep damage and precipitation in the austenitic ductile cast iron D5S at 750 °C
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The austenitic ductile cast iron D5S, used for exhaust manifolds in heavy-duty truck engines, was creep tested at 750 °C in air under constant load (CL) with initial stresses of 18.5-50 MPa. Four of the specimens (20-50 MPa) were tested to rupture (22-5476 h) and the remaining one (18.5 MPa) was interrupted in the secondary creep regime after 6494 h. Secondary creep was generally predominant at lower stresses whereas primary and tertiary creep were predominant at higher stresses. The damage was characterized by various types of voids/creep cavities around the graphite nodules which were interlinked by shear cracks upon fracture. The results show that significant amounts of finely dispersed Cr7C3 carbides, larger Fe2Ni2Si particles, and an unknown carbide precipitated during creep exposure. Dislocation substructures which had developed during creep were revealed through EBSD.
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| 18. |
- Öberg, Christian, et al.
(författare)
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Monotonic and Cyclic Creep of Cast Materials for Exhaust Manifolds
- 2019
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Ingår i: SAE International Journal of Materials & Manufacturing. - : SAE INT. - 1946-3979 .- 1946-3987. ; 12:2, s. 149-162
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Tidskriftsartikel (refereegranskat)abstract
- Cast materials are creep tested between 600 and 900 degrees C using three methods: (i) tensile testing at different strain rates, (ii) stress relaxation during thermal cycling and (iii) traditional creep tests at constant load. Comparisons are made between fast and slow methods and between monotonic and cyclic deformation modes. The tested materials, SiMo51, SiMo1000, Ni-resist D5S and HK30, are used for exhaust manifolds in heavy-duty diesel engines. The fast and cheap methods, (i) and (ii), were used on all materials, while the tedious and costly method, (iii), was used on SiMo51 only. The creep rates from monotonic tensile tests and stress relaxations during thermal cycling agree well. There is no difference between monotonic and cyclic creep rates, and cyclic rates are practically unchanged with the number of thermal cycles. No or small differences in creep rates are observed when comparing tension and compression, although three of the materials include large graphite nodules. At 700 degrees C, a Norton plot for SiMo51 shows coinciding results for tensile test and compressive stress relaxations, whereas the minimum creep rates from constant load tests fall one order of magnitude lower, but with the same slope. For all materials, the Norton creep parameters are evaluated with accurate reproduction of the experimental data. For HK30, two sets of parameters are needed because of deformation hardening.
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| 19. |
- 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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