| 1. |
- Xiang, Shengmei, 1991-
(författare)
-
Oxidation, Creep and Fatigue Synergies in Cast Materials for Exhaust Manifolds
- 2021
-
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.
|
|
| 2. |
- Öberg, Christian, 1989-
(författare)
-
Creep Behavior of High Temperature Cast Materials for Exhaust Applications
- 2020
-
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.
|
|
| 3. |
- Kasedde, Hillary, 1984-
(författare)
-
Characterization of Raw Materials for Salt Extraction from Lake Katwe, Uganda
- 2013
-
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.
|
|
