Oxidation, Creep and Fatigue Synergies in Cast Materials for Exhaust Manifolds
Time: Wed 2021-03-31 09.00
Subject area: Materials Science and Engineering
Doctoral student: Shengmei Xiang , Strukturer
Opponent: Professor Johan Ahlström, Chalmers University of Technology
Supervisor: Professor Joakim Odqvist, Strukturer; PhD Baohua Zhu, Scania AB; Professor Stefan Jonsson, Egenskaper
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.