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High-Temperature Corrosion-Fatigue of Cast Alloys for Exhaust Manifolds
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-3281-6835
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 45
Series
TRITA-ITM-AVL ; 2018:45
Keywords [en]
High temperature, corrosion fatigue, low-cycle fatigue, cast alloys, high-temperature oxidation, intermetallic phase
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-235170ISBN: 978-91-7729-944-8 (print)OAI: oai:DiVA.org:kth-235170DiVA, id: diva2:1248664
Presentation
2018-10-05, N111, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180917

Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-20Bibliographically approved
List of papers
1. High-Temperature Corrosion-Fatigue Behavior of Ductile Cast Irons for Exhaust Manifolds Applications
Open this publication in new window or tab >>High-Temperature Corrosion-Fatigue Behavior of Ductile Cast Irons for Exhaust Manifolds Applications
2018 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 925, p. 369-376Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Switzerland: Trans Tech Publications Inc., 2018
Keywords
corrosion fatigue, ductile cast iron, high-temperature oxidation, controlled atmosphere
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-235168 (URN)10.4028/www.scientific.net/MSF.925.369 (DOI)2-s2.0-85050033154 (Scopus ID)
Conference
11th International Symposium on the Science and Processing of Cast Iron, Jönköping, September 4-7, 2017
Note

QC 20180917

Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-17Bibliographically approved
2. Comparison of the high-temperature corrosion-fatigue resistance of cast alloys for exhaust manifolds
Open this publication in new window or tab >>Comparison of the high-temperature corrosion-fatigue resistance of cast alloys for exhaust manifolds
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keywords
Corrosion fatigue, ductile cast iron, cast stainless steel, high temperature, low-cycle fatigue, controlled atmosphere, sigma phase
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-235169 (URN)
Note

QC 20180918

Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-18Bibliographically approved

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