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Oxidation and corrosion fatigue aspects of cast exhaust manifolds
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Mechanical Metallurgy.ORCID iD: 0000-0001-5212-2227
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Emission regulations for heavy-duty diesel engines are becoming increasingly restrictive to limit the environmental impacts of exhaust gases and particles. Increasing the specific power output of diesel engines would improve fuel efficiency and greatly reduce emissions, but these changes could lead to increased exhaust gas temperature, increasing demands on the exhaust manifold material. This is currently the ferritic ductile cast iron alloy SiMo51, containing about 4 wt% Si and ~1 wt% Mo, which operates close to its fatigue and oxidation resistance limits at peak temperature (750C). To ensure high durability at higher temperatures, three different approaches to improving the life of exhaust manifolds were developed in this thesis.

The first approach was to modify SiMo51 by adding different combinations of Cr and Ni to improve its high-temperature strength and oxidation resistance, or by applying a thermal barrier coating (TBC) to reduce the material temperature and thereby improve fatigue life. In the second approach, new materials for engine components, e.g. austenitic ductile iron and cast stainless steel, were investigated for their high-temperature fatigue and oxidation properties. In order to identify the most suitable alloys for this application, in the third the environmental effects of the corrosive diesel exhaust gas on the fatigue life of SiMo51 were investigated.

The high-temperature oxidation resistance of SiMo51 at 700 and 800C in air was found to be improved by adding Cr, whereas Ni showed adverse effects. The effects of solid-solution hardening from Ni and precipitation hardening from Cr were low at 700C, with improvements only at lower temperatures. Applying a TBC system, providing thermal protection from a ceramic topcoat and oxidation protection from a metallic bond coat, resulted in only small reductions in material temperature, but according to finite element calculations still effectively improved the fatigue life of a turbo manifold. Possible alternative materials to SiMo51 identified were austenitic cast ductile iron Ni-resistant D5S and austenitic cast stainless steel HK30, which provided high durability of exhaust manifolds up to 800 and 900C, respectively. Corrosion fatigue testing of SiMo51 at 700C in diesel exhaust gas demonstrated that the corrosive gas reduced fatigue life by 30-50% compared with air and by 60-75% compared with an inert environment. The reduced fatigue life was associated with a mechanism whereby the crack tip oxidized, followed by crack growth. Thus another potential benefit of TBC systems is that the bond coat may reduce oxidation interactions and further improve fatigue life.

These results can be used for selecting materials for exhaust applications. They also reveal many new research questions for future studies. Combining the different approaches of alloy modification, new material testing and improving the performance using coatings widened the scope of how component life in exhaust manifolds can be improved. Moreover, the findings on environmental interactions on SiMo51 fatigue provide a completely new understanding of these processes in ductile irons, important knowledge when designing components exposed to corrosive environments. The novel facility developed for high-temperature corrosion fatigue testing can be useful to other researchers working in this field. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 87 p.
Keyword [en]
cast exhaust manifolds, high-temperature corrosion, high-temperature low-cycle fatigue, high-temperature corrosion fatigue, ductile cast irons, cast stainless steel
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-166274OAI: oai:DiVA.org:kth-166274DiVA: diva2:810259
Public defence
2015-05-29, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Note

QC 20150507

Available from: 2015-05-07 Created: 2015-05-06 Last updated: 2015-05-07Bibliographically approved
List of papers
1. Influence of Cr and Ni on High-Temperature Corrosion Behavior of Ferritic Ductile Cast Iron in Air and Exhaust Gases
Open this publication in new window or tab >>Influence of Cr and Ni on High-Temperature Corrosion Behavior of Ferritic Ductile Cast Iron in Air and Exhaust Gases
2013 (English)In: Oxidation of Metals, ISSN 0030-770X, E-ISSN 1573-4889, Vol. 80, no 5-6, 455-466 p.Article in journal (Refereed) Published
Abstract [en]

Due to an expected temperature increase of the exhaust gases in heavy-duty engines in order to meet future emission regulations, there is a need to develop materials that can operate at higher temperatures. The exhaust manifold in the hot end of the exhaust system is specifically affected since the most common material today, SiMo51, is already operating close to its limits. Accordingly, the effects of Cr and Ni-additions on the high-temperature corrosion resistance of this material in air and exhaust gases were examined. It was found that the addition of 0.5 and 1 wt% Cr improved the oxidation resistance in air at 700 and 800 A degrees C by the formation of an SiO2 barrier layer as well as a Cr-oxide at the oxide/metal interface. However, no Cr-oxide was detected after exposure to exhaust gases, probably due to a water vapor-assisted evaporation of Cr from the oxide. The addition of 1 wt% Ni resulted in a deteriorated SiO2 barrier layer and reduced oxidation resistance.

Keyword
Ferritic ductile cast iron, Water-assisted evaporation, Exhaust manifold
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122014 (URN)10.1007/s11085-013-9389-8 (DOI)000327081200002 ()2-s2.0-84890119737 (Scopus ID)
Funder
Vinnova
Note

QC 20131217

Available from: 2013-05-08 Created: 2013-05-08 Last updated: 2017-12-06Bibliographically approved
2. High-temperature corrosion of materials for cast exhaust components
Open this publication in new window or tab >>High-temperature corrosion of materials for cast exhaust components
2014 (English)In: 7th European Corrosion Congress, 2014Conference paper, Published paper (Other academic)
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-166269 (URN)
Conference
7th European Corrosion Congress, Pisa 2014
Funder
VINNOVA, 2009-01433
Note

QC 20150507

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2015-05-07Bibliographically approved
3. High-temperature mechanical- and fatigue properties of cast alloys intended for use in exhaust manifolds
Open this publication in new window or tab >>High-temperature mechanical- and fatigue properties of cast alloys intended for use in exhaust manifolds
2014 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 616, 78-87 p.Article in journal (Refereed) Published
Abstract [en]

In the present work materials for use in exhaust manifolds of heavy-duty diesel engines were tested in air from 20 to 1000 degrees C with respect to mechanical properties. Two cast irons, SiMo51 and Ni-resist D5S, four austenitic cast steels, HF, A3N, HK30 and HK-Nb, and one ferritic cast steel, 1.4509 were studied. The experimental work included thermal conductivity, thermal expansion, uniaxial stress-strain testing, low-cycle fatigue testing up to 30,000 cycles and fractography. Below 500 degrees C, SiMo51 is superior. At higher temperatures, a transition from elastic to plastic strain dominance was observed for the cast irons, reducing their performance. Carbide-forming elements increase heat conductivity and result in a dendrite-like fracture surfaces during fatigue testing. The austenitic steels are superior only at higher temperatures.

Keyword
Low-cycle fatigue, High-temperature mechanical properties, Ductile cast iron, Cast stainless steel, Exhaust manifolds, Physical properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-156112 (URN)10.1016/j.msea.2014.08.014 (DOI)000343352400010 ()2-s2.0-84906506637 (Scopus ID)
Funder
Vinnova, 2009-01433
Note

QC 20150109

Available from: 2015-01-09 Created: 2014-11-21 Last updated: 2017-12-05Bibliographically approved
4. High-temperature corrosion fatigue of a ferritic ductile cast iron in inert and corrosive environments at 700˚C
Open this publication in new window or tab >>High-temperature corrosion fatigue of a ferritic ductile cast iron in inert and corrosive environments at 700˚C
2015 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336Article in journal (Refereed) Accepted
Abstract [en]

In the present work, low-cycle fatigue testing of a ferritic ductile cast iron named SiMo51 has been carried out in three atmospheres: argon, air and a synthetic diesel exhaust-gas at 700°C. The fatigue life was reduced up to 80% in the worst case. Two crack growth mechanisms were observed and directly linked to oxidation. At weak oxidation, a nodule-to- nodule crack growth occurred. At strong oxidation, crack growth occurred through oxidized material in front of the crack tip. 

Keyword
high-temperature LCF, corrosion fatigue, ductile cast iron, crack growth mechanisms
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-166272 (URN)
Conference
10th International Symposium on the Science and Processing of Cast Iron
Funder
VINNOVA, 2012-01690
Note

QP 201505

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2017-12-04Bibliographically approved
5. Evaluation of internal thermal barrier coatings for exhaust manifolds
Open this publication in new window or tab >>Evaluation of internal thermal barrier coatings for exhaust manifolds
Show others...
2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 272, 198-212 p.Article in journal (Refereed) Published
Abstract [en]

Seven different thermal barrier coatings (TBC) intended for coating the inside of an exhaust manifold to reduce its material temperature were studied. They comprised five plasma-sprayed (mullite, forsterite, La2Zr2O7, 8YSZ, and nanostructured 8YSZ) and two sol-gel composite (one sprayed and one dipped) coatings, which were examined for their thermal insulation properties and oxidation and spallation resistance. Thermal cyclic tests in air and in exhaust gas in a diesel test engine showed that thermal expansion mismatch between substrate and TBC was most crucial for TBC lifetime. Moreover, thermal modeling indicated that it is possible to reduce the material temperature by 50 °C, which is important for improving the fatigue life of exhaust manifolds. This reduction can be obtained with a 0.2 mm thick TBC with thermal conductivity close to 0.1 W/m K, or a 3–6 mm thick TBC with thermal conductivity 1.5–3 W/m K. 

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Exhaust manifold, SiMo51, thermal barrier coating, sol-gel composite, plasma-sprayed coatings, thermal modeling
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-166273 (URN)10.1016/j.surfcoat.2015.04.005 (DOI)000355055600024 ()2-s2.0-84927596466 (Scopus ID)
Funder
VINNOVA, 2012-01690
Note

QC 20150507

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2017-12-04Bibliographically approved

Open Access in DiVA

Thesis(4100 kB)