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Monotonic and Cyclic Creep of Exhaust Manifold Materials and their effects on Predicted Thermo-mechanical Fatigue Life
KTH, School of Industrial Engineering and Management (ITM). 8912104273. (Unit of properties)ORCID iD: 0000-0001-8673-9073
Scania CV.
Volkswagen.
KTH, School of Industrial Engineering and Management (ITM). (Unit of properties)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Creep properties of SiMo51, SiMo1000, Ni-resist D5S and HK30, materials used for exhaust manifolds in heavy-duty diesel-engines, are examined by three methods; sequential tensile tests at high temperature, varying the strain rate; relaxation of stresses and strains during isothermal holds of thermal cycling with fixed grips; and traditional creep testing. The results for tensile tests and relaxations coincide while the creep tests show values, one order of magnitude lower.

The creep can be modelled by Norton’s law and the cyclic influence, including strain reversals, is low. Simulations with/without taking creep into account show that predicted fatigue life for an exhaust manifold, subjected to engine tests with cycling between hot and cold gases, generally increase, with a few exceptions in some points, and that critical spots of low fatigue life are removed because of reduced mean stresses.

Keywords [en]
Creep, constant-load test, tensile test, thermal cycling, cast materials, cast iron, exhaust materials
National Category
Engineering and Technology Natural Sciences
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-228005OAI: oai:DiVA.org:kth-228005DiVA, id: diva2:1206306
Note

Financier is Scania CV. 

QC 20180517

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-05-17Bibliographically approved
In thesis
1. Thermal Cycling, Creep- and Tensile Testing of Cast Exhaust Materials at Elevated Temperatures
Open this publication in new window or tab >>Thermal Cycling, Creep- and Tensile Testing of Cast Exhaust Materials at Elevated Temperatures
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

An exhaust manifold of a truck engine is subjected to tough conditions. As the truck is started, operated and shut down, it becomes subjected to thermal cycling up to around 800°C. At such high temperatures, corrosion, fatigue and creep are active degradation mechanisms. As can be imagined, the interplay between the three complicates materials selection. It is desired to have a versatile grade of high durability which is not too expensive. At the moment, a ferritic, ductile cast iron designated SiMo51 is used for the application. However, due to the rough conditions, it is considered to be on the verge of its operational limit. As a consequence, there is an ongoing search for candidate materials. In this study, the ductile cast irons SiMo51, SiMo1000, D5S and the cast steel HK30 have been included.

In the past, there have been several studies describing corrosion and fatigue of the cast materials used for exhaust manifolds. However, on the subject of creep of cast materials, little is known. The present study aims to reveal creep tendencies of cast materials and to do it in several ways. More precisely, three creep-testing methods were employed: the conventional constant-load creep-test, the “Sequential tensile test (STT)” and “Stress relaxations with thermal cycling (SRTC)”. The first one is the traditional one. It is tedious, usually lasting months or years. The second one is a tensile test in which the strain rate is changed in sequences as specimen deformation proceeds. Here, the idea is that a slow tensile test is not different from a conventional creep test. In the third one, stress relaxations are provoked as a specimen is thermally cycled in a locked state. Since stress relaxations are a consequence of creep deformation, the relaxation data gathered from isothermal holds can be directly compared to results from the isothermal constant-load creep-test and STT. When thermally cycled in a locked state, the materials display a loop character in σ, ε and T which provides extensive information about the mechanical properties over the selected temperature interval.

In a logarithmic Norton plot, the creep strain rate is plotted as a function of stress. By plotting STT-data in such Norton plots, it was shown that the creep behaviour of the included materials is well represented by Norton’s law. Furthermore, it was found that the creep strain rates and stress relaxations, measured during isothermal holds in SRTC, in several cases show perfect coincidence with tensile test data obtained through STT. At 700°C, data from all three tests were inserted in the same Norton plot. At higher stress levels, the SRTC-curve follows the STT-curve and at lower stresses, when the creep regime is entered, it bends down and unites with data obtained by the constant-load creep tests. Additionally, it was seen that a relatively high degree of pre-deformation can give a critical stress below which creep deformation stops completely.

Place, publisher, year, edition, pages
STOCKHOLM: KTH Royal Institute of Technology, 2018. p. 38
Series
TRITA-ITM-AVL ; 2018:22
Keywords
creep, constant-load, thermal cycling, tensile test, cast materials, cast iron, exhaust materials.
National Category
Engineering and Technology
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-228022 (URN)978-91-7729-822-9 (ISBN)
Presentation
2018-06-13, Hörsalen, by 117, Granparksvägen 10, Södertälje, 13:00 (English)
Opponent
Supervisors
Note

The Financier is Scania CV. 

Available from: 2018-05-17 Created: 2018-05-16 Last updated: 2018-05-17Bibliographically approved

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