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The influence of microstructure on mechanical and tribological properties of lamellar and compacted irons in engine applications
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0003-2698-5445
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lamellar graphite iron (LGI) is commonly used in diesel engine applications such as piston rings–cylinder liner where an excellent combination of physical and tribological properties is essential to avoid scuffing and bore polishing issues. The excellent tribological behaviour of LGI alloys is related to the graphite lamellas, which act as solid lubricant agents by feeding onto the tribosurfaces under sliding conditions. However, increasingly tighter emissions and fuel economy legislations and the higher demands on enhanced power and durability have encouraged both engine designers and manufacturers to introduce pearlitic compacted graphite irons (CGI) as an alternative material replacing LGI, although the poor machinability of pearlitic CGI alloys compared to the LGI remains a challenge.

The focus of this study is placed on investigating how the microstructure of LGI and CGI alloys affects their mechanical and tribological properties. This was initially undertaken by investigating representative, worn lamellar cast iron piston rings taken from a two-stroke large-bore heavy-duty diesel engine. As known that it is tribologically essential to keep the graphite open under sliding conditions, in particular under starved lubrication regimes or unlubricated conditions to avoid scuffing issues; however, this study revealed the closure of a majority of graphite lamellas; profoundly for those lamellas that were parallel to sliding direction; due to the severe matrix deformation caused by abrasion. Both microindentation and microscratch testing, which were used to crudely simulate the abrasion under starved lubricated condition in combustion chamber, suggested a novel mechanism of activating the graphite lamellas to serve as lubricating agents in which the matrix deformation adjacent to the graphite initially resulted in fracturing and then extrusion of the graphite lamellas.

Additionally, in order to investigate the relation between matrix constituents, mechanical properties and machinability of cast iron materials, solution-strengthened CGI alloys were produced with different levels of silicon and section thicknesses. The results showed significant improvements in mechanical properties and machinability while deteriorating the ductility. Moreover, multiple regression analysis, based on chemical composition and microstructural characteristics was used to model the local mechanical properties of high Si ferritic CGI alloys, followed by implementing the derived models into a casting process simulation which enables the local mechanical properties of castings with complex geometries. Very good agreement was observed between the measured and predicted microstructure and mechanical properties.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2016. , 80 p.
Series
JTH Dissertation Series, 17
Keyword [en]
Cast iron, Si solution-strengthened CGI, microstructure, mechanical properties, modelling and simulation, tribology, abrasive wear, scratch testing
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-32052ISBN: 978-91-87289-18-7OAI: oai:DiVA.org:hj-32052DiVA: diva2:1040024
Public defence
2016-11-25, E1405, Jönköping University, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2016-10-26Bibliographically approved
List of papers
1. The relationship between flake graphite orientation, smearing effect, and closing tendency under abrasive wear conditions
Open this publication in new window or tab >>The relationship between flake graphite orientation, smearing effect, and closing tendency under abrasive wear conditions
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 317, no 1–2, 153-162 p.Article in journal (Refereed) Published
Abstract [en]

Abstract Plastic deformation of the matrix during the wear process results in closing the graphite flakes. In this study, the relationship between the deformation of the matrix and the closing tendency of flake graphite was investigated, both qualitatively and quantitatively. Two representative piston rings, which belonged to the same two-stroke marine engine but were operated for different periods of time, were studied. Initial microstructural observations indicated a uniform distribution of graphite flakes on unworn surfaces, whereas worn surfaces demonstrated a tendency towards a preferred orientation. Approximately 40% of the open flakes of the unworn surfaces were closed during sliding, which may result in the deterioration of the self-lubricating capability of cast iron. Moreover, flakes within the orientation range of 0 to 30° relative to the sliding direction showed a maximum closing tendency when subjected to sliding. The closing tendency gradually decreased as the angle increased, approaching a minimum between 30 and 70°. A slight increase in the closing tendency was observed for flakes with orientations between 70 and 90°. A similar trend was observed on both rings. Furthermore, SEM and EDS analysis indicated substantial deformation of the matrix in the area around the flakes. An insignificant corrosion attack was observed on both worn piston ring surfaces.

Keyword
Grey cast iron, Piston ring-cylinder liner, Flake graphite orientation, Sliding wear, Abrasive wear, Graphite closing tendency
National Category
Metallurgy and Metallic Materials Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hj:diva-24210 (URN)10.1016/j.wear.2014.05.015 (DOI)000341340700015 ()2-s2.0-84902973160 (ScopusID)
Projects
European project entitled Helios
Funder
EU, FP7, Seventh Framework Programme, 265861
Available from: 2014-07-07 Created: 2014-07-03 Last updated: 2016-11-29Bibliographically approved
2. A study on graphite extrusion phenomenon under the sliding wear response of cast iron using microindentation and microscratch techniques
Open this publication in new window or tab >>A study on graphite extrusion phenomenon under the sliding wear response of cast iron using microindentation and microscratch techniques
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 320, no 1-2, 120-126 p.Article in journal (Refereed) Published
Abstract [en]

This study focuses on the graphite flakes extrusion mechanism during microindenting and microscratching of cast iron. Observations on the graphite response under abrasive conditions revealed that the matrix deformation which is occurred during a sliding wear condition could have a significant influence on its lubricating performance. Simple microindentation and microscratch tests were conducted to explore the lamellar graphite contribution to tribofilm formation under abrasive wear conditions. The results obtained showed that induced plastic deformation which developed adjacent to the graphite compressed the lamellas and in turn resulting in extrusion of the graphite from its natural position. Further investigations on both indentation and scratch tests indicated that, surprisingly, the graphite began to be fractured and extruded from the centre of graphite lamellas, irrespective of the lamella size. Additionally, a mechanism was proposed to explain the self-lubricating and the extrusion behaviour of the lamellar graphite as a result of indentation.

Place, publisher, year, edition, pages
Lausanne: Elsevier, 2014
Keyword
Lamellar graphite iron; Graphite extrusion; Sliding wear; Abrasive wear; Microindentation testing; Microscratch testing
National Category
Tribology
Identifiers
urn:nbn:se:hj:diva-24779 (URN)10.1016/j.wear.2014.09.002 (DOI)000345156400015 ()2-s2.0-84907556394 (ScopusID)
Projects
Helios
Funder
EU, FP7, Seventh Framework Programme, 265861
Available from: 2014-09-21 Created: 2014-09-21 Last updated: 2016-10-26Bibliographically approved
3. Cast iron and the self-lubricating behaviour of graphite under abrasive wear conditions
Open this publication in new window or tab >>Cast iron and the self-lubricating behaviour of graphite under abrasive wear conditions
2014 (English)Conference paper, Abstract (Refereed)
Abstract [en]

Cast iron is assessed as a self-lubricating material under sliding conditions. This is due to the graphite particles distributed in the matrix, which come out from their pocket, and form a tribofilm between the mating surfaces, and by which improve the tribological characteristics. In this study, the directionality and the interaction between the graphite and matrix material was investigated by microindentation and microscratch techniques. The results showed that the graphite is fractured and pushed out from the middle of graphite lamellas as a result of indentation. It was also observed that the graphite orientation below the surface intensely influenced the pushing out behaviour. For the graphite oriented toward the indenter position, the effect was more pronounced. Moreover, it was found that a scratch test can be used to investigate and explain the graphite pushing out tendency. The result was used to explain the directionality and closing tendency of the graphite lamellas during sliding.

Keyword
lamellar graphite iron, graphite self-lubricating behaviour, tribological performance, microindentation test, microscratch test
National Category
Tribology
Identifiers
urn:nbn:se:hj:diva-25199 (URN)
Conference
10th International Symposium on the Science and Processing of Cast Iron – SPCI10
Funder
EU, European Research Council, 265861 (Helios)
Available from: 2014-12-01 Created: 2014-12-01 Last updated: 2016-10-26Bibliographically approved
4. Effect of interaction between lamellar graphite and cat-fines on tribological behaviour of cast iron under abrasion
Open this publication in new window or tab >>Effect of interaction between lamellar graphite and cat-fines on tribological behaviour of cast iron under abrasion
2015 (English)In: Proceeding of ITC, 2015Conference paper (Refereed)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-32049 (URN)
Conference
6th International Tribology Conference, Tokyo, Japan, 16-20th September 2015
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2016-10-26Bibliographically approved
5. Mechanical properties of solid solution-strengthened CGI
Open this publication in new window or tab >>Mechanical properties of solid solution-strengthened CGI
Show others...
2016 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 29, no 1-2, 98-105 p.Article in journal (Refereed) Published
Abstract [en]

Despite the increased usage of pearlitic compacted graphite iron (CGI) in heavy vehicle engines, poor machinability of this material remains as one of the main technical challenges as compared to conventional lamellar iron. To minimise the machining cost, it is believed that solution-strengthened CGI material with a ferritic matrix could bring an advantage. The present study focuses on the effect of solution strengthening of silicon and section thickness on tensile, microstructure and hardness properties of high-Si CGI materials. To do so, plates with thicknesses from 7 to 75 mm were cast with three different target silicon levels 3.7, 4.0 and 4.5 wt%. For all Si levels, the microstructure was ferritic with a very limited pearlite content. The highest nodularity was observed in 7 and 15 mm plate sections, respectively, however, it decreased as the plate thickness increased. Moreover, increasing Si content to 4.5 wt% resulted in substantial improvement up to 65 and 50% in proof stress and tensile strength, respectively, as compared to pearlitic CGI. However, adding up Si content to such a high level remarkably deteriorated elongation to failure. For each Si level, results showed that the Young’s modulus and tensile strength are fairly independent of the plate thickness (30–75 mm), however, a significant increase was observed for thin section plates, particularly 7 mm plate due to the higher nodularity in these sections.

Keyword
Compacted graphite iron, High-Si ferritic CGI, Mechanical properties, Solid solution strengthening, Ferrite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-30841 (URN)10.1080/13640461.2015.1106781 (DOI)000377468800016 ()2-s2.0-84978389185 (ScopusID)
Funder
VINNOVA
Note

Special Issue: Special Issue featuring papers from SPCI10

Available from: 2016-06-23 Created: 2016-06-23 Last updated: 2016-10-26Bibliographically approved
6. Modelling and simulation of local mechanical properties of high silicon solution-strengthened ferritic CGI materials
Open this publication in new window or tab >>Modelling and simulation of local mechanical properties of high silicon solution-strengthened ferritic CGI materials
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-32050 (URN)
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2016-10-26
7. Machinability of solid solution-strengthened compacted graphite iron: Influence of the microstructure, mechanical properties and cutting conditions on tool wear response
Open this publication in new window or tab >>Machinability of solid solution-strengthened compacted graphite iron: Influence of the microstructure, mechanical properties and cutting conditions on tool wear response
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-32051 (URN)
Available from: 2016-10-26 Created: 2016-10-26 Last updated: 2016-10-26

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