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Microstructure Formation During Solidification and Solid State Transformation in Compacted Graphite Iron
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Compacted graphite iron (CGI) is rapidly becoming an attractive alternative material for engine components in the automotive industry, replacing lamellar graphite iron (LGI) in applications where high mechanical strength is desired. However, the gain in mechanical strength comes with a cost; thermal conductivity, process control and machining are three areas that are more challenging for CGI. This generates a need for research regarding various aspects concerning CGI. In this thesis the microstructure formation during solidification and solid state transformation will be the focus of interest.

The phase transformations relevant for microstructure formation of importance to properties in CGI were studied. Experiments were performed in an industrial foundry giving this research direct relevance to regular production of CGI castings.

Solidification of the grey (graphite/austenite) eutectic will be discussed, focusing on some relevant aspects influencing the graphite morphology of CGI. The formation of graphite nodules has been investigated by studying colour-etched microstructures. In a material containing mainly CGI cells it was found that nodules form either early during solidification as a consequence of high undercooling or late in the solidification sequence due to a combination of high undercooling and segregation of nodularising elements. Solidification of the white (cementite/austenite) eutectic was studied using chill wedges and the influence of some alloying elements on the amount of carbides was examined. To further enhance the understanding of carbide formation in CGI a commercial casting simulation software was used to correlate real castings to simulations. It was found that the alloying elements investigated influence the carbide formation in a similar way as in other graphitic cast irons and that high nodularity CGI is more prone to chill formation than low nodularity CGI. The solid state transformation was studied and a deterministic model was developed. The model divides a eutectic cell into layers, in order to take into account segregation of alloying elements, which was observed to be influential for the ferrite growth. Moreover, the effect of alloying elements on mechanical properties (tensile properties and hardness) was evaluated. Properties were correlated to microstructural features originating from both solidification and solid state transformations. The trends found generally confirmed previous results regarding properties in graphitic cast irons.

Place, publisher, year, edition, pages
Gothenburg: Chalmers University of Technology , 2011. , 50 p.
Series
Doktorsavhandlingar vid Chalmers tekniska högskola, ISSN 0346-718X ; 3206
Keyword [en]
Cast Iron, CGI, Microstructure Formation, Mechanical Properties, Modelling, Solidification, Solid State Transformation
Identifiers
URN: urn:nbn:se:hj:diva-15694ISBN: 978-91-7385-525-9 (print)OAI: oai:DiVA.org:hj-15694DiVA: diva2:430923
Public defence
2011-05-17, E1405, Gjuterigatan 5, 55111, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2011-08-08 Created: 2011-07-13 Last updated: 2011-12-14Bibliographically approved
List of papers
1. The Influence of alloying elements on Chill Formation in CGI
Open this publication in new window or tab >>The Influence of alloying elements on Chill Formation in CGI
2010 (English)In: Science and processing of cast iron IX: selected peer reviewed papers from the Ninth International Symposium on Science and Processing of Cast Iron, Luxor-Egypt, November 10-13, 2010 / [ed] Nofal, A. and Waly, M., Stafa-Zürich: Trans Tech Publications , 2010, 126-131 p.Conference paper, Published paper (Refereed)
Abstract [en]

The influence of alloying elements on the chill formation in Compacted Graphite Iron (CGI) is investigated. Chill wedges cast in an industrial foundry were used to investigate the chill formation. A total number of 19 chemical compositions were studied, including three trials of varying nodularity treatment level; four trials of varying copper content; four trials of varying silicon content; four trials of varying tin content and four trials of varying carbide promoter content. Three wedges were cast for each alloy composition, of which one was used for measuring the temperature at three different heights in the wedge.

Contrary to some previous reports, the results indicate that low-nodularity CGI is not more prone to chill formation (columnar white) than high-nodularity CGI. Trends regarding the effect of alloying elements on chill formation are shown to generally be in agreement with previous work on spheroidal graphite iron and lamellar graphite iron. Most of the samples also show carbide formation in centre line areas of the wedge (inverse chill), this occurrence is also discussed in the paper.

 

Place, publisher, year, edition, pages
Stafa-Zürich: Trans Tech Publications, 2010
Series
Key Engineering Materials, ISSN 1013-9826 ; 457
Keyword
CGI, Chill Formation, Chill Wedges, Microstructure Formation, Simulation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hj:diva-14225 (URN)9780878492275 (ISBN)
Conference
Science and Processing of Cast Iron IX
Available from: 2011-01-07 Created: 2011-01-07 Last updated: 2017-08-14Bibliographically approved
2. Modeling of Ferrite Growth in Compacted Graphite Iron
Open this publication in new window or tab >>Modeling of Ferrite Growth in Compacted Graphite Iron
2009 (English)In: Modeling of Casting, Welding and Advanced Solidification Processes - XII, Warrendale, Pennsylvania, USA: The Minerals, Metals & Materials Society (TMS) , 2009, 505-512 p.Conference paper, Published paper (Other academic)
Abstract [en]

In the present work the ferrite growth in Compacted Graphite Iron (CGI) has been modeled. The relative amounts of ferrite and pearlite are of vital importance for the mechanical properties of cast iron. Thus, the possibility to predict the matrix microstructure in CGI by means of simulation would be an indispensable tool for foundries and design engineers. The increasing use of CGI, especially for automotive applications, further creates a need for this research.

 

This study shows that graphite in CGI grows in cells similar to eutectic grey iron cells. In cast irons the ferrite nucleates on the graphite and thereafter grows around the graphite particles until the formation of pearlite interrupts further ferrite formation. The complexity of the graphite particles in CGI imply that the ferrite will grow in different stages. A microstructure study of CGI reveals that ferrite nucleates on compacted graphite particles where low amounts of pearlite stabilizing elements can be found. Growth of ferrite then proceeds until it impinges with ferrite nucleated on adjacent graphite particles. Further ferrite growth takes place radially away from the center of the CGI cell until pearlite nucleation interrupts its growth. At some point during this process ferrite nucleates on graphite nodules in the last to freeze areas of the microstructure giving a contribution to the ferrite content. In the present work growth models of ferrite in nodular cast iron are adapted to CGI.

Place, publisher, year, edition, pages
Warrendale, Pennsylvania, USA: The Minerals, Metals & Materials Society (TMS), 2009
Keyword
Cast Iron, CGI, Ferrite Growth, Modeling, Eutectoid Transformation
Identifiers
urn:nbn:se:hj:diva-10590 (URN)978-0-87339-742-1 (ISBN)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-08-14Bibliographically approved
3. Influence of alloying elements on microstructure and mechanical properties of CGI
Open this publication in new window or tab >>Influence of alloying elements on microstructure and mechanical properties of CGI
2010 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 23, no 2, 97-110 p.Article in journal (Refereed) Published
Abstract [en]

The influence of alloying elements and cooling conditions on compacted graphite iron is investigated. To investigate the influence of different alloying elements a sampling cup, designed for thermal analysis, and tensile test bars were cast. A total of 19 casting trials were performed and for each of the trials three different cooling conditions were studied for both the sampling cup and the tensile test bars. Mg has a major influence on the graphite morphology and causes a substantial increase in nodularity. Cu, Si and Sn only cause minor changes in the nodularity, but on the other hand affect the mechanical properties primarily by altering the pearlite content of the material. Cr and Mo are investigated mainly with respect to their carbide promoting properties. However, no columnar white structure was found in the microstructure. The relationship between the microstructure and the thermal history caused by differences in alloying content is also studied.

Place, publisher, year, edition, pages
Maney Publishing, 2010
Keyword
CGI, cast iron, microstructure formation, mechanical properties, alloying elements
Identifiers
urn:nbn:se:hj:diva-10874 (URN)10.1179/136404609X12505973098972 (DOI)
Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved
4. The influence of copper on microstructure and mechanical properties of compacted graphite iron
Open this publication in new window or tab >>The influence of copper on microstructure and mechanical properties of compacted graphite iron
2009 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 22, no 1-4, 164-167 p.Article in journal (Refereed) Published
Abstract [en]

The influence of copper content (0.26 to 1.31 wt-%) on microstructure formation and mechanicalproperties of compacted graphite iron (CGI) has been evaluated through standard metallographicanalysis, colour etching techniques and tensile testing of machined test bars. Theproperties investigated are yield strength, tensile strength and elongation. The castings weremade in an industrial environment from a combination of CGI returns, pig iron, cast iron- and steelscrap. A total of four heats were cast in specially designed sampling cups (3 different coolingrates), chill wedges as well as tensile test bars machined from sand moulded cylinders (20, 45and 85 mm in diameter). The results clearly illustrate the combined effect of copper and coolingrate on nodularity, chilling tendency as well as pearlite content. A discussion concerning the effectof graphite morphology on the ferrite growth is also included.

Place, publisher, year, edition, pages
London, UK: Maney Publishing, 2009
Keyword
CGI, Copper, Microstructure, Mechanical Properties, Chilling tendency, Nodularity
Identifiers
urn:nbn:se:hj:diva-10620 (URN)10.1179/136404609X367597 (DOI)
Available from: 2009-10-12 Created: 2009-10-12 Last updated: 2017-12-12Bibliographically approved
5. Literature Review of Microstructure Formation in Compacted Graphite Iron
Open this publication in new window or tab >>Literature Review of Microstructure Formation in Compacted Graphite Iron
2010 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 23, no 3, 185-192 p.Article, review/survey (Refereed) Published
Abstract [en]

The literature concerning microstructure formation in Compacted Graphite Iron (CGI) is reviewed. CGI has an intermediate graphite morphology between Lamellar Graphite Iron (LGI) and Spheroidal Graphite Iron (SGI). The formation of compacted graphite morphology is controlled by small changes in alloy composition. Several important factors influencing the formation of the as-solidified microstructure, as well as the microstructure formation during the eutectoid transformation are also reviewed. The focus of this review is to compare mechanisms in which CGI differs from LGI and SGI. The effects of microstructure on some properties that are relevant in commercial applications are included in the study. Additionally the relatively few models for microstructure formation are also summarised.

Keyword
CGI, literature review, microstructure formation, mechanical properties, modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hj:diva-10871 (URN)10.1179/136404609X12535244328378 (DOI)
Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved

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