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Compacted graphite iron: On solidification phenomena related to shrinkage defects
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6938-037X
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An improved understanding of solidification of compacted graphite iron (CGI) is necessary to predict shrinkage related casting defects. Spheroidal graphite is not only found in ductile iron (SGI) but usually also in CGI, but its characteristics and role in this material is less understood. Uncertainties also remain regarding the segregation of alloying elements in cast irons. This is important because of its important role in the solidification process. The dendritic austenite structure has received little attention in cast iron research. A good understanding of the development of this structure is important to understand feeding of melt through the solidifying material.

Nodularity is a measure of the amount of spheroidal versus compacted graphite in the microstructure. At a lower nodularity, the compacted graphite tips were found to grow in contact with the melt for a longer time before being encapsulated in the austenite. Moreover, as the nodularity was reduced, the subpopulation of larger spheroidal graphite gradually disappeared, reducing the bimodal size distribution to unimodal.

Segregation of Si, Mn and Cu in SGI and CGI with a solidification time of near 10 min was found to be rather predictable under assumptions of no diffusion in austenite and complete mixing in the melt. Gradients of these elements contribute to a decrease in the driving force for diffusion of carbon into the austenite from the liquid, which is important for the growth of graphite which is separated from the liquid by austenite.

During solidification of a near-eutectic CGI, the carbon concentration of austenite was found to deviate considerably from local equilibrium with graphite during solidification. This is important to consider in growth models for graphite by diffusion of carbon through a barrier of austenite.

Micropores were shown to have displaced liquid from the solid structure at a late stage of solidification when solidification was slowing down and the temperature of the casting was falling at an increasing rate.

The development of dendritic austenite in a near eutectic CGI was investigated. The contact area between liquid and the dendritic structure exceeded the contact area between liquid and eutectic cells through the dominant part of solidification. This highlights the importance of good understanding of the development of this structure in order to predict feeding of melt through the solidifying material. The coarsening of the structure was found to proceed at a higher rate compared to studies under isothermal condition. The dendritic structure continued to grow in parallel with the eutectic by a combination of thickening and dendritic growth.

Abstract [sv]

En förbättrad förståelse av stelningsförloppet hos kompaktgrafitjärn (CGI) är nödvändig för att förutbestämma krymprelaterade gjutdefekter. Nodulär grafit förekommer inte bara i segjärn (SGI) utan vanligtvis även i CGI, men dess karaktär och roll är mindre känd för detta material. Osäkerheter kvarstår även kring segring av legeringsämnen i gjutjärn, vilket spelar en viktig roll under dess stelningsförlopp. Dendritisk austenitstruktur har fått relativt lite uppmärksamhet bland forskare inom gjutjärn. God kunskap om dess utveckling är viktigt för att förstå matning av smälta genom stelningsförloppet.

Nodularitet är ett mått på andelen nodulär kontra kompakt grafitmorfologi i mikrostrukturen. Vid lägre nodularitet visades kompaktgrafitens extremiter växa i kontakt med smältan en längre period innan de inneslöts av austenit. Gruppen av större nodular försvann dessutom gradvis vid lägre nodularitet, vilket reducerade den starkt bimodala storleksfördelningen till unimodal.

Segring av Si, Mn och Cu i SGI och CGI med stelningstid på nära 10 min fanns förutbestämbar under antaganden om obetydlig diffusion i austenit och omedelbar blandning med smältan. Gradienter av dessa element bidrar till en sänkt drivkraft för diffusion av kol mot austenitens inre, vilket är viktigt för tillväxt av nodulär grafit.

Under stelning av ett nära eutektiskt CGI fanns kolhalten i austenit avvika avsevärt från lokal jämvikt med grafit under stelningen. Detta är viktigt att ta hänsyn till i modeller för tillväxt av grafit genom diffusion av kol genom en barriär av austenit.

En studie av mikroporer visade att dessa trängt undan smälta mellan stelnad struktur i ett sent stadie av stelningen, med avtagande stelningshastighet och ökande temperaturfall.

Utvecklingen av dendritstruktur i ett nära eutektiskt kompaktgrafitjärn undersöktes. Smältans kontaktyta mot denna struktur överskred kontaktyta mot eutektiska stelningskroppar genom större delen av stelningen. Detta visar att en god förståelse för dendritstrukturens utveckling genom stelningen är viktig för att beskriva matning av smälta genom stelnande struktur. Förgrovning av strukturen visades fortskrida i högre takt genom stelningen jämfört med studier vid isotermiskt tillstånd. Dendritstrukturen fortsatte växa parallellt med eutektisk stelning genom en kombination av homoepitaxial och dendritisk tillväxt.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2019. , p. 69
Series
JTH Dissertation Series ; 046
Keywords [en]
Compacted graphite iron, Spheroidal graphite iron, Solidification, Microsegregation, Porosity
Keywords [sv]
Kompaktgrafitjärn, segjärn, stelning, mikrosegregation, porositet
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-43661ISBN: 978-91-87289-49-1 (print)OAI: oai:DiVA.org:hj-43661DiVA, id: diva2:1315408
Public defence
2019-06-12, E1405, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2019-05-13 Created: 2019-05-13 Last updated: 2019-05-22Bibliographically approved
List of papers
1. Size distribution of graphite nodules in hypereutectic cast irons of varying nodularity
Open this publication in new window or tab >>Size distribution of graphite nodules in hypereutectic cast irons of varying nodularity
2018 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 49, no 5, p. 2487-2504Article in journal (Refereed) Published
Abstract [en]

An SGI was machined into 400 g cylindrical pieces and remelted in an electrical resistance furnace protected by Ar gas to produce materials ranging from SGI to CGI. The graphite morphology was controlled by varying the holding time at 1723 K (1450 °C) between 10 and 60 minutes. The discrete sectional size distribution of nodules by number density was measured on cross sections of the specimens and translated to volumetric distribution by volume fraction. Subpopulations of nodules were distinguished by fitting Gaussian distribution functions to the measured distribution. Primary and eutectic graphite, were found to account for most of the volume of nodular graphite in all cases. For holding times of 40 minutes and greater, corresponding to nodularity roughly below 40 pct, the primary subpopulation was very small and difficult to distinguish, leaving eutectic nodules as the dominant subpopulation. The mode and standard deviation of the two subpopulations were roughly independent of nodularity. Moreover, the nodular and vermicular graphite were segregated in the microstructure. In conclusion, the results suggest that the parallel development of the vermicular eutectic had small influence on the size distribution of eutectic graphite nodules.

Place, publisher, year, edition, pages
Springer, 2018
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35530 (URN)10.1007/s11663-018-1274-z (DOI)000444765500033 ()2-s2.0-85048767236 (Scopus ID)HOA JTH 2018 (Local ID)HOA JTH 2018 (Archive number)HOA JTH 2018 (OAI)
Note

Included in licentiate thesis by Björn Domeij (2017), On the solidification of compacted and spheroidal graphite irons, as manuscript.

Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2019-05-13Bibliographically approved
2. Inferring the development of microsegregation and microstructure in Spheroidal and Compacted Graphite Iron using EPMA‐WDS
Open this publication in new window or tab >>Inferring the development of microsegregation and microstructure in Spheroidal and Compacted Graphite Iron using EPMA‐WDS
2017 (English)In: Solidification Processing 2017: Proceedings of the 6th Decennial International Conference on Solidification Processing / [ed] Zhongyun Fan, Brunel Centre for Advanced Solidification Technology (BCAST) , 2017, no 1, p. 455-458Conference paper, Published paper (Refereed)
Abstract [en]

Microsegregation is closely related to the solidification characteristics and microstructure development of a material. In this paper, the microsegregation of Si, Cu, Mn, and P was investigated on a spheroidal and a compacted graphite iron using a modified EPMA equipped with WDS spectrometers. On the basis of the approximated local equilibrium eutectic temperature, the solidification sequence of the matrix was estimated. The inferred microstructure development appeared to correspond well to published interrupted solidification experimental results. Solute profiles and effective partition coefficients were constructed using the solidification sequence. While the spatial microsegregation patterns clearly differed between the two materials, solute profiles and effective partition coefficients were very similar. For Si, Cu and Mn, the solute profiles corresponded reasonably with simulation results produced using the Scheil-Gulliver module of the Thermo-Calc software with the TCFE7 databank, indicating back-diffusion of these elements is negligible for SGI and CGI with solidification times up to 10 min. The effective partition coefficients for Si, Cu and Mn were fairly constant until about 90% of the matrix had solidified, after which they appeared to approach unity.

Place, publisher, year, edition, pages
Brunel Centre for Advanced Solidification Technology (BCAST), 2017
Keywords
Cast Iron, Microsegregation, Electron microprobe analysis
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35531 (URN)978-1-908549-29-7 (ISBN)
Conference
6th Decennial International Conference on Solidification Processing, 25th-28th July 2017, Beaumont Estate, Old Windsor, UK
Note

Included in thesis in manuscript form.

Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2019-05-13Bibliographically approved
3. Solidification Chronology of the Metal Matrix and a Study of Conditions for Micropore Formation in Cast Irons Using EPMA and FTA
Open this publication in new window or tab >>Solidification Chronology of the Metal Matrix and a Study of Conditions for Micropore Formation in Cast Irons Using EPMA and FTA
2018 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 925, p. 436-443Article in journal (Refereed) Published
Abstract [en]

Microsegregation is intimately coupled with solidification, the development of microstructure, and involved in the formation of various casting defects. This paper demonstrates how the local composition of the metal matrix of graphitic cast irons, measured using quantitative electron microprobe analysis, can be used to determine its solidification chronology. The method is applied in combination with Fourier thermal analysis to investigate the formation of micropores in cast irons with varying proportions of compacted and spheroidal graphite produced by remelting. The results indicate that micropores formed at mass fractions of solid between 0.77 and 0.91, which corresponded to a stage of solidification when the temperature decline of the castings was large and increasing. In 4 out of the 5 castings, pores appear to have formed soon after the rate of solidification and heat dissipation had reached their maximum and were decreasing. While the freezing point depression due to build-up of microsegregation and the transition from compacted to spheroidal type growth of the eutectic both influencing solidification kinetics and the temperature evolution of the casting, the results did not indicate a clear relation to the observed late deceleration of solidification.

Place, publisher, year, edition, pages
Trans Tech Publications, 2018
Keywords
Solidification, Microsegregation, EPMA, Microporosity, Cast Iron
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-40787 (URN)10.4028/www.scientific.net/MSF.925.436 (DOI)XYZ ()2-s2.0-85050037946 (Scopus ID)
Conference
11th International Symposium on the Science and Processing of Cast Iron, SPCI-XI 2017, Jönköping, Sweden, 4-7 September 2017
Available from: 2018-06-25 Created: 2018-06-25 Last updated: 2019-05-13Bibliographically approved
4. Quantification of dendritic austenite after interrupted solidification in a hypoeutectic lamellar graphite iron
Open this publication in new window or tab >>Quantification of dendritic austenite after interrupted solidification in a hypoeutectic lamellar graphite iron
2016 (English)In: Metallography, Microstructure, and Analysis, ISSN 2192-9270, Vol. 5, no 1, p. 28-42Article in journal (Refereed) Published
Abstract [en]

This paper presents an unconventional etching technique to reveal the microstructure in a hypoeutectic lamellar graphite iron that has been quenched after isothermal heat treatment in the proeutectic semi-solid temperature region. A technique for quantifying the dendrite microstructure using the aforementioned etching technique involving a combination of a raster graphics editor and an image analysis software is outlined. The agreement between this quantification technique with regard to volume fraction and surface area per unit volume of the dendritic austenite and corresponding point counting and line intercept techniques is analyzed. The etching technique was found useful but sporadic tinting of martensite was problematic. Some measurements showed significant systematic disagreement which correlated with the coarseness of the measured dendrites. Most systematic disagreement is attributed to difficulties in defining the dendrite boundary in the analogues and much of the random disagreement to easily identified discrepancies between the analogue and the micrograph.

Place, publisher, year, edition, pages
New York: Springer, 2016
Keywords
Cast iron, Quantitative metallography, Color metallography, Microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28880 (URN)10.1007/s13632-015-0250-0 (DOI)000377604200007 ()2-s2.0-84960404906 (Scopus ID)
Projects
Spofic II
Funder
VINNOVA, 2013-04720
Note

One of five papers selected for the 2016 Editor’s Choice.

Available from: 2016-01-07 Created: 2016-01-07 Last updated: 2019-05-13Bibliographically approved
5. The evolution of dendritic austenite in a solidifying near‐eutectic compacted graphite iron under varying cooling conditions
Open this publication in new window or tab >>The evolution of dendritic austenite in a solidifying near‐eutectic compacted graphite iron under varying cooling conditions
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-43658 (URN)
Note

Submitted to journal.

Available from: 2019-05-13 Created: 2019-05-13 Last updated: 2019-05-13
6. The distribution of carbon during solidification of a compacted graphite iron
Open this publication in new window or tab >>The distribution of carbon during solidification of a compacted graphite iron
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-43659 (URN)
Available from: 2019-05-13 Created: 2019-05-13 Last updated: 2019-05-13

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