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Size distribution of graphite nodules in hypereutectic cast irons of varying nodularity
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6938-037X
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-6339-4292
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-3024-9005
2018 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916Article in journal (Refereed) Epub ahead of print
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: urn:nbn:se:hj:diva-35530DOI: 10.1007/s11663-018-1274-zISI: XYZScopus ID: 2-s2.0-85048767236OAI: oai:DiVA.org:hj-35530DiVA, id: diva2:1095913
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: 2018-07-03
In thesis
1. On the solidification of compacted and spheroidal graphite irons
Open this publication in new window or tab >>On the solidification of compacted and spheroidal graphite irons
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

A good understanding of the solidification process of a cast material is essential to understand how the combination of alloy composition and the casting process variables combines into the solid cast component and its performance. The wrong combination may result in poor performance or casting defects. Spheroidal graphite has been well researched in ductile irons (SGI) where it is predominant. Spheroidal graphite is also present in smaller amounts in compacted graphite irons (CGI), but its nature and role in this material is less understood. Recent associations of spheroidal graphite in CGI with shrinkage porosity problems highlights the need for better understanding in this area. The importance of the dendritic austenite structure to the properties and solidification behaviour of cast irons has been highlighted in recent research. However, progress is to a degree limited by lack of practical means to characterize the structure.

In the present work, the transition of a cast iron from SGI to CGI though remelting was studied. As the fraction of SG dropped, the tips of the compacted graphite tended to lose contact with the melt at a later stage. After this occurred, solidification appeared to be dominated by spheroidal graphite. Compacted and spheroidal graphite was found to solidify mostly segregated, and the increased recallescence induced by a higher fraction of compacted graphite displayed small influence on the size distribution of spheroidal graphite apart from the total number and fraction. The partitioning of Si, Mn and Cu in SGI and CGI was found to agree well with each other, as well as with theoretical predictions under the assumptions of zero diffusion of the elements in the solid. This shows that the proportions of spheroidal and compacted graphite has small or no influence on the evolution of these elements in the melt during solidification. A method for characterization of the dendritic austenite in quenched cast irons was introduced and evaluated. The method includes a technique for producing a visual contrast between the ledeburite matrix and the dendritic austenite, and a scheme for producing binary images from the resulting micrographs which are suitable for automatic image analysis. Measurements of the volume fraction and surface area per unit volume of the dendritic austenite structure using the introduced method was found to agree reasonably with traditional point counting and line intercept techniques. The difficulty in finding the exact boundary was proposed to be the major source of systematic disagreement.

Abstract [sv]

En god förståelse för stelningsprocessen av ett gjutet material är väsentligt för att förstå hur kombinationen av legeringens kemiska sammansättning och gjutprocessens variabler resulterar i den stelnade komponenten och dess prestanda. Fel kombination kan resultera i sänkt prestanda eller gjutdefekter. Sfärisk grafit är väl studerad i segjärn (SGI) där grafitmorfologin är dominant. Mindre mängder sfärisk grafit är dock närvarande även i kompaktgrafit, där dess karaktär och roll är mindre känd. Samband mellan sfärisk grafit i kompaktgrafitjärn och krympporositet understryker behovet av bättre förståelse inom detta område. Dessutom har betydelsen av den tidiga dendritiska austenitstrukturen för senare delen av stelningen uppmärksammats. Utveckling inom detta område är dessvärre till en grad begränsad av bristen på kunskap om praktiska metoder för att karaktärisera dess struktur.

I detta arbete studerades övergången från segjärn till kompaktgrafit genom omsmältning. Vid sänkt fraktion sfärisk grafit visade sig kompaktgrafiten tappa kontakten med smältan vid senare stadie av stelningen. Efter detta inträffade, dominerades stelningen till synes av tillväxt av sfärisk grafit. Kompaktgrafit och sfärisk grafit bildades i huvudsak segregerade. Ökad rekallesens till följd av ökad fraktion kompaktgrafit visade sig ha låg inverkan på storleksfördelningen av eutektisk sfärisk grafit bortsett från dess totala antal och fraktion. Omfördelningen av Si, Mn och Cu mellan stelnad matris och smälta i segjärn och kompaktgrafitjärn fanns stämma bra överens med varandra, samt med teoretiska värden med antagande om försumbar diffusion i stelnad matris. Detta visar att proportionerna av sfärisk och kompaktgrafit har liten eller ingen inverkan på halten av dessa ämnen i smältan under stelningen. En metod för karaktärisering av den dendritiska austenitstrukturen i släckt gjutjärn introducerades och utvärderades. Metoden inkluderar en teknik för att åstadkomma kontrast mellan ledeburitmatrisen och den dendritiska austeniten, och en teknik för att producera binära bilder från resulterande mikroskopbilder som är lämpliga för automatisk bildanalys. Mätningar av volymfraktion och yta per volymenhet av dendritstrukturen genom tillämpning av den introducerade metoden visade rimlig överensstämmelse med traditionella punktfraktion‐ och linjetekniker. Svårigheten att hitta den exakta gränslinjen mellan dendritisk struktur och ledeburit föreslogs vara den huvudsakliga källan till systematisk oöverensstämmelse.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. p. 48
Series
JTH research report, ISSN 1404-0018 ; 24
Keywords
Compacted graphite iron, Spheroidal graphite iron, Solidification, Microsegregation, Kompaktgrafitjärn, segjärn, stelning, mikrosegregation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35533 (URN)9789187289255 (ISBN)
Supervisors
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-16Bibliographically approved

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Domeij, BjörnHernando, Juan CarlosDiószegi, Attila

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