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On the solidification of compacted and spheroidal graphite irons
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0001-6938-037X
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. , 48 p.
Series
JTH research report, ISSN 1404-0018 ; 24
Keyword [en]
Compacted graphite iron, Spheroidal graphite iron, Solidification, Microsegregation
Keyword [sv]
Kompaktgrafitjärn, segjärn, stelning, mikrosegregation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-35533ISBN: 9789187289255 OAI: oai:DiVA.org:hj-35533DiVA: diva2:1095932
Supervisors
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-16Bibliographically 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
2016 (English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35530 (URN)
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-16Bibliographically 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
2016 (English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35531 (URN)
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-16Bibliographically approved
3. 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, 28-42 p.Article 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
Keyword
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
Available from: 2016-01-07 Created: 2016-01-07 Last updated: 2017-07-03Bibliographically approved

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