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Quantification of dendritic austenite after interrupted solidification in a hypoeutectic lamellar graphite iron
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.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
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. Vol. 5, no 1, 28-42 p.
Keyword [en]
Cast iron, Quantitative metallography, Color metallography, Microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-28880DOI: 10.1007/s13632-015-0250-0ISI: 000377604200007Scopus ID: 2-s2.0-84960404906OAI: oai:DiVA.org:hj-28880DiVA: diva2:891457
Projects
Spofic II
Funder
VINNOVA, 2013-04720
Available from: 2016-01-07 Created: 2016-01-07 Last updated: 2017-07-03Bibliographically approved
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. 48 p.
Series
JTH research report, ISSN 1404-0018 ; 24
Keyword
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
2. Morphological characterization of primary austenite in cast iron
Open this publication in new window or tab >>Morphological characterization of primary austenite in cast iron
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Automotive industry products portfolio includes a wide variety of complex‐shaped cast iron products, such as truck engine components, that need to withstand a constant trend of higher demands, especially urged by stricter environmental regulations on emissions. Combined with this continued demand on properties improvement, cast iron industry faces a process problem related to the lack of understanding of solidification and mechanisms behind defect formation.

Casting products are highly affected by the product design and the manufacturing method itself, which governs the final microstructure and hence the final mechanical properties. Wall thickness of the moulding material strongly influences the solidification time, varying the microstructural coarseness, resulting in a component with different properties depending on the local shape of the casting.

The main objective of this work is the characterization of the primary austenite microstructure and its coarsening process, which has been poorly documented in cast iron literature, to allow the prediction and control of these microstructural features present in the casting.

The microstructural evolution of the primary austenite in hypoeutectic lamellar graphite iron (LGI) is studied under isothermal coarsening conditions. The dendritic microstructure suffered major morphological changes that included dendrite fragmentation, globularization, and coalescence. Empirical relations based on morphological parameters are introduced to predict the microstructural evolution of primary austenite. A novel technique for colour‐etching and semi‐automatic image analysis for the characterization of quenched dendritic microstructures in cast iron is presented. A new experimental technique for production of graphitic iron with varying nodularity is presented as a solution to control the production of compacted (CGI) and spheroidal graphite iron (SGI) under laboratory conditions. The nodularity evolution is controlled as a function of the holding time and the residual Mg, allowing the study of the primary solidification and primary microstructures of hypoeutectic CGI and SGI in future investigations.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. 50 p.
Series
JTH Dissertation Series, 23
Keyword
Lamellar Graphite Iron, Solidification, Primary Austenite, Microstructure Evolution, Dendritic coarsening, Compacted Graphite Iron, Magnesium Fading, Nodularity
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-35585 (URN)9789187289248 (ISBN)
Supervisors
Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-08-16Bibliographically approved

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