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Revisiting the graphite nodule in ductile iron
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-7527-719X
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-6339-4292
The Ohio State University, Columbus, OH, United States.
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-3024-9005
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2019 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 161, p. 66-69Article in journal (Refereed) Published
Abstract [en]

The growth mechanism of graphite nodules in ductile iron was experimentally investigated using high-resolution 3D tomography of an individual graphite nodule in a near-eutectic ductile iron. The dual beam scanning electron microscopy (FIB-SEM) technique was used for this purpose. Iron particles elongated in the radial direction were observed inside a graphite nodule. Some micro-voids were detected inside the nodule, mostly located at the end of the iron particles. These observations were compared with established theories about the growth of graphite nodules and iron entrapment/engulfment in between the graphite sectors during solidification of ductile iron. 

Place, publisher, year, edition, pages
Elsevier, 2019. Vol. 161, p. 66-69
Keywords [en]
3D reconstruction, FIB, Growth mechanism, Solidification, Ductility, Graphite, Iron, Scanning electron microscopy, 3-d tomographies, Graphite nodules, Growth mechanisms, High resolution, Iron Particles, Micro voids, Radial direction, Cast iron
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:hj:diva-41986DOI: 10.1016/j.scriptamat.2018.10.018ISI: 000450375500015Scopus ID: 2-s2.0-85055115924Local ID: JTHMaterialISOAI: oai:DiVA.org:hj-41986DiVA, id: diva2:1261326
Available from: 2018-11-07 Created: 2018-11-07 Last updated: 2019-04-29Bibliographically approved
In thesis
1. The role of primary austenite morphology in cast iron
Open this publication in new window or tab >>The role of primary austenite morphology in cast iron
2019 (English)Doctoral 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. Urged by strict environmental regulations on emissions, these components constantly need to combine higher demands on performance with lighter designs. As a result, cast iron industry continuously faces new challenges related to solidification of new alloys, component designs and casting processes.

Complex shapes, variations in the thickness of the casting and the molding material strongly influence the solidification time for a component, thus varying its microstructural coarseness and hence showing different properties depending on the local shape of the casting.

This work increases our understanding of the morphological evolution of primary austenite occurring during isothermal coarsening at the semi-solid state. New experimental techniques have been developed to show that primary austenite coarsens according to the Ostwald ripening model in lamellar (LGI), compacted (CGI)and spheroidal (SGI) graphite iron. Significant morphological changes occur after long coarsening times, including dendrite fragmentation and coalescence. The quantitative characterization of the morphological changes during coarsening is accurately described by morphological parameters, i.e., Mγ, DIDHyd and DγNN.

Subsequently, the impact of primary austenite morphology on the eutectic microstructures in CGI and SGI has been investigated. It was observed that the eutectic microstructures are not significantly affected by the surface area of primary austenite and the size of the interdendritic regions. Fraction, nodularity, shape distribution of graphite particles and the number of nodules and eutectic cells are similar as a function of coarsening time. These results suggest that the nucleation frequency and growth of eutectic microstructures are not significantly influenced by the morphology of primary austenite.

Furthermore, miniaturized tensile tests demonstrated that the UTS in CGI is directly related to the primary austenite morphology. The UTS decreases with the increasing coarseness of primary austenite, showing an inverse linear relation to Mγ, DIDHyd and DγNN. These results demonstrate the strong impact of primary austenite morphology on UTS when the eutectic and eutectoid microstructures are similar, emphasizing the importance of incorporating the morphology of primary austenite in our models.

Abstract [sv]

Fordonsindustrins produktportfölj innehåller en mängd olika gjutjärnskomponenter med komplicerad geometri, exempelvis komponenter till lastbilsmotorer. Skärpt lagstiftning beträffande olika miljöpåverkande utsläpp har höjt kraven på sådana komponenter vad gäller både mekaniska egenskaper och vikt. Gjutjärnsgjuterierna måste ständigt hantera nya utmaningar beträffande stelning av ny legeringar, komponentkonstruktion och gjutprocesser.

Den lokala stelningstiden kommer att variera med varierande tjocklek på såväl gjutstycke som formmaterial vilket ger stora variationer i mikrostrukturens grovlek. Denna mikrostrukturvariation leder till väsentliga skillnader i mekaniska egenskaper inom ett gjutstycke med komplex geometri.

I detta arbete studeras hur den primära austenitens morfologi förändras under så kallad isoterm förgrovning i gjutjärn i delvis stelnat tillstånd. Nyutvecklade experimentella metoder har använts för att visa att austeniten i gjutjärn med lamellärgrafit (LGI), vermikulär/kompakt grafit (CGI) samt sfärisk grafit (SGI) förgrovas i enlighet med Ostwalds modell för förgrovning. Austenitmorfologin förändras högstavsevärt efter långa hålltider vid relevanta temperaturer för förgrovning, även genom dendritfragmentering och koalescens. Morfologiförändringarna kankvantifieras tämligen väl med morfologiparametrarna Mγ, DIDHyd och DγNN.

Vidare har den primära austenitens påverkan på den eutektiska mikrostrukturen i CGI och SGI studerats. En observation var att den eutektiska mikrostrukturen inte påverkas av ytarean av den primära austeniten eller av storleken av de interdendritiska områdena. Fraktionen grafit, nodulariteten, grafitens formfördelning, antalet noduler samt antalet eutektiska celler påverkas på likartat sätt av förgrovningsprocessen. Detta antyder att kärnbildningsfrekvensen och tillväxten av eutektisk struktur inte påverkas i någon större utsträckning av den primära austenitens morfologi.

Dragprovning av stavar i miniatyrformat visade att dragbrottgränsen (UTS) hos CGI är direkt beroende av den primära austenitens morfologi. UTS är linjärt avtagande med ökande Mγ, DIDHyd och DγNN vilket visar att den primära austenitens morfologi har en stark påverkan på gjutjärns mekaniska egenskaper. Alltså bör de modeller som används för att prediktera gjutjärns egenskaper innehålla den primära austenitens morfologi.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2019. p. 74
Series
JTH Dissertation Series ; 040
Keywords
Solidification, Cast Iron, Primary Austenite, Microstructure Evolution, Coarsening, LGI, CGI, SGI, UTS, stelning, gjutjärn, primär austenit, mikrostrukturbildning, förgrovning, LGI, CGI, SGI, UTS
National Category
Materials Engineering
Identifiers
urn:nbn:se:hj:diva-43553 (URN)978-91-87289-42-2 (ISBN)
Public defence
2019-05-21, E1405, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-04-29Bibliographically approved

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Ghassemali, EhsanHernando, Juan CarlosDiószegi, AttilaJarfors, Anders E.W.

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