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The morphological evolution of primary austenite during isothermal coarsening
Jönköping University, School of Engineering, JTH, Materials and Manufacturing.ORCID iD: 0000-0002-6339-4292
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: 0000-0002-3024-9005
2017 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 131, p. 492-499Article in journal (Refereed) Published
Abstract [en]

The morphological evolution of primary austenite in an industrial hypoeutectic lamellar cast iron was studied under isothermal conditions for coarsening times varying from 0 min to 96 h. The dendritic austenite structure formed during the primary solidification suffered major morphological changes during the isothermal coarsening process. After a sufficient coarsening time, dendrite fragmentation, globularization, and coalescence of austenite were studied using electron backscatter diffraction (EBSD) technique. This study confirmed that the secondary dendrite arm spacing (SDAS) is an inappropriate length scale to describe the primary austenite coarsening process for longer times. The application of shape independent quantitative parameters confirmed the reduction of the total interfacial area during microstructural coarsening. The modulus of the primary austenite, Mγ, which represents the volume-surface ratio for the austenite phase, and the spatial distribution of the austenite particles, measured as the nearest distance between the center of gravity of neighboring particles, Dγ, followed a linear relation with the cube root of coarsening time during the whole coarsening process. The mean curvature of the austenite interface, characterized through stereological relations, showed a linear relation to Mγ and Dγ, allowing the quantitative characterization and modeling of the complete coarsening process of primary austenite.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 131, p. 492-499
Keywords [en]
Dendrite fragmentation, Dendritic coarsening, EBSD, Microstructure evolution, Primary austenite, Austenite, Cast iron, Isotherms, Microstructure, Dendrite fragmentations, Electron backscatter diffraction technique, Micro-structure evolutions, Microstructural coarsening, Quantitative characterization, Secondary dendrite arm spacing, Coarsening
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-36890DOI: 10.1016/j.matchar.2017.07.030ISI: 000411535600054Scopus ID: 2-s2.0-85026387205OAI: oai:DiVA.org:hj-36890DiVA, id: diva2:1133573
Note

Included in licentiate thesis in submitted form.

Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2019-04-29Bibliographically approved
In thesis
1. 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. p. 50
Series
JTH Dissertation Series ; 23
Keywords
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
2. 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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Hernando, Juan CarlosGhassemali, EhsanDiószegi, Attila

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