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On the deformation behavior and cracking of ductile iron; effect of microstructure
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-5635-8023
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the effect of microstructural variation on the mechanical properties and deformation behavior of ductile iron. To research and determine these effects, two grades of ductile iron, (i) GJS-500-7 and (ii) high silicon GJS-500-14, were cast in a geometry containing several plates with different section thicknesses in order to produce microstructural variation. Microstructural investigations as well as tensile and hardness tests were performed on the casting plates. The results revealed higher ferrite fraction, graphite particle count, and yield strength in the high silicon GJS-500-14 grade compared to the GJS-500-7 grade.

To study the relationship between the microstructural variation and tensile behavior on macroscale, tensile stress-strain response was characterized using the Ludwigson equation. The obtained tensile properties were modeled, based on the microstructural characteristics, using multiple linear regression and analysis of variance (ANOVA). The models showed that silicon content, graphite particle count, ferrite fraction, and fraction of porosity are the major contributing factors that influence tensile behavior. The models were entered into a casting process simulation software, and the simulated microstructure and tensile properties were validated using the experimental data. This enabled the opportunity to predict tensile properties of cast components with similar microstructural characteristics.

To investigate deformation behavior on micro-scale, a method was developed to quantitatively measure strain in the microstructure, utilizing the digital image correlation (DIC) technique together with in-situ tensile testing. In this method, a pit-etching procedure was developed to generate a random speckle pattern, enabling DIC strain measurement to be conducted in the matrix and the area between the graphite particles. The method was validated by benchmarking the measured yield strength with the material’s standard yield strength.

The microstructural deformation behavior under tensile loading was characterized. During elastic deformation, strain mapping revealed a heterogeneous strain distribution in the microstructure, as well as shear bands that formed between graphite particles. The crack was initiated at the stress ranges in which a kink occurred in the tensile curve, indicating the dissipation of energy during both plastic deformation and crack initiation. A large amount of strain localization was measured at the onset of the micro-cracks on the strain maps. The micro-cracks were initiated at local strain levels higher than 2%, suggesting a threshold level of strain required for micro-crack initiation.

A continuum Finite Element (FE) model containing a physical length scale was developed to predict strain on the microstructure of ductile iron. The material parameters for this model were calculated by optimization, utilizing the Ramberg-Osgood equation. The predicted strain maps were compared to the strain maps measured by DIC, both qualitatively and quantitatively. To a large extent, the strain maps were in agreement, resulting in the validation of the model on micro-scale.

In order to perform a micro-scale characterization of dynamic deformation behavior, local strain distribution on the microstructure was studied by performing in-situ cyclic tests using a scanning electron microscope (SEM). A novel method, based on the focused ion beam (FIB) milling, was developed to generate a speckle pattern on the microstructure of the ferritic ductile iron (GJS-500-14 grade) to enable quantitative DIC strain measurement to be performed. The results showed that the maximum strain concentration occurred in the vicinity of the micro-cracks, particularly ahead of the micro-crack tip.

Abstract [sv]

Denna avhandling fokuserar på effekten av variationer i mikrostrukturen på mekaniska egenskaper och deformationsbeteende hos segjärn. För att undersöka dessa effekter, två olika sorter av segjärn, (i) GJS-500-7 och (ii) högkisellegerad GJS-500-14, gjutits till plattor av olika tjocklekar för att generera mikrostrukturvariationen. Mikrostrukturundersökning, samt drag- och hårdhetsprov gjordes på de gjutna plattorna. Resultaten visade att en högre ferritfraktion, grafitpartikelantal och sträckgräns i den högkisellegerade GJS-500-14-sorten jämfört med GJS-500-7.

För att studera förhållandet mellan mikrostrukturell variation och spännings-töjningsbeteendet på makroskala, modellerades detta med hjälp av Ludwigson-ekvationen. De erhållna spännings-töjningsegenskaperna modellerades baserat på mikrostrukturell karaktäristika genom multipel linjärregression och variansanalys (ANOVA). Modellerna visade att kiselhalt, grafitpartikelantal, ferritfraktion och porfraktion var de viktigaste bidragande faktorerna. Modellerna implementerades i ett simuleringsprogram för gjutningsprocessen. Resultatet från simuleringen validerades med hjälp av experimentella data som inte ingick i underlaget för regressionsanalysen. Detta möjliggjorde att prediktera spännings-töjningsbeteendet och dess variation hos gjutna segjärns komponenter med liknande sammansättning och gjutna tjocklekar som användes i denna studie.

För att kunna undersöka deformationsbeteendet på mikroskala utvecklades en metod för kvantitativ mätning av töjning i mikrostrukturen, genom DIC-tekniken (digital image correlation) tillsammans med in-situ dragprovning. I denna metod utvecklades en grop-etsningsprocess för att generera ett slumpvis prickmönster, vilket möjliggjorde DIC-töjningsmätning i matrisen och i området mellan grafitpartiklarna med tillräcklig upplösning. Metoden validerades genom benchmarking av den uppmätta sträckgränsen mot materialets makroskopiska sträckgräns mätt med konventionell dragprovning.

Det mikrostrukturella deformationsbeteendet under dragbelastning karakteriserades. Under elastisk deformation avslöjade töjningsmönstret en heterogen töjningsfördelning i mikrostrukturen, och bildandet av skjuvband mellan grafitpartiklar. Sprickbildning initierades vid låg spänning och redan vid de spänningsnivåer som ligger vis ”knät” på dragprovningskurvan, vilket indikerar energidissipering genom både begynnande plastisk deformation och sprickbildning. Den lokala töjningen vis sprickinitiering skedde då den lokala töjningen översteg 2%, vilket indikerar att detta skulle kunna vara en tröskelnivå för den töjning som erfordras för initiering av mikro-sprickor.

En kontinuum Finita Element (FE) modell utvecklades för att prediktera töjningen hos ett segjärn och dess fördelning i segjärns mikrostruktur. Materialparametrarna för denna modell optimerades genom att anpassa parametrarna i Ramberg-Osgood ekvationen. De predikterade töjningsfördelningarna jämfördes med de experimentell uppmätta töjningsmönstren uppmätta med DIC, både kvalitativt och kvantitativt. Töjningsmönstren överensstämde i stor utsträckning, vilket resulterade i att modellerna kunde anses vara validerade på mikronivå.

För att kunna mäta töjningsmönster under dynamiska förlopp på mikronivå utvecklades en metod för att skapa prickmönster och att utföra in-situ CT provning i ett svepeletronmikroskop (SEM). Prickmönstret skapades genom avverkning med en fokuserad jonstråle (FIB), och provades på det ferritiska segjärnet (GJS-500-14 grad). Resultaten visade att maximal töjningskoncentration fanns i närheten av mikrosprickorna, framförallt framför sprickspetsen.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2017. , 75 p.
Series
JTH Dissertation Series, 27
Keyword [en]
Spherical graphite iron, component casting, high silicon ductile iron, digital image correlation (DIC), in-situ tensile testing, in-situ cyclic testing, DIC pattern generation, pit etching, micro-scale deformation, micro-crack, finite element analysis (FEA), focused ion beam (FIB) milling
Keyword [sv]
segjärn, komponentgjutning, högkisellegerat segjärn, digital image correlation (DIC), insitu dragprovning, in-situ cyklisk provning, DIC-mönstergenerering, grop-etsning, mikroskalig deformation, mikrosprickor, finite element analys (FEA), fokuserad jonstråle (FIB) avverkning
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-36852ISBN: 978-91-87289-28-6 (print)OAI: oai:DiVA.org:hj-36852DiVA: diva2:1131392
Public defence
2017-08-08, E1405 (Gjuterisalen), Tekniska Högskolan, Jönköping, 10:00
Opponent
Supervisors
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-08-29Bibliographically approved
List of papers
1. Characterization and modeling of the mechanical behavior of high silicon ductile iron
Open this publication in new window or tab >>Characterization and modeling of the mechanical behavior of high silicon ductile iron
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2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 708, 159-170 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the effect of the solidification conditions and silicon content on the mechanical properties of ductile iron and presents empirical models for predicting the tensile behavior based on the microstructural characterizations. Two ductile iron grades of GJS-500-7 and GJS-500-14 were cast with silicon content of 2.36% and 3.71%, respectively. The cast geometry consisted of six plates with different thicknesses that provided different cooling rates during the solidification. Microstructure analysis, tensile and hardness tests were performed on the as-cast material. Tensile behavior was characterized by the Ludwigson equation. The tensile fracture surfaces were analyzed to quantify the fraction of porosity. The results showed that graphite content, graphite nodule count, ferrite fraction and yield strength were increased by increasing the silicon content. A higher silicon content resulted in lower work hardening exponent and strength coefficient on the Ludwigson equation. The results for 0.2% offset yield and the Ludwigson equation parameters were modeled based on microstructural characteristics, with influence of silicon content as the main contributing factor. The models were implemented into a casting process simulation to enable prediction of microstructure-based tensile behavior. A good agreement was obtained between measured and simulated tensile behavior, validating the predictions of simulation in cast components with similar microstructural characteristics.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Casting simulation Component casting Ludwigson equation parameters Silicon content Spherical graphite iron
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36849 (URN)10.1016/j.msea.2017.09.115 (DOI)2-s2.0-85030708426 (Scopus ID)
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-10-18Bibliographically approved
2. Development of a pattern making method for strain measurement on microstructural level in ferritic cast iron
Open this publication in new window or tab >>Development of a pattern making method for strain measurement on microstructural level in ferritic cast iron
2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The current paper focuses on development of a method for studying micro-scale strains on the microstructure of ferritic cast iron. For this purpose, in-situ tensile tests were done under the optical microscope combined with digital image correlation (DIC). Critical in this development was to be able to achieve a reliable high spatial resolution of strain around microstructural features, such as graphite particles. Measurement of local strain fi elds in cast iron materials have so far been relying on displacement of naturally occurring microstructure patterns such as graphite particles, which limits the spatial resolution of strain measurement. In order to increase the spatial resolution of the measured strain, a pit etching procedure was applied to generate a random speckle pattern on the ferritic matrix. Th e critical challenges of in-situ investigation of microstructural deformation were identifi ed as speckle pattern quality and accurate selection of subset size and strain window size. Th e traceability of this method was studied by benchmarking the measured elastic modulus with that obtained from full-scale tensile test. Th e elastic modulus calculated from average strains, measured by DIC, showed a good agreement with material’s elastic modulus. Th is validates the measured localized strain values and can be used as a validation for modeling of local deformation.

Keyword
Ferritic cast iron, Digital image correlation (DIC), In-situ tensile test, Pit etching, Subset size, Strain window size
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28326 (URN)
Conference
23rd International Conference on Processing and Fabrication of Advanced Materials (PFAM-XXIII), Roorkee, India, December 5-7, 2014
Available from: 2015-11-16 Created: 2015-11-16 Last updated: 2017-08-14Bibliographically approved
3. Microstructural strain distribution in ductile iron: Comparison between finite element simulation and digital image correlation measurements
Open this publication in new window or tab >>Microstructural strain distribution in ductile iron: Comparison between finite element simulation and digital image correlation measurements
2016 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 655, 27-35 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a study on microstructural deformation of a ferritic–pearlitic ductile iron, utilizing in-situ tensile testing, digital image correlation (DIC) and finite element analysis (FEA). For this purpose, the in-situ tensile test and DIC were used to measure local strain fields in the deformed microstructure. Furthermore, a continuum finite element (FE) model was used to predict the strain maps in the microstructure. Ferrite and pearlite parameters for the FE-model were optimized based on the Ramberg–Osgood relation. The DIC and simulation strain maps were compared qualitatively and quantitatively. Similar strain patterns containing shear bands in identical locations were observed in both strain maps. The average and localized strain values of the DIC and simulation conformed to a large extent. It was found that the Ramberg–Osgood model can be used to capture the main trends of strain localization. The discrepancies between the simulated and DIC results were explained based on the; (i) subsurface effect of the microstructure; (ii) differences in the strain spatial resolutions of the DIC and simulation and (iii) abrupt changes in strain prediction of the continuum FE-model in the interface of the phases due to the sudden changes in the elastic modulus.

Keyword
Microstructural deformation; Ductile iron; Digital image correlation (DIC); In-situ tensile test; Finite elements analysis (FEA)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28330 (URN)
Available from: 2015-11-17 Created: 2015-11-17 Last updated: 2017-12-01Bibliographically approved
4. Micro-Crack Initiation in High-Silicon Cast Iron during Tension Loading
Open this publication in new window or tab >>Micro-Crack Initiation in High-Silicon Cast Iron during Tension Loading
2015 (English)In: TMS2015 Supplemental Proceedings, The Minerals, Metals, and Materials Society, 2015, John Wiley & Sons, 2015, 947-953 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
Micro-crack, local strain, ferritic cast iron, digital image correlation (DIC), in-situ tensile testing
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28327 (URN)10.1002/9781119093466.ch115 (DOI)9781119082415 (ISBN)
Conference
144th Annual Meeting and Exhibition: Connecting the Global Minerals, Metals, and Materials Community, TMS 2015, Walt Disney World Orlando, United States, 15 March 2015 through 19 March 2015
Available from: 2015-11-16 Created: 2015-11-16 Last updated: 2017-08-14Bibliographically approved
5. Strain localization and crack formation effects on stress-strain response of ductile iron
Open this publication in new window or tab >>Strain localization and crack formation effects on stress-strain response of ductile iron
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2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 702, 265-271 p.Article in journal (Refereed) Published
Abstract [en]

The strain localization and crack formation in ferritic-pearlitic ductile iron under tension was investigated by in-situ tensile tests. In-situ tensile tests under optical microscope were performed and the onset of the early ferrite-graphite decohesions and micro-cracks inside the matrix were studied. The results revealed that early ferrite-graphite decohesion and micro-cracks inside the ferrite were formed at the stress range of 280–330 MPa, where a kink occurred in the stress-strain response, suggesting the dissipation of energy in both plastic deformation and crack initiation. Some micro-cracks initiated and propagated inside the ferrite but were arrested within the ferrite zone before propagating in the pearlite. Digital Image Correlation (DIC) was used to measure local strains in the deformed micrographs obtained from the in-situ tensile test. Higher strain localization in the microstructure was measured for the areas in which the early ferrite-graphite decohesions occurred or the micro-cracks initiated.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Cracking, Digital image correlation, Graphite-matrix decohesion, In-situ tensile test, Micro-crack, Crack initiation, Ductility, Ferrite, Ferritic steel, Graphite, Image analysis, Pearlite, Strain, Strain measurement, Tensile testing, D. digital image correlation (DIC), De-cohesion, Digital image correlations, Local strains, Strain localizations, Stress range, Stress-strain response, Cracks
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36848 (URN)10.1016/j.msea.2017.07.018 (DOI)000407983500030 ()2-s2.0-85024504785 (Scopus ID)JTHMaterialIS (Local ID)JTHMaterialIS (Archive number)JTHMaterialIS (OAI)
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-09-07Bibliographically approved
6. Microstructural strain mapping during in-situ cyclic testing of ductile iron
Open this publication in new window or tab >>Microstructural strain mapping during in-situ cyclic testing of ductile iron
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36851 (URN)
Note

Manuscript, to be submitted for journal publication, 2017.

Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-08-14

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Kasvayee, Keivan Amiri

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