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The effect of inoculation on the thermal expansion/contraction during solidification of gray cast iron
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-7474-2053
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2014 (English)In: 6th International Conference on Solidification and Gravity, 2014, p. 447-451Conference paper, Published paper (Refereed)
Abstract [en]

Inoculation of casting used to improve the microstructure and the properties of the component. Depending upon the area of application, gray cast iron has different microstructure and mechanical properties. The type and amount of the inoculation result in shape and orientation differences of the flake graphite. The Linear Variable Differential Transformer (LVDT) shows a variation in displacements change during the solidification. Temperature measurement used to analyze cooling curves and microstructural analysis of sample to examine the physical differences. The microstructural study used for describing the variation in area fraction and shape of graphite. The experimental result indicates contraction in austenite formation region, and expansion in eutectic formation region. The effect of the inoculant and the superheat temperature shows a variation in degree of expansion/contraction and cooling rates of the experiments. Effective inoculation results in homogenizing the eutectic structure, reducing the undercooled and interdendritic graphite throughout the structure. Inoculation of the alloy results in higher expansion in eutectic formation region. Differences in eutectic formation region were observed due to variation in equilibrium point, and it needs carful investigation in future works.

Place, publisher, year, edition, pages
2014. p. 447-451
Keyword [en]
Austenite, Eutectic, Flake graphite, Inoculation and LVDT, Eutectics, Graphite, Mechanical properties, Microstructure, Phase diagrams, Temperature measurement, Austenite formation, Eutectic structures, Inoculation results, Linear variable differential transformer, Microstructural analysis, Microstructure and mechanical properties, Solidification
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-167049DOI: 10.4028/www.scientific.net/MSF.790-791.447Scopus ID: 2-s2.0-84901476188ISBN: 9783038350934 (print)OAI: oai:DiVA.org:kth-167049DiVA, id: diva2:812998
Conference
2 September 2013 through 5 September 2013, Miskolc, Lillafured
Note

QC 20150521

Available from: 2015-05-21 Created: 2015-05-21 Last updated: 2017-02-28Bibliographically approved
In thesis
1. On the Volume Changes during the Solidification of Cast Irons and Peritectic Steels
Open this publication in new window or tab >>On the Volume Changes during the Solidification of Cast Irons and Peritectic Steels
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis work deals with the volume changes during the solidification of cast irons and peritectic steels. The volume changes in casting metals are related to the expansion and/or contraction of the molten metal during solidification. Often, different types of shrinkage, namely macro- and micro-shrinkage, affect the casting quality. In addition to that, exposure of the metal casting to higher contraction or expansion during the solidification might also be related to internal strain development in samples, which eventually leads to surface crack propagation in some types of steel alloys during continuous casting. In consequence, a deep understanding of the mechanisms and control of the solidification will improve casting quality and production.

All of the experiments during the entire work were carried out on laboratory scale samples. Displacement changes during solidification were measured with the help of a Linear Variable Displacement Transformer (LVDT). All of the LVDT experiments were performed on samples inside a sand mould. Simultaneously, the cooling curves of the respective samples during solidification were recorded with a thermocouple. By combining the displacement and cooling curves, the volume changes was evaluated and later used to explain the influence of inoculants, carbon and cooling rates on volume shrinkages of the casting. Hypoeutectic grey cast iron (GCI) and nodular cast iron (NCI) with hypo-, hyper- and eutectic carbon compositions were considered in the experiments from cast iron group. High nickel alloy steel (Sandvik Sanbar 64) was also used from peritectic steel type. These materials were melted inside an induction furnace and treated with different types of inoculants before and during pouring in order to modify the composition.

Samples that were taken from the LVDT experiments were investigated using a number of different  methods in order to support the observations from the displacement measurements:  Differential Thermal Analysis (DTA), to evaluate the different phase present; Dilatometry, to see the effect of cooling rates on contraction for the various types of alloys; metallographic studies with optical microscopy; Backscattered electrons (BSE) analysis on SEM S-3700N, to investigate the different types of oxide and sulphide nuclei; and bulk density measurements  by applying Archimedes' principle. Furthermore, the experimental volume expansion during solidification was compared with the theoretically calculated values for GCI and NCI.

It was found that the casting shows hardly any shrinkage during early solidification in GCI, but in the eutectic region the casting expands until the end of solidification. The measured and the calculated volume changes are close to one another, but the former shows more expansion. The addition of MBZCAS (Si, Ca, Zr, Ba, Mn and Al) promotes more flake graphite, and ASSC (Si, Ca, Sr and Al) does not increase the number of eutectic cells by much. In addition to that, it lowers the primary austenite fraction, promotes more eutectic growth and decreases undercooled graphite and secondary dendritic arm spacing (SDAS). As a result, the volume expansion changes in the eutectic region. The expansion during the eutectic growth increase with an increase in the inoculant weight percentage. At the same time, the eutectic cells become smaller and increase in number. The effect of the inoculant and the superheat temperature shows a variation in the degree of expansion/contraction and the cooling rates for the experiments. Effective inoculation tends to homogenize the eutectic structure, reducing the undercooled and interdendritic graphite throughout the structure.

In NCI experiments, it was found that the samples showed no expansion in the transversal direction due to higher micro-shrinkages in the centre, whereas in the longitudinal direction the samples shows expansion until solidification was complete.   The theoretical and measured volume changes agreed with each other. The austenite fraction and number of micro-shrinkage pores decreased with increase in carbon content. The nodule count and distribution changes with carbon content. The thermal contraction

of NCI is not influenced by the variation in carbon content at lower cooling rates. The structural analysis and solidification simulation results for NCI show that the nodule size and count distribution along the cross-sections at various locations are different due to the variation in cooling rates and carbon concentration. Finer nodule graphite appears in the thinner sections and close to the mold walls. A coarser structure is distributed mostly in the last solidified location. The simulation result indicates that finer nodules are associated with higher cooling rate and a lower degree of microsegregation, whereas the coarser nodules are related to lower cooling rate and a higher degree of microsegregation. As a result, this structural variation influences the micro-shrinkage in different parts.

The displacement change measurements show that the peritectic steel expands and/or contracts during the solidification. The primary austenite precipitation during the solidification in the metastable region is accompanied by gradual expansion on the casting sides. Primary δ-ferrite precipitation under stable phase diagram is complemented by a severe contraction during solidification. The microstructural analysis reveals that the only difference between the samples is grain refinement with Ti addition. Moreover, the severe contraction in solidification region might be the source for the crack formation due to strain development, and further theoretical analysis is required in the future to verify this observation.

Place, publisher, year, edition, pages
Stockholm: Kungliga Tekniska högskolan, 2017. p. 50
Keyword
Volume changes; Solidification; Peritectic steel; Micro- and Macro-shrinkage; LVDT; GCI; NCI; Hypo- and Hypereutectic; Austenite; DTA; Dilatometry; BSE; Nuclei; Flake graphite; SDAS; Eutectic cells; Undercooled graphite; Simulation; Cooling rate; Microsegregation; austenite; Metastable region; δ-ferrite; Stable phase diagram; Grain refinement; Strain.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-202558 (URN)978-91-7729-299-9 (ISBN)
Public defence
2017-04-07, B1, Brinellvägen 23, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20170228

Available from: 2017-02-28 Created: 2017-02-27 Last updated: 2017-03-06Bibliographically approved

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