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On the Solidification and Structure Formation during Casting of Large Inserts
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.ORCID iD: 0000-0002-7474-2053
2017 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916Article in journal (Other academic) [Artistic work] Submitted
Abstract [en]

Graphite nodule count and size distributions for Boiling Water Reactor (BWR) and Pressure Water Reactor (PWR) inserts have been investigated by taking samples from heights of 2160 and 1150 mm, respectively. In each cross-section, two locations were taken into consideration for both the microstructural and solidification modeling. The numerical solidification modeling was performed in a two-dimension model by considering nucleation and cell growth in a eutectic ductile cast iron. The microstructural result reveals that the nodule size and count distribution along the cross-sections are different in each location for both inserts. Finer nodule graphites appear in the thinner sections and close to the mold walls. Coarser structure is distributed mostly in the last solidified location. The simulation result describes the finer nodules are related with higher cooling rate and lower degree of microsegregation, whereas the coarser are related to lower cooling rate and a higher degree of microsegregation. The solidification time interval and the last solidifying locations for BWR and PWR are also different. 

Place, publisher, year, edition, pages
2017.
Keyword [en]
Microstructure; Solidification modeling; Nucleation; Eutectic ductile cast iron; Degree of microsegregation
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-202557OAI: oai:DiVA.org:kth-202557DiVA: diva2:1077424
Note

QC 20170228

Available from: 2017-02-27 Created: 2017-02-27 Last updated: 2017-11-29Bibliographically 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. 50 p.
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)
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Supervisors
Note

QC 20170228

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

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