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Effect of Crystallinity on the Thermal Diffusivity of Mould Fluxes for the Continuous Casting of Steels
KTH, Superseded Departments, Materials Science and Engineering.
KTH, Superseded Departments, Materials Science and Engineering.
KTH, Superseded Departments, Materials Science and Engineering.
2004 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 44, no 4, 691-697 p.Article in journal (Refereed) Published
Abstract [en]

The thermal diffusivities of mould flux having the glassy and crystalline states were measured as a function of temperature by the laser flash method to obtain the relationship between the thermal diffusivity and the degree of crystallization. The thermal diffusivities of the liquid mould flux were also measured to compare the data with those of the solid samples. The thermal diffusivity increases roughly linearly with an increase in the degree of crystallization. The thermal diffusivities of glassy and liquid samples having the ratios of NBO/T=1.48, 1.65 and 2.11 exhibit roughly the same values of 4.6*10-7 m2 s-1 and 4.0*10-7 m2 s-1, respectively. It is considered that because the silicate network is largely broken down, there is not a significant change of the structure for these samples.

Place, publisher, year, edition, pages
2004. Vol. 44, no 4, 691-697 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-5406OAI: oai:DiVA.org:kth-5406DiVA: diva2:9765
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2017-11-21Bibliographically approved
In thesis
1. Experimental Studies of Thermal Diffusivities concerning some Industrially Important Systems
Open this publication in new window or tab >>Experimental Studies of Thermal Diffusivities concerning some Industrially Important Systems
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The main objective of this industrially important work was to gain an increasing understanding of the properties of some industrially important materials such as CMSX-4 nickel base super alloy, 90Ti.6Al.4V alloy, 25Cr:6Ni stainless steel, 0.7% carbon steel, AISI 304 stainless steel-alumina composites, mould powder used in continuous casting of steel as well as coke used in blast furnace with special reference to the thermal diffusivities. The measurements were carried out in a wide temperature range covering solid, liquid, glassy and crystalline states.

For CMSX-4 alloy, the thermal conductivities were calculated from the experimental thermal diffusivities. Both the diffusivities and conductivities were found to increase with increasing temperature. Microscopic analysis showed the presence of intermetallic phases γ´ such as Ni3Al below 1253 K. In this region, the mean free path of the electrons and phonons is likely to be limited by scattering against lattice defects. Between 1253 K and solidus temperature, these phases dissolved in the alloy adding to the impurities in the matrix, which, in turn, caused a decrease in the thermal diffusivity. This effect was confirmed by annealing the samples at 1573 K. The thermal diffusivities of the annealed samples measured at 1277, 1403 and 1531 K were found to be lower than the thermal diffusivities of non-annealed samples and the values did not show any noticeable change with time. It could be related to the attainment of equilibrium with the completion of the dissolution of γ´ phase during the annealing process. Liquid CMSX-4 does not show any change of thermal diffusivity with temperature. It may be attributed to the decrease of the mean free path being shorter than characteristic distance between two neighbouring atoms.

Same tendency could be observed in the case of 90Ti.6Al.4V alloy. Since the thermal diffusivity increases with increasing temperature below 1225 K and shows slight decrease or constancy at higher temperature. For 25Cr:6Ni stainless steel, the thermal diffusivity is nearly constant up to about 700 K. Beyond that, there is an increase with temperature both during heating as well as cooling cycle. On the other hand, the slope of the curve increases above 950 K, which can be due to the increase of bcc phase in the structure. 0.7% carbon steel shows a decrease in the thermal diffusivity at temperature below Curie point, where the structure contains bcc+ fcc phases. Above this point the thermal diffusivity increases, where the structure contains only fcc phase. The experimental thermal conductivity values of these alloys show good agreement with the calculated values using Mills model.

Thermal diffusivity measurements as a function of temperature of sintered AISI 304 stainless steel-alumina composites having various composition, viz, 0.001, 0.01, 0.1, 1, 2, 3, 5, 7, 8 and 10 wt% Al2O3 were carried out in the present work. The thermal diffusivity as well as the thermal conductivity were found to increase with temperature for all composite specimens. The thermal diffusivity/conductivity decreases with increasing weight fraction of alumina in the composites. The experimental results are in good agreement with simple rule of mixture, Eucken equation and developed Ohm´s law model at weight fraction of alumina below 5 wt%. Beyond this, the thermal diffusivity/ conductivity exhibits a high discrepancy probably due to the agglomeration of alumina particles during cold pressing and sintering.

On the other hand, thermal diffusivities of industrial mould flux having glassy and crystalline states decrease with increasing temperature at lower temperature and are constant at higher temperature except for one glassy sample. The thermal diffusivity is increased with increasing crystallisation degree of mould flux, which is expected from theoretical considerations.

Analogously, the thermal diffusivity measurements of mould flux do not show any significant change with temperature in liquid state. It is likely to be due to the silicate network being largely broken down.

In the case of coke, the sample taken from deeper level of the pilot blast furnace is found to have larger thermal diffusivity. This can be correlated to the average crystallite size along the structural c-axis, Lc, which is indicative of the higher degree of graphitisation. This was also confirmed by XRD measurements of the different coke samples. The degree of graphitisation was found to increase with increasing temperature. Further, XRD and heat capacity measurements of coke samples taken from different levels in the shaft of the pilot blast furnace show that the graphitisation of coke was instantaneous between 973 and 1473 K.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006
Keyword
laser flash, thermal diffusivity, heat conduction, phonon, electron contribution, crystallisation degree, graphitisation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-3864 (URN)91-7178-251-6 (ISBN)
Public defence
2006-03-16, Sal F3, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100629Available from: 2006-03-03 Created: 2006-03-03 Last updated: 2010-06-29Bibliographically approved
2. Property measurements towards understanding process phenomena
Open this publication in new window or tab >>Property measurements towards understanding process phenomena
2005 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

The main objective of this industrially important work was to gain an increasing understanding of the properties of materials such as CMSX-4 nickel base super alloy, mould powder used in continuous casting of steel and coke used in blast furnace, with special reference to the thermal diffusivities. The measurements were carried out in a wide temperature range, solid, liquid, glassy and crystalline states.

For CMSX-4 alloy, the thermal conductivities were calculated from the experimental thermal diffusivities. Both the diffusivities and conductivities were found to increase with increasing temperature. Microscopic analysis showed the presence of intermetallic phases such as NiTi and NiTi2 below 1253 K. In this region, the mean free path of the phonons is likely to be limited by scattering against lattice defects. Between 1253 K and solidus temperature, these phases dissolved in the alloy adding to the impurities in the matrix, which, in turn, caused a decrease in the thermal diffusivity. This effect was confirmed by annealing the samples at 1573 K. The thermal diffusivities of the annealed samples measured at 1277, 1403 and 1531 K were found to be lower than the thermal diffusivities of non-annealed samples and the values did not show any noticeable change with time. It could be related to the attainment of equilibrium with the completion of the dissolution of γ and γ´ phases during the annealing process.

Liquid CMSX-4 does not show any change of thermal diffusivity with temperature. It may be attributed to the decreasing the mean free path being shorter than characteristic distance between two neighbouring atoms.

On the other hand thermal diffusivities of mould powder having glassy and crystalline states decrease with increasing temperature at lower temperature and are constant at higher temperature except for one glassy sample.

Analogously, the thermal diffusivity measurements of mould powder did not show any significant change with temperature in liquid state. It is likely to be due to the silicate network being largely broken down.

The thermal diffusivity is increased with increasing crystallisation degree of mould powder, which is expected from theoretical considerations.

The coke sample, taken from deeper level of the blast furnace, is found to have larger thermal diffusivity. This could be correlated to the average crystallite size along the structural c-axis, Lc, which is indicative of the higher degree of graphitisation. This was also confirmed by XRD measurements of the different coke samples. The degree of graphitisation was found to increase with increasing temperature. Further, XRD measurements of coke samples taken from different levels in the shaft of the blast furnace show that the graphitisation of coke was instantaneous between room temperature and 1473 K.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. x, 27 p.
Keyword
Materials science, Materialvetenskap
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-300 (URN)91-7178-029-7 (ISBN)
Presentation
2005-06-03, B1, Brinellvägen 23, Stockholm, 10:00
Note
QC 20101123Available from: 2005-07-15 Created: 2005-07-15 Last updated: 2010-11-23Bibliographically approved

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