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Ferrite Formation Dynamics and Microstructures in Inclusion Engineered Steels with Ti2O3 and TiN Additions
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.ORCID iD: 0000-0003-0533-6729
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-1102-4342
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The dynamics of intragranular ferrite (IGF) formation in inclusion engineered steels with Ti2O3 and TiN additions were investigated using in-situ high temperature confocal laser scanning microscopy (CLSM). Furthermore, the chemical composition of the inclusions and the final microstructures after continuous cooling was investigated using electron probe microanalysis (EPMA) and electron backscatter diffraction (EBSD), respectively. The results show that there is a significant effect of the chemical composition of the inclusions, the cooling rate and the prior austenite grain size on the phase fractions and the starting temperatures of IGF and grain boundary ferrite (GBF) formation. The fraction of IGF is larger in the steel with Ti2O3 addition compared to the steel with TiN addition after the same thermal cycle has been imposed. This is because the TiOx phase provides more potent nucleation sites for IGF than the TiN phase does. The fraction of IGF in the steels was highest after at an intermediate cooling rate of 70 ºC/min since competing phase transformations were avoided, however, the IGF was refined with increasing cooling rate. In addition, the IGF fraction increases and the starting temperature of GBF decreases with the increasing prior austenite grain size, however, the starting temperature of IGF keeps almost the value when the grain size changes.

Keyword [en]
in situ confocal laser scanning microscopy (CLSM); intragranular ferrite; Ti-oxide; TiN; inclusion engineering; steels; phase transformation.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-162283OAI: oai:DiVA.org:kth-162283DiVA: diva2:797688
Note

QS 2015

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2015-03-25Bibliographically approved
In thesis
1. Microstructure and Inclusion Characteristics in Steels with Ti-oxide and TiN Additions
Open this publication in new window or tab >>Microstructure and Inclusion Characteristics in Steels with Ti-oxide and TiN Additions
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Non-metallic inclusions in steels are generally considered to be detrimental for mechanical properties. However, it has been recognized that certain inclusions, such as Ti-oxide and TiN, can serve as potent nucleation sites for the formation of intragranular ferrite (IGF) in low-alloy steels. The formation of IGF could improve the toughness of the coarse grained heat affected zone (CGHAZ) of weld metals. Thus, the present thesis mainly focuses on the effect of size of nucleation sites on the IGF formation. Quantitative studies on the composition, size distribution and nucleation probability for each size of the inclusions as well as the area fraction, starting temperature and morphology of an IGF have been carried out.

In the present work, the Ti-oxide and TiN powders were mixed with metallic powders. The mixed powders were heated up to the liquid state and cooled with a slow cooling rate of 3.6 ºC/min. These as-cast steels with Ti-oxide and TiN additions were used to simulate the IGF formation in the CGHAZ of weld metals. Specifically, the inclusion and microstructure characteristics in as-cast steels have been investigated. The results show that the nucleant inclusion was identified as a TiOx+MnS phase in steels with Ti2O3 additions and as a TiN+Mn-Al-Si-Ti-O+MnS phase in steels with TiN additions. In addition, the TiOx and TiN phases are detected to be the effective nucleation sites for IGF formation. It is clearly shown that an increased inclusion size leads to an increased probability of IGF nucleation. This probability of IGF nucleation for each inclusion size of the TiOx+MnS inclusions is clearly higher than that of the complex TiN+Mn-Al-Si-Ti-O+MnS inclusions. In addition, the area fraction of IGF in the steels with Ti2O3 additions is larger than that of the steels with TiN additions. This result agrees with the predicted tendency of the probability of IGF nucleation for each inclusion size in the steels with Ti2O3 and TiN additions.

In order to predict the effective inclusion size for IGF formation, the critical diameters of the TiO, TiN and VN inclusions, which acted as the nucleation sites of IGF formation, were also calculated based on the classical nucleation theory. The critical diameters of TiO, TiN and VN inclusions for IGF formation were found to be 0.192, 0.355 and 0.810 μm in the present steels. The calculation results were found to be in agreement with the experiment data of an effective inclusion size. Moreover, the effects of the S, Mn and C contents on the critical diameters of inclusions were also calculated. It was found that the critical diameter of the TiO, TiN and VN inclusions increases with an increased content of Mn or C. However, the S content doesn’t have a direct effect on the critical diameter of the inclusions for IGF formation. The probability of IGF nucleation for each inclusion size slightly decreases in the steel containing a higher S content. This fact is due to that an increased amount of MnS precipitation covers the nucleant inclusion surface.

In the as-cast experiment, it was noted that an IGF can be formed in steels with Ti2O3 and TiN additions with a cooling rate of 3.6 ºC/min. In order to control the microstructure characteristics, such as the area fraction and the morphology of an IGF, and to investigate the starting temperature of IGF and grain boundary ferrite (GBF) formation, the dynamic transformation behavior of IGF and GBF was studied in-situ by a high temperature confocal laser scanning microscope (CLSM). Furthermore, the chemical compositions of the inclusions and the morphology of IGF after the in-situ observations were investigated by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) which equipped wavelength dispersive spectrometer (WDS). The results show that the area fraction of IGF is larger in the steels with Ti2O3 additions compared to the steels with TiN additions, after the same thermal cycle has been imposed. This is due to that the TiOx phase provides more potent nucleation sites for IGF than the TiN phase does. Also, the area fraction of IGF in the steels is highest after at an intermediate cooling rate of 70 ºC/min, since the competing phase transformations are avoided. This fact has been detected by using a hybrid methodology in combination with CLSM and differential scanning calorimetry (DSC). In addition, it is noted that the morphology of an IGF is refined with an increased cooling rate.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xii, 77 p.
Keyword
intragranular ferrite; oxide metallurgy; inclusion engineering; titanium nitride; titanium oxide; inclusion size; heterogeneous nucleation; critical diameter; confocal laser scanning microscopy (CLSM); differential scanning calorimetry (DSC); continuous cooling transformation (CCT) diagram.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-162284 (URN)978-91-7595-484-4 (ISBN)
Public defence
2015-04-15, B2, Brinellvägen 23, KTH, Stockholm, 12:00 (English)
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Supervisors
Note

QC 20150325

Available from: 2015-03-25 Created: 2015-03-24 Last updated: 2015-07-01Bibliographically approved

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Mu, Wangzhong

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