Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Microstructure and Inclusion Characteristics in Steels with Ti-oxide and TiN Additions
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.ORCID iD: 0000-0003-0533-6729
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 [en]
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: urn:nbn:se:kth:diva-162284ISBN: 978-91-7595-484-4 (print)OAI: oai:DiVA.org:kth-162284DiVA: diva2:797693
Public defence
2015-04-15, B2, Brinellvägen 23, KTH, Stockholm, 12:00 (English)
Opponent
Supervisors
Note

QC 20150325

Available from: 2015-03-25 Created: 2015-03-24 Last updated: 2015-07-01Bibliographically approved
List of papers
1. Inclusion and Microstructure Characteristics in Steels with TiN Additions
Open this publication in new window or tab >>Inclusion and Microstructure Characteristics in Steels with TiN Additions
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The quantitative analysis of inclusion and microstructure characteristics in the steels with TiN additions has been studied. The typical inclusion was detected to be a TiN+Mn-Al-Si-Ti-O+MnS phase. This identification was based on the measurements of scanning electron microscopy with energy dispersive X-ray spectrometer (SEM-EDS), electron probe microanalysis (EPMA) which equipped wavelength dispersive X-Ray spectroscopy (WDS) and equilibrium calculations by using the commercial software Thermo-Calc.. TiN was found to be the effective nucleation site for the formation of intragranular ferrite (IGF). Furthermore, the increased inclusion size led to the increased probability of IGF nucleation, and this probability of IGF nucleation was slightly decreased with the increased sulfur content. This tendency is in agreement with the tendency of the area fraction of IGF in the steels containing different sulfur contents. 

Keyword
intragranular ferrite; titanium nitride; inclusion size; nucleation; sulfur
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-162275 (URN)
Note

QS 2015

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2015-03-25Bibliographically approved
2. Effect of Sulfur Content on Inclusion and Microstructure Characteristics in Steels with Ti2O3 and TiO2 Additions
Open this publication in new window or tab >>Effect of Sulfur Content on Inclusion and Microstructure Characteristics in Steels with Ti2O3 and TiO2 Additions
2014 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 54, no 12, 2907-2916 p.Article in journal (Refereed) Published
Abstract [en]

The effect of the sulfur content on the inclusion and microstructure characteristics of steels where Ti2O3 and TiO2 have been added was studied. Based on the microscopic examinations, it is found in the steel samples with Ti2O3 additions that the area fraction of intragranular ferrite decreases from 52.68% to 39.09% as the sulfur content increases from 0.009 mass.% to 0.030 mass.%. In the steel samples with TiO2 additions, this value also decreases from 49.05% to 36.26% as the sulfur content increases. The nucleant inclusion was identified as a TiOx+MnS phase based on SEM-EDS measurements as well as on equilibrium calculations with thermodynamic calculation software, Thermo-Calc. Also, TiOx was found to be the nucleation site for an intragranular ferrite formation. Moreover, the nucleation probability increases with an increased inclusion size. It is also noted that the nucleation probability decreases slightly with an increased sulfur content. The minimum size of TiOx+MnS inclusions for an IGF nucleation is about 0.85 mu m in the present samples. Furthermore, this minimum size of TiOx inclusions is shifted to a size of about 0.5 mu m by excluding the depth of a MnS layer. In addition, the effective nucleation size range of TiOx inclusions in the steels, where Ti2O3 and TiO2 had been added, is smaller than that of TiN+Mn-Al-Si-Ti-O inclusions in steel samples where TiN had been added.

Keyword
intragranular ferrite, titanium oxide, inclusion size, nucleation, sulfur
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-159638 (URN)10.2355/isijinternational.54.2907 (DOI)000347278400032 ()2-s2.0-84920365258 (Scopus ID)
Note

QC 20150205

Available from: 2015-02-05 Created: 2015-02-05 Last updated: 2017-12-05Bibliographically approved
3. Prediction of Intragranular Ferrite Nucleation in Steels with Ti-oxide and TiN Additions
Open this publication in new window or tab >>Prediction of Intragranular Ferrite Nucleation in Steels with Ti-oxide and TiN Additions
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The present work provides a method to calculate the critical diameters of TiO, TiN and VN inclusions for IGF nucleation. It is noted that the critical diameters of TiO, TiN and VN inclusions for IGF nucleation were calculated to be 0.192, 0.355 and 0.810 μm. The calculation result was in agreement with the experiment data in the steels with TiN and Ti-oxide additions. Moreover, it is the first attempt to predict the critical diameters of inclusions in the steels containing different contents of Mn and S. The critical diameters were increased with the increase of Mn content. However, the S content does not have a direct effect on the critical diameters for IGF nucleation.

Keyword
intragranular ferrite; heterogeneous nucleation; critical diameter; TiO; TiN; VN
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-162278 (URN)
Note

QS 2015

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2015-03-25Bibliographically approved
4. Effect of Carbon Content on the Probability of the Intragranular Ferrite Formation
Open this publication in new window or tab >>Effect of Carbon Content on the Probability of the Intragranular Ferrite Formation
2015 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344XArticle in journal (Refereed) Submitted
Abstract [en]

The effect of the carbon content on the probability of the intragranular ferrite (IGF) formation was investigated in the present work. The TiN inclusion was detected to be the effective nucleation site for the IGF formation in the Fe-0.2 mass.% C alloy and the Fe-0.4 mass.% C alloy. It is noted that the probability of the IGF formation for each inclusion size is decreased with the increase of carbon content. Moreover, the critical diameters of the TiN, TiO and VN inclusions in the steels with different carbon contents were calculated based on the classical nucleation theory. The calculated critical diameter was also found to be decreased with the increase of carbon content. This is in agreement with the experiment results. Finally, the decrease of the probability of IGF formation for each inclusion size is due to a larger amount of pearlite formation in the steel contains a higher carbon content, which was detected by differential scanning calorimetry (DSC) measurements.

Place, publisher, year, edition, pages
Wiley: , 2015
Keyword
carbon content; intragranular ferrite (IGF); inclusion size; nucleation; differential scanning calorimetry (DSC).
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-162279 (URN)
Note

NQC 2015

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2017-12-04Bibliographically approved
5. Ferrite Formation Dynamics and Microstructures in Inclusion Engineered Steels with Ti2O3 and TiN Additions
Open this publication in new window or tab >>Ferrite Formation Dynamics and Microstructures in Inclusion Engineered Steels with Ti2O3 and TiN Additions
Show others...
(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
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:nbn:se:kth:diva-162283 (URN)
Note

QS 2015

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2015-03-25Bibliographically approved
6. Combination of in situ microscopy and calorimetry to study austenite decomposition in inclusion engineered steels
Open this publication in new window or tab >>Combination of in situ microscopy and calorimetry to study austenite decomposition in inclusion engineered steels
Show others...
2015 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344XArticle in journal (Refereed) Published
Abstract [en]

In situ high-temperature confocal laser scanning microscopy and differential scanning calorimetry studies of ferrite formation in inclusion engineered (Ti2O3 and TiN) steels have been performed. The applied methodology allows distinction between intragranular ferrite, grain boundary ferrite, and pearlite. The effect of the inclusions and cooling rates on the initiation of phase transformation and the final microstructure is discussed. It is concluded that the applied hybrid methodology could provide vital details of solid-state phase transformations within the field of inclusion engineering.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2015
Keyword
In situ confocal laser scanning microscopy (CLSM); Differential scanning calorimetry (DSC); Inclusion engineering; Ti-oxide; TiN.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-162270 (URN)10.1002/srin.201500008 (DOI)000369823600002 ()2-s2.0-84955372049 (Scopus ID)
Note

QC 20160307

Available from: 2015-03-24 Created: 2015-03-24 Last updated: 2017-12-04Bibliographically approved

Open Access in DiVA