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Aspects of Structural Evolution in Cemented Carbide – Carbide Size, Shape and Stability
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-4556-032X
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cemented carbide is a composite material used in applications like cutting tools and rock drilling inserts. The material commonly consists of WC grains embedded in a Co-rich binder phase and the material properties strongly depend on the WC grain size. Hence, to tailor the properties it is important to understand the fundamental mechanisms of grain coarsening. At the same time, the higher demands on material properties today also require new solutions. In the present work, some different aspects of structural evolutions in cemented carbides have been investigated.

The first part of the work considers WC grain coarsening by means of size, size distribution and shape. Some efforts of the work have been to evaluate the effects of C-activity and initial WC powder size and distribution on the coarsening behavior in the material using different characterization techniques, e.g. scanning electron microscopy, and electron backscattered diffraction. Additionally, two earlier developed models are used and evaluated with the experimental data. The results indicate that the C-activity will affect size, size distribution and shape of the WC grains. It was also observed that the initial WC powder size and size distribution will have a large influence on the WC grain coarsening. The statistical shape was found to fit a spherical approximation but for individual grains both faceted and non-faceted shapes was observed. Steps and planar defects were observed supporting that the nucleation of new atomic layers is the main rate limiting mechanism for grain coarsening.

The second part of this work considers the carbide phase stability in the (Ti,Zr)C system. The phase stability was investigated after synthesizing and aging a mixed (Ti,Zr)C using X-ray diffraction and different types of electron microscopy techniques. A decomposed lamellar structure was found with a composition variation of approximately 10% between the 50-75 nm thick lamellas. The experimental investigations were supported by computational work and the results were in good agreement. Additionally, two cemented carbide related systems were studied. A miscibility gap was found in the two investigated systems, (Ti,Zr,W)(C,N)-Co or Fe-graphite, and the effect of N2-gas pressure was investigated suggesting a critical N2-gas pressure below 0.1 bar.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xii, 72 p.
Keyword [en]
Cemented carbide, (Ti, Zr)C, Coarsening, Phase separation, C-activity, initial WC powder size, electron backscattered diffraction, microscopy, X-ray diffraction
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-133933ISBN: 978-91-7501-944-4 (print)OAI: oai:DiVA.org:kth-133933DiVA: diva2:664213
Public defence
2013-12-13, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131118

Available from: 2013-11-18 Created: 2013-11-13 Last updated: 2013-11-18Bibliographically approved
List of papers
1. Effect of carbon activity and powder particle size on WC grain coarsening during sintering of cemented carbides
Open this publication in new window or tab >>Effect of carbon activity and powder particle size on WC grain coarsening during sintering of cemented carbides
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2014 (English)In: International journal of refractory metals & hard materials, ISSN 0958-0611, E-ISSN 2213-3917, Vol. 42, 30-35 p.Article in journal (Refereed) Published
Abstract [en]

Liquid-phase sintering is an important step in the production of cemented carbides. During sintering, the average WC grain size increases, leading to a coarser structure, which affects the performance of the final product. The coarsening occurs by dissolution of small grains and growth of large grains. In the present work, the effect of high carbon activity during sintering on the WC grain coarsening has been evaluated using electron backscattered diffraction (EBSD) and the results have been compared with a previous work where sintering was performed at a lower carbon activity. A more homogeneous grain size distribution was observed in alloys sintered at a high carbon activity. In addition, the effect of the initial powder particle size distribution was investigated. It was found that the coarsening rate of a WC powder with an initial small average grain size is significantly higher as compared to the coarsening rate for a powder with a larger initial average grain size. The results obtained emphasize the importance of considering the complete particle size distribution in order to predict coarsening.

Keyword
Cemented carbides, Grain coarsening, Carbon activity, Initial powder particle size, Size distribution, EBSD
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133934 (URN)10.1016/j.ijrmhm.2013.10.004 (DOI)000329553500005 ()2-s2.0-84887499010 (Scopus ID)
Funder
Vinnova
Note

QC 20131118

Available from: 2013-11-13 Created: 2013-11-13 Last updated: 2017-12-06Bibliographically approved
2. On the three-dimensional structure of WC grains in cemented carbides
Open this publication in new window or tab >>On the three-dimensional structure of WC grains in cemented carbides
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2013 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 13, 4726-4733 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, the size distribution and shape of WC grains in cemented carbides (WC-Co), with different Co contents, have been investigated in three dimensions. Direct three-dimensional (3-D) measurements, using focused ion beam serial sectioning and electron backscattered diffraction (EBSD), were performed and a 3-D microstructure was reconstructed. These measurements were supplemented by two-dimensional (2-D) EBSD and scanning electron microscopy on extracted WC grains. The data from 2-D EBSD collected on planar sections were transformed to three dimensions using a recently developed statistical method based on an iterative inverse Saltykov procedure. This stereological analysis revealed that the assumed spherical shape of WC grains during the Saltykov method is reasonable and the estimated 3-D size distribution is qualitatively in good agreement with the actual distribution measured from 3-D EBSD. Although the spherical assumption is generally fair, the WC grains have both faceted and rounded surfaces. This is a consequence of the relatively low amount of liquid phase during sintering, which makes impingements significant. Furthermore, the observed terraced surface structure of some WC grains suggests that 2-D nucleation is the chief coarsening mechanism to consider.

Keyword
Three-dimensional EBSD, Three-dimensional characterisation, Carbides, Coarsening, Inverse Saltykov
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-125544 (URN)10.1016/j.actamat.2013.05.008 (DOI)000321601400003 ()2-s2.0-84879422588 (Scopus ID)
Funder
Vinnova
Note

QC 20130813

Available from: 2013-08-13 Created: 2013-08-09 Last updated: 2017-12-06Bibliographically approved
3. Microstructure, grain size distribution and grain shape in WC-Co alloys sintered at different carbon activities
Open this publication in new window or tab >>Microstructure, grain size distribution and grain shape in WC-Co alloys sintered at different carbon activities
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2014 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0958-0611, Vol. 43, 205-211 p.Article in journal (Refereed) Published
Abstract [en]

The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC-Co alloys sintered at 1410 C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M6C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC-Co-eta and WC-Co-graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC-Co-graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology.

Keyword
Carbon activity, Cemented carbide, Coarsening, Electron backscatter diffraction (EBSD), WC shape
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133936 (URN)10.1016/j.ijrmhm.2013.12.007 (DOI)000333789700032 ()2-s2.0-84891399234 (Scopus ID)
Funder
Vinnova
Note

QC 20140203. Updated from submitted to published.

Available from: 2013-11-13 Created: 2013-11-13 Last updated: 2014-05-05Bibliographically approved
4. Abnormal grain growth in cemented carbides - Experiments and simulations
Open this publication in new window or tab >>Abnormal grain growth in cemented carbides - Experiments and simulations
2011 (English)In: International journal of refractory metals & hard materials, ISSN 0958-0611, E-ISSN 2213-3917, Vol. 29, no 4, 488-494 p.Article in journal (Refereed) Published
Abstract [en]

The grain size and the grain size distribution are two of the most important factors when tailoring the mechanical properties of cemented carbides. In the present work the effect on the growth behavior when adding some abnormal grains in an initial fine grained powder is studied. It is clearly seen that abnormal grains in a fine grained matrix lead to faster grain growth and a higher average grain size.

Keyword
Cemented carbide, grain growth, abnormal grain growth, bimodal, sintering, modeling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-32427 (URN)10.1016/j.ijrmhm.2011.02.008 (DOI)000291138600013 ()2-s2.0-79955530750 (Scopus ID)
Note

QC 20110701

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved
5. Synthesis and phase separation of (Ti,Zr)C
Open this publication in new window or tab >>Synthesis and phase separation of (Ti,Zr)C
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2014 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 66, 209-218 p.Article in journal (Refereed) Published
Abstract [en]

Synthesis and phase separation of (Ti,Zr)C were investigated in the present work. The (Ti,Zr)C phase was synthesized at 2200 C and subsequently aged at 1300 C for different times. The microstructure was investigated using X-ray diffraction and electron microscopy, and supplemented by first-principles calculations. The (Ti,Zr)C phase separates into a lamellar nanostructure with alternating Ti- and Zr-rich face-centered cubic domains as well as non-stoichiometric TiC and ZrC. The lamellar structure is a consequence of phase separation within the miscibility gap that is directionally constrained by high coherency stresses, as indicated by the first-principles calculations. Moreover, the increased hardness due to the phase separation suggests that the mixed carbide could be used as a strengthening constituent in, for example, cemented carbides.

Keyword
Carbides, Electron microscopy, First-principles calculations, Phase separation, X-ray diffraction
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133937 (URN)10.1016/j.actamat.2013.11.074 (DOI)000331422600021 ()2-s2.0-84891772946 (Scopus ID)
Funder
Vinnova
Note

QC 20140203. Updated from submitted to published.

Available from: 2013-11-13 Created: 2013-11-13 Last updated: 2017-12-06Bibliographically approved
6. Influence of nitrogen Gas pressure on the miscibility Gap in the Ti-Zr carbonitride system
Open this publication in new window or tab >>Influence of nitrogen Gas pressure on the miscibility Gap in the Ti-Zr carbonitride system
2012 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 32, 11-15 p.Article in journal (Refereed) Published
Abstract [en]

The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)-Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N-2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N-2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N-2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim.

Keyword
Cemented carbides, TiC, ZrC, Miscibility gap
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-95088 (URN)10.1016/j.ijrmhm.2011.12.014 (DOI)000302841400003 ()2-s2.0-84858336869 (Scopus ID)
Note
QC 20120522Available from: 2012-05-22 Created: 2012-05-14 Last updated: 2013-11-18Bibliographically approved

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Citation style
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  • ieee
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