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WC grain growth during sintering of cemented carbides: Experiments and simulations
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cemented carbides are composite materials consisting of a hard carbide and a ductile binder. They are powdermetallurgically manufactured, where liquid-phase sintering is one of the main steps. The most common cemented carbide consists of WC and Co and it is widely used for cutting tools. Two of the most important factors controlling the mechanical properties are the WC grain size and the grain size distribution and thus it is of great interest to understand the grain growth behavior.

In this thesis the grain growth during sintering at 1430 °C is studied both experimentally and through computer simulations. The grain growth behavior in cemented carbides cannot be explained from the classical LSW-theory. The WC grains have a faceted shape necessitating growth by 2-D nucleation of new atomic layers or surface defects. A new model based on 2-D nucleation, long-range diffusion and interface friction is formulated.

Three powders having different average sizes are studied and both experiments and simulations show that a fine-grained powder may grow past a coarse-grained powder, indicating that abnormal grain growth has taken place in the fine-grained powder. Fine-grained powders with various fractions of large grains are also studied and it is seen that a faster growth is obtained with increasing fraction of large grains and that an initially slightly bimodal powder can approach the logaritmic normal distribution after long sintering times.

The grain size measurements are performed on 2-D sections using image analysis on SEM images or EBSD analysis. Since the growth model is based on 3-D size distributions the 2-D size distributions have to be transformed to 3-D, and a new method, Inverse Saltykov, is proposed. The 2-D size distribution is first represented with kernel estimators and the 3-D size distribution is optimized in an iterative manner. In this way both negative values in the 3-D size distribution and modifications of the raw data are avoided.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , vii, 42 p.
Keyword [en]
Cemented carbide, Grain growth, Abnormal grain growth, Image analysis, Modeling, 3-D grain size distribution, Inverse Saltykov
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-32430ISBN: 978-91-7415-915-8OAI: oai:DiVA.org:kth-32430DiVA: diva2:410627
Public defence
2011-05-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110426Available from: 2011-04-26 Created: 2011-04-14 Last updated: 2011-04-26Bibliographically approved
List of papers
1. Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis
Open this publication in new window or tab >>Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis
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2008 (English)In: International Journal of Refratory metals and Hard Materials, ISSN 0263-4368, Vol. 26, no 5, 449-455 p.Article in journal (Refereed) Published
Abstract [en]

The WC carbide grain size is important for the technological properties of cemented carbide cutting tools. in the present work the WC carbide grain size distribution is determined after milling and sintering for 0.25, 1 and 8 h at 1430 degrees C. The WC grain size distribution, both for the powder after milling and the sintered specimens, is determined by two different methods, i.e. image analysis on scanning electron microscopy (SEM) images and electron backscatter diffraction (EBSD). It should be noted that in this work the 2D grain size distribution is considered. The EBSD analysis clearly shows that the special 12 boundaries are present in the powder and that their fraction decreases during sintering and particularly during the early stages. When the 12 boundaries are omitted in the EBSD analysis the results of the grain size measurements for the two methods agree quite well.

Keyword
carbides, grain size, image analysis, Sigma 2 boundaries, cemented carbides, co alloys, boundaries
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-17683 (URN)10.1016/j.ijrmhm.2007.10.004 (DOI)000257611900010 ()2-s2.0-44649122750 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-04-26Bibliographically approved
2. Inverse Saltykov analysis for particle-size distributions and their time evolution
Open this publication in new window or tab >>Inverse Saltykov analysis for particle-size distributions and their time evolution
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 3, 874-882 p.Article in journal (Refereed) Published
Abstract [en]

In this work a new method for transforming 2-D to 3-D size distributions is proposed. A representation of the 2-D size distributions is constructed from the data of measured radii with a statistical method called the kernel density estimator. The method yields a smooth density estimation that is more accurate than the classic histogram. The 3-D distribution is optimized from the 2-D density estimate in an iterative manner. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keyword
Grain size, Image analysis, 3-D distribution, Carbides
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-30977 (URN)10.1016/j.actamat.2010.09.046 (DOI)000286690100003 ()2-s2.0-78650680735 (ScopusID)
Note
QC 20110310Available from: 2011-03-10 Created: 2011-03-07 Last updated: 2011-04-26Bibliographically approved
3. Carbide grain growth in cemented carbides
Open this publication in new window or tab >>Carbide grain growth in cemented carbides
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 5, 1912-1923 p.Article in journal (Refereed) Published
Abstract [en]

Abnormal grain growth is often observed in cemented carbides during sintering, but cannot be understood in terms of the classical LSW theory. In this work the grain growth behavior during sintering at 1430 °C is studied both experimentally and by means of computer simulations. A model based on several processes—2-D nucleation of growth ledges, mass transfer across the interface and long-range diffusion coupled in series—is formulated and the equations are solved numerically. Both computer simulations and experimental studies reveal that the grain growth behavior is strongly influenced by the initial size distribution.

Keyword
Abnormal grain growth, Carbides, Grain growth, Modeling, Sintering, 2-d nucleations, Cemented carbides, Experimental studies, Grain growth behavior, Long-range diffusion, LSW theory, Model-based, Carbide tools, Computational methods, Computer simulation, Grain size and shape
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-31450 (URN)10.1016/j.actamat.2010.11.056 (DOI)000287775400006 ()2-s2.0-79551497846 (ScopusID)
Note
QC 20110316Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-04-26Bibliographically 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 (ScopusID)
Note

QC 20110701

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2016-04-27Bibliographically approved
5. Carbide grain growth: the effect of grain size distributions
Open this publication in new window or tab >>Carbide grain growth: the effect of grain size distributions
(English)Manuscript (preprint) (Other academic)
Abstract [en]

It is important to understand the WC grain growth behavior in order to obtain good mechanical properties when manufacturing cemented carbides. In the present study the WC grain growth for dierent initial grain size distributions during sintering is studied theoretically using a model based on three processes; 2-D nucleation of atomic planes, long-range diusion and interface friction. Simulations clearly show that the initial grain size distribution inuences the growth behavior significantly. Bimodal powders show a more dramatic grain growth than powders having a lognormalgrain size distribution.

Keyword
Cemented carbide, grain growth, abnormal grain growth, simulation
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-32429 (URN)
Note
QC 20110426Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2011-04-26Bibliographically approved

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