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Influence of microstructure on fatigue and ductility properties of tool steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2008 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Fatigue and ductility properties in various tool steels, produced by powder metallurgy, spray forming or conventionally ingot casting, have been analysed experimentally and successfully compared to developed models. The models are able to predict the fatigue limit and cause for fatigue fracture, and strain- and stress-development until fracture during the ductile fracture process respectively. Total fracture in a tool steel component, both in fatigue and ductility testing, is caused by a propagating crack initiated by particles, i.e. carbides or non-metallic inclusions. The models are based on experimentally observed size distributions.

The axial fatigue strength at two million cycles was determined for various tool steels. The fracture surface of each test bar broken was examined in SEM to determine the cause for fatigue failure, i.e. a single carbide or inclusion particle or a cluster of carbides, and the size of the particle. The particles act as stress concentrators where a crack is easily initiated when the material is subjected to alternating stresses. The developed models calculate the probability that at least one particle will be present in the material which is larger than the threshold level for crack initiation at a certain stress range.

The ductility testing was performed on various tool steels by four-point bending under static load. The load and displacement until total fracture were recorded and the maximum strain and stress acting in the material were calculated. The fracture surface of each broken test bar was examined in SEM, though the crack initiating area appears different compared to a fatigue failure. Ductile fracture is caused by a crack emanating from voids nucleated around many particles in a joint process and then linked together. By finite element modelling of void initiation and propagation in 2D of an experimentally observed carbide microstructure for each tool steel, successful comparisons with experiments were performed. Carbides were modelled as cracked when larger than a certain size, based on fracture surface observations, and the matrix cracked above a pre-defined plastic deformation level. The stresses and strains at total failure were in good agreement between model and experiments when evaluated.

The use of these developed models could be a powerful tool for optimisation of fatigue and ductility properties for tool steels. With good fatigue and ductility properties normal failures appearing during operation of a tool steel product could be minimised. By theoretical tests in the developed models of various carbide microstructures the optimum mechanical properties could be achieved with a minimum of experiments performed.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 29, vi p.
Keyword [en]
Tool steels, fatigue, ductility, carbides, carbide cluster, inclusion, size distribution, modelling, FEM
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4624ISBN: 978-91-7178-859-7 (print)OAI: oai:DiVA.org:kth-4624DiVA: diva2:13127
Presentation
2008-02-18, Benediktsalen, KIMAB, Drottning Kristinas väg 48, Stockholm, 10:30
Opponent
Supervisors
Note
QC 20101119Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2010-11-19Bibliographically approved
List of papers
1. How carbides and inclusions influence fatigue strength in tool steels
Open this publication in new window or tab >>How carbides and inclusions influence fatigue strength in tool steels
2006 (English)In: 7th International Tooling Conference, 2006Conference paper, Published paper (Other academic)
Keyword
Tool steel, fatigue, carbides, carbide cluster, inclusion, size distribution
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7940 (URN)
Note
QC 20101119Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2010-11-19Bibliographically approved
2. Experimental Testing and Computer Simulations of Ductile Fracture in Tool Steels
Open this publication in new window or tab >>Experimental Testing and Computer Simulations of Ductile Fracture in Tool Steels
2012 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 83, no 1, 91-99 p.Article in journal (Refereed) Published
Abstract [en]

Ductility was determined in experimental four-point bending tests of smooth specimens of tool steel. The tool steels had different contents of carbides and carbide sizes and with a hardness of approximately 60HRc. Two of the materials tested were produced powder metallurgically, one was spray formed and one was conventionally uphill ingot cast. Carbide size distribution analysis was performed on planar polished sections of each material. Correlation between carbide microstructure and ductility performance was obtained. The fracture mechanisms were investigated with fractography. A 3D FE-model was used to simulate the four-point bending tests and thereby analyse the matrix flow curve. Also the strain at failure was analysed for each material when simulations were performed based on experimental data. SEM-images of the materials carbide microstructure were used to create 2D FE-models. The models simulated crack initiation and propagation by removing elements in the steel matrix as the plastic strain reached a critical level. With three variants, simulations of crack initiation and propagation at carbides were investigated. That was carbides with no cohesion to matrix, carbides fixed to the matrix and carbides with internal cracks. Comparison of strains at failure for the 2D and the 3D FE-models showed good correlation.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keyword
Tool steel, FEM, four-point bending, ductility, carbide microstructure, crack
National Category
Metallurgy and Metallic Materials
Research subject
SRA - Production; SRA - Production
Identifiers
urn:nbn:se:kth:diva-7941 (URN)10.1002/srin.201100220 (DOI)000298743000009 ()2-s2.0-84855379629 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20120206

Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2016-04-26Bibliographically approved
3. Fatigue Strength of Conventionally Cast Tool Steels and its Dependence of Carbide Microstructure
Open this publication in new window or tab >>Fatigue Strength of Conventionally Cast Tool Steels and its Dependence of Carbide Microstructure
2012 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 83, no 1, 83-90 p.Article in journal (Refereed) Published
Abstract [en]

The axial fatigue strength at two million cycles was experimentally determined for two conventionally cast tool steels and successfully compared with results from a fatigue limit model. Specimens were tested both in the rolling and transverse direction and showed large differences in fatigue properties due to the segregated carbide microstructure. Rolling direction specimens experienced higher fatigue strength than the transverse direction specimens. This is due to smaller carbides present in the load affected cross section of the rolling direction fatigue test bars compared to the cross section of the transverse direction fatigue test bars. Fractographic analysis of failed specimens showed that large carbides had caused fatigue failure, which was also predicted by the model. Measured size distributions of carbides and inclusions were used as input data in the model. The probability that at least one particle will be present in the material volume having a size larger than the threshold value for crack propagation was calculated.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keyword
Tool steel, fatigue, carbide microstructure, size distribution, model
National Category
Metallurgy and Metallic Materials
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-7942 (URN)10.1002/srin.201100219 (DOI)000298743000008 ()2-s2.0-84855384146 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20120206

Available from: 2008-01-31 Created: 2008-01-31 Last updated: 2016-04-26Bibliographically approved

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