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Study of precipitation in martensitic Fe-C-Cr alloys during tempering: Experiments and modelling
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-4825-7430
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Understanding the precipitation reaction is very important since precipitation hardening is one of the most effective strengthening mechanisms in metallic alloys. In martensitic steels, a tempering heat treatment is often performed. During tempering various new phases are precipitated and the spatial and temporal evolution of these precipitates strongly influences the properties of the steel, such as strength/ductility, creep, fatigue and hot corrosion resistance. Therefore, the possibility of quantitative modelling of the precipitation process will provide many opportunities for advanced materials and process design and optimization as well as service life assessments. The Fe-C-Cr system forms the basis for tool steels and is consequently used in many applications such as e.g. metal forming operations. They are characterized by a high hardness and good toughness, even at elevated temperatures.In the present work, the as-quenched martensitic microstructures of four Fe-C-Cr alloys with varying Cr and C contents were characterized by Light Optical Microscopy (LOM) and Electron Microscopy. The effects of Cr and C on the morphology of martensite were investigated. It was found that Cr addition had a similar effect as C on the martensitic morphology and on the ratio of high-angle grain boundary (HAGB) to low-angle grain boundary (LAGB). However, the micro-hardness was unaffected by the Cr addition whilst it was strongly influenced by the C addition.In addition, a quantitative experimental characterization of the precipitates formed during tempering of the martensite was performed. The Langer-Schwartz theory combined with the Kampmann-Wagner-Numerical (KWN) method, as implemented in the software TC-PRISMA, was used to predict the precipitation of carbides after tempering in one of the model alloys: Fe-0.15C-4.0Cr (mass%). The microstructure characterization of the as-quenched material provided vital input parameters for the modelling work and a comparison was made between the modelling predictions and the experimental results. The effect of parameters such as dislocation density, grain size and interfacial energy on the precipitation of carbides was discussed.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xi, 34 p.
Keyword [en]
Fe-C-Cr alloy, Microstructure, Precipitates, Tempering of martensite, Electron microscopy, Modelling.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-176430ISBN: 978-91-7595-756-2 (print)OAI: oai:DiVA.org:kth-176430DiVA: diva2:866846
Presentation
2015-12-04, Kuben N111, Brinellvägen 23, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20151105

Available from: 2015-11-05 Created: 2015-11-03 Last updated: 2015-11-10Bibliographically approved
List of papers
1. Microstructure of Martensite in Fe-C-Cr and its Implications for Modelling of Carbide Precipitation during Tempering
Open this publication in new window or tab >>Microstructure of Martensite in Fe-C-Cr and its Implications for Modelling of Carbide Precipitation during Tempering
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2014 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 54, no 11, 2649-2656 p.Article in journal (Refereed) Published
Abstract [en]

The microstructure of as-quenched martensite in four Fe-C-Cr alloys (0.15C-1Cr, 0.15C-4Cr, 1C-1Cr, 1C-4Cr, mass%) has been investigated. Moreover, the microstructures served as input for setting up modeling of carbide precipitation during tempering of martensite. The modelling was conducted using the Langer-Schwartz approach and the software TC-PRISMA, which retrieves thermodynamic data from the Thermo-Calc databank. It was found that the martensite in the low carbon steels is predominantly lath martensite with units arranged parallel to each other. On the other hand, the plate martensite dominates the microstructure in the high carbon steels. The ratio of high-angle to low-angle grain boundaries was found to increase with increasing Cr in the low carbon steels, which indicates that Cr has a similar effect as C on the lath martensite microstructure, however, the micro-hardness remained unaffected by the addition of Cr. Finally, the precipitation modeling clearly demonstrates the importance of proper definition of the initial microstructure for predictive modelling. Parameters such as dislocation density and frequency of high-angle grain boundaries have a drastic effect on e.g. the mean size of carbides.

Keyword
steels, martensite, microstructure, precipitation, electron microscopy, modeling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-158349 (URN)10.2355/isijinternational.54.2649 (DOI)000345485100030 ()2-s2.0-84917739189 (Scopus ID)
Note

QC 20150121

Available from: 2015-01-21 Created: 2015-01-07 Last updated: 2017-12-05Bibliographically approved
2. Quantitative modeling and experimental verification of carbide precipitation in a martesnsitic Fe-0,16 wt.%C-4.0 wt.%Cr alloy
Open this publication in new window or tab >>Quantitative modeling and experimental verification of carbide precipitation in a martesnsitic Fe-0,16 wt.%C-4.0 wt.%Cr alloy
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(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-176470 (URN)
Note

QS 2015

Available from: 2015-11-05 Created: 2015-11-05 Last updated: 2015-11-10Bibliographically approved

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Hou, Ziyong

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