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Modelling and Characterisation of the Martensite Formation in Low Alloyed Carbon Steels
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected.

The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel.

The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of.

Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017. , 110 p.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keyword [en]
Dilatometry, Hardness, EBSD, Martensite, Austenite, Kurdjumov-Sachs, Phase transformation, Modelling
National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-62369ISBN: 978-91-7583-839-7 (print)ISBN: 978-91-7583-840-3 (electronic)OAI: oai:DiVA.org:ltu-62369DiVA: diva2:1079666
Presentation
2017-04-28, TDB, Luleå, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2017-03-10 Created: 2017-03-09 Last updated: 2017-04-10Bibliographically approved
List of papers
1. Simplified Implementation of the Koistinen-Marburger Model for Use in Finite Element Simulations
Open this publication in new window or tab >>Simplified Implementation of the Koistinen-Marburger Model for Use in Finite Element Simulations
2016 (English)In: Proceedings of the 11th international congress on thermal stresses, Severino: Edizioni Paguro , 2016, , 4 p.107-110 p.Conference paper, (Other academic)
Place, publisher, year, edition, pages
Severino: Edizioni Paguro, 2016. 4 p.
Keyword
Martensite, Modelling, Phase Transformation
National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-62366 (URN)978-88-99509-14-9 (ISBN)
Conference
Proceedings of the 11th international congress on thermal stresses, Salerno, Italy, June 5–9, 2016
Available from: 2017-03-09 Created: 2017-03-09 Last updated: 2017-03-15Bibliographically approved
2. Investigation of Parent Austenite Grains from Martensite Structure using EBSD in a Wear Resistant Steel
Open this publication in new window or tab >>Investigation of Parent Austenite Grains from Martensite Structure using EBSD in a Wear Resistant Steel
2017 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 10, 453Article in journal (Refereed) Published
Keyword
Dilatometry, Hardness, EBSD, Martensite, Austenite, Kurdjumov-Sachs
National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-62368 (URN)10.3390/ma10050453 (DOI)
Available from: 2017-03-09 Created: 2017-03-09 Last updated: 2017-05-11Bibliographically approved
3. Influence of prior deformation in austenite on the martensite formation in a low-alloyed carbon steel
Open this publication in new window or tab >>Influence of prior deformation in austenite on the martensite formation in a low-alloyed carbon steel
(English)Article in journal (Refereed) Submitted
Keyword
Steel, Martensite, Phase transformation, Modelling, Thermal strain
National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-62367 (URN)
Available from: 2017-03-09 Created: 2017-03-09 Last updated: 2017-04-03

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