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Post weld-treatment of laser welded AHSS by application of quenching and partitioning technique
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-9100-7982
Department for Materials Science, Functional Materials, Saarland University.
2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 698, p. 174-182Article in journal (Refereed) Published
Abstract [en]

Two-step quenching and partitioning (Q&P) treatment was applied on specimens of an advanced high strength steel (AHSS) after laser welding, for post welding treatment. In order to avoid formation of brittle martensite phase, which usually form due to very high cooling rate of laser welding. To simulate the effect of different Q&P parameters after welding in the most critical part of HAZ, several cycles were performed in Gleeble simulator and analyzed in advance. Subsequently some of the cycles were repeated after laser welding by using an induction heater close to the weld. Different techniques including SEM, EBSD and XRD were used to analyze the microconstituents of the structure and mechanical properties were investigated by micro-hardness measurements across the weld, tensile and impact toughness tests. The final structure consists of controlled amount of tempered martensite with precipitates, bainite laths and small amount of fresh martensite depending on the thermal cycles. In addition, samples heated at a temperature between Ms and Bs (in this case 540C) showed the best mechanical properties. Therefore, this technique not only improves the microstructure and mechanical properties of the fusion zone (FZ) and heat affected zone (HAZ) but gives also a quick industrial processing method for post welding treatments.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 698, p. 174-182
Keywords [en]
AHSS, Quenching and partitioning, Microstructure, Mechanical properties
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-63370DOI: 10.1016/j.msea.2017.05.053ISI: 000405251900021Scopus ID: 2-s2.0-85019392617OAI: oai:DiVA.org:ltu-63370DiVA, id: diva2:1095785
Note

Validerad; 2017; Nivå 2; 2017-05-24 (andbra)

Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2019-09-13Bibliographically approved
In thesis
1. Increasing phase transformation rate in advanced high strength steel applications
Open this publication in new window or tab >>Increasing phase transformation rate in advanced high strength steel applications
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Ökning avfasomvandlingshastigheten för avancerade höghållfasta stål
Abstract [en]

The bainite transformation rate has been shown to increase by starting the heat treatment with partial martensite transformation after austenitization. Based on this fact, a process very similar to “Quenching and Partitioning” (Q&P) is used to produce a fine-grained complex microstructure of martensite, bainite and retained austenite with outstanding mechanical properties in a very short time. During this process, different mechanisms including bainite transformation, carbon partitioning, carbide precipitation, grain growth may occur. All these mechanisms can affect the mechanical properties such as strength, ductility and toughness. Investigation of the different mechanisms influencing the properties and subsequent optimization of these is important. In this work, different mechanisms of the Q&P heat treatment process and its practical industrial applications have been investigated. 

Firstly, the implementation of a Q&P method directly after laser welding for a few seconds to substitute any post welding treatment has been studied. To investigate the feasibility, limitations, and advantages of this method for a low-carbon low-silicon high strength steel, the microstructure and mechanical properties by both modelling and experimental approach were studied. Promising results show that this method can decrease the ordinary post-welding treatment time from a few minutes to a few seconds, and in addition improve the mechanical properties of the fusion zone and the heat affected zone to the same or even higher values in comparison with the base material.

In the second part of this work, the effect of quenching and partitioning on the microstructure and mechanical properties of a high carbon steel has been studied. The aim with this part was to optimize the phase transformation rate for production of ultra-high strength steel by controlling its microstructural evolution. The results show that it is possible to get good strength values also for high carbon steels by Q&P treatment. In addition, the approach with process control maps can give a good overview of which properties can be achieved by this method. Hardness value of over 700 HV, and tensile strength of up to 2.5 GPa with a relatively good ductility of 4-6% has been achieved by quenching to room temperature and partitioning for less than one minute at 400 °C resulting in a microstructure consisting of martensite and retained austenite. In a nutshell, the approach to bainite transformation with pre-existing martensite shorten the processing time for development of advanced high strength steels significantly. This method is also possible to be used in industrial processes as in welding.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Advanced high strength steel, Quenching and partitioning, welding
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-72569 (URN)978-91-7790-324-6 (ISBN)978-91-7790-325-3 (ISBN)
Public defence
2019-05-10, E231, Luleå, 10:00 (English)
Opponent
Supervisors
Note

This thesis is the result of a collaboration between Luleå University of Technology and Saarland University that aims toward a double degree.

Available from: 2019-03-05 Created: 2019-03-04 Last updated: 2019-04-16Bibliographically approved

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Publisher's full textScopushttp://www.sciencedirect.com/science/article/pii/S092150931730669X

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Forouzan, FarnooshVuorinen, Esa

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