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Increasing phase transformation rate in advanced high strength steel applications
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-5390-7701
2019 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Ökning avfasomvandlingshastigheten för avancerade höghållfasta stål (Swedish)
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 [en]
Advanced high strength steel, Quenching and partitioning, welding
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-72569ISBN: 978-91-7790-324-6 (print)ISBN: 978-91-7790-325-3 (electronic)OAI: oai:DiVA.org:ltu-72569DiVA, id: diva2:1293408
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
List of papers
1. Microstructure analysis and mechanical properties of Low alloy High strength Quenched and Partitioned Steel
Open this publication in new window or tab >>Microstructure analysis and mechanical properties of Low alloy High strength Quenched and Partitioned Steel
Show others...
2017 (English)In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 258, p. 574-578Article in journal (Refereed) Published
Abstract [en]

Gleeble study of the quenching and partitioning (Q&P) process has been performed onDomex 960 steel (Fe, 0.08 %C, 1.79 %Mn, 0.23 %Si, 0.184 %Ti, and 0.038 %Al). The effect ofdifferent Q&P conditions on microstructure and mechanical properties were investigated. The aimof the process is to produce a fine grained microstructure for better ductility and controlled amountsof different micro-constituents to increase the strength and toughness simultaneously. Threedifferent quenching temperatures, three partitioning temperatures and three partitioning times havebeen selected to process the 27 specimens by Gleeble® 1500. The specimens were characterized bymeans of OM, SEM, XRD, hardness and impact tests. It was found that, fine lath martensite withretained austenite is achievable without high amount of Si or Al in the composition although lack ofthese elements may cause the formation of carbides and decrease the available amount of carbon forpartitioning into the austenite. The hardness increases as the quenching temperature is decreased,however, at highest partitioning temperature (640◦C) the hardness increases sharply due to extensiveprecipitate formation.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2017
Keywords
AHSS, Gleeble, Quenching and partitioning (Q&P), X-ray, mechanical properties
National Category
Other Materials Engineering
Research subject
Engineering Materials; Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-60479 (URN)10.4028/www.scientific.net/SSP.258.574 (DOI)2-s2.0-85009754153 (Scopus ID)
Conference
8th conference of Materials Structure & Micromechanics of Fracture (MSMF8), Brno, Czech republic, 27-29 June, 2016
Note

Konferensartikel i tidskrift

Available from: 2016-11-16 Created: 2016-11-16 Last updated: 2019-03-04Bibliographically approved
2. Post weld-treatment of laser welded AHSS by application of quenching and partitioning technique
Open this publication in new window or tab >>Post weld-treatment of laser welded AHSS by application of quenching and partitioning technique
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
Keywords
AHSS, Quenching and partitioning, Microstructure, Mechanical properties
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-63370 (URN)10.1016/j.msea.2017.05.053 (DOI)000405251900021 ()2-s2.0-85019392617 (Scopus ID)
Note

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

Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2019-09-13Bibliographically approved
3. Effect of Carbon Partitioning, Carbide Precipitation, and Grain Size on Brittle Fracture of Ultra-High-Strength, Low-Carbon Steel after Welding by a Quenching and Partitioning Process
Open this publication in new window or tab >>Effect of Carbon Partitioning, Carbide Precipitation, and Grain Size on Brittle Fracture of Ultra-High-Strength, Low-Carbon Steel after Welding by a Quenching and Partitioning Process
2018 (English)In: Metals, ISSN 2075-4701, Vol. 8, no 10, article id 747Article in journal (Refereed) Published
Abstract [en]

To improve the weld zone properties of Advanced High Strength Steel (AHSS), quenching and partitioning (Q&P) has been used immediately after laser welding of a low-carbon steel. However, the mechanical properties can be affected for several reasons: (i) The carbon content and amount of retained austenite, bainite, and fresh martensite; (ii) Precipitate size and distribution; (iii) Grain size. In this work, carbon movements during the partitioning stage and prediction of Ti (C, N), and MoC precipitation at different partitioning temperatures have been simulated by using Thermocalc, Dictra, and TC-PRISMA. Verification and comparison of the experimental results were performed by optical microscopy, X-ray diffraction (XRD), Scanning Electron Microscop (SEM), and Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Scanning Diffraction (EBSD) analysis were used to investigate the effect of martensitic/bainitic packet size. Results show that the increase in the number density of small precipitates in the sample partitioned at 640 °C compensates for the increase in crystallographic packets size. The strength and ductility values are kept at a high level, but the impact toughness will decrease considerably.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
low-carbon AHSS, Q&P, toughness, modelling, precipitation, martensite packet
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-71070 (URN)10.3390/met8100747 (DOI)2-s2.0-85053830157 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-10-05 (svasva)

Available from: 2018-10-02 Created: 2018-10-02 Last updated: 2019-03-04Bibliographically approved
4. Optimization of Quenching Temperature to Minimize the Micro Segregation Induced Banding Phenomena in Quenching and Partitioning (Q&P) Steels
Open this publication in new window or tab >>Optimization of Quenching Temperature to Minimize the Micro Segregation Induced Banding Phenomena in Quenching and Partitioning (Q&P) Steels
2019 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 90, no 1, article id 1800281Article in journal (Refereed) Published
Abstract [en]

Mn, Cr, and Si are favorable elements for designing the quenching and partitioning (Q&P) steels while the microsegregation of them is a common phenomenon in the steels. This segregation makes the bands of enriched and depleted Mn–Cr regions, which affects the Ms temperature of the bands and consequently influence the volume fraction of initial martensite, retained austenite, and secondary fresh martensite in different bands. This issue leads to non-homogeneity in the microstructure and mechanical properties. In this study, the optimization method to minimize the inhomogeneity by selection of the quenching temperature is demonstrated.

Place, publisher, year, edition, pages
John Wiley & Sons, 2019
Keywords
banding, hardness, microsegregation, microstructure, quenching and partitioning (Q&P)
National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-71024 (URN)10.1002/srin.201800281 (DOI)000454790200006 ()2-s2.0-85053542723 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-01-25 (johcin) 

Available from: 2018-09-28 Created: 2018-09-28 Last updated: 2019-03-04Bibliographically approved
5. Process Control Maps to Design an Ultra-High Strength-Ductile Steel
Open this publication in new window or tab >>Process Control Maps to Design an Ultra-High Strength-Ductile Steel
2019 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 35, no 10, p. 1173-1184Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
ultrahigh strength, high carbon low alloy steel, Q&P, ductility, map
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-73102 (URN)10.1080/02670836.2019.1615752 (DOI)000469142200001 ()2-s2.0-85066881284 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-19 (johcin)

Available from: 2019-03-04 Created: 2019-03-04 Last updated: 2019-06-19Bibliographically approved

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Forouzan, Farnoosh

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Citation style
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