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Effect of Carbon Content on Variant Pairing in Bainitic Low Alloy Steel
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures. Husqvarna Group, Huskvarna, 561 82, Sweden.ORCID iD: 0000-0002-1968-4364
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.ORCID iD: 0000-0003-1102-4342
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.ORCID iD: 0000-0002-7656-9733
2022 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 53, no 9, p. 3418-3427Article in journal (Refereed) Published
Abstract [en]

Variant pairing in bainite was evaluated in four different commercial low alloy steels with medium to high carbon content. The steels investigated were austempered in the temperature range 275 °C to 450 °C to obtain a bainitic microstructure. It was found that the V1–V6 is the most frequent variant pairing at lower temperature, while it gradually decreases towards intermediate temperatures, and at the highest austempering temperatures the variant pairing with low misorientation boundaries such as V1–V4 and V1–V8 is the most frequent. The preferred variant pairing is the least pronounced after austempering of steels with higher carbon contents at intermediate temperature. Nonetheless, a continuous increase of variant pairing within the same Bain group was observed with increasing austempering temperature for all steels. Furthermore, it was observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increases with increasing austempering temperature for all steels.

Place, publisher, year, edition, pages
Springer Nature , 2022. Vol. 53, no 9, p. 3418-3427
Keywords [en]
Bainite, Carbides, Temperature, Austempering temperature, Electron back scatter diffraction, Highest temperature, Intermediate temperatures, Medium carbon bainitic steel, Medium-carbon steels, Temperature changes, Varying temperature, Low carbon steel, variant pairs
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-320870DOI: 10.1007/s11661-022-06757-4ISI: 000824841800001Scopus ID: 2-s2.0-85133588434OAI: oai:DiVA.org:kth-320870DiVA, id: diva2:1707883
Funder
KTH Royal Institute of Technology
Note

QC 20221109

Available from: 2022-11-01 Created: 2022-11-01 Last updated: 2023-12-07Bibliographically approved
In thesis
1. Thermal Stability and Mechanical Properties of Bainitic Steel
Open this publication in new window or tab >>Thermal Stability and Mechanical Properties of Bainitic Steel
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For material design it is important to understand and establish the relations between the processing of the steel, the resulting microstructure and how that correlate to the properties of the steel. The main focus of this thesis has been to increase this knowledge for low alloy bainitic steels with medium to high carbon content. This can be used to optimize the material properties in order to obtain a better performance of the final product. It may also be used to achieve a better and more efficient process and thus it also contributes to sustainability. Further, it is also important knowledge when substantial changes in an existing production is required or driven due to changes in regulations and the increasing demand for more sustainable processes. This may require further changes in one or several of the processes in order for the material to maintain its properties and performance.

The present thesis has in depth investigated the effects of austempering temperatures and carbon content, where the microstructure of the bainitic steels was characterized and the different contributions to the strength were investigated. The contributions investigated were grain size, dislocation density, precipitations and dissolved alloying elements. The contributions were characterized through extensive experimental work coupled with complementary thermodynamic calculations. Further, the effects of tempering were examined and compared to each of the strengthening contributions. For the steels investigated, the effects from a varying carbon content were in focus. This since carbon plays a very important role in steels, especially when the other alloying contents are kept low. It was shown that dislocation density and coarseness of the bainitic microstructure is affected by the austempering temperature. However, it was only the dislocation density which were affected by the austempering time. It was also concluded that the decrease in dislocation density was the main reason for the hardness decrease during tempering up to 375 °C. Further it was shown that an increase in carbon content increased the hardness of the bainite, and slightly improved the tempering resistance at shorter times. However, the increased carbon content also resulted in longer austempering times to reach a fully bainitic structure, especially at the lower austempering temperatures. 

 

Finally, to better understand the bainitic microstructure and the transformation related to austempering temperature, the crystallographic relations in bainite were investigated by detailed electron backscatter diffraction and analysis of the variant pairing. The investigations were performed over the austempering temperature range of 275-450 °C for the bainitic transformation, in low alloyed steels with a varying carbon content. The variant pairing was also correlated to the corresponding bainitic microstructure. It could be concluded that variant pairing had two distinct changes when the austempering temperature was varied in the steel with a medium carbon content. This differs from the literature were two types of bainite is commonly referred to, lower and upper bainite. Further it was shown that a continuous increase of variant pairing within the same Bain group could be observed with increasing austempering temperature. It was also observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increased linearly with an increasing austempering temperature for bainite.

Abstract [sv]

För materialdesign är det viktigt att förstå och att kunna säkerställa sambanden mellan framställningen av stålet, den resulterande mikrostrukturen och hur detta korrelerar med stålets egenskaper. Huvudfokus i denna avhandling har varit att öka just denna kunskap, särskilt för låglegerade bainitiska stål med medelhögt till högt kolinnehåll. Denna kunskap kan användas för att optimera materialegenskaperna för att i sin tur optimera prestandan hos slutprodukten. Kunskapen kan också användas för att uppnå en bättre och mer effektiv process och där igenom bidra till bättre hållbarhet. Vidare är det viktig kunskap när betydande förändringar i en befintlig produktion krävs eller påskyndas till följd av förändringar i regelverk ofta drivet av ökande krav/efterfrågan på mer hållbara processer. Detta kan även kräva ytterligare förändringar i en eller flera av processerna för att komponenterna ska bibehålla sina ursprungliga egenskaper och prestanda.

Denna avhandling har ingående undersökt effekterna av austempereringstemperaturer och kolhalt, där strukturen hos de bainitiska stålen karakteriserats och de olika bidragen till materialets hårdhet undersökts. Bidragen som undersökts är kornstorlek, dislokationsdensitet, utskiljningar och inlösta legeringsämnen. Bidragen karaktäriserades genom ett omfattande experimentellt arbete i kombination med kompletterande termodynamiska beräkningar. Vidare undersöktes effekterna av anlöpning och jämfördes med vart och ett av de olika hårdhetsbidragen. För de undersökta stålen stod effekterna från en varierande kolhalt i fokus. Detta eftersom kolet spelar en mycket viktig roll i stålet, speciellt när övriga legeringshalter är låga. Effekterna från den varierande kolhalten på både de mekaniska egenskaper och stålets mikrostrukturen undersöktes. Det visades att dislokationsdensiteten och grovleken hos den bainitiska mikrostrukturen påverkas av austempereringstemperaturen. Det var dock endast dislokationstätheten som påverkades av austempereringstid och slutsatsen var att minskning i dislokationsdensitet var huvudorsaken till hårdhetsminskningen under anlöpning upp till 375 °C. Vidare visades att en ökning av kolhalten också ökade hårdheten hos bainiten och förbättrade anlöpningsmotståndet något vid kortare anlöpningstider. En ökande kolhalt resulterade också i längre austempereringstider för att nå en helt bainitisk struktur, speciellt vid de lägre austempereringstemperaturerna.

 

Slutligen, för att bättre förstå den bainitiska mikrostrukturen och dess omvadling relaterad till austempereringstemperatur, undersöktes de kristallografiska relationerna i bainiten. Detta gjordes genom en detaljerad elektronbackscatter-diffraktion och analys av variantpar. Undersökningarna utfördes över austempereringstemperatur området för den bainitiska omvandlingen, i låglegerade stål med varierande kolhalt. Variantparen korrelerads sedan till dess motsvarande bainitiska mikrostruktur. Man kunde dra slutsatsen att variantparen har två distinkta skiften när austempereringstemperaturen ökades i stålet med medelhög kolhalt. Detta skiljer sig från litteraturen där det vanligtvis hänvisas till två typer av bainit, lägre och övre bainit. Vidare visades att en kontinuerlig ökning av variantpar inom samma Bain-grupp kunde observeras med ökande austempereringstemperatur. Det observerades också att avvikelsen från det teoretiska orienteringsförhållandet Kurdjumov–Sachs ökade linjärt med en ökande austempereringstemperatur för bainit.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. p. 165
Series
TRITA-ITM-AVL ; 2022:34
Keywords
Bainite, Austempering, Low alloyed steel, Tempering, Variant pairing, Material characterization
National Category
Metallurgy and Metallic Materials
Research subject
Metallurgical process science; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-320871 (URN)978-91-8040-401-3 (ISBN)
Public defence
2022-12-09, F3 / https://kth-se.zoom.us/webinar/register/WN_7TF6bfCPTca3-DZ5KGx8hA, Lindstedtsvägen 26, Stockholm, 14:00 (English)
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Supervisors
Available from: 2022-11-11 Created: 2022-11-03 Last updated: 2023-12-07Bibliographically approved

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Ståhlkrantz, AdamHedström, PeterBorgenstam, Annika

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