Change search
ReferencesLink to record
Permanent link

Direct link
Structure and properties of advanced fine grained steels produced using novel thermal treatments
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fine grained advanced steels exhibit favourable mechanical properties for applications requiring high strength, ductility and impact toughness. These properties result from a microstructure containing a fine distribution of several phases including ferrite, austenite, martensite and bainite. The bainite phase is in the form of fine lamellas of ferrite and carbon-enriched austenite which due to proper control of the chemical composition is lacking the nanometre scaled carbides associated with traditional bainite. The mechanisms of bainite phase transformations in steels have been debated since the naming of bainite in 1934, and range from diffusion-controlled (reconstructive) to diffusionless (displacive). Interest in the manufacture and application of fine grained advanced multiphase steels can be dated back to the 1970s, and it has been intensified after the turn of the century with the industrial production and application of such steels. The structure and mechanical properties of fine grained advanced steels produced using novel thermal treatments are described. The results of in-situ x-ray diffraction studies of the austenite to bainite transformation process provide information about the effect of carbon redistribution on the formation of transformation products. Transformation microstructures created by various thermal treatments are characterised using optical and scanning electron microscopy, which reveal the presence of martensite, bainite (in various forms), ferrite and retained austenite. Microstructural control is found to be possible by quenching the steels from the austenite phase to temperatures below the start temperature of martensite formation (determined by steel composition). The quenching is followed by isothermal treatment for varying times at temperatures related to the formation of transformation products (bainite in particular). This combined heat treatment also increases the rate of phase transformation in comparison with isothermal treatments. Thus multiphase microstructures are produced, which are found to possess favourable mechanical properties, in particular tensile strength, toughness and wear resistance. In press hardening of 0.26wt-carbon steels, by using the combined heat treatment described, the yield strength was found to be comparable to existing materials and the ductility was found to be higher. Welding tests of medium carbon steel with control of the post weld time temperature cycle in accordance with the combined heat treatment process minimized the risk of brittle phase formation in the weld as well as the heat affected zone. In addition the wear resistance in sliding as well as in rolling-sliding was shown to be better for austempered medium carbon high-silicon steels with fine grained ferritic austenitic microstructure in comparison with conventional steels. The presented work can be regarded as a contribution to the current world-wide effort to develop the 3rd generation advanced high-strength steels, which are expected to be a substantial part of the structural steel market before the end of the present decade. The switch to such steels in a large number of applications in manufacturing of transport vehicles and other products, will be comparable with the switch from plain carbon steels to HSLA steels in the seventies and eighties.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2012. , 43 p.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-18717Local ID: 9fe62e45-f669-45df-a58e-7819602198a8ISBN: 978-91-7439-530-3OAI: oai:DiVA.org:ltu-18717DiVA: diva2:991728
Note
Godkänd; 2012; 20121023 (esa); DISPUTATION . Ämne: Konstruktionsmaterial/Engineering Materials Opponent: Professor David Porter, Dept of Mechanical Engineering, Oulu University, Oulu, Finland Ordförande: Docent Marta-Lena Antti, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 20 december 2012, kl 10.00 Plats: E231, Luleå tekniska universitetAvailable from: 2016-09-29 Created: 2016-09-29Bibliographically approved

Open Access in DiVA

fulltext(12692 kB)15 downloads
File information
File name FULLTEXT01.pdfFile size 12692 kBChecksum SHA-512
d64c206b4a7d8e88690db27dbe1112de1ffa1acbed75628bf7a6a59670e84bceff1013f6931d606211d94ad6a0c80ce5ed98d5e491fa109496e34cc1f22db5dc
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Vuorinen, Esa
By organisation
Material Science

Search outside of DiVA

GoogleGoogle Scholar
Total: 15 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 4 hits
ReferencesLink to record
Permanent link

Direct link