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A Microstructural Investigation of Roll Formed Austenitic Stainless Steel
Dalarna University, SE-791 88 Falun, Sweden.
2013 (English)In: Sheet Metal 2013, Trans Tech Publications, 2013, Vol. 549, p. 364-371Conference paper, Published paper (Refereed)
Abstract [en]

Due to high production rates and the possibility to form complex geometries roll forming has become an increasingly popular forming process for sheet metal. Increasing quantities of high strength steels are used today but can be difficult to form due to their low ductility. One way to partly overcome this problem is to heat the steel in the forming area thus locally increasing the ductility. In the present study partially heated cold rolled high strength AISI 301 type austenitic stainless steel was investigated using electron backscattered diffraction (EBSD), and the results were compared to microhardness measurements. The results show that partial heating will give an almost complete reverse martensite transformation, i.e. martensite (α′) transforms to austenite (γ), close to the surfaces and grain growth in the middle of the steel sheet. The extension of the heat affected zone can be determined using either microhardness or EBSD measurements. Both these measurements can be used to determine the position of the neutral layer after roll forming. The hardness measurement cannot distinguish between microstructural features but the results are in good agreement with the EBSD results for volume fraction of α′-martensite. A major advantage of using EBSD is the possibility to characterize and follow the microstructural development when heating and roll forming.

Place, publisher, year, edition, pages
Trans Tech Publications, 2013. Vol. 549, p. 364-371
Series
Key Engineering Materials, ISSN 1013-9826 ; 549
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-227655DOI: 10.4028/www.scientific.net/KEM.549.364ISI: 000321156900046Scopus ID: 2-s2.0-84877869662ISBN: 9783037856710 OAI: oai:DiVA.org:kth-227655DiVA, id: diva2:1205017
Conference
15th International Conference on Sheet Metal, SheMet 2013; Belfast; United Kingdom; 25 March 2013 through 27 March 2013
Note

QC 20180514

Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-05-14Bibliographically approved
In thesis
1. Experimental Studies of Deformation Structures in Stainless Steels using EBSD
Open this publication in new window or tab >>Experimental Studies of Deformation Structures in Stainless Steels using EBSD
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the focus has been the study of deformation structures in stainless steels by using electron backscatter diffraction (EBSD). Via increased knowledge of the evolution of the substructure during deformation, the design and control of the manufacturing process can be improved.

A relation was found between the active deformation mechanisms, the evolution of low angle boundaries (LABs) and the strain hardening rate. When deformation twinning was an active deformation mechanism in an austenitic stainless steel with lower stacking fault energy (SFE), the strain hardening rate was maintained up to large strains due to formation of LABs. The deformation twin boundaries acted as new obstacles for dislocation slip which in turn increased the formation of LABs even further. During deformation by slip in an austenitic stainless steel with a higher SFE, the strain hardening rate instead decreased when LABs were formed. A high value of SFE promotes dislocation cross slip which in turn increases annihilation of dislocations leading to a minor increase in LAB formation.

Deformation structures formed in surface grains during in situ tensile tests were found to develop at lower strains than in bulk grains obtained from interrupted conventional tensile tests. This behavior is consistent with the fact that dislocations sources and deformation twinning operate at approximately half the stress on a free surface as compared to the bulk.

The deformation structures were quantified by measuring size distributions for entities bounded by LABs and high angle boundaries (HABs). The size distributions were found to be well described by bimodal lognormal distribution functions. The average size for the distribution of small grains and subgrains correlated well with the mean free distance of dislocation slip and to the strain hardening.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. p. 63
Series
TRITA-ITM-AVL ; 2018:24
Keyword
EBSD, Austenitic stainless steels, Duplex stainless steel, In situ tensile test, Grain boundaries, Grain rotation, Grain size distribution, Texture, Strain hardening, Structure-property relationship, High strain rate, Wire rod rolling, Roll forming
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-227663 (URN)978-91-7729-772-7 (ISBN)
Public defence
2018-06-05, B2, Brinellvägen 23, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Note

QC 20180514

Available from: 2018-05-14 Created: 2018-05-09 Last updated: 2018-05-14Bibliographically approved

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