Toughness properties at multi-layer laser beam welding of high-strength steels
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RESEARCH PAPER
Toughness properties at multi-layer laser beam welding of high-strength steels Joerg Volpp 1
&
Pär Jonsén 1 & Anandkumar Ramasamy 2 & Bert Kalfsbeek 2
Received: 29 July 2020 / Accepted: 28 September 2020 # The Author(s) 2020
Abstract The material characteristics of high toughness and high strength in steel are usually not available at the same time. However, it would be an advantage if high-strength steels would show high impact toughness also at lower temperatures for applications in critical surroundings. In this paper, an approach of multi-layer welding of high-strength steel is presented in order to increase the weld-metal toughness using wire material in combination with thermal cycle modifications. Promising interlocking microstructures were found after multiple tempering of the previously applied structure at homogeneously distributed material in the weld seam. It was found that short thermal cycles during laser processing lead to insufficient time for carbon diffusion, which leads to remaining ferrite structures in contrast to the prediction of welding transformation diagrams. The additionally applied heating cycles during multi-layer laser welding induce the formation of interlocking microstructures that help to increase the weld seam toughness. Keywords High-strength steel . Narrow-gap multi-layer welding . Laser welding . Weld metal toughness . Interlocking microstructures
1 Introduction High-strength steel (HSS) processing becomes more and more interesting for reducing the weight of parts at comparable strength values, e.g. for ship building [1], building bridges [2], pressure vessel, gas/oil transportation line pipes, and offshore constructions [3–5]. However, typically at higher strength values of the steel grade, the toughness at low working temperatures decreases [6]. This can be a drawback and a possible reason for avoiding the use of the steel in critical environment. In general, steel grades with high strength values (up to 1100 MPa) are produced by quenching and tempering processes. Thereby, the material is reheated above austenization Recommended for publication by Commission IV - Power Beam Processes * Joerg Volpp [email protected] 1
Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-971 87 Luleå, Sweden
2
Lincoln Electric Europe, 6534 AD Nijmegen, The Netherlands
temperature in order to dissolve the carbon in the austenite. Fast cooling (quenching) is usually used to avoid the formation of ferrite and pearlite, which would require a diffusion process. The carbon can remain dissolved, and after cooling, the microstructure mainly transforms into martensite. During welding of HSS, complex heat treatment is present leading to microstructural changes and variations of strength and toughness values. Typical welding processes include submerged-arc, SAW; gas-metal-arc, GMAW; or laser-archybrid welding, LAHW. Typically, long cooling times are present, e.g. for GMAW [7] or SAW [8], which can induce the diffusion processes that can lead to sof
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