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Licensed Unlicensed Requires Authentication Published by De Gruyter June 19, 2017

Effect of post-weld heat treatment on fusion boundary microstructure in dissimilar metal welds for subsea service

Wirkung der Wärmebehandlung nach dem Schweißen auf die Gefügeausbildung entlang der Schmelzlinie von Mischschweißverbindungen im Unterwasserbetrieb
Carolin Fink and Boian T. Alexandrov
From the journal Materials Testing

Abstract

Microstructural features at the fusion boundary in dissimilar metal welds significantly influence the susceptibility to local hydrogen embrittlement under subsea service conditions. Two subsea dissimilar metal welds using nickel-based filler metal on low alloy steel substrate were studied in this work in order to investigate the nature and microstructural evolution of the dissimilar interface during post-weld heat treatment (PWHT), and to determine resistance to hydrogen embrittlement. The delayed hydrogen cracking test (DHCT) was utilized to determine hydrogen-assisted cracking (HAC) susceptibility. Thermokinetic modeling was conducted to study phase formation and carbon diffusion across the dissimilar interface. Diffusion calculations incorporated initial compositional gradients after welding and the nonmoving phase boundary during PWHT. Resistance to HAC was in good agreement with previously obtained DHCT results for the tested dissimilar metal welds. Tempered martensite in the heat-affected zone of the steel substrate was observed as a function of PWHT temperature. Fresh martensite with high local hardness formed during cooling in highly diluted weld metal regions. The cell model incorporated in DICTRA effectively predicted differences in carbon concentration profiles across the dissimilar interface and type of carbide precipitation as a function of PWHT procedure and steel substrate.

Kurzfassung

Die Gefügeausbildung entlang der Schmelzlinie von Mischschweiβverbindungen ist von groβer Bedeutung für deren Anfälligkeit gegenüber Wasserstoffversprödung bei Betrieb im Offshore- und Unterwasserbereich. Im Rahmen dieser Studie wurden zwei typische Mischverbindungen aus Ni-Basis-Schweiβgut auf niedriglegiertem Stahl bezüglich Veränderungen im Gefüge der Bindezone durch Wärmebehandlung und auf Beständigkeit gegenüber Wasserstoffversprödung untersucht. Als Prüfverfahren zur Beurteilung der wasserstoffunterstützten Kaltrissbeständigkeit der Mischverbindungen wurde der Delayed Hydrogen Cracking Test (DHCT) herangezogen. Proben mit horizontal zur Belastungsrichtung verlaufender Schmelzlinie wurden elektrolytisch mit Wasserstoff beladen und gleichzeitig einer konstanten Zugbeanspruchung ausgesetzt. Die Standzeit bis zum Versagen der Proben wurde als quantitatives Testkriterium herangezogen. Die Diffusion von Kohlenstoff und die Bildung von Carbiden im Bereich der Schmelzlinie während der Wärmebehandlung wurde mittels thermodynamischer Simulationen in ThermoCalc und DICTRA untersucht. Die Kaltrissprüfergebnisse zeigen eine gute Übereinstimmung mit früheren DHCT-Tests an den untersuchten Mischverbindungen. In der Wärmeeinflusszone der Stahlgrundwerkstoffe wird die Martensitphase in Abhängigkeit von der Höhe der Wärmebehandlungstemperatur unterschiedlich stark angelassen. In Bereichen entlang der Schmelzlinie mit hoher Aufmischung mit dem Ni-Basisschweiβgut bildet sich bei der Abkühlung frischer Martensit mit hoher Härte. Das Simulationsmodel in DICTRA beschreibt Unterschiede im Kohlenstoffprofil und der Ausscheidung von Carbiden in der Bindezone der Mischverbindungen in Abhängigkeit der Wärmebehandlung und des niedriglegierten Stahlgrundwerkstoffes.


*Correspondence Address, Dr.-Ing. Carolin Fink, Assistant Professor, Welding Engineering Program, Department of Materials Science and Engineering, 1248 Arthur E. Adams Drive, Columbus, OH 43221, USA, E-mail:

Dr. Carolin Fink is Assistant Professor in the Department of Materials Science and Engineering and the Welding Engineering Program at The Ohio State University, Columbus, USA. Her research interests include weld cracking and materials degradation phenomena, in particular elevated temperature cracking and liquid metal embrittlement, welding metallurgy and weldability of nickel-based alloys, welding of dissimilar materials and weldability testing. In 2015, she joined The Ohio State University as a postdoctoral researcher in the welding engineering group. In 2016, she was awarded the Henry Granjon Prize of the International Institute of Welding (IIW) in recognition of her PhD research on ductility-dip cracking in nickel-based alloys. She received her PhD in Mechanical Engineering from the Otto-von-Guericke University Magdeburg in Germany, and is a certified International Welding Engineer (IWE).

Dr. Boian T. Alexandrov is Associate Professor at the Welding Engineering Program of The Ohio State University, Columbus, USA. His research interests are in the area of physical metallurgy with a strong focus on welding metallurgy. He has developed innovative methods for weldability evaluation, phase transformation analysis, and metallurgical characterization that enabled quantification of nonequilibrium phenomena and response to processing in advanced alloys, and led to resolution of major technology-restricting weldability problems in the power generation and oil and gas sectors.


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Published Online: 2017-06-19
Published in Print: 2017-06-01

© 2017, Carl Hanser Verlag, München