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Influence of welding and additive manufacturing thermal cycles on microstructure and properties of super duplex stainless steel base and weld metal studied by a physical simulation technique
University West, Department of Engineering Science, Division of Welding Technology.
2018 (English)Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesisAlternative title
Inverkan av termiska cykler på utskiljning av sekundärfaser och egenskaper vid svetsning och additiv tillverkning i fysiskt simulerade super duplext rostfritt grundmaterial och svetsgods (Swedish)
Abstract [en]

Super duplex stainless steel (SDSS) is known for its excellent combination of high corrosion resistance and strength. However, the main limitations in SDSS applications are the risk of getting an imbalanced ferrite and austenite fraction and the sensitivity to form intermetallic phases at temperatures between approximately 600-1000 °C. During welding of SDSS, the welding parameters must be selected carefully in order to avoid formation of intermetallic phases such as sigma phase due to slow cooling or multiple reheating. In addition, special care is needed to have a balanced ferrite and austenite fraction. In this work, the influence of multiple thermal cycles and cooling rate on the microstructure of SDSS base and weld metal was investigated. A novel heat treatment method, using a stationary TIG arc, was performed to produce the samples. The test discs were composed of base and weld metal. Five samples were produced including 1, 5, and 15 passes of rapid cooling, one sample with medium cooling and one with slow cooling. The cooling time between 1000-700 °C was aimed at being similar for the pair of 5 passes and medium cooling and the other pair of 15 passes and slow cooling. The material was characterized by light optical microscopy and scanning electron microscopy while hardness mapping and sensitization testing were employed to evaluate the properties. It was revealed that sigma phase was more prone to precipitate in the weld compared to the base metal when exposed to high ageing temperature and repeated thermal cycles. Slow cooling was found to promote sigma phase precipitation more than multiple reheating in the weld metal. In the base metal, a minor difference was found between the slowly cooled and multiple reheated samples. Rapid cooling in multiple reheating generated nitrides in the fusion boundary zone. As more reheating passes were applied, the amount of nitrides decreased. Multipass reheating cycles also resulted in increased hardness and nitrogen depletion adjacent to the fusion boundary zone causing the ferrite content to increase. Based on this study, less sigma phase precipitation in the weld metal was achieved when using multiple reheating passes with low heat input instead of a few passes with high heat input, providing equal accumulative heating time between 1000-700 °C. However, the influence of multiple reheating on the hardness was larger compared to slow cooling.

Place, publisher, year, edition, pages
2018. , p. 40
Keywords [en]
Super duplex stainless steel, Weld metal, Sigma phase, Secondary austenite, Arc heat treatment, Welding, Wire arc additive manufacturing Hardness, Sensitization
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:hv:diva-12712Local ID: EXP800OAI: oai:DiVA.org:hv-12712DiVA, id: diva2:1232011
Subject / course
Technology
Educational program
Produktionsteknik
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Available from: 2018-07-26 Created: 2018-07-10 Last updated: 2018-07-26Bibliographically approved

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