Alternative nickel based filler metals for use in the girth welding of bi-metallic pipelines

Maltin, C. A. and Galloway, A. M. and Mweemba, M (2014) Alternative nickel based filler metals for use in the girth welding of bi-metallic pipelines. Metallurgical and Materials Transactions A. ISSN 1073-5623 (Submitted)

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Abstract

The highest grade of carbon steel which may be used as a substrate material in bi-metallic pipelines is being limited to API 5L X65 by the as welded yield strength of the filler metals currently employed for girth welding. An investigation has been carried out to assess the suitability of three alternative nickel based filler metals for this application: Inconel 617, Inconel 718 and Incoloy 945X. Their viability has been determined through microstructural characterisation and mechanical testing, focusing on tensile strength and impact toughness. Investigations have been carried out into the precipitation of eutectic type phases within the weld metal and the behaviour of alloying elements resulting in such precipitation. The data presented have indicated that of the three weld metals, Inconel 617 exhibits superior as-welded properties and provides a yield strength sufficient to overmatch the specified minimum yield strength of API 5L X65 pipeline. It has been determined that although Nb additions are essential to promote the precipitation strengthening, its segregation to the interdendritic region during solidification leads to the formation of an incoherent eutectic type precipitate. As a result of the lower volume fraction of eutectic type precipitate, Inconel 617 also demonstrated superior impact toughness properties.