Dissimilar Metal Welds Environmental Fracture Mechanics Testing

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Project Code: 32900

Start date and planned duration: February 2019, 36 months

Objectives

Quantify how environmental parameters affect susceptibility to failure of dissimilar metal welds (DMWs).
Produce a set of guidelines on how to approach environmental fracture mechanics testing of dissimilar metal welds which can be reliably used for engineering critical assessment (ECA).
Establish, via a programme of advanced mechanical testing and characterisation, the micro-mechanical response of interfacial zones, and assess the impact of these on the response to hydrogen embrittlement.

Project Outline

Dissimilar metal welds are commonly used in the oil and gas industry for subsea applications. Such components are subjected to cathodic protection (CP) whilst subsea in order to prevent corrosion of nearby ferritic materials. However, as a consequence of the applied CP, hydrogen is able to evolve at the surface of exposed (uncoated) metallic surfaces, whereupon it can diffuse into the material. Hydrogen accumulation in critical concentrations at a dissimilar fusion boundary can, under certain circumstances, cause cracks to initiate. The vast majority of subsea dissimilar metal welds continue to provide successful service; however, occasional failures continue to be observed.

TWI has developed a methodology for ranking environmental performance, based on single edge notched bend testing (SENB), to assess dissimilar metal welds (DMWs). These ranking methods aid materials selection, as they provide an economic means of shortlisting candidate materials. Currently, however, there are no guidelines on how to conduct a fracture mechanics assessment on a dissimilar joint, such that test data can be used, directly, in a design or fitness-for-service assessment.

The effects of various fracture mechanics testing parameters on the susceptibility to environmental fracture will be investigated, allowing data to be generated, which can be used in environmental ECAs. whilst, simultaneously, revealing the factors influencing failure of joints in the field. The project will develop best-practice guidelines for environmental fracture mechanics testing of DMWs.

Industry Sectors

Benefits to Industry

Guidance on assessment of DMW joints will allow industry to create improved designs and carry out more effective ECA for structures containing DMWs for subsea applications. The outcomes are also expected to be of value for DMW applications in other environments, including the nuclear industry.

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