Carbon Diffusion Across the Bimetallic Interface of Welded Clad Pipes
- Dag Lindholm (Institute for Energy Technology)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- clad pipes, computer simulations, welding, carbon diffusion
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- 20 since 2007
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Clad pipes are manufactured both with and without a thin nickel interlayer. Diffusion of carbon from the base material to the clad, which degrades the structure adjacent to the interface and increases the local embrittlement, is effectively reduced by this interlayer. However, a majority of the existing subsea clad pipes are without this interlayer, and diffusion of carbon at the bimetallic interface is an issue that still needs to be addressed. This paper presents a methodology for calculation of carbon diffusion, or any other elements, that accounts for the combined heat exposure in the production process of the clad pipes and in an eventual repair welding. In the welding simulation the local temperatures are logged and the resulting file is modified afterwards to include data, if available, from the diffusion bonding process. In the diffusion calculation temperatures are not calculated but read from this log file. Identical thermal history in the welding simulation and subsequent diffusion simulation is therefore ensured with diffusion coefficients being dependent on the temperature and on the type of microstructure. A case study, that concludes that the diffusion bonding process is the major contributor to diffusion of carbon, demonstrates the methodology.
Utilization of clad and lined pipes, where a thin layer of corrosion resistant alloy (CRA) provides corrosion resistance inside a conventional carbon steel pipe, is with increasing frequency considered as an economically viable alternative for corrosion management in many new oil and gas developments. Different production techniques are used to manufacture these pipes. While lined pipes are manufactured by mechanical expansion of a CRA pipe to the backing pipe, production of clad pipes is by metallurgical bonding of the clad to the backing material.
Microstructure and mechanical properties of such pipes are influenced by the method these are manufactured. Temperature, pressure and time are key parameters which influence on the diffusion of the atomic species. Diffusion of major alloying elements in steels like carbon that migrates from the base metal to the clad and chromium diffusion to the grain boundaries, influences on the structure and the ductility of the backing steel at the base-clad interface. Changes in the composition at and adjacent to the bimetallic interface and formation of chromium carbides at the grain boundaries in the clad, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (sensitization), is a well-known problem that can occur during welding or improper heat treatment.
|File Size||3 MB||Number of Pages||8|