Optimization of Corrosion Resistance and Weldability: A Challenge for Superduplex Filler Metals

Abstract

Abstract The SDSS have been increasingly used in the offshore industry due to their excellent mechanical properties and corrosion resistance properties to the media they are exposed to. The SDSS are mainly used in subsea components such as Christmas Trees, Manifolds, BOP and also for Topside Modules in Platforms. A well balanced chemical composition guarantees the outstanding corrosion and mechanical properties of SDSS material. Ferrite forming elements such as Cr, Mo ensure a high resistance to chloride containing media as well as a high strength. Austenite forming elements Ni and Ni balance the microstructure to 50%. However, the thermal instability increases with the amount of elements and restricts the weldability compared to usual stainless weld metal of the 3xx series. In particular the GMA-, and SA welding process asks for a specific approach to minimize the tendency to pore formation. This report describes the influence of a fine tuned chemical composition and welding parameters to achieve a sound weld metal along with an optimum in corrosion resistance. 1 Superduplex Stainless Steels Superduplex Stainless Steels are materials characterized by a microstructure of austenite and ferrite, figure 1. Driven mainly by the offshore industry to increase the corrosion resistance and the mechanical properties enhancement of the conventional Duplex Steels has led to the so called Superduplex Stainless Steels. Main (Super) Duplex Steels are UNS 35750 and 35760. An overview of the chemical composition of usual Duplex steels is given in table 1. These steels are featured, compared to the conventional Duplex grades, by increased contents in Cr (22–25%), Mo (2–4%) and N (0,2 -0,35%), with possible additions of Cu and W. These increased contents lead to a higher corrosion equivalent (Pitting-Index PREN =% Cr + 3,3 % Mo + (0,5 % W) + 16 % N). This does not only improve the pitting (crevice) corrosion resistance in the as welded condition but is beneficial for a higher strength level as well. Superduplex Stainless Steels show a PREn > 40. The heat affected zone (HAZ) was already experienced as the weakest part in conventional Duplex joints. A sufficient high N-content (> 0,20%) is badly required, to control embrittlement by grain grow and increased ferrite contents in the heat affected zone, even more for Superduplex Stainless steels.