Pipe fittings in duplex stainless steel with deviation in quality caused by sigma phase precipitation

Pipe fittings in duplex stainless steel (grade 2205) are used both top-side and subsea by the oil industry offshore. Precipitation processes during heat treatment of such fittings may result in various detrimental intermetallic phases and carbides. Among these defects, the sigma phase has a particular interest since it already in small amounts may cause a considerable reduction in mechanical properties and corrosion resistance of the material. To study the influence of sigma phase content on the stress–strain behaviour and ductility under quasi-static and dynamic loading conditions, tensile tests at room temperature (+20 °C) and low temperatures (−46 °C, −10 °C) were carried out for duplex stainless steel with varying levels of sigma phase (∼0 vol.%, 1–5 vol.%, ∼10 vol.%). Specimens pre-charged with hydrogen were also tested to study the influence of sigma phase on cathodic protected subsea components. In addition, fracture toughness tests (CTOD) on single-edge notch bend specimens and Charpy impact tests on V-notched specimens at various temperatures were conducted to establish the influence of sigma phase on the toughness of the material.

Research highlights▶ Charpy impact tests at various temperatures on V-notched duplex steel grade 2205 specimens show that the material has lost most of its impact toughness already at about 1% sigma. ▶ However, the impact toughness may not be the most representative measure of the material's capability of dissipating energy under accidental impact situations on top-sides offshore. ▶ To study the influence of sigma content on the stress–strain behaviour and ductility in uniaxial tension, quasi-static and dynamic tensile tests at room temperature and low temperatures were carried out for the duplex steel with varying levels of sigma phase. ▶ It is shown that the effect of sigma phase is far less severe with respect to tensile ductility than impact toughness.