MECHANICHAL PROPERTIES AND CORROSION RESISTANCE OF DUPLEX STAINLESS STEEL FORGINGS WITH LARGE WALL THICKNESSES

Abstract

Duplex stainless steels are widely used in chemical process, oil and gas industry. One of the main applications are disconnect able mooring systems (SWIVEL connections) for offshore Floating Production Storage and Offloading units (FPSO). Due to the increasing demand for energy ever larger cast or forged parts are required in the offshore industry. It is a well known fact that in larger duplex stainless steel forgings precipitation of detrimental phases can take place, such as Sigma phase or nitrides. Impact strength at low temperature and corrosion resistance will be influenced by these precipitations. Therefore mechanical properties and corrosion resistance of ring rolled products with large wall thicknesses are the main objective of this report. Hence, mechanical properties including impact strength were determined as a function of specimen test location depth in a part that failed during acceptance testing. Additionally two test rings with identical wall thickness and different forging ratios were investigated in the same manner. Corrosion resistance tests were performed according to ASTM G48 and in some cases Critical Pitting Temperature (CPT) was established. The microstructure was examined via Optical Microscopy as well as Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). A Focused Ion Beam (FIB) device was used to micromachine TEM foils at evident sites in SEM specimens.