API X-80 steel is extensively used as a piping material in oil and gas industries with good combination of strength, toughness and resistance to H2S damage. Drill riser, for example, is a conduit providing a temporary extension of a subsea oil well to surface drilling facility and is in large part made of welded X-80 steel pipe. The riser system is subject to fatigue loading from wave and tidal motion, vortex induced vibration (VIV) and operating loads at low temperatures in corrosive environments. The reliability of drill riser system is extremely important and therefore the effects of environmental factors, including temperature and seawater, on fatigue crack propagation (FCP) behavior of X-80 steels, both BM (base metal) and WM (weld metal), need to be well understood. In this study, S-N fatigue and FCP tests were conducted on X-80 steel, including BM and WM, in air at 25 and −50°C and an R ratio of 0.1. The FCP tests were also performed on X-80 steel BM in artificial sea water at both cathodically and anodically controlled voltages. The fatigue behavior of X-80 steel at various testing conditions are discussed based on detailed optical micrographic and SEM fractographic observations.

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