An Investigation into the Corrosion Fatigue Behaviour of High Strength Carbon Steel Tensile Armour Wires

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Abstract

The corrosion fatigue behaviour of high strength carbon steel tensile armour wires that are used in flexible risers has been explored. An investigation of the corrosion fatigue failure mechanisms for two different sets of corrosion fatigue tested high strength steel wires has been carried out. The two different tensile armour wires were 12 mm x 4 mm and 12 mm x 7 mm. The wires had been corrosion fatigue tested in up to three different seawater environments, namely aerated, CO2 saturated to 1 bar absolute and 100 mbar absolute H2S-CO2 balance to 1 bar absolute.The corrosion fatigue failure investigation included undertaking statistical analysis of fatigue crack and corrosion pit data to establish the effects of environment, applied stress, R-ratio and microstructure due to degree of cold drawing on the corrosion fatigue behaviour. The 12 mm x 4 mm has fine grain martensite-pearlite structure with anisotropic microstructure in the transverse plane. The 12 mm x 7 mm has larger grain martensite-pearlite structure with equiaxed microstructure in the transverse plane. The corrosion fatigue crack path for the two tensile armour wires exhibits transgranular and intergranular cracking due to variations in R-ratio and microstructure.The analysis identified that a significant amount of localised corrosion pitting was present on the surface of both the 12 mm x 4 mm and 12 mm x 7 mm high strength carbon steel tensile armour wires and that many corrosion fatigue cracks had initiated from these geometric discontinuities.A method was developed in order to apply an optical image correlation technique to a sample immersed in seawater. The research has shown that digital image correlation may be applied for in-situ imaging of a corroding and dynamically deforming surface within a seawater environment. The technique demonstrated the establishment of localised surface strain around the corrosion pits during mechanical loading. The results of the surface strain mapping show that the interaction between multiple corrosion pits is consistent with a significant increase in surface strain when compared to a single surface pit acting alone. The results also show that a small single stress raiser can exhibit a high surface stress concentration when compared to a larger one as the strain is dependent upon the geometry of the pit. The highest strain concentration is at the edge of the pit, parallel to the loading direction. The results show the interaction that multiple pits have with each other, the effect they have on surface strains and how they and other types of stress raiser lead to premature failure of components. Further to this the effects of residual stress on crack nucleation were considered. Fatigue cracks initiate at the surface of the high strength carbon steel tensile armour wire therefore surface measurements were carried out to establish the effects of environment and applied load on the development of residual stress fields.The 12 mm x 4 mm wire shows some correlation between applied stress range and surface residual stress measurements with. For the 12 mm x 4 mm wire corrosion fatigue tested in aerated seawater the surface residual stress becomes increasing compressive with an increase in applied stress. For the 12 mm x 4 mm wire corrosion fatigue tested in CO2 saturated seawater the surface residual stress appears to be independent of applied stress. However for the 12 mm x 7 mm carbon steel tensile armour wire there is no correlation between the applied stress range and the surface residual stress. The differences in surface residual stress may be due to the differences in R-ratio, microstructure and level of cold drawing due to the Bauschinger effect.Surface residual stress measurements have been used to explore the effects of the shakedown process on the high strength carbon steel tensile armour wires prior to corrosion fatigue testing. They show that at a high applied stress range the shakedown process readily develops a compressive residual stress on the surface of the carbon steel wire. This is mostly the case for the low applied stress range; however care should be taken when considering the effects of shakedown on a lower stress range in so far as it may not completely remove the tensile residual stress.Through thickness residual stress measurements show a similar distribution of residual stress fields throughout the high strength carbon steel tensile armour wires independent of the applied stress range and environment.

Keyword(s)

carbon steel; corrosion; fatigue; flexible riser; tensile armour

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