Hydrogen embrittlement in subsea pipelines made from X70: Effect of plastic deformation on hydrogen diffusion
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As a part of the DEEPIT project at SINTEF, hydrogen permeation tests have been carried out on API X70 pipeline steel. The effect of plastic deformationon hydrogen diffusion properties has been evaluated, using a worldwide unique permeation apparatus at NTNU. The apparatus is a purposebuild advanced version the standard hydrogen cell presented by Devanathan and Stachurski  in 1962. API X70 steel has been tested both in base metal (BM) and simulated heat affected zone (HAZ) conditions. The heat treatment was done to simulate the coarse grain microstructure that occurs close to welds. These two conditions of API X70 steel were tested as two different series of specimens,within whom four different pre-strain levels were applied. All specimens were tested at ambient temperature, in addition to the most strained HAZ specimen being tested at elevated temperatures. The recorded permeation transients were analyzed using the framework of the international standard ISO17081:2004 . The effective hydrogen diffusion coefficient (Deff ) at ambient temperature measured for the API X70BM specimens in this work varied from 2:79 × 10–7 cm2 s –1 to 2:50 × 10−6cm2 s−1, and for the HAZ specimens from 2:13 × 10−7 cm2 s−1 to 8:02 × 10−7cm2 s−1. This is in agreement with reported values for comparable steels. The Deff of the HAZ specimens were in general lower than the Deff of the BM specimens, at similar pre-strain levels up to 1%. At 3% pre-strain level, the Deff of the BM and HAZ specimens were almost identical, showing that the effect of 3% pre-strain is much more significant than of the heat treatment. Due to the many dislocations arisen during pre-straining, the Deff generally decreased with increasing pre-strain level, for both BM and HAZspceimens. The sub-surface hydrogen concentration (C0) was found to be unaffected by the pre-strain level, and the values varied from 0.08 ppm to 0.20 ppm forall specimens, regardless of specimen type and pre-strain level. The HAZ specimen with the highest pre-strain level (3%) was in additionto 25 ºC, also tested at 50 °C and 70 °C. The results from this testing was plottet as the temperature dependence of Deff , and the Arrhenius line was estimated, using regression. Deff was found to respond to temperature effects according to the Arrhenius equation. The number of reversible traps (Nr) were found to be 1:28 × 1019 sites cm−3, and the binding energy (Eb)40:29 kJ mol−1. This work is suggested to be extended to cover trap calculations for all specimens. This will need more experimental work, as all specimens have to be tested at elevated temperatures. This extended work would be an important contribution to the understanding of the effects of plastic deformation on hydrogen diffusion.