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High strength low alloy steels are widely applied as pipelines for crude oil and natural gas transportation and, currently, new approaches to alloy design, in addition to the use of advanced steelmaking and processing techniques, have become essential for obtaining structures that resist to hydrogen damage, which is the main cause of pipelines failure in H 2 Srich environments. The main objective of the present work is to evaluate the influence of microstructural features on hydrogen diffusivity in two API X65 steels, with different Mn contents. One of the steel plates has been recently developed for usage in sour environments, is on its experimental stage and has a low Mn content. The other one is a commercial plate steel, with high Mn content, developed for sweet applications. Both steel plates were characterized in its three sections, in relation to the rolling direction: longitudinal, transverse and top surface of the plate (parallell to the rolling direction). After that, samples obtained from each section of the plates were submitted to hydrogen permeation tests; the low Mn steel was also analysed with EBSD, for texture determination. The low Mn steel presents a homogeneous microstructure through plate thickness, composed of refined ferrite and small pearlite islands. The high Mn steel has a heterogeneous microstructure through the plate thickness, composed of ferrite and pearlite bands, and presents centerline segregation. Hydrogen permeation tests showed that the D eff obtained for the low Mn steel sections are slightly higher than for the high Mn steel. Another two important parameters that were calculated for both steels are the subsurface hydrogen concentration, C 0 , and the number of traps per unit volume, N t. Contrary to what was expected, the low Mn steel presented the higher C 0 and N t values. Thermal dessorption spectroscopy analysis confirmed that the low Mn steel traps more H atoms than the high Mn one. These results, along with the similar D eff values, were related to the presence of nanoprecipitates of microalloying elements, that cannot be detected via optical and scanning electron microscopy. Additionally, also for both steels, the D eff values varied in function of the analyzed section as it follows: D eff longitudinal ≅ D eff transverse > D eff top. In order to better understand this anisotropic behavior, a new diffusion coefficient, which was called diffusion coefficient at the steady state, D ss , was determined. D ss considers that all the trapping sites are saturated, enabling, thus, the evaluation of physical obstacles to H diffusion. For the high Mn steel, the D ss varied in the same matter as the D eff : D ss longitudinal ≅ D ss transverse > D ss top; this behavior was associated with the microstructural banding present in the material. For the low Mn steel, the D ss exhibited a different behavior: D ss transverse > D ss longitudinal ≥ D ss top, suggesting that H diffusion can be aided by grain boundaries while the trapping sites are being filled and that crystallographic textur...

In the present work, an ODS 12 Cr steel was characterized using Electron Microscopy techniques, in an as-received condition and after annealing treatments between 773 K and 1573 K. Results show a complex microstructure, with the presence of fine Y–Ti–O nanoparticles dispersed in the matrix. After annealing at 1573 K, the average diameter of Y–Ti–O nanoparticles increases from ~ 4 to ~ 7 nm and partial recrystallization occurs. The trapping behavior of deuterium in the steel in its as-received state and annealed at 1573 K was investigated. Samples were exposed to low-energy deuterium plasma and analyzed with thermal desorption spectroscopy, after waiting times of 1 day and 25 days. The samples measured 1 day after exposure released a higher total amount of deuterium than the ones measured after 25 days. The effect of waiting time is explained by the release of deuterium, at 300 K, from sites with low activation energy for detrapping, Ed. In the as-received condition, part of the deuterium detrapped at 300 K was re-trapped by high-Ed sites. For the samples in the annealed condition, the redistribution of deuterium from low-Ed to high-Ed sites was not observed, but the total amount of deuterium released was higher.