Effect of Microstructure on Hydrogen Permeation in X70 Steel
Creators: Sun, Lu
Publisher: name: University of Alberta Library
Year: 2023

<p>The susceptibility of pipeline steel to hydrogen induced degradation is closely related to the interaction between hydrogen and some microstructural features of the steel such as precipitates, inclusions, phase type and grain size.  These microstructural features can act as hydrogen traps (irreversible or reversible) and delay hydrogen diffusion process. Thermomechanical controlled processing (TMCP), by which advanced pipeline steels are manufactured, have great influence on the formation and distribution of these microstructural features and hence the trapping ability of the pipeline steels.In the present study, two groups of X70 pipeline steel with different microstructures were obtained by different TMCP.  The microstructure of each steel was characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD). Meanwhile, the effective diffusion coefficient (Deff) and hydrogen trap density (Nt) were calculated by t-lag method and curve-fitting method based on the hydrogen permeation curves obtained by Devanathan-Stachurski (DS) electrochemical hydrogen permeation test. Furthermore, a trap efficiency model was established to evaluate the trapping ability of the phases presented in X70 steel; and a hydrogen diffusion numerical simulation model was developed to determine the diffusivity, trap density and binding energy based on experimental hydrogen permeation data. It is shown that nano-sized NbC precipitates are irreversible hydrogen traps that can decrease the amount of diffusible hydrogen. Also, it is found that a great amount of these fine precipitates was generated during coiling process, the count density of which increased with decreased cooling rate after runout table. Conversely, large (~ 100 nm) TiN precipitates (including complex NbC/TiN precipitates), were not effective hydrogen traps under the test condition.Non-metallic inclusions (NMIs), mostly CaS and Al2O3, were found acting as strong irreversible hydrogen traps. Finer NMIs trap hydrogen more efficiently due to their larger specific surface area. And it is shown that an increase in overall rolling deformation resulted in a finer NMI size distribution.In addition, pocket/block boundaries of lath low bainite are high angle grain boundaries (HAGBs) that act as irreversible traps. The fraction of these HAGBs is greatly affected by prior austenite grain (PAG) size and can be controlled by rough rolling process. A trapping efficiency model was established to correlate the characterization results to the hydrogen permeation test results. According to the modelling result, the reversible trapping efficiency of the phases in X70 steel rank in the order of M/A &gt; bainite &gt; ferrite.At last, a numerical simulation model was established and gives Deff values that are very close to those calculated by time-lag method. According to the model, the average binding energy of irreversible traps in 3E-Coiled is higher at surface region than near centerline region.</p>