Hydrogen permeation behavior in relation to microstructural evolution of low carbon bainitic steel weldments

The microstructures of high strength bainitic steel weldments were characterized, and their hydrogen permeability was investigated by the electrochemical permeation technique. The effective diffusion coefficient in the welded joint increases with heat input, while the hydrogen concentration at the sub-surface decreases with increasing heat input, mainly due to coarsening grain and inclusion sizes. Compared with as-welded specimens, the tempered membranes have lower effective diffusion coefficient and higher density of hydrogen traps due to newly formed carbides acting as hydrogen trapping sites that are derived from the decomposition of martensite-austenite constituents. These were further confirmed by the partial permeation technique.