In the current study, the effect of hydrogen atoms on the intergranular failure of α-iron is examined by a molecular dynamics (MD) simulation. The effect of hydrogen embrittlement on the grain boundary (GB) is investigated by diffusing hydrogen atoms into the grain boundaries using a bicrystal body-centered cubic (BCC) model and then deforming the model with a uniaxial tension. The Debye Waller factors are applied to illustrate the volume change of GBs, and the simulation results suggest that the trapped hydrogen atoms in GBs can therefore increase the excess volume of GBs, thus enhancing intergranular failure. When a constant displacement loading is applied to the bicrystal model, the increased strain energy can barely be released via dislocation emission when H is present. The hydrogen pinning effect occurs in the current dislocation slip system, {112}. The hydrogen atoms facilitate cracking via a decrease of the free surface energy and enhance the phase transition via an increase in the local pressure. Hence, the failure mechanism is prone to intergranular failure so as to release excessive pressure and energy near GBs. This study provides a mechanistic framework of intergranular failure, and a theoretical model is then developed to predict the intergranular cracking rate.

In this paper, the microstructure and mechanical properties at the interface of Incoloy825/X65 bimetallic composite pipe achieved by explosive welding were systematically studied. Results show that a periodic wavy bonding interface was formed between the Incoloy825 and X65 due to the jets caused by the high-impact stresses during explosive welding, which was beneficial for improving the bonding strength of the Incoloy825 and X65. Atomic diffusion and localized melting occurred near the interface during explosive welding. Some fine crystal grains were generated in the vicinity of the interface due to the strong plastic deformation and dynamic recrystallization, whereas some elongated grains growing in the perpendicular direction to the interface were found in the wider localized melted zone. Room temperature nanoindentation test showed that due to the existence of fine grains resulting from the recrystallization at the interface, the hardness of the interface was higher than that of the base metals. Voids and defects were prone to appearing in the vortex regions near the interface, where the weak point in the interface was located.

For the long-term service of X65/Incoloy 825 bimetallic composite pipe girth welds in the H₂S environment, the corrosion damage behavior was investigated. Characterizations of welded joints were conducted by OM, SEM, XRD, and EDS. The pitting corrosion’s 3D surface morphology of welded joints in NACE-B solution saturated with H₂S was investigated. The results show that composition segregation occurs in the welding process, and the Laves phase appears between dendrites, which lead to the decrease in corrosion resistance. The maximum depth of the corrosion pit is 2.46 μm after 30 days, 4.54 μm after 60 days, and 10.94 μm after 180 days. The cell automata (CA) model of corrosion damage was established based on the MATLAB program, and the basic elements of the cell automata model were determined. Through the simulation of the electrochemical reaction and diffusion process, the influence of different parameters of the pitting corrosion morphology was determined. The simulation results were compared with experimental data, which are available for the prediction of the morphology and size of the corrosion pits on the sample surface after 300 days. The results will be beneficial for the long-term service of the bimetallic composite pipe.