The strength and plasticity balance of F/B dual-phase X80 pipeline steels strongly depends on deformation compatibility between the soft phase of ferrite and the hard phase of bainite; thus, the tensile strength of ferrite and bainite, as non-negligible factors affecting the deformation compatibility, should be considered first. In this purely theoretical paper, an abstract representative volume elements (RVE) model was developed, based on the mesostructure of an F/B dual-phase X80 pipeline steel. The effect of the yield strength difference between bainite and ferrite on tensile properties and the strain hardening behaviors of the mesostructure was studied. The results show that deformation first occurs in ferrite, and strain and stress localize in ferrite prior to bainite. In the modified Crussard-Jaoul (C-J) analysis, as the yield strength ratio of bainite to ferrite (${\sigma }_{y,B}/{\sigma }_{y,F}$) increases, the transition strain associated with the deformation transformation from ferrite soft phase deformation to uniform deformation of ferrite and bainite increases. Meanwhile, as the uncoordinated deformation of ferrite and bainite is enhanced, the strain localization factor (SLF) increases, especially the local strain concentration. Consequently, the yield, tensile strength, and yield ratio (yield strength/tensile strength) increase with the increase in ${\sigma }_{y,B}/{\sigma }_{y,F}$. Inversely, the strain hardening exponent and uniform elongation decrease.

Hydrogen, as a clean energy source, has gradually become an important choice for the energy transformation in the world. Utilizing existing natural gas pipelines for hydrogen-blended transportation is one of the most economical and effective ways to achieve large-scale hydrogen transportation. However, hydrogen can easily penetrate into the pipe material during the hydrogen-blended transportation process, causing damage to the properties of the pipe. The heat-affected zone (HAZ) of the weld, being the weakest part of the pipeline, is highly sensitive to hydrogen embrittlement. The microstructure and properties of the grains in the heat-affected zone undergoes changes during the welding process. Therefore, this paper divides the HAZ of X80 welded pipes into three sub-HAZ, namely the coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ, to study the hydrogen behavior. The results show that the degree of hydrogen damage in each sub-HAZ varies significantly at different strain rates. The coarse-grained HAZ has the highest hydrogen embrittlement sensitivity at low strain rates, while the intercritical HAZ experiences the greatest hydrogen damage at high strain rates. By combining the microstructural differences within each sub-HAZ, the plastic damage mechanism of hydrogen in each sub-HAZ is analyzed, with the aim of providing a scientific basis for the feasibility of using X80 welded pipes in hydrogen-blended transportation.

Impact toughness of the weld metal is one of the important factors affecting the quality of hot bends, which is strongly dependent on the microstructure transformation during hot bending and tempering. In this study, three kinds of weld metals with different Ni contents were selected, and then the effects of tempering temperature on the microstructure impact toughness of weld metals for hot bends were investigated by simulation conducted on a Gleeble-3500 thermal simulator. The results show that the nonmetallic inclusion particles in weld metals can become the nuclear core of acicular ferrite like in as-welded metal. So, the overlapping acicular ferrite microstructure is obtained in the weld metal after direct cooling from the reheating temperature. During tempering, the overlapping acicular ferrite microstructure is degenerated, and martensite/austenite (M/A) constituents in the acicular ferrite microstructure decompose into ferrites and carbides. The resulting carbide particles mainly distribute along the acicular ferrite grain boundaries. With the increase of the tempering temperature, the carbide particles coarsen, which decreases the impact toughness of the weld metal of hot bends. Addition of Ni to weld metals can refine the acicular ferrite and improve the impact toughness.

To obtain high strength and excellent deformability for ferrite/bainite dual‐phase (F/B DP) pipeline steel for gas pipelines based on strain‐based design, the volume fractions of ferrite and bainite should be considered first. In this work, abstract representative volume elements (RVE) of finite element models (FEMs) of mesostructure for F/B DP pipeline steel with volume fractions of bainite between 30% and 58% were established, and the effects of volume fraction of bainite on the tensile properties and deformation compatibility were studied. Results show that the stress and strain in the mesostructure were primarily distributed in the bainite and ferrite, respectively, and strain concentration occurs at the ferrite/bainite interface. With increasing volume fractions of bainite, the strain localization factor (SLF) and strain ratio of ferrite with bainite (ε_F/ε_B) decrease, which can improve the deformation compatibility of the F/B DP pipeline steel. However, the stress ratio of bainite with ferrite (σ_B/σ_F) and the contributions of bainite to stress and strain sequentially increase, and, as a result, the strength increases and the ductility decreases. Therefore, a balance of strength and deformability can be obtained when the optimal volume fraction of bainite is in the range of 40% to 48%.