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Hydrogen-induced damage is an inevitable challenge in pipeline safety applications, especially, the fusion welded joints owing to microstructure heterogeneity caused by welding process. In this work, X100 pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 300–600 rpm under water cooling, and the relationship among the microstructure, hydrogen diffusivity, and hydrogen embrittlement (HE) behavior of the nugget zone (NZ) were studied. The NZ at 600 rpm had the highest effective hydrogen diffusion coefficient (D eff) of 2.1 × 10−10 m2/s because of the highest dislocation density and lowest ratio of effective grain boundary. The D eff decreased with decreasing rotation rate due to the decrease of dislocation density and the increase of ratio of effective grain boundary, and the lowest D eff of 1.32 × 10−10 m2/s was obtained at 300 rpm. After hydrogen charging, the tensile strength of all specimens decreased slightly, while the elongation decreased significantly. As the rotation rate decreased, the elongation loss was obviously inhibited, and ultimately a lowest elongation loss of 31.8% was obtained at 300 rpm. The abovementioned excellent mechanical properties were attributed to the fine ferrite/martensite structure, low D eff , and strong {111}//ND texture dramatically inhibiting hydrogen-induced cracking initiation and propagation. • FSW of X100 steel was performed in water cooling, producing defect-free weld. • The diffusion behavior of hydrogen in the NZ was studies. • The effect of hydrogen on deformation and fracture behavior was analyzed. • The NZ with fine ferrite/martensite present high HE resistance. [ABSTRACT FROM AUTHOR]

The X100 pipeline steel was successfully friction stir welded at rotation speed of 600 rpm and welding speed of 50 mm/min using W-Re stirring tool. Microstructural analysis and Charpy v-notch impact-tests of the welds were carried out in nugget zone (NZ). In this case, the NZ exhibits inhomogeneous microstructure. Fine granular bainite was observed on the advanced side (AS) of the NZ as a result of mechanical stabilization effect, whereas the coarse lath-bainite with few granular bainite was observed on the retreating side (RS), which resulted from coarse prior austenite grains due to the lower cooling rate. Furthermore, a transition region containing intersecting bainite laths appeared between the two zones above. Generally, it was found that boron, which dissolved into the nugget zone during FSW, contributed the formation of classical lath-bainite in hard zone when FSWing pipeline steel using the PCBN stirring tool. The Charpy v-notch impact-tests results show that the whole NZ achieved good impact toughness and the poor impact toughness microstructure was successfully avoided in our study. [ABSTRACT FROM AUTHOR]

Herein, we address the scientific gap in the relationship among the microstructure, crystallography, and toughness in the nugget zone (NZ) of friction stir welded X80 pipeline steel at various heat input parameters of 300, 400, and 600 rpm. The decreased heat input results in refined prior austenite accompanied with an increase in the Z-parameter. Lath bainite (LB) and granular bainite (GB) appear at a high heat input and a bulk of GB appears at a medium heat input, whereas at a low heat input, a complex phase of LB, GB, and ferrite is obtained. The highest ratio of the bainite packet boundaries was identified at a low heat input, which is attributed to the existence of three Bain groups. The strongest shear texture was detected in the NZ at a medium heat input. An excellent toughness of up to 92.5% of that of the base metal was achieved at a low heat input owing to the highest ratio of packet boundaries, finest martensite-austenite constituents, and soft second phase of ferrite. [ABSTRACT FROM AUTHOR]

The medium-Mn steels with high strength and ductility can be used as a potential material in the engineering fields. However, the weld in the medium-Mn steel joint has extremely low ductility due to the existence of full martensite. In this work, a superior tensile strength of 1453 MPa and elongation of 32.7% was obtained in the weld of friction stir welded (FSWed) medium-Mn steel. This should be related to high volume fraction of austenite (24.4%) far higher than that of basal metal (15.1%). The formation mechanism of exceptionally high fraction of austenite was studied based on the transformation kinetics. The volume fraction of austenite in the weld predicted by classical model was only 15.4% much lower than the experimentally measured value, indicating that this model is not suitable for FSW. We applied various assumptions about the diffusivity of Mn in austenite and ferrite during simulation. Consequently, when the Mn diffusivity of austenite and ferrite was multiplied by 4000 and 1500 times, respectively, the simulated austenite volume fraction was well consistent with the experimental results. This was attributed to that the high density of dislocations introduced by strong plastic deformation of FSW promoted the Mn diffusion and the austenitic reverse transformation. • The weld contained a high volume fraction of austenite (24.4%) far higher than that of the basal metal. • The formation mechanism of exceptionally high fraction of austenite was studied based on the transformation kinetics. • A superior tensile strength of 1453 MPa and elongation of 32.7% was obtained in the weld of FSW medium-Mn steel. • The contribution of various strengthening mechanisms to yield strength was evaluated. [ABSTRACT FROM AUTHOR]

The high Mn steel (Mn ≥ 20 wt.%) is widely applied because of excellent cryogenic properties. In the fusion-welded high-Mn steel joint, the Mn vapor and segregation degrade the joint quality. Friction stir welding (FSW) produces a high-Mn steel joint without defect, but the nugget zone (NZ) exhibits a poor plasticity. In this work, the preheating was subjected to FSW high-Mn steel to improve the plasticity of the NZ. Compared with the as-welded NZ, the as-preheated NZ contained slightly coarser austenite with lower density of dislocations (6.7 × 1014 m−2), and had a lower stacking fault energy (SFE) (47.18 mJ m−2) at room temperature. The lower SFE caused the deformation mechanism of dislocation slip and active twinning-induced plasticity (TWIP) in the as-preheated NZ owing to the lower back stress and critical twinning stress. Subsequently, a high strength and plasticity product of 62 GPa% was achieved in the as-preheated NZ, which was consistent with high strain hardening rate. At cryogenic temperature of −196 °C, the SFE of the as-preheated NZ decreased to 35.66 mJ m−2, but more active TWIP besides dislocation slip was found during tension. Therefore, the as-preheated NZ exhibited excellent strength and plasticity product of 91.9 GPa%. Furthermore, the yield strength of the as-preheated NZ was slightly reduced, which was attributed to the decreased of dislocation density and grain growth. • The preheating was introduced into the friction stir welded high Mn steel. • The SFE of austenite in the as-welded and as-preheated NZs at room and cryogenic temperature was calculated. • The deformation behavior of the as-welded and as-preheated NZs at room and cryogenic temperature was studied. • The contribution of various strengthening mechanisms to yield strength was evaluated. • The as-preheated-320 NZ presented excellent strength and plasticity product at room and cryogenic temperature. [ABSTRACT FROM AUTHOR]

The medium-Mn steels have a great application potential in the engineering fields due to its excellent strength and toughness. However, the presence of coarse martensite and segregation of Mn in the fusion-welded medium-Mn steel joint severely deteriorated toughness and ductility. In this work, 6 mm thick medium-Mn steel plates were joined by friction stir welding, and then were intercritically annealed to enhance the toughness and ductility of the joint. The as-welded nugget zone (NZ) contained lath martensite with low proportion of high angle boundaries (HABs), leading to the joint with lower elongation of 22.5% and the impact energy of 73.6 J/cm2. However, the as-annealed NZ consisted of ferrite and austenite because partial martensite changed to reversed austenite, while other martensite became ferrite during annealing. In addition, the proportion of HABs in the as-annealed NZ increased obviously. In comparison with as-welded joint, the impact energy and elongation of the as-annealed joint was increased by 70.7% and 44.4%, respectively. The abovementioned excellent mechanical properties were attributed to the high proportion of HABs and a large amount of ultra-fine reversed austenite with high thermal and mechanical stability, which provided a significant transformation induced plasticity effect, obviously inhibiting crack initiation and propagation. This systematic work provided a reference for welding similar materials containing retained austenite. [ABSTRACT FROM AUTHOR]

In this study, the crystallography of the nugget zone of bainitic steel was examined by applying friction stir welding (FSW) and submerging in various cooling mediums. In air, the variants mainly occurred in a single Bain group because of the plastic accommodation of transformation strain. However, in water and a mixture of water and CO 2 , the variants generated in two and four close-packed (CP) groups, respectively, and this was attributed to the self-accommodation of transformation strain. In this case, the variant selection was inhibited by both the severe deformation during FSW and low temperature of cooling medium. Furthermore, the reduced cooling-medium temperature resulted in refined bainitic and martensitic blocks with an increased fraction of high-misorientation variant pairs owing to the increased transformation driving force. • Previous studies on the FSW bainitic steel were focused on the phase transformation, and the crystallography was ignored. • The crystallographic characteristics and its effect on microstructure refinement for FSW bainitic steel was studied. • In air, the plastic deformation in austenite easily accommodates the transformation strain, causing the variant selection. • In water and water+CO 2 , refined blocks were obtained with an increased fraction of high-misorientation variant pairs. [ABSTRACT FROM AUTHOR]

A 6 mm thickness medium-Mn ultrahigh strength steel was friction stir welded (FSW) with various rotation speed at constant of welding speed in this study. The microstructural evolution and impact toughness of the stir zone were investigated in a systematic manner. The microstructural observation results suggested that the metastable austenite in base metal disappeared after FSW, and the stir zone of FSW samples contained various morphology of martensite (i.e., well tempered martensite, less tempered martensite and common martensite). The content of well tempered martensite and less tempered martensite in the stir zone was decreased with the increase of rotation speed, and the martensite blocks and prior austenite grain size were gradually coarsened. It is suggested that the plastic deformation and the small prior austenite grain size promoted the formation of well tempered martensite in the stir zone at low rotation speed. The well tempered martensite could effectively release the stress concentration by interfaces separation with other martensite of different morphology during impact test, which greatly benefited the impact toughness. Furthermore, the small martensite blocks could also improve the impact toughness. The impact toughness of the stir zone was decided by the martensite blocks size and the content of different morphology of martensite phase. • The stir zone of FSW samples contained various morphology of martensite. • The martensite morphology varies greatly at different welding parameters. • Massive well tempered martensite formed in the stir zone at low rotation speed. • The well tempered martensite can effectively release the stress concentration. [ABSTRACT FROM AUTHOR]