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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]

A lean duplex stainless steel was friction stir processed at 300 rpm-100 mm/min using a W-Re stirring tool. The microstructural evolution on the advancing side, the center, and the retreating side of the stir zone were studied by a combination of electron microscopy and electron backscattered diffraction. The study indicated that the direction of fiber structure on the advancing side, center, retreating side were 0°, 90°, 45° relative to the processing direction because of the stirring effect of the tool. Given that the strain rate and temperature were different on the advancing side, center and retreating side, the phase fraction, misorientation, texture components varied in different parts of the stir zone. The deformation mostly occurred in the center and retreating side and the material on the advancing side almost did not deform during tensile test. As a result, strain induced martensitic transformation occurred in the center and retreating side. The voids initiated at the interface of ferrite and transformed martensite on the retreating side, and cracks also propagated along this interface during tensile test. Furthermore, because of the 45° direction of the fiber structure on the retreating side, the tensile strength of the FSP sample was slightly less than the base metal. [ABSTRACT FROM AUTHOR]

A high strength pipeline steel was successfully friction stir processed at high, medium and low heat input processing parameters. The microstructural evolution on advancing side and retreating side of the stir zone was studied using a combination of optical microscopy, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. The microstructural observations suggested that the prior austenite grain size, bainite morphology, texture components and impact toughness were distributed asymmetrically on the advancing side and retreating side at high and medium heat input. This was caused by asymmetric distribution and change in temperature and strain rate. While, the entire stir zone at low heat input was characterized by uniform microstructure and excellent impact toughness. Therefore, the microstructure and impact toughness on the advancing side and retreating side was closely related to processing parameters. [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]

AISI 201 stainless steel plate was friction stir processed (FSP) at rotation speeds in the range of 500–1000 rpm with traverse speed of 100 mm/min, and rotation speed of 1000 rpm with traverse speed of 50 mm/min using W-Re stirring tool. The microstructural evolution was studied by optical microscopy, scanning electron microscopy, electron backscattered diffraction, electron probe micro analyzer and transmission electron microscopy. The results suggested that the final grain structure in the stir zone (SZ) originated from the competition between continuous recrystallization (CDRX) and discontinuous recrystallization (DDRX). The grain size and the content of δ-ferrite in SZ were closely related to the processing parameters, and a band structure contained 6.5% δ-ferrite formed. Furthermore, the δ-ferrite formed in SZ significantly promoted the occurrence of DDRX and strong (111) [112] annealing texture formed in δ-ferrite. No sigma phase was observed in the SZ, and the density of dislocations and sub-structures was found to be seriously affected by the processing parameters and locations of SZs. The relationship between mechanical properties and structure is elucidated. [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]