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The effect of grain boundary character distribution (GBCD) on hydrogen-induced cracks (HICs) initiation and propagation in nickel-saving austenitic stainless steel was investigated at different strain rates with electrochemical hydrogen charging. The fracture strength and elongation of specimens decreased as the strain rates decreased due to more hydrogen atoms being transported by mobile dislocations. HICs are more likely to initiate at random grain boundaries (RGBs) because of poor deformation coordination of adjacent grains at RGBs after hydrogen charging. Special boundaries (SBs) deflected HICs and hindered the propagation of HICs owing to a more stable boundary structure and less strain localization during slow strain rate tensile (SSRT) deformation with hydrogen charging. Furthermore, highly efficient distributions of SBs contribute to great resistance to hydrogen embrittlement. • Higher content of SBs reduced the elongation and strength degeneration of material after H-charging. • With the increase of SB, the fracture mode changes from intergranular to quasi-cleavage. • SBs are beneficial to delay hydrogen permeation. • HICs prefer to initiate on RGBs due to poor deformation coordination after H-charging. • HICs are deflected and terminated by special boundaries, a higher fraction of f J2 /(1-f J3) effectively inhibit HICs propagation. [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]

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 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]

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]