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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]

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]