Fig.2. Schematic illustration of different positions of X90 bend.
To investigate the microstructure characterizes, small samples were cut from the pipe part. Standard grinding and polishing technique were employed to obtain mirror surface. Then, the polished surface was etched with a solution of 4 vol.% natal to reveal the microstructure via Leica DMIRM optical microscope (OM) and JSM-5900LV scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was carried out to identify the phase composition, and examine the dislocation distribution. The TEM foils were cut from the samples, mechanical grinding to about 50 μm thickness. 3 mm disks were punched from the foils and twin-jet electropolished with a solution of 75% methanol + 25% nitric acid as electrolyte at -30 ℃. Foils were examined with a JEOL2010 TEM operated at 200 kV.
Electron back-scatter diffraction (EBSD) analysis was performed on a FEI NANO SEM 430 field-emission scanning electron micro-scope (FE-SEM) in order to study the grain boundaries and local misorientation. Local misorientation was used to evaluate small strain gradients in material. The data were post-processed with Channel 5 software provided by Oxford HKL Technology®. The specimens were electrolytically polished a solution of 650 ml alcohol, 100 ml chlorate, and 50 ml distilled water at room temperature for 20 s. The EBSD maps were obtained using a step size of 0.15 μm, area over 60 μm × 60 μm.
Results and discussion