Residual Stress Assessment and its Effect on SCC of Pipelines Steel in Acidic Soil Environment
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EXPERIMENTAL PROCEDURE A seamless API 5L X52 pipeline steel was used. The dimensions of the pipeline were 8 inches in diameter and 0.437 inches in nominal wall thickness. The chemical composition was shown elsewhere [7]. The girth welds were obtained by qualified welders under a qualified welding procedure according to API 1104 standard, using the SMAW process with V-bevel at 30° as is shown in Figure 1. To simulate multiple welding repairs, the repaired weld was removed and welded again, to obtain a second, third and fourth welding repair. The specimens according to the number of repairs were identified as 0R (as-welded), 1R, 2R, 3R and 4R respectively. The repair was made on the whole circumference of the pipe specimen. a) 30°
b)
c)
30°
2 mm
Figure 1. a) Schematic of the V-bevel at 30°C, b) notch after been removed the metal to repair and c) piece of pipe after been welded. Optical and scanning electron microscopy (SEM) observations of the microstructure for the different welding repairs were carried out. Additionally, SEM analysis of the failure zone was performed.
Residual stress measurement was carried out in order to determine its magnitude and stress distribution generated during the repairs through an equipment of X-ray diffraction Phillips brand X-PERT model, which radiation source has Chromium (Cr Kα). The residual stresses measurement was made inside of the weld joint in five points located at: a) welding center, b) 5 mm away from the center of the weld bead to both ends c) 35 mm away from the center of welding toward both ends, as illustrated in Figure 2a. The diffraction peak used to measure residual stresses was a ferrite peak (211) located at an angle of 156.81° in axis 2θ. For each point measurements were taken at an angle of: a) ψ= 0 ° (measured in the circumferential direction and b) ψ = 90 ° (measured in the axial direction). In the calculation of the residual stresses are using an elastic modulus of 201 GPa and a Poisson's ratio of 0.3 (values tabulated in software for steel). The susceptibility to stress corrosion cracking (SCC) in girth welds of seamless API X52 steel pipe containing multiple shielded metal arc welding (SMAW) repairs and one aswelded condition were evaluated using slow strain rate tests (SSRT) according to NACE TM0198 standard. The SSRT were performed in air and NS4 solution at pH=3 at room temperature and at constant elongation rate of 1.0 x10-6 s-1. Cylindrical tensile specimens were transversal machined to the direction of the application to the girth weld as is shown in Figure 2b.
Figure 2. a) Location of the residual stress measurements, b) Cross section of the welding and SSRT specimen. The NS4 solution contain NaHCO3 0.483g/L, CaCl2 0.181 g/L, MgSO4 0.131 g/L and KCl 0.122 g/L. The pH solution was around 8.5 and after was adjusted with HCl.
RESULTS AND DISCUSSION Microstructure Microstructural characterization through optical microscopy of the welding joint is shown in Figure 3. This characterization was made in the three different regions of the weldments. The
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