    Previous study evaluated residual stress in a circumferential “V”-groove butt joint of a heat-recovery steam generator (HRSG) pipeline; the material was ASTM A-335-Grade P22. Aim had been to check on the influence over creep-relaxation previously found out for a tee made of the same material. The butt joint had been operating for the same period of 200,000 hours, same temperature of 528°C at almost a half pressure (0.46 Kg/mm^2^ vs. 1.06 Kg/mm^2^). X-ray diffraction (XRD) technique applied close to the weld highlighted anomalously high stress-level on the outer surface for all four butt-joint samples tested. Residual-stress over 400 MPa measured along the cylinder’s tangential direction was statically not acceptable. On the inner surface where deposited beads may have tempered adjacent base material, measurement via hole-drilling (HD) technique showed a symmetrical plane-state residual-stress of 199 MPa. It was consistent with that measured via XRD on the outer surface in the cylinder’s longitudinal direction. Supposing a case of incomplete post heating planned for the weld may have explained the occurrence of being much higher than 40 MPa, value predicted after 200,000 hours. Similar influence over creep results found out for the tee and the butt joint had validated modeling welding simulation considered for both joints. A comprehensive new series of XRD tests aims now at measuring residual stress across the cylinder’s wall, both inner and outer sides. The shallow layer considered has thickness sufficient for building a map of measurements covering different depths and locations on the surface. Incremental material removals are of .05 mm over the test area. The experimental plan includes also HD tests supporting the XRD ones applied at 20 mm from the weld, which was the reference location in previous study. Further analysis of microstructure (through metallographic methods and the optical microscope) should help understand whether and how the material is far from the ideal (homogeneous, isotropic and linear-elastic). This point is important for checking on both material’s behavior and methods’ applicability. Longitudinal flaws detected on the surface affect microstructure of adjacent base metal, causing XRD results’ dispersion: regardless of that, preliminary comparison with previous measurements roughly shows stress level increasing similarly across the cylinder’s wall from the inner side on: it appears to confirm some kind of obstacle to the creep relaxation. Average stress values, however, appear lower than previous measurements, showing better compatibility to the analysis results. 

 


Residual-Stress, Material Characterization in P22 Hrsg-Pipeline Butt Joint

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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