Abstract Scope |
Erosion- corrosion is one the most significant cause for the failure of material and steam pipe bend joints in power plant applications. Analysis has been conducted to verify the mechanism of erosion-corrosion (or flow accelerated corrosion) on the welds. In addition to the metallographic analysis, chemical analysis, phase analysis, and computational fluid dynamics (CFD) simulation of the flow patterns has been conducted. The CFD flow patterns have been linked with the erosion factor toward erosion-corrosion. The upstream pipe weld at the 6 o’clock location has been identified as the most damaged location, where the condensed water had the maximum velocity. The failure is found to start from root pass welds or downstream HAZ of steam pipe. In the early stage of erosion-corrosion, material undercutting caused by flowing medium mechanical erosion was mainly concentrated in the protruding root pass reinforcement position. Several recommendations about the welding procedure and bend design during the piping fabrication process have been made. Lower root pass reinforcement, narrower root gap, thinner filler rod and longer bend section can decrease the sudden changes in the flow pattern. The optimum welding process parameters are required for root pass welds of steam pipe. Smaller heat input resulted in finer grains, higher hardness and more heterogeneous chemical composition while higher heat input resulted in coarse equiaxed grains, a lower hardness, and a more homogenous chemical composition. Erosion will dominate in higher heat input welds while corrosion will dominate in lower heat input welds. |