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The subject boiler is a vertical type with a capacity of 2063 ton/hr steam. Its operating and design pressure are 2608psi (18MPa) and 2928psi (20.2MPa) respectively while maximum outlet operating temperature is 1008oF (542.2oC). Water wall tubes are seamless tubes made of SA 210 and SA 213 T2 steels with nominal outer diameter of 2.5-3.0"(63.5-76.2mm) and wall thickness of 0.25-0.313" (6.35-7.95mm). After 77360 operating hours, some water wall tubes have experienced leakage. Leaked tubes were subjected to different non-destructive and destructive examinations to clarify cause of failure. Visual investigation showed through-thickness hole in base metal, close to weld region of twoleaked tubes. Optical and scanning electron microscopic examinations of leaked zones disclosed complete decomposition of pearlite into ferrite and spheroidal carbides, creep voids, and grain boundary micro-cracking. It is concluded that failure of the subject boiler tubes is attributed mainly to long term localized overheating. This could be related to heat tinted area (oxidation) formed on tube outer surface by heating (arc strike) during welding. During operation, oxidized or scale layerformed on outer surface of heat-tinted area will be exfoliated due to thermal stresses. Repeated cycles of thermal oxidation and exfoliation of scale layer will result in localized thinning and subsequently, localized overheating leading to acceleration of microstructure degradation. Thinning of water wall-tubes together with its microstructure degradation willresult in increase in boiler pressure above the maximum safe working pressure leading to complete failure or leakage of tubes. It was recommended that boiler water wall tubes to be checked using different nondestructive methods during the nearest shut down and all thinned zones of tubes to be replaced. In order to decrease the possibility of such failure in future, proper welding procedure should be applied so that heat-tinted areas due to heating or arc strike on tube outersurface to be avoided.

Weld Heat Tented Area Induced Failure of Boiler Water Wall Tubes in Power Generation Plant