Role of oxide notching and degraded alloy microstructure in remarkably premature failure of steam generator tubes

Failure analysis investigation was carried out on a ruptured primary superheater tube of a boiler unit in a power plant. The rupture of 1Cr–0.5Mo steel tube had caused steam leakage and led to boiler shutdown after only 17,520 h of operation. The physical and mechanical characteristics of the alloy and the oxide scales in the area of failure were characterized by visual inspection, optical microscopy and microhardness testing, whereas the chemical variations were examined by optical emission spectrometry and energy dispersive spectrometry (EDS). A distinct surface/sub-surface zone of decarburization that was observed both on the inner and outer sides of the tube was developed before the tubes were put to the service. The fracture was attributed to a prolonged overheating as a result of the oxide scale build-up, causing loss of creep strength. The decarburized surface/sub-surface facilitated crack initiation/propagation.