Metallurgical evaluation of creep-failed superheater tubes

The present study is focused on the metallurgical factors induced during high-temperature operation of a pipeline that resulted in its short-time failure. The pipeline is used as superheater in a lignite power-plant unit and is constructed by an array of steel tubes jointed together by circumferential weldments. Systematic recording of data over one year of service indicated that local material’s failures were occurring, on an average, every 15 days and localised either at curved areas of the tubes, or in the neighbourhood of circumferential weldments. Visual inspection, macrographic examination and microscopic observations of representative failed parts revealed microstructural features indicative of creep rupture. In both cases, the material failed due to the rapid growth and coalescence of creep voids, initiated at a service temperature equal to 43% of the steel melting point.