The failure analysis of 9–12% chromium steel tubes, containing about 2.5% molybdenum, is discussed in the present study. The component is used in a steam power plant boiler as a high-temperature superheater tube and has been in service for about 100,000 h. The failure occurred without appreciable wall thinning. Specimens were taken from the region beneath the fracture surface and investigated by optical and electron microscopes. The microstructure was composed of ferrite and grain boundary particulate carbides. The results indicated that the fracture was initiated because of the bending of the tube near the anchor and propagation of the crack through the interfaces between massive carbides and matrix (sensitized zone). Final fracture has occurred as a result of an overload due to the decreasing of load carrying section produced by crack propagation.

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