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The failure mechanism of TP347H austenitic heat-resistant steel tube serviced in boiler superheater in a 600 MW subcritical thermal power unit is presented in this paper. Through microstructure observation, energy-dispersive spectrometry analysis, and selected electron diffraction, the mechanism of TP347H superheater bend tube failure is found to be attributed to intergranular corrosion cracking, more M23C6 and primary NbC(N) phase precipitating and coarsening along the grain boundary and formation of high content of slip lines and α′-martensite structure. The α′-martensite kept the Kurdjumov–Sachs (K-S) orientation relationships with parent austenite phase (γ-parent), which generated in TP347H bend tube may be related to the deformation-induced martensite (DIM) during rolling and cold working. The α′-martensite nucleated mainly at the grain boundary intersection between the twin crystal and γ-parent and grew inwards along the twin band. The dislocation density at the intersection is larger, and the secondary NbC(N) phase inhibited the nucleation of α'-martensite. [ABSTRACT FROM AUTHOR]

Failure cause and service properties of 20Cr1Mo1VNbTiB bolts in a 660 MW supercritical thermal power unit have been studied. The bolts fractured with evidence of material aging. Chemical analysis, microstructure characterization, mechanical properties testing, and energy-dispersive spectrometry were used for the analysis. The creep cavity was found in the microstructure with material serviced under 566 °C for about 30000 h. Some creep cavities accumulate and interconnect to form intergranular cracks. The cracks expand under the torsional force and lead to fracture failure. The precipitated phase in 20Cr1Mo1VNbTiB mainly includes TiN(C), NbC, and Mo-rich carbides. TiN(C) was the square shape and distributed on the matrix. NbC was a fine spherical shape and mainly distributed in the crystal. The Mo-rich carbide was elliptic in shape and primarily distributed in the grain boundaries. The coarse TiN(C) precipitates were relatively stable during service. The size of TiN(C) does not change and cannot be eliminated by reheat treatment. More quantity and larger Mo-rich carbides precipitated at grain boundaries after service, which led to the impact energy decrease. To avoid the same fracture accident, the 20Cr1Mo1VNbTiB bolts are not recommended for use in supercritical units with a steam temperature of 566 °C and above. [ABSTRACT FROM AUTHOR]

Failure analysis of a fractured high strength bolt used in a MW Class wind turbine blade connection system was conducted to clarify the failure cause and to determine the prevention method. Applying detailed mechanical characteristics and metallurgical investigations, the bolts failure cause can be attributed to bending fatigue. Evidenced by the nut polished markings, insufficient tightening was considered as the important reason for the bolt failure. The thread microstructure indicated that the fractured bolt thread was rolled rather than machined. However, rolling induced cracks locating on the thread crest and non-uniformly distributed decarburized layer on the bolt thread root existed. Thus, it can be concluded that the bolt failure was caused by the insufficient tightening and improper manufacturing procedure. [ABSTRACT FROM AUTHOR]