Failure Analysis and Service Properties of 20Cr1Mo1VNbTiB Bolt in 660 MW Supercritical Thermal Power Unit.

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]