AN INVESTIGATION OF THE FAILURE OF LOW-PRESSURE STEAM-TURBINE BLADES

An analysis of the failure of LP turbine blades of a 210 MW thermal po wer plant has been presented in this paper. The blade material is of 1 2% Cr steel with tempered martensitic microstructure. Microstructural analysis as well as hardness and tensile tests did not indicate any de gradation in terms of microstructure and mechanical properties, physic al discontinuities were observed in the braze joint which might have b een formed due to improper brazing operation. Failure of the brazed jo ints between the blade and lacing rod was found to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed that the cracks were initiated from various points on the blade surface, which were at the interface with the lacing rod. S triations and beach marks were also observed which indicated the occur rence of high cyclic loading on the blades. Frequency data obtained fr om plant indicated the possibility of excessive vibration generated du e to fluctuation in grid frequency during operation. Thus, the situati on was aggravated due to a resonant condition of vibration, facilitati ng the propagation of cracks which were initiated earlier. (C) 1998 El sevier Science Ltd. All rights reserved.