Abstract: Aiming at the problem that the penultimate free-stand blade fractured frequently in two 50MW steam turbo-generators of a power plant after five years of operation, this paper makes an in-depth analysis on the causes of the accidents and searches for the root cause of the problem. Through analyzing the blade material and fracture, checking the blade frequency, calculating the strength of the blade and the blade root by means of mathematical modeling and finite element analysis, understanding the stress distribution and defects of the blade, comparing the requirements of design standards, the paper revealed the root cause of the blade root fracture and based on this, it presented several sugges tions for improvement. This provides a theoretical basis for the set revamp at the following stage.

Key words: steam turbo-generator, blade fracture, strength calculation, stress concentration