Corrosion cracks were found at the bottom of the first hook of the L-4 stage of a low-pressure steam turbine rotor after about 230,000 hours operation with about 300 times of starts and stops of a thermal power plant. They developed in the circumferential direction, showing a groove line on the bottom surface of the first hook. Cross sectional observation showed that they had branched and blunted shapes, and X-ray cartography analysis results showed that they filled with corrosion products, and sulfur concentration was detected. In order to verify that they were caused by stress corrosion cracking (SCC), bent-beam stress-corrosion tests with four-point loaded specimens (ASTM G40 method) were performed in a 0.25wt%H2SO4+NH4OH solution with a pH controlled at 9.8 to simulate the corrosion environment of the steam turbine, and the test temperature was set at 80°C. After 6,000-hour and 10,000-hour tests, the cross sections of SCC specimens were inspected with a scanning electron microscope (SEM). Branched and blunted cracks were observed and the crack shapes were similar to those of the corrosion cracks occurred in the steam turbine. Sulfur concentration was also observed in the blunted cracks of the specimens by X-ray cartography analysis. Based on the test results, it can be assumed that the corrosion cracks developed from stress corrosion cracking (SCC) in the low-pressure steam turbine. It is thus necessary to periodically inspect not only the L-1, 2 stages of the wet and corrosion zone, but also the L-3, 4 stages of dry and salt zone in low-pressure steam turbines.

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