The case describes NDT inspection and metallurgical investigation performed on gas-turbine 4 th stage blades after one blade failure in an ENEL combined cycle power plant. The failure occurred in the blade root in a zone called shank. NDT inspection by LT performed after failure on all 4 th stage blades in the platform zone corresponding to the location of failure showed defects on seven other blades in the same shank area. Metallurgical investigations were performed on the failed blade and on another defected one. Chemical composition and stress rupture test results agreed with the specification limits for the material (UNS N07750), no origin defects were detected in the shanks of the two blades. The fracture showed a surface aspect typical of fatigue (HCF), corresponding to an estimated number of approximately 7000 cycles. ENEL has further validated an UT technique for an inspection of blade platform "in situ", without disassembling of the blades row verifying it once after the removal of the blades, controlling them by LT also. The UT control has been performed on 4 gas turbines, revealing reliable and applicable "in situ". One gas turbine only was found affected by shank cracks.
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