Failure Investigation of an Industrial Turbine Blade

A 30 MW industrial turbine set suffered failure during operation. The unit was shutdown and the turbine casing was opened to realize that a blade failed at the root. The root portions of the failed blade which were locked in the rotor were dismantled and sent to the laboratory for detailed failure analysis. Chemical analysis indicated that blades were manufactured as per specification. Microstructural characterization confirmed the presence of uniform tempered martensite, which was in conformance with the material standards. Visual examination and stereomicroscopic examination were carried out and it was observed that one of the root portions showed crack propagation features originating from the clamping pin location while the other portions indicated overload failure kind of features. Field Emission Scanning Electron Microscopic (FESEM) investigation confirmed the location of crack origin at the interface of clamping pin. Fretting debris was not observed at the crack origin eliminating the possibility of fretting fatigue failure mechanism. Elemental analysis using Energy-Dispersive X-ray spectroscopy (EDS) at the crack origin revealed the presence of elements such as Cl, S, Ca, etc. This led to suspicion on the chemistry of water utilized for generation of steam for running the turbine. Upon detailed review of water chemistry, it was confirmed that the same deviated from specified values. It can, therefore, be concluded that crack origin location being a highly stressed region in conjunction with the presence of elements such as Cl, S, Ca, etc., could have caused stress corrosion cracking leading to crack initiation. Subsequent load hunting of the turbine led to fast propagation of this crack to failure. The comprehensive procedure followed for carrying out failure investigation and detailed results obtained are discussed in this paper.