The assignment of financial liability for turbine blade failures in steam turbines rests on the ability to determine the damage mechanism or mechanisms responsible for the failure. A discussion is presented outlining various items to look for in a post-turbine blade failure investigation. The discussion centers around the question of how to determine whether the failure was a fatigue induced failure, occurring in accordance with normal life cycle estimates, or whether outside influences could have initiated or hastened the failure.

When a steam turbine was put out of service, cracks were noticed on many of the blades in the low pressure section round the stabilization bolts and perpendicular to the blade axis. The blades were made from chrome alloy steel X20-Cr13 (Material No. 1.402). When the bolts were brazed into the blades inadmissible localized overheating of the steel must have occurred, which resulted in transformation stresses and hence reduced deformability. The cracks arose as a consequence of careless brazing. Whether the cracks should be considered as stress cracks over their entire extent or partially as fatigue cracks produced by vibration in the operation of the turbine as a result of steplike growing of microcracks could not be deduced from the fracture surfaces. Microfractography showed that the cracks developed in stages.

The intermediate pressure (IP) turbine of a thermal generating station is driven by steam from the boiler's reheater. On one particular IP turbine, a thick deposit was found on the insides of the rotor blade shrouds in two instances two years apart. The source of the deposits was not known; bulk chemical analysis had simply shown that iron was a major component. Optical microscopy and electron microprobe analysis were used to identify the deposits. In the first instance, the deposit was found to be debris that was left in the reheater tubes during boiler modification and swept to the turbine by the steam. There were still some of these debris particles present when the incident two years later was investigated but generally the second deposit was found to be of two layer oxide particles which were shown to have spalled from 2-14% chromium reheater tube surfaces.

In an electric power station, seven turbine blades out of 112 broke or cracked within 8 to 14 months after commencement of operation. The blades in question were all located on the last running wheel in the low pressure section of a 35,000 kW high pressure condensing turbine. They were milled blades without binding wires and cover band. They did not fracture at the fastening, i.e. the location of highest bending stress, but in a central region which was 165 to 225 mm away from the gripped end. The blades were fabricated from a stainless heat-treatable chromium steel containing 0.2C and 13.9Cr. Microstructural examination showed the blades were destroyed by flexural vibrations which evidently reached their maximum amplitude at the location of fracture. Erosion of the inlet edge, possibly in connection with vibration-induced corrosion cracking, contributed to fracture.

This article focuses on the life assessment methods for elevated-temperature failure mechanisms and metallurgical instabilities that reduce life or cause loss of function or operating time of high-temperature components, namely, gas turbine blade, and power plant piping and tubing. The article discusses metallurgical instabilities of steel-based alloys and nickel-base superalloys. It provides information on several life assessment methods, namely, the life fraction rule, parameter-based assessments, the thermal-mechanical fatigue, coating evaluations, hardness testing, microstructural evaluations, the creep cavitation damage assessment, the oxide-scale-based life prediction, and high-temperature crack growth methods.

A cracked, martensitic stainless steel, low-pressure turbine blade from a 623 MW turbine generator was found to exhibit fatigue cracks during a routine turbine inspection. The blade was cracked at the first notch of the fir tree and the cracks initiated at pits induced by chloride attack. Examination of the blade microstructure at the fracture origins revealed oxide-filled pits and transgranular cracks. The oxide filled cracks appeared to have originated at small surface pits and probably propagated in a fatigue or corrosion-fatigue fracture mode. It was recommended that the sources of the chlorides be eliminated and that the remaining blades be inspected at regular maintenance intervals for evidence of cracking.

Two blade-detachment failures in large (600 kW) wind turbine generators were investigated. In the first case, bolt failures were established as the initial failure event. A fatigue crack reached a critical length, fast fracture developed and was then arrested as the bolt unloaded. Crack growth resumed when loading increased with cracking or fracture of adjacent bolts. The problem was identified as one of insufficient preload on the bolts. In the second failure on a different unit, a retaining nut on a blade assembly split, allowing a roller bearing to slide off a shaft and a blade to separate at its attachment hub. The failure was observed to be by fatigue. It was determined that pieces of the outer retaining rib (or flange) on the bearing inner cage had fractured by fatigue and were trapped between the nut and the bearing, producing excessive cyclic loading on the nut by a wedging action as the blade pitch adjusted during a revolution. Fatigue of the rim occurred as a result of inadequate lubrication in the bearing, which led to load transfer across the rollers, onto the rim.

This article focuses on common failures of the components associated with the flow path of industrial gas turbines. Examples of steam turbine blade failures are also discussed, because these components share some similarities with gas turbine blading. Some of the analytical methods used in the laboratory portion of the failure investigation are mentioned in the failure examples. The topics covered are creep, localized overheating, thermal-mechanical fatigue, high-cycle fatigue, fretting wear, erosive wear, high-temperature oxidation, hot corrosion, liquid metal embrittlement, and manufacturing and repair deficiencies.

Three blades from 45,000 kW, 3,000 rpm turbine were received for examination, comprising the root of blade 28, blade 89 showing a crack in one of the root teeth, and blade 106 which was free from defects. Microscopic examination of the blade material showed it to be a ferritic stainless steel of the type commonly used for turbine blades. A number of non-metallic inclusions were present which had been drawn into threads in rolling; these appeared to consist largely of duplex silicates. The failure of blade 28 was the result of the development of a creeping crack. Magnetic crack examination of blade 89 revealed a crack in a tooth in an identical position to the start of the crack in blade 28 but on the opposite, i.e., steam inlet, side of the blade. Similar examination of blade 106 did not reveal any cracks. Cracking was associated with unsatisfactory bedding of the blade teeth on the faces of the wheel grooves. It was concluded that the blade failures were due primarily to over-loading of the individual blade teeth due to incorrect fitting in the wheel. Vibration was an important contributory factor, as it resulted in the imposition of fluctuating stresses on the overloaded teeth. Non-metallic inclusions in the blade material playing a minor part.

Erosion of solid surfaces can be brought about solely by liquids in two ways: from damage induced by formation and subsequent collapse of voids or cavities within the liquid, and from high-velocity impacts between a solid surface and liquid droplets. The former process is called cavitation erosion and the latter is liquid-droplet erosion. This article emphasizes on manifestations of damage and ways to minimize or repair these types of liquid impact damage, with illustrations.

Numerous cracks observed on the surface of a forged A470 Class 4 alloy steel steam turbine rotor disc from an air compressor in a nitric acid plant were found to be the result of caustic induced stress-corrosion cracking (SCC). No material defects or anomalies were observed in the disc sample that could have contributed to crack initiation or propagation or secondary crack propagation. Chlorides detected in the fracture surface deposits were likely the primary cause for the pitting observed on the disc surfaces and within the turbine blade attachment area. It was recommended that the potential for water carryover or feedwater induction into the turbine be addressed via an engineering evaluation of the plant's water treatment procedures, steam separation equipment, and start-up procedures.

This article describes the visual, fractographic, and metallographic evidence typically encountered when analyzing stress rupture of turbine airfoils. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries.

Rotor blades in the compressor section of a J79 engine had failed. Optical, stereoscopic, microhardness testing, and SEM examinations were conducted to determine the cause. The blades were made of STS403 and were used uncoated. They were damaged over an extensive area, from the 15th through the 17th compressor stages, as were stator vanes and casing sections. The fractured surface of the 17th blade showed multiple origins along with secondary cracking and extensive propagation that preceded separation. The metallographic analysis of the microstructure suggested work hardening. Based on the results, the cause of the fractured blade was high-amplitude fatigue due to severe stall. After normal engine usage of five months, the blade fractured sending fragments throughout the combustion and turbine sections.

Wind turbine blades are secured by a number of high-strength bolts. The failure of one such bolt, which caused a turbine blade to detach, was investigated to determine why it fractured. Based on the results of a detailed analysis, consisting of stress calculations, chemical composition testing, metallurgical examination, mechanical property testing, and fractographic analysis, it was determined that the bolt failed by fatigue accelerated by stress concentration at low temperatures. The investigation also provided suggestions for avoiding similar failures.

A number of rotating blades in a diffuser at a sugar beet processing plant fabricated from rectangular bars cut from rolled carbon-manganese steel plate fractured brittlely. However, apparently identical blades underwent significant plastic deformation without fracture. Inspection of both fractured and bent blades revealed similar preexisting cracks at the toes of bar attachment welds. Metallographic examination of the bent and the fractured bars revealed they had been cut parallel and transverse, respectively, to the rolling direction of the steel plate. Due to the combined effects of the low fracture toughness of the plate on planes parallel in the rolling direction, the presence of the preexisting cracks, and the relatively large section thickness of the bars, the bars whose lengths were transverse to the rolling direction fractured brittlely when subjected to impact loads. Had the poor transverse properties of thick-section plate been recognized, and all the bars properly cut with respect to the rolling direction, the premature fractures would not have occurred.

Several compressor diaphragms from five gas turbines cracked after a short time in service. The vanes were constructed of type 403 stainless steel, and welding was performed using type 309L austenitic stainless steel filler metal. The fractures originated in the weld heat-affected zones of inner and outer shrouds. A complete metallurgical analysis was conducted to determine the cause of failure. It was concluded that the diaphragms had failed by fatigue. Analysis suggests that the welds contained high residual stresses and had not been properly stress relieved. Improper welding techniques may have also contributed to the failures. Use of proper welding techniques, including appropriate prewelding and postwelding heat treatments, was recommended.