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.

Gas turbines and other types of combustion turbomachinery are susceptible to hot corrosion at elevated temperatures. Two such cases resulting in the failure of a gas turbine component were investigated to learn more about the hot corrosion process and the underlying failure mechanisms. Each component was analyzed using optical and scanning electron microscopy, energy dispersive spectroscopy, mechanical testing, and nondestructive techniques. The results of the investigation provide insights on the influence of temperature, composition, and microstructure and the contributing effects of high-temperature oxidation on the hot corrosion process. Preventative measures are also discussed.

An investigation was conducted to better understand the time-dependent degradation of thermal barrier coated superalloy components in gas turbines. First-stage vanes are normally subjected to the highest gas velocities and temperatures during operation, and were thus the focus of the study. The samples that were analyzed had been operating at 1350 °C in a gas turbine at a combined-cycle generating plant. They were regenerated once, then used for different lengths of time. The investigation included chemical analysis, scanning electron microscopy, SEM/energy dispersive spectroscopy, and x-ray diffraction. It was shown that degradation is driven by chemical and mechanical differences, oxide growth, depletion, and recrystallization, the combined effect of which results in exfoliation, spallation, and mechanical thinning.

New type 321 corrosion-resistant steel heat shields were cracking during welding operations. A failure analysis was performed. The cause was found to be chloride induced stress-corrosion cracking. Packaging was suspected and confirmed to be the cause of the chloride contamination. A contributing factor was the length of time spent in the packaging, 21 years.

While an attempt was being made to start the diesel engine of a dragline type excavator, a severe explosion took place, the operator unfortunately being killed. The engine was normally started by compressed air from an air bottle or receiver charged to a pressure of 350 psi by a small auxiliary engine driven compressor fitted on the excavators. On an occasion when this had broken down, the air bottle was replenished by the engineer-in-charge from a cylinder of compressed air. The compressor engine remained unserviced and when the starting bottle again became depleted, the excavator operator re-charged it himself from another cylinder without the knowledge of the engineer. The engine was started satisfactorily, and the bottle charged again to 350 psi in readiness for the start on the following day. It was while an attempt was being made to start the engine on this occasion that the explosion took place. The cylinder used by the operator for re-charging the receiver contained not air but oxygen.

Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

Several diesel-engine rocker levers (malleable iron similar to ASTM A 602, grade M7002) failed at low hours in overspeed, over-fuel, highly loaded developmental engine tests. Identical rocker levers had performed acceptably in normal engine tests. The rocker levers were failing through the radius of an adjusting screw arm. The typical fracture face exhibited two distinct modes of crack propagation: the upper portion indicated overload at final fracture, whereas the majority of the fracture suggested a fatigue fracture. Investigation (visual inspection, 1.5x/30x/60x magnification, and nital etched 300x magnification) supported the conclusion that the rocker levers failed in fatigue, with casting defects, or spiking, acting as stress raisers to initiate failures in highly loaded engine tests. Recommendations included shot peening of the levers as an interim measure to reduce the possibility of failure and redesign to increase the cross-sectional area of the levers.

A rocket-motor case made of consumable-electrode vacuum arc remelted D-6ac alloy steel failed during hydrostatic proof-pressure testing. Close visual examination, magnetic-particle inspection, and hardness tests showed cracks that appeared to have occurred after austenitizing but before tempering. Microscopic examinations of ethereal picral etched sections indicated that the cracks appeared before or during the final tempering phase of the heat treatment and that cracking had occurred while the steel was in the as-quenched condition, before its 315 deg C (600 deg F) snap temper. Chemical analysis of the cracked metal showed a slightly higher level of carbon than in the component that did not crack. X-ray diffraction studies of material from the fractured dome showed a very low level of retained austenite, and chemical analysis showed a slightly higher content of carbon in the metal of the three cracked components. Bend tests verified the conclusion that the most likely mechanism of delayed quench cracking was isothermal transformation of retained austenite to martensite under the influence of residual quenching stresses. Recommendations included modifying the quenching portion of the heat-treating cycle and tempering in the salt pot used for quenching, immediately after quenching.

Within the first few months of operation of an 8 km (5 mile) long 455 mm (18 in.) diam high-pressure steam line between a coal-fired electricity-generating plant and a paper mill, several of the Inconel 600 bellows failed. The steam line operated at 6030 kPa (875 psi) and 420 deg C (790 deg F). Metallographic sections, energy-dispersive x-ray spectra, chemical analyses, tensile tests, and Auger microscope analyses showed the failed bellows met the specifications for the material. However, investigation also showed entire oxide thickness was contaminated with relatively large amounts of sodium, calcium, potassium, aluminum, and sulfur, alkali, alkali earth, and other contaminants that completely permeated even the thin oxides on the fracture surfaces. Additional investigation of the purity of the steam itself as reported by the power plant showed that corrosion and cracks were ultimately caused by the steam. While under normal operation, the steam's purity posed no problem to the material, during boiler cleaning operations, the generating plant had allowed contamination to get into the steam line.

Cracks formed on cylinder inserts from a water-cooled locomotive diesel engine, on the water side in the neck between the cylindrical part and the collar. Cracks were revealed by magnetic-particle inspection. As a rule, several parallel cracks had appeared, some of which were very fine. The part played by corrosion in the formation of the cracks was demonstrated with the help of metallographic techniques. The surface regions of the cracks widened into funnel form, which is a result of the corrosive influence of the cooling water. Actual corrosion pits could not be found indicating that the vibrational stresses had a greater share in the damage than the corrosive influence. Cracks appeared initially only in those engines in which no corrosion inhibitor had been added to the cooling water. The cracking was caused by corrosion fatigue. The combined presence of a corrosive medium and cyclical operating stress was needed to cause cracks. No cracks appeared when corrosion inhibitor was added to the cooling water.

The crankshaft of a six cylinder, 225-hp diesel engine driving a small locomotive was examined. About nine months after installation a fall in oil pressure was traced to damage to No. 5 crank pin bearing. A small lip present on one side of the discontinuity apparently served to scrape the bearing material. The defect was stoned smooth, a new bearing fitted, and the engine returned to service. The engine performed satisfactorily for a further twelve months until fracture of the crankshaft through the No. 5 crank pin supervened. The fracture revealed a complex torsional fatigue failure. Microscopic examination revealed that the pin had been hard chromium plated and that the plating followed the curved edge of the outer extremity of the defect. This crank pin contained an inherent defect in the form of a slag inclusion or crack situated at the surface. That the crack only showed itself after a period of service suggests that initially it may have been slightly below the surface of the machined pin and some slight extension outwards took place in service.

A failed spiral gear and pinion set made from 4320H Ni-Cr-Mo alloy steel operating in a high-speed electric traction motor gear unit driving a rapid transit train were submitted for analysis. The pinion was intact, but the gear had broken into two sections that resulted when two fractured areas went through the body of the gear. Wheel mileage of the assembly was 34,000 miles at the time of failure. All physical and metallurgical characteristics were well within specified standards, and both parts should have withstood normal loading conditions. The primary mode of failure was tooth bending fatigue of the gear from the reverse direction near the toe end. The cause of failure was a crossed-over tooth bearing condition that placed loads at the heel end when going forward and at the toe end when going in reverse. The condition was too consistent to be a deflection under load; therefore, it most likely was permanent misalignment within the assembly.

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200 to 250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.

Vibration analysis can be used in solving both rotating and nonrotating equipment problems. This paper presents case histories that, over a span of approximately 25 years, used vibration analysis to troubleshoot a wide range of problems.

A pilot-valve bushing fractured after only a few hours of service. In operation, the bushing was subjected to torsional stresses with possible slight bending stresses. A slight misalignment occurred in the assembly before fracture. The bushing was made of 8617 steel and was case hardened to a depth of 0.13 to 0.4 mm (0.005 to 0.015 in.) by carbonitriding. Specifications required that the part be carbonitrided, cooled, rehardened by quenching from 790 deg C (1450 deg F), then tempered at about 175 deg C (350 deg F). Visual examination, hardness testing, and metallographic and microstructural investigation supported the conclusion that the bushing fractured in fatigue because of a highly stressed case-hardened surface of unsatisfactory microstructure and subsurface nonmetallic inclusions. Cracks initiated at the highly stressed surface and propagated across the section as a result of cyclic loading. The precise cause of the unsatisfactory microstructure of the carbonitrided case could not be determined, but it was apparent that heat-treating specifications had not been closely followed. Recommendations included that inspection procedures be modified to avoid the use of steel containing nonmetallic stringer inclusions and that specifications for carbonitriding, hardening, and tempering be rigorously observed.

An investigation of wear and failure of babbitt bushes was completed in this study. The results showed that wear at dry sliding of babbitt obtained by plasma spraying was less than that of babbitt in the as-cast state and after a deformation heat treatment. The failure of babbitt bushes was caused by a simultaneous and interrelated exhibition of fatigue and wear processes that depend considerably on cohesion strength between the bush and the bearing base and accumulation of defects on the contact surface between the bush and the shaft.

A pinion gear made of AMS 6470 steel, nitrided all over, lost internal splined teeth due to wear. Spline failure of the power turbine gear caused an engine overspeed and disintegration. Excessive spline wear resulted from a new coupling being mated during overhaul with a worn gear spline. Wear on the spline teeth flanks of the coupling was attributed to severe wear on the mating gear (internal) spline teeth. The assigned cause was an inadequate maintenance procedure which resulted in a wear-damaged component being retained in the power train during engine overhaul. To prevent reoccurrence, specific inspection criteria were issued defining maximum limits for spline wear. A procedure and requirements were specified for installing the coupling and pinion gear at the next overhaul.

The damage to a screw on the head of a 1.8 liter personal car engine was nucleated as the result of common disadvantageous environmental influences and reversed loads leading to corrosion fatigue.

The circumstances surrounding the in-service failure of a cast Ni-base superalloy (Alloy 713LC) second stage turbine blade and a cast and coated Co-base superalloy (MAR-M302) first stage air-cooled vane in two turbine engines used for marine application are described. An overview of a systematic approach, analyzing the nature of degeneration and failure of the failed components, utilizing conventional metallurgical techniques, is presented. The topographical features of the turbine blade fracture surface revealed a fatigue-induced crack growth pattern, where crack initiation had taken place in the blade trailing edge. An estimate of the crack-growth rate for the stage II fatigue fracture region coupled with the metallographic results helped to identify the final mode of the turbine blade failure. A detailed metallographic and fractographic examination of the air-cooled vane revealed that coating erosion in conjunction with severe hot-corrosion was responsible for crack initiation in the leading edge area.