Following leakage which developed within the furnace of a horizontal multi-tubular type boiler, examination revealed a series of cracks adjacent to the stiffening rings in the first plain furnace ring. The fire-side surface of the sample was coated with a layer of oxide scale. Microscopical examination of sections through the cracks showed them to be filled with oxide and to be of the multi-branched type, having blunt terminations. The general nature of the cracks was characteristic of cracking from thermal or corrosion fatigue, as results from the operation of varying stresses in an oxidizing or corrosive environment. The cracking in this particular case was due principally to the inordinately large gap between the components. Additionally, several of the sealing welds of the tubes to the back tube plate were cracked in a radial manner, and it would appear that in addition, abnormal thermal conditions may well have been experienced intermittently in service.

After some 87,000 h of operation, failure took place in the bend of a steam pipe connecting a coil of the third superheater of a steam generator to the outlet steam collector. The unit operated at 538 deg C and 135 kPa, producing 400 t/h of steam. The 2.25Cr-1Mo steel pipe in which failure took place was 50.8 mm in diam with a nominal wall thickness of 8 mm. It connected to the AISI 321 superheater tube by means of a butt weld and was one of 46 such parallel connecting tubes. The Cr-Mo tubing was situated outside the heat transfer zone of the superheater. The overall sequence of failure involved overheating of the Cr-Mo outlet tubes, heavy oxidation, oxide cracking on thermal cycling, thermal fatigue cracking plus oxidation, creep-controlled crack growth, and rapid plastic deformation and rupture. This failure was indicative of excess temperature of the steam coming from the heat transfer zone of the coil. It showed that many damage mechanisms may combine in the transition from fracture initiation to final failure. The presence of grain boundary sliding as an indication of creep damage was useful in the characterization of the stress level as high and showed that the process of creep was not operative throughout the life of the equipment.

An outer fan-duct assembly of titanium alloy Ti-5Al-2.5Sn (AMS 4910) for a gas-turbine fan section cracked 75 mm (3 in.) circumferentially through a repair weld in an arc weld in the front flange-duct segment. Examination of the crack with a binocular microscope revealed no evidence of fatigue. A blue etch-anodize inspection showed the presence of an alpha case along the edges of the repair weld. The alpha case, a brittle oxide-enriched layer, forms when welds are inadequately shielded from the atmosphere during deposition. The brittleness of this layer caused transgranular cracks to form and propagate in tension under the thermal stresses created by the repair-weld heat input. The crack resulted from contamination and embrittlement of a repair weld that had received inadequate gas shielding. Thermal stresses cracked the oxide-rich layer that formed. The gas-shielding accessories of the welding torch were overhauled to ensure that leak-in or entrainment of air was eliminated. Also, the purity of the shielding-gas supplies was rechecked to make certain that these had not become contaminated.

During disassembly of an engine that was to be modified, a fractured turbine blade was found. When the fracture was examined at low magnification, it was observed that a fatigue fracture had originated on the concave side of the leading edge and had progressed slightly more than halfway from the leading edge to the trailing edge on the concave surface before ultimate failure occurred in dynamic tension. Analysis (including visual inspection, SEM, and 250x/500x micrographic examination) supported the conclusions that the blades failed due to thermal fatigue. Recommendations included application of a protective coating to the blades, provided the coating was sufficiently ductile to avoid cracking during operation to prevent surface oxidation. Such a coating would also alleviate thermal differentials, provided the thermal conductivity of the coating exceeded that of the base metal. It was also determined that directionally solidified blades could minimize thermal fatigue cracking by eliminating intersection of grain boundaries with the surface. However, this improvement would be more costly than applying a protective coating.

During 5.7 years of service, dye penetrant inspection of Inconel 800H pigtail connections regularly showed cracks at weld toes. Weld repairs were not able to prevent reoccurrence but often aggravated the condition. Samples containing small, but detectable, reducer-to-pigtail cracks showed intergranular cracks originating at weld toes and filled with oxidation product, which precluded determination of the cracking mechanism. All weldments exhibited high degrees of secondary precipitates, with original fabrication welds exhibiting higher apparent levels than repair welds. SEM/EDS analysis showed base metal grain boundary precipitates to be primarily chromium carbides, but some titanium carbides were also observed. Failure was believed to result from the synergism of thermally driven tube distortion, which resulted in over-stress, and from the intergranular oxidation products and intergranular carbides which contributed to cracking. It was recommended that stresses be reduced and /or that materials and components be changed. Refinements in welding procedures and implementation of preweld/postweld heat treatments were recommended also.

A return bend (made from ASTM A213, grade T11, ferritic steel) from a triolefin-unit heater ruptured after two years in service. The unit operated at 2410 kPa, with a hydrocarbon feed stream (85% propylene) entering at 260 to 290 deg C and leaving at 425 to 480 deg C. The fracture was found to terminate at the welds that joined the bend to the pipeline. A high concentration of both small and large inclusions was exhibited by the metallographic examination of the steel near the fracture. Branched cracks similar to those produced by stress corrosion of steel were observed in a section through the fireside edge of the fracture surface. Scale was observed over most of the crack path which acted as a stress raiser. The effect of the oxide was magnified during thermal cycles because of differential thermal expansion, with the steel having a greater expansion coefficient than the scale. It was recommended that material that is intended for critical applications where failure cannot be tolerated should be non-destructively examined.

A fuel-nozzle-support assembly showed transverse indications after fluorescent liquid-penetrant inspection of a repair-welded area at a fillet on the front side of the support neck adjacent to the mounting flange. Visual examination disclosed an irregular crack. The crack through the neck was sectioned; examination showed that the crack had extended through the repair weld. The crack had followed an intergranular path. The crack was opened, and binocular-microscope examination of the fracture surface showed that the surface contained dendrites with discolored oxide films that were typical of exposure to air when very hot. Several additional subsurface cracks, typical of hot tears, were observed in and near the weld. There had been too much local heat input in making the repair weld. The result was localized thermal contraction and hot tearing. The cracking of the repair weld was attributed to unfavorable welding practice that accentuated thermal contraction stresses and caused hot tearing. Recommendations involved use of a small-diameter welding electrode, a lower heat input, and deposition in shallow layers that could be effectively peened between passes to minimize internal stress.

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.

A forging die in a 250-ton press producing brass valves began to show signs of fatigue after a few thousand hits. By the time it reached 30,000 hits, the die was badly damaged and was submitted for analysis along with one of the last forgings produced. The investigation included visual and macroscopic inspection, metallographic and chemical analysis, SEM imaging, optical profilometry, mechanical property testing, and EDX analysis. The die was made of chromium hot-work tool steel and the forgings were made of CuZn39Pb3 heated to an initial working temperature 700 deg C. The entire surface of the die was covered with fatigue cracks and many fillets had been plastically deformed. Several other types of damage were also observed, including areas of oxidation, corrosion pits, voids, abrasive wear, die adhesion, and thermal fatigue. Fatigue cracking was the primary cause of failure with significant contributions from the other damage mechanisms.

A CrN coated restrike punch was made of WR-95 (similar to H-11), which was fluidized bed nitrided. The coated punch was used on hot Inconel at about 1040 deg C (1900 deg F). However, a water-soluble graphite coolant was used to maintain the punch temperature at 230 deg C (450 deg F). Visual and binocular inspection at 64+ revealed presence of cracks and complete washout of coating in the working area of the failed punch. Comparison of metallographic cross sections of used and unused punches revealed a significant microstructural transformation in case of the used punch. Presence of a yellow porous layer was clearly evident between the nitrided layer and the coating, in case of the used punch. Cracks were observed to propagate from the outer surface into the bulk. Oxidation was evident along the cracks. The microstructural transformation observed in the case of the used punch was a clear indication of high temperature exposure (due to insufficient cooling) during application. The most probable cause of failure was thermal fatigue.

Three examples of corrosion-fatigue cracking from the toes of substantial fillet welds applied to seal-leaking riveted seams in steam accumulators are described. In the first case, this practice resulted in a disastrous explosion; in the second, which involved two identical vessels at the same location, cracking in course of development was discovered during internal inspection. Microscope examination of several specimens cut to intersect a crack showed it to be typical of corrosion-fatigue; it was in the form of a broad fissure, contained oxide deposits, and the termination was blunt-ended. The two cases not only serve to illustrate the danger of applying fillet welds to seal the lap edges of riveted seams, but point to the inadvisability of employing riveted construction for vessels intended for service under conditions involving frequent pressure and thermal fluctuations, as it is extremely difficult to maintain the tightness of riveted seams under these conditions. Such vessels are now almost exclusively of all-welded construction

Alloy UNS N08800 (Alloy 800) tubes of the steam superheating coils of two hydrocracker charge heaters in a refinery failed prematurely in service. Failure analysis of the tubes indicated that the failures could be attributed to thermal fatigue as a result of temperature fluctuations as well as restriction to movement. Fatigue cracks initiated intergranularly from both the flue gas and steam sides. Enhanced general and grain boundary oxidation coupled with age hardening of the alloy led to the formation of incipient intergranular cracks that acted as sites for the initiation of the fatigue cracks.

A turbine vane made of cast cobalt-base alloy AMS 5382 (Stellite 31; composition: Co-25.5Cr-10.5Ni-7.5W) was returned from service after an undetermined number of service hours because of crack indications on the airfoil sections. This alloy is cast by the precision investment method. Analysis (visual inspection, 100x/500x metallographic examination of sections etched with a mixture of ferric chloride, hydrochloric acid, and methanol, and bend tests) supported the conclusions that cracking of the airfoil sections was caused by thermal fatigue and was contributed to by low ductility due to age hardening, subsurface oxidation related to intragranular carbides, and high residual tensile macrostresses. No further conclusions could be drawn because of the lack of detailed service history, and no recommendations were made.

The first-stage blades in a model 501D5 gas turbine had 16 cooling holes. After 32,000 h of service, the blades exhibited cracking at the cooling holes. The blade material was wrought Udimet 520 alloy, with nominal composition of 57Ni-19Cr-12Co-6Mo-1W-2Al-3Ti-0.05C-0.005B. The cooling holes' surface was not coated. Investigation supported the conclusions that the cracking at the cooling holes was due to grain-boundary oxidation and nitridation at the cooling hole surface, embrittlement and loss of local ductility of the base alloy, temperature gradient from the airfoil surface to the cooling holes, which led to relatively high thermal stresses at the holes located at the thicker sections of the airfoil, and stress concentration of 2.5 at the cooling hole and the presence of relatively high total strain (an inelastic strain of 1.2%) at the cooling hole surface. Recommendations include applying the specially designed methods given in this case study to estimate the metal temperature and stresses in order to predict the life of turbine blades under similar operating conditions.

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.

An experimental high-temperature rotary valve was found stuck due to growth and distortion after approximately 100 h. Gas temperatures were suspected to have been high due to overfueled conditions. Both the rotor and housing in which it was stuck were annealed ferritic ductile iron similar to ASTM A395. Visual examination of the rotor revealed unusually heavy oxidation and thermal fatigue cracking along the edge of the gas passage. Material properties, including microstructure, composition, and hardness, of both the rotor and housing were evaluated to determine the cause of failure. The microstructure of the rotor was examined in three regions. The shaft material, the heavy section next to the gas passage and the thin edge of the rotor adjacent to the gas passage. The excessive gas temperatures were responsible for the expansion and distortion that prevented rotation of the rotor. Actual operating temperatures exceeded those intended for this application. The presence of transformation products in the brake-rotor edge indicated that the lower critical temperature had been exceeded during operation.

A Ti-6Al-4V alloy pressure vessel failed during a proof-pressure test, fracturing along the center girth weld. The girth joints were welded with the automatic gas tungsten arc process utilizing an auxiliary trailing shield attached to the welding torch to provide inert-gas shielding for the exterior surface of the weld. A segmented backup ring with a gas channel was used inside the vessel to shield the weld root. The pressure vessel failed due to contamination of the fusion zone by oxygen, which resulted when the gas shielding the root face of the weld was diluted by air that leaked into the gas channel. Thermal stresses cracked the embrittled weld, exposing the crack surfaces to oxidation before cooling. One of these cracks caused a stress concentration so severe that failure of the vessel wall during the proof test was inevitable. A sealing system at the split-line region of the segmented backup ring was provided, and a fine-mesh stainless steel screen diffuser was incorporated in the channel section of the backup ring to prevent air from leaking in. A titanium alloy color chart was furnished to permit correlation of weld-zone discoloration with the degree of atmospheric contamination.

A short fracture section of a forged and normalized Ck 35 (DIN 17200) steel slide showed three distinct zones: a dark colored crystalline area, an incipient crack propagating into a far advanced, rubbed fracture surface, and a fine crystalline final break. Metallographic examination showed the dark incipient crack was present before the last heat treatment and was oxidized and decarburized prior to the conclusion of the annealing process. The crack ran perpendicular to the fiber, so it was not formed before or during forging. It was a thermal stress crack produced during flame cutting of the middle section of the slide. The initial crack acted as a sharp notch favoring the formation of the fatigue fracture which lead to the failure of the slide.

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.

High temperature corrosion may occur in numerous environments and is affected by factors such as temperature, alloy or protective coating composition, time, and gas composition. This article explains a number of potential degradation processes, namely, oxidation, carburization and metal dusting, sulfidation, hot corrosion, chloridation, hydrogen interactions, molten metals, molten salts, and aging reactions including sensitization, stress-corrosion cracking, and corrosion fatigue. It concludes with a discussion on various protective coatings, such as aluminide coatings, overlay coatings, thermal barrier coatings, and ceramic coatings.