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 failure occurred in buried brass (92% Cu, 8% Zn) piping used to carry drinking water in wet clay soil after less than two years in service. Investigation (visual inspection, chemical analysis of both the pipe surface and water, and a comparison of the corrosion failure of power station condenser tubing cooled by seawater for two copper alloys, an aluminum brass alloyed with arsenic (UNS C68700, ASTM B111, or Cu-Zn-20Al DIN17660), and a cupronickel 70-30 alloy with iron added (C71500, ASTM B111, or Cu-Ni-30Fe DIN17665)) supported the conclusion that the failure was caused by microbial induced corrosion by sulfate-reducing bacteria. No recommendations were made.

Type 316L (UNS S31603) austenitic stainless steel piping was installed as part of a storm-sewer treatment collection system in a manufacturing facility. Within six months of start-up, leaks were discovered. Investigation (on-site current flow testing, visual inspection, water tests, and 5x/10x images etched in ASTM 89 reagent) supported the conclusion that the pitting in the austenitic stainless steel pipe was believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low-flow and stagnant conditions present in the piping were primary contributors to the pit progression. Recommendations included replacing the pipe. Several alloys, nonmetallic materials, and lining materials were proposed for coupon testing to determine which would operate best in an environment with high levels of aerobic bacteria.

Linear indications on the outer surface of a cross in a piping system were revealed by dye-penetrant examination. The cross was specified to be SA403 type WP 304 stainless steel. The cross had been subjected to induction-heating stress improvement. The linear indications on the cross were located in wide bands running circumferentially below the cross-to-cap weld and above the cap-to-discharge-pipe weld. The material was found to conform to the requirements both in terms of hardness and strength. Intergranular cracks filled with oxide were observed on metallographic analysis of a sectioned and oxalic acid etched sample. The grain size was found to exceed the ASTM standard. No indications of sensitization were observed during testing with practice A of ASTM A 262. Definitive evidence of contaminants to support SCC as the failure mechanism was not disclosed during analysis. It was concluded that overheating or burning of the forging, which classically results in large grain size, intergranular fractures, and fine oxide particles dispersed throughout the grains was the possible reason for the failure.

The outlet-piping system of a steam-reformer unit failed by extensive cracking at four weld locations. The welded system consisted of Incoloy 800 (Fe-32Ni-21Cr-0.05C) pipe and fittings. The exterior surfaces of the system were insulated with rock wool that did not contain weatherproofing. On-site visual examination and magnetic testing indicated severe external corrosion of most of the piping. The system showed extensive cracking in weld HAZ. One specimen indicated that corrosion extended to a depth of 3.2 mm and cracks were seen at the edge of the cover bead and in the HAZ of the weld. Metallographic examination showed that cracking was intergranular and that adjacent grain boundaries had undergone deep intergranular attack. Examination at higher magnification revealed heavy carbide precipitation, primarily at grain boundaries, indicating that the alloy had been sensitized, which resulted from heating during welding. Electron probe x-ray microanalysis showed the outside surface of the tube did not have the protective chromium oxide scale normally found on Incoloy 800. The inside surface of the tube had a thin chromium oxide protective scale. This evidence supported the conclusions that the deep oxidation greatly decreased the strength of the weld HAZ and cracking followed.

Copper piping in a closed-loop water heater system was found to be corroded with MIC and erosion of the weak oxide layer. Investigation (visual inspection, bacterial corrosion cultures, and 20x/400x micrographs) supported the conclusion that the corrosion was caused by microbiological attack, specifically sulfur-reducing bacteria. No recommendations were made.

A 680,000 kg (750 ton) per day ammonia unit was shut down following a fire near the outlet of the waste heat exchanger. The fire had resulted from leakage of ammonia from the type 316 stainless steel outlet piping. The outlet piping immediately downstream from the waste heat exchanger consisted of a flange made from a casting, and a reducing cone, a short length of pipe, and a 90 deg elbow, all made of 13 mm thick plate. A liner wrapped with insulation was welded to the smaller end of the reducing cone. All of the piping up to the flange was wrapped with insulation. Investigation (visual inspection, 10x unetched images, liquid-penetrant inspection, and chemical analysis of the insulation) supported the conclusion that the failure occurred in the area of the flange-to-cone weld by SCC as the result of aqueous chlorides leached from the insulation around the liner by condensate. Recommendations included eliminating the chlorides from the system, maintaining the temperature of the outlet stream above the dewpoint at all times, or that replacing the type 316 stainless steel with an alloy such as Incoloy 800 that is more resistant to chloride attack.

The onset of leakage adjacent to two butt welds in a 2 in. bore pipe was traced to the development of fine cracks. The pipe carried 40% sodium hydroxide solution. The actual temperature was not known, but the pipeline was steam traced at a pressure of 30 psi, equivalent to a temperature of 130 deg C (266 deg F). Magnetic crack detection revealed circumferential crack-like indications situated a short distance from the butt weld. Cracking originated on the bore surfaces of the tube and was of an intergranular nature reminiscent of caustic cracking in steam boilers. The strength of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point magnitude, were responsible. As all other welds were suspect, the remedy was to remove the joints and to reweld followed by local stress relief.

An austenitic stainless steel (type 316/316L stainless steel, schedule 40, 64 mm (2.5 in.) diam and larger) piping network used in the fire-sprinkler system in a large saltwater passenger and car ferry failed by rapid leaking. Operating conditions involved stagnant seawater at ambient temperatures. The pipe was in service for four weeks when three leaks appeared. Investigation (visual inspection and photographic images) supported the conclusion that the failure was caused by attack and corrosion damage of Cl ions in conditions that were ideal for three modes of highly accelerated pitting of austenitic stainless steel: the bottom surface, weld or HAZ pits, and crevices. Recommendations included proper material selection for piping, flanges, and weld rods with greater corrosion resistance. Proper filtering to prevent entrained abrasives and timely breakdown inspections were also advised.

A large diameter steel pipe reinforced by stiffening rings with saddle supports was subjected to thermal cycling as the system was started up, operated, and shut down. The pipe functioned as an emission control exhaust duct from a furnace and was designed originally using lengths of rolled and welded COR-TEN steel plate butt welded together on site. The pipe sustained local buckling and cracking, then fractured during the first five months of operation. Failure was due to low cycle fatigue and fast fracture caused by differential thermal expansion stresses. Thermal lag between the stiffening rings welded to the outside of the pipe and the pipe wall itself resulted in large radial and axial thermal stresses at the welds. Redundant tied down saddle supports in each segment of pipe between expansion joints restrained pipe arching due to circumferential temperature variations, producing large axial thermal bending stresses. Thermal cycling of the system initiated fatigue cracks at the stiffener rings. When the critical crack size was reached, fast fracture occurred. The system was redesigned by eliminating the redundant restraints and by modifying the stiffener rings to permit free radial thermal breathing of the pipe.

The carbon steel feedwater piping at a waste-to-energy plant was suffering from wall thinning and leaking after being in service for approximately six years. Metallographic examination of ring sections removed front the piping revealed a normal microstructure consisting of pearlite and ferrite. However, the internal surface on the thicker regions of the rings exhibited significant deposit buildup, where the thinned regions showed none. No significant corrosion or pitting was observed on either the internal or external surface of the piping. The lack of internal deposits on the affected areas and the evidence of flow patterns indicated that the wall thinning and subsequent failure were caused by internal erosion damage. The exact cause of the erosion could not be determined by the appearance of the piping. Probable causes of the erosion include an excessively high velocity flow through the piping, extremely turbulent flow, and/or intrusions (weld backing rings or weld bead protrusions) on the internal surface of the pipes. Increasing the pipe diameter and decreasing the intrusions on the internal surface would help to eliminate the problem.

A carbon steel piping cross-tee assembly which conveyed hydrogen sulfide (H7S) process gas at 150 to 275 deg C (300 to 585 deg F) with a maximum allowable operating pressure of 3 MPa (450 psig) ruptured at the toe of one of the welds at the cross after several years of service. The failure was initially thought to be the result of thermal fatigue, and the internal surfaces exhibited the “elephant hide” pattern characteristic of thermal fatigue. However metallographic failure analysis found that this pattern was the result of corrosion rather than thermal fatigue. Corrosion caused failure at this location because the weld was abnormally thin as fabricated. Thus, failure resulted from inadequate deposition of weld metal and subsequent wall thinning from internal corrosion. It was recommended that the cross-tee be replaced with a like component, with more careful attention to weld quality.

The failure mode of through-wall cracking of a butt weld in a 5083-O aluminum alloy piping system in an ethylene plant was identified as mercury liquid metal embrittlement. As a result of this finding, 226 of the more than 400 butt welds in the system were ultrasonically inspected for cracking. One additional weld was found that had been degraded by mercury. A welding team experienced in repairing mercury contaminated piping was recruited to make the repairs. Corrective action included the installation of a sulfur-impregnated charcoal mercury-removal bed and replacement of the aluminum equipment that was in operation prior to the installation of the mercury-removal bed.

One inch diam Type 304 stainless steel piping was designed to carry containment atmosphere samples to an analyzer to monitor hydrogen and oxygen levels during operational and the design basis accident conditions that are postulated to occur in a boiling water reactor. Only one of six lines in the system had thru-wall cracks. Shallow incipient cracks were detected at the lowest elevations of one other line. The balance of the system had no signs of SCC attack. Chlorides and corrosion deposits in varying amounts were found throughout the system. The failure mechanism was transgranular, chloride, stress-corrosion cracking. Replacement decisions were based on the presence of SCC attack or heavy corrosion deposits indicative of extended exposure time to chloride-contaminated water. The existing uncracked pipe, about 75 percent of the piping in the system, was retained despite the presence of low level surface chlorides. Controls were implemented to insure that temperatures are kept below 150 deg F, or, walls of the pipe are moisture-free or the cumulative wetted period will never exceed 30 h.

A 75 cm OD x 33 mm thick pipe in a horizontal section of a hot steam reheat line ruptured after 15 years in service. The failed section was manufactured from rolled plate of material specification SA387, grade C. The longitudinal seam weld was a double butt-weld that was V-welded from both sides and failure was found to propagate along the longitudinal seam and its HAZ. The fracture surface near the inner wall of the pipe was found to have a bluish gray appearance, while the fracture surface near the outer wall was rust colored (oxides). The transverse-to-the-weld specimen from the longitudinal seam weld was revealed to have lower elongation and a shear type failure rather than the cup-cone failures. It was concluded that the welded longitudinal seam exhibited embrittlement. A low-ductility intergranular fracture that progressed through the weld metal was revealed by scanning electron microscopy. The cracks were revealed to be in existence for some time before the final failure which was indicated by the extent and amount of corrosion products. It was concluded that low ductility was responsible for the original initiation of cracks in the pipe.

Nuclear power plants typically experience two or three high-cycle fatigue failures of stainless steel socket-welded connections in small bore piping during each plant-year of operation. This paper discusses fatigue-induced failure in socket-welded joints and the strategy Texas Utilities Electric Company (TU Electric) has implemented in response to these failures. High-cycle fatigue is invisible to proven commercial nondestructive evaluation (NDE) methods during crack initiation and the initial phases of crack growth. Under a constant applied stress, cracks grow at accelerating rates, which means cracks extend from a detectable size to a through-wall crack in a relatively short time. When fatigue cracks grow large enough to be visible to NDE, it is likely that the component is near the end of its useful life. TU Electric has determined that an inspection program designed to detect a crack prior to the component leaking would involve frequent inspections at a given location and that the cost of the inspection program would far exceed the benefits of avoiding a leak. Instead, TU Electric locates these cracks by visually monitoring for leaks. Field experience with fatigue-induced cracks in socket-welded joints has confirmed that visual monitoring does detect cracks in a timely manner, that these cracks do not result in catastrophic failures, and that the plant can be safely shut down in spite of a leaking socket-welded joint in a small bore pipe. Historical data from TU Electric and Southwest Research Institute are presented regarding the frequency of failures, failure locations, and the potential causes. The topics addressed include 1) metallurgical and fractographic features of fatigue cracks at the weld toe and weld root; 2) factors that are associated with fatigue, such as mechanical vibration, internal pulsation, joint design, and welding workmanship; and 3) implications of a leaking crack on plant safety. TU Electric has implemented the use of modified welding techniques for the fabrication of socket-welded joints that are expected to improve their ability to tolerate fatigue.

This article discusses pressure vessels, piping, and associated pressure-boundary items of the types used in nuclear and conventional power plants, refineries, and chemical-processing plants. It begins by explaining the necessity of conducting a failure analysis, followed by the objectives of a failure analysis. Then, the article discusses the processes involved in failure analysis, including codes and standards. Next, fabrication flaws that can develop into failures of in-service pressure vessels and piping are covered. This is followed by sections discussing in-service mechanical and metallurgical failures, environment-assisted cracking failures, and other damage mechanisms that induce cracking failures. Finally, the article provides information on inspection practices.

Graphitization, the formation of graphite nodules in carbon and low alloy steels, contributes to many failures in high-temperature environments. Three such failures in power-generating systems were analyzed to demonstrate the unpredictable nature of this failure mechanism and its effect on material properties and structures. In general, the more randomly distributed the nodules, the less effect they have on structural integrity. In the cases examined, the nodules were found to be organized in planar arrays, indicating they might have an effect on material properties. Closer inspection, however, revealed that the magnitude of the effect depends on the relative orientation of the planar arrangement and principle tensile stress. For normal orientation, the effect of embrittlement tends to be most severe. Conversely, when the orientation is parallel, the nodules have little or no effect. The cases examined show that knowledge is incomplete in regard to graphitization, and the prediction of its occurrence is not yet possible.