An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

Catastrophic pitting corrosion occurred in type 304L stainless steel pipe flange assemblies in an industrial food processor. During regular service the pumped medium was pureed vegetables. In situ maintenance procedures included cleaning of the assemblies with a sodium hypochlorite solution. It was determined that the assemblies failed due to an austenite-martensite galvanic couple activated by a chlorine bearing electrolyte. The martensitic areas resulted from a transformation during cold-forming operations. Solution annealing after forming, revision of the design of the pipe flange assemblies to eliminate the forming operation, and removal of the source of chlorine were recommended.

An elbow of 70 mm OD and 10 mm wall thickness made from St 35.29, and exposed to 315 atmospheres internal pressure in an oil hydraulic shear installation, cracked lengthwise after a short operating period. Because the stress was not sufficient to explain the fracture of this elbow under this pressure, an investigation was conducted to establish whether material or processing errors had occurred. Microscopic examination showed that a ferritic-pearlitic structure in select locations was very fine-grained. Other signs of fast cooling as compared to normally formed structure of the core zone were noted. It was also possible that the pipe was resting on a cold plate during bending or that it came in touch with a cold tool. This apparently caused the strains at the transition to the cross-sectional part that had been cooled more slowly. The location of the crack at just this point gave rise to the conclusion that it was formed either by the sole or contributive effect of these stresses.

Stainless steel pipe (273-mm OD x 8-mm wall thickness) used in the fabrication of large manifolds developed crack-like decohesions during a routine inductive bending procedure. The imperfections, which were found near the outside diameter, were around 3 mm in length oriented in the circumferential direction and penetrated nearly 2 mm into the pipe wall. The pipes were made of titanium-stabilized austenitic stainless steel X6CrNiMoTi17-12-2. Six hypotheses were considered during the investigation, which ultimately concluded that the failure was caused by liquation cracking due to overheating.

Leakage was detected in a malleable iron elbow (ASTM A 47, grade 35018) after only three months in service. Life expectancy for the elbow was 12 to 24 months. The piping alternately supplied steam and cooling water to a tire-curing press. The supply line and elbow were subjected to 14 heating and cooling cycles per hour for at least 16 h/day, or a minimum of 224 cycles/day. Steam and water pressure were 1035 kPa (150 psi) and 895 kPa (130 psi) respectively, and water-flow rate was estimated to be 1325 L/min (350 gal/min) based on pump capacity. Water-inlet temperature was 10 to 15 deg C (50 to 60 deg F) and outlet temperature was 50 to 60 deg C (120 to 140 deg F). The pH of the water was 6.9. Investigation (visual inspection, chemical analysis, and 67x nital etched micrographs) supported the conclusion that the elbows had been given the usual annealing and normalizing treatment for ferritizing malleable iron. This resulted in lower resistance to erosion and corrosion than pearlitic malleable iron. Recommendations included replacing the elbows with heat-treated fittings with a pearlitic malleable microstructure.

A 460 mm (18 in.) diam suction line to the main feed water pump for a nuclear power plant failed in a violent, catastrophic manner. Samples of pipe, elbow, and weld materials (ASTM A106 grade B carbon steel, ASTM A234 grade WPB carbon steel, and E7018 carbon steel electrode, respectively) from the suction line were analyzed. Evidence of overall thinning of the elbow and pipe material and ductile tearing of fractures indicated that the feed water pipe failed as a result of an erosion corrosion mechanism, which thinned the wall sufficiently to cause rapid, ductile tearing of the material after its design stress had been exceeded. It was recommended that steel with a higher chromium content be used to mitigate the erosion corrosion potential in the lines and that more rigorous nondestructive (ultrasonic) examinations be performed.

An SB407 alloy 800H tube failed at a 100 deg bend shortly after startup of a new steam superheater. Three bends failed and one bend remote from the failure area was examined. Visual examination showed that the fracture started on the outside surface along the inside radius of the bend and propagated in a brittle, intergranular fashion. Chemical analysis revealed that lead contamination was a significant factor in the failure and phosphorus may have contributed. The localized nature of the cracks and minimum secondary cracking suggested a distinct, synergistic effect of applied tensile stress with the contamination. Stress analysis found that stress alone was not enough to cause failure; however the operating stresses in the 100 deg bends were higher than at most other locations in the superheater Reduced creep ductility may be another possible cause of failure. Remedial actions included reducing the tube temperature, replacing the Schedule 40 100 deg bends with Schedule 80 pipe, and solution annealing the pipe after bending.

A main steam pipe was found to be leaking due to a large circumferential crack in a pipe-to-fitting weld in one of two steam leads between the superheater outlet nozzles and the turbine stop valves (a line made of SA335-P22 material). The main crack surface was found to be rough, oriented about normal to the outside surface, and had a dark oxidized appearance. The cracking was found to be predominantly intergranular. Distinct shiny bands that etched slower than the remainder of the sample at the top of each individual weld bead were revealed by microscopic examination. These bands were found contain small cracks and microvoids. A mechanism of intergranular creep rupture at elevated temperature was identified as a result of a series of stress-rupture and tensile tests. It was revealed by the crack shape that cracking initiated on the pipe exterior, then propagated inward and in the circumferential direction in response to a bending moment load. It was concluded that the primary cause of failure was the occurrence of bending stresses that exceeded the stress levels predicted by design calculations and that were higher than the maximum allowable primary membrane stress.

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.

A cadmium-plated 4340 Ni-Cr-Mo steel ballast elbow assembly was submitted for failure analysis to determine the element or radical present in an oxidation product found inside the elbow assembly. Energy-dispersive x-ray analysis in the SEM showed that iron was the predominant species, presumably in an oxide form. The inside surface had the appearance of typical corrosion products. Hardness measurements indicated that the 4340 steel was heat treated to a strength of approximately 862 MPa (125 ksi). It was concluded that the oxide detected on the ballast elbow was iron oxide. The possibility that the corrosion products would eventually create a blockage of the affected hole was great considering the small hole diameter (4.2 mm, or 0.165 in.). It was recommended that a quick fix to stop the corrosion would be to apply a corrosion inhibitor inside the hole. This, however, would cause the possibility of inhibitor buildup and the eventual clogging of the hole. A change in the manufacturing process to include a cadmium plating on the hole inside surface was recommended. This was to be accomplished in accordance with MIL specification QQ-P-416, Type II, Class 1. A material change to 300-series stainless steel was also recommended.

An irrigation pipe made of medium-density PE failed during service. This pipe was subjected to severe cyclic-bending strain of the order of 6% while under tensile stress of approximately 6.9 MPa (1000 psi) and a hoop stress of approximately 6.2 MPa (900 psi), far more stringent conditions than those encountered in most applications of PE pipes. Visual inspection and reflected-light optical micrographs were used to plot bandwidth as a function of crack length. The conclusion was that, contrary to the dominant belief that pipe failure initiates from surface defects, a critical size flaw within the pipe wall can also initiate failure as it did in this case. Recommendations included that similarity criteria should be established between the fracture behavior of a component in service and that observed in the laboratory.

During a hydraulic test on one of the boilers in a range, leakage occurred from the lower surface of a horizontal S bend in the main steam pipe between the drum connector box and the junction valve. The pipe in question was 15 in. bore and had been in service for about 50 years. Specimens were prepared for microscopical examination to include the defective zone and a section through a circumferential crack. The defective zone was found to contain numerous inclusions of slag and oxides of globular form. Regions surrounding the inclusions were decarburized, the indications being that this region of the plate had been heated to an excessively high temperature. A corrosion-fatigue fissure was at one location, this having originated at the internal surface of the pipe and run into an inclusion in the defective zone. The failure resulted from the development of corrosion-fatigue fissures which originated at a zone containing defects introduced at the time of manufacture. These may have had their origin in the ingot from which the plate was rolled or, alternatively, be indicative of a zone which suffered overheating and local burning at the time the forge weld was made.

Several of the aluminum alloy 6061-T6 drawn seamless tubes (ASTM B 234, 2.5 cm (1.0 in.) OD with wall thickness of 1.7 mm (0.065 in.)) connecting an array of headers to a system of water-cooling pipes failed. The tubes were supplied in the O temper. They were bent to the desired curvature, preheated, then solution treated, water quenched, and then aged for 8 to 10 h. Analysis (visual inspection, slow-bend testing, 65x macrographic analysis, macroetching, spectrographic analysis, hardness tests, microhardness tests, tension tests, and microscopic examination) supported the conclusions that bending of the connector tubes in the annealed condition induced critical strain near the neutral axis of the tube, which resulted in excessive growth of individual grains during the subsequent solution treatment. Recommendations included bending the connector tubes in the T4 temper as early as possible after being quenched from the solution temperature. The tubes should be stored in dry ice after the quench until bending can be done. The tubes should be aged immediately after being formed. Flattening and slow-bend tests should be specified to ensure that the connector tubes had satisfactory ductility.

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.

Steel pipes, used for years in a food factory soft water preheater, were found to leak as a result of corrosion. The pipes, made of 18/8 steel, were immersed in steam maintained at 0.5 atm and 150 deg C. They carried desalinated process water, heating it to approximately 100 deg C. Inspection revealed a reddish-brown coating on the outside of the pipe with a few flat pitting holes and incipient cracks. Corrosion was also observed on the inner walls of the pipe, consisting of rust patches with pitting scars, branching out to predominantly transcrystalline cracks. In this case, leaking appears to be the result of a combination of pitting and stress corrosion, most likely due to chlorides. The factory was recommended to use molybdenum-alloyed steels (type 18/10 or 18/12) which are more resistant to local disruption of passivating films and pitting than molybdenum-free types such as 18/8.

Austenitic stainless steel (X 10 Cr-Ni-Mo-Ti 18 10, Material No. 1.4571) cooling coils were found leaking in 15 spots after eight weeks of service in an apparatus in which ammonium sulfide solution was converted into ammonium sulfate. The external temperature of the coil was approximately 175 deg C and it was cooled by water at 3 atm. Examination of two sections of the coil showed pinhead size pitting cavities at the exterior surface and partially parallel and partially angled array of fine cracks on external as well as the internal surfaces of the bend. Metallographic examination conducted on longitudinal and transverse sections showed predominantly transcrystalline cracks, originated from the pits at the external surfaces of the pipe. Their appearance suggested they were stress corrosion cracks that occur in austenitic steels under the combined effect of stresses and certain corrosion agents, especially chlorides. If chlorides were absent, hydrogen sulfide which causes similar pitting and is capable of causing cracks could be suspected. Favorable state of stresses, which could be residual or due to heat treating, bending or straightening operations, would be recommended for better behavior of the container.

An 850 ft. long steam main working at a pressure of 120 psi ruptured. Two lengths of pipe were submitted for examination, one containing the rupture and the other from an unaffected part removed to facilitate repair. The rupture surfaces were smooth, suggesting that the failure had taken place either at a weld in the pipe or at a discontinuity in the material. Microscopic examination through the joints at the ends of the rupture confirmed that the pipe had been made from strip and the edges lap-welded. The second case concerned an 8 in. diam pipe in which a longitudinal defect was discovered running the entire length. Examination of a section through the pipe containing the defect, showed this to be a scarf-welded pipe, only about half the section of the scarf having been satisfactorily welded together. It was concluded that both pipes had been defective at the time of manufacture and that service conditions had served to extend the defects.

Several elbow subassemblies comprising segments of oil-line assemblies that recycled aircraft-engine oil from pump to filter broke in service. The components of the subassemblies were made of aluminum alloy 6061-T6. Two subassemblies were returned to the laboratory to determine cause of failure. In one, the threaded boss had separated from the elbow at the weld. In the other, the failure was by fracture of the elbow near the flange. The separation of the threaded boss from the elbow was due to a poor welding procedure. Crack propagation was accelerated by fatigue caused by cyclic service stresses. The fracture of the second elbow near the flange was caused by overaging during repair welding of the boss weld. Satisfactory weld penetration was achieved by improved training of the welders plus more careful inspection. Repair welding was prohibited, to avoid recurrence of overaging from the welding heat. Additional support for the oil line was installed to reduce vibration and minimize fatigue of the elbow.

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.