A power plant using two steam generators (vertical U-tube and shell heat exchangers, approximately 21 m (68 ft) high with a steam drum diameter of 6 m (20 ft)) experienced a steam generator tube rupture. Each steam generator contained 11,012 Inconel alloy 600 (nickel-base alloy) tubes measuring 19 mm OD, nominal wall thickness of 1.0 mm (0.042 in.), and average length of 18 m (57.75 ft). The original operating temperature of the reactor coolant was 328 deg C (621 deg F). A tube removal effort was conducted following the tube rupture event. Investigation (visual inspection, SEM fractographs, and micrographs) showed evidence of IGSCC initiating at the OD and IGA under ridgelike deposits that were analyzed and found to be slightly alkaline to very alkaline (caustic) in nature. Crack oxide analysis indicated sulfate levels in excess of expected values. The analysis supported the conclusion that that the deposits formed at locations that experienced steam blanketing or dryout at the higher levels of the steam generators. Recommendations included steam generator water-chemistry controls, chemical cleaning, and reduction of the primary reactor coolant system temperature.

Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

A welded elbow assembly (AISI type 321 stainless steel, with components joined with ER347 stainless steel filler metal by gas tungsten arc welding) was part of a hydraulic-pump pressure line for a jet aircraft. The other end of the tube was attached to a flexible metal hose, which provided no support and offered no resistance to vibration. The line was leaking hydraulic fluid at the nut end of the elbow. Investigation supported the conclusion that failure was by fatigue cracking initiated from a notch at the root of the weld and was propagated by cyclic loading of the tubing as the result of vibration and inadequate support of the hose assembly. Recommendations included changing the joint design from a cylindrical lap joint to a square-groove butt joint. Also, an additional support was recommended for the hose assembly to minimize vibration at the elbow.

The case and stiffener of an inner-combustion-chamber case assembly failed by completely fracturing circumferentially around the edge of a groove arc weld joining the case and stiffener to the flange. The assembly consisted of a cylindrical stiffener inserted into a cylindrical case that were both welded to a flange. The case, stiffener, flange, and weld deposit were all of nickel-base alloy 718. It was observed that a manual arc weld repair had been made along almost the entire circumference of the original weld. Investigation (visual inspection, 0.5x macrographs, and 10x etched with 2% chromic acid plus HCl views) supported the conclusions that failure was by fatigue from multiple origins caused by welding defects. Ultimate failure was by tensile overload of the sections partly separated by the fatigue cracks. Recommendations included correct fit-up of the case, stiffener, and flange and more skillful welding techniques to avoid undercutting and unfused interfaces.

A pipe in a chip conveyor cracked at the toe of an exterior fillet weld connecting a flange to the pipe. The chip conveyor consisted of several spool sections. Each section was made up of a length of low-alloy steel pipe and two flanges, which were welded to each end. The composition specified for the pipe steel was 0.25C-0.98Mn-3.52Ni-1.34Cr-0.24Mo, which approximates a 9300 steel with high molybdenum. Investigation supported the conclusion that the conveyor pipe failed by brittle fracture, which was attributed to the stresses induced in forcing the circular flange over the elliptical section of the pipe. The toe of the weld and the adjacent undercut were stress raisers that determined the point of major crack origin. Under residual stress, the internal point of incomplete fusion also initiated additional cracks. Recommendations included ensuring a proper fit between an elliptical flange and pipe end to eliminate the cracking.

While undergoing vibration testing, a type 347 stainless steel inlet header for a fuel-to-air heat exchanger cracked in the header tube adjacent to the weld bead between the tube and header duct. Investigation (visual inspection and liquid penetrant inspection) supported the conclusion that the crack in the header tube was the result of a stress concentration at the toe of the weld joining a doubler collar to the tube. The stress concentration was caused by undercutting from poor welding technique and an unfavorable joint design that did not permit a good fit-up. Recommendations included manufacturing the doubler collar so that it could be placed in intimate contact with the header duct, and a revised weld procedure was recommended to result in a smaller, controlled, homogeneous weld joint with less distortion.

An amusement ride failed when a component in the ride parted, permitting it to fly apart. The ride consisted of a central shaft supporting a spider of three arms, each of which was equipped with an AISI 1040 steel secondary shaft about which a circular platform rotated. The main shaft rotated at about 12 rpm and the platforms at a speed of 20 rpm. The accident occurred when one of the secondary shafts on the amusement ride broke. The point of fracture was adjacent to a weld that attached the shaft to a 16 mm thick plate, which in turn bore the platform support arms. Investigation (visual inspection, 0.4x magnification, and stress analysis) supported the conclusion that a likely cause for the fatigue failure was the combination of residual stresses generated in welding and centrifugal service stresses from operation that were accentuated by areas of stress concentration at the undercut locations. Without the excessive residual stress, the shaft dimensions appeared ample for the service load. Recommendations included applying the fillet weld with more care to avoid undercutting. The residual stresses could be minimized by pre-weld and post-weld heat application.

Two tubular AISI 1025 steel posts (improved design) in a carrier vehicle failed by cracking at the radius of the flange after five weeks of service. The posts were two of four that supported the chassis of the vehicle high above the wheels. The original design involved a flat flange of low-carbon low-alloy steel that was welded to an AISI 1025 steel tube, and the improved design included placing the welded joint of the flange farther away from the flange fillet. Investigation (visual inspection and chemical analysis) supported the conclusion that the failures in the flanges of improved design were attributed to fatigue cracks initiating at the aluminum oxide inclusions in the flange fillet. Recommendations included retaining the improved design of the flange with the weld approximately 50 mm (2 in.) from the fillet, but changing the metal to a forging of AISI 4140 steel, oil quenched and tempered to a hardness of 241 to 285 HRB. Preheating to 370 deg C (700 deg F) before and during welding with AISI 4130 steel wire was specified. It was also recommended that the weld be subjected to magnetic-particle inspection and then stress relieved at 595 deg C (1100 deg F), followed by final machining.

During the final shop welding of a large armature for a direct-current motor (4475 kW, or 6000 hp), a loud bang was heard, and the welding operation stopped. When the weld was cold, nondestructive evaluation revealed a large crack adjacent to the root weld. Investigation showed the main crack had propagated parallel to the fusion boundary along the subcritical HAZ and was associated with long stringers of type II manganese sulfide (MnS) inclusions. This supported the conclusion that the weld failed by lamellar tearing as a result of the high rotational strain induced at the root of the weld caused by the weld design, weld sequence, and thermal effects. Recommendations included removing the old weldment to a depth beyond the crack and replacing this with a softer weld metal layer before making the main weld onto the softer layer.

A railway tank car developed a fracture in the region of the sill and shell attachment during operation at -34 deg C (-30 deg F). On either side of the sill-support member, cracking initiated at the weld between a 6.4 mm thick frontal cover plate and a 1.6 mm thick side support plate. The crack then propagated in a brittle manner upward through the side plate, through the welds attaching the side plate to a 25 mm (1 in.) thick shell plate (ASTM A212, grade B steel), and continued for several millimeters in the shell plate before terminating. Other plates involved were not positively identified but were generally classified as semi-killed carbon steels. Investigation (visual inspection, hardness testing, chemical analysis, Charpy V-notch testing, and drop-weight testing) supported the conclusions that the fracture was initiated by weld imperfections and propagated in a brittle manner as a result of service stresses acting on the plate having low toughness at the low service temperatures encountered. Recommendations included that the specifications for the steel plates be modified to include a toughness requirement and that improved welding and inspection practices be performed to reduce the incidence of weld imperfections.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

Post-service destructive evaluation was performed on two commercially pure zirconium pump impellers. One impeller failed after short service in an aqueous hydrochloric acid environment. Its exposed surfaces are bright and shiny, covered with pockmarks, and peppered with pitting. Uniform corrosion is evident and two deep linear defects are present on impeller blade tips. In contrast, the undamaged impeller surfaces are covered with a dark oxide film. This and many other impellers in seemingly identical service conditions survive long lives with little or no apparent damage. No material or manufacturing defects were found to explain the different service performance of the two impellers. Microstructure, microhardness and material chemistry are consistent with the specified material. Examination reveals the damage mechanism to be corrosion-enhanced cavitation erosion, the most severe form of erosion corrosion. Cavitation damage to the protective oxide film caused the zirconium to lose its normally outstanding corrosion resistance. The root cause of the impeller failure is most likely the introduction of excessive air into the pump due to low liquid level, a bad seal or inadequate head. Corrosion pitting, crevice corrosion, and solidification cracks (casting defect) also contributed to the failure.

Cracks, with no other damage, were observed in a niobium alloy (Nb-106) part when it was pulled from several months of protective storage for assembly into a rocket nozzle. SEM views showed the cracks to be intergranular, with contaminant particles on a large number of the grain facets. EDX analysis showed they consisted of niobium and fluorine. Plastic replicas, prepared by standard TEM techniques, were analyzed with selected-area electron diffraction, showing a pattern match for niobium tetrafluoride. Auger analyses showed electron spectra containing peaks representing carbon, oxygen, nitrogen, fluorine, and chlorine. Investigation into the processing history of the part showed the tenacious oxide film formed by the affinity of niobium for oxygen - even when heat treated in a vacuum – was removed with a combination of strong acids: nitric, hydrochloric, hydrofluoric, and lactic, resulting in the contaminants found on the surface. Thus, residues of the cleaning acid on the part had caused SCC during storage, with the tensile stresses necessary to generate SCC assumed to have been residual stresses from the heat treatment. Recommendation was made that more stringent cleaning procedures to remove any trace of the cleaning acids be used.

Stainless steel liners (AISI type 321) used in bellows-type expansion joints in a duct assembly installed in a low-pressure nitrogen gas system failed in service. The duct assembly consisted of two expansion joints connected by a 32 cm (12 in.) OD pipe of ASTM A106 grade B steel. Elbows made of ASTM A234 grade B steel were attached to each end of the assembly, 180 deg apart. A 1.3 mm (0.050 in.) thick liner with an OD of 29 cm (11 in.) was welded inside each joint. The upstream ends were stable, but the downstream ends of the liners remained free, allowing the components to move with the expansion and contraction of the bellows. Investigation (visual inspection, hardness testing, and 30x fractographs) supported the conclusion that the liners failed in fatigue initiated at the intersection of the longitudinal weld forming the liner and the circumferential weld by which it attached to the bellows assembly. Recommendations included increasing the thickness of the liners from 1.3 to 1.9 mm (0.050 to 0.075 in.) in order to damp some of the stress-producing vibrations.

One main undercarriage axle made of high strength alloy steel was subjected to simulated fatigue test for 6000 h of service. After only 300 h it broke in two along the sharp radius. The fracture revealed a coarse, irregular, and brittle surface before final fracture by thick angular shear lip zone. The presence of micropores in the cleavage facets as well as at the grain boundaries and hairline type crack indications under SEM examination were all suggestive of hydrogen embrittlement. On the basis of investigation results and observations, it was concluded that the transverse breakage of the axle had occurred intergranularly in a brittle manner, possibly, initiated by a shallow zone of fatigue along the sharp radius acting as stress riser.

A weld that attached the head to the shell of a preheater containing steam at 1.4 MPa (200 psi) and was used in the manufacture of paper cracked in service. The original joint contained a 6.4 by 50 mm backing ring that had been tack welded to the inside surface of the shell in a position to project beyond the fully beveled top edge of the shell. The projecting edge of the ring fitted against a deep undercut on the inner corner of the rim of the head. The internal 90-deg angle in this undercut was sharp, with almost no fillet. A bevel from the lower edge of the undercut to the outside of the head completed the groove for the circumferential attachment weld. Investigation (visual inspection and actual size views etched in hot 50% hydrochloric acid) supported the conclusion that cracking occurred in the HAZ in the head of the original design, originating in the sharp corner of the undercut, which was an inherent stress raiser. Recommendations included revised joint design to ensure full root penetration.

Failed ferrous components were analyzed from a crane that operated on an offshore platform. The crane failed during operation and fell into the sea. The brake spring on the boom hoist was found to have fractured in four places. The spring contained a line defect (seam) that was the source of each crack. The fracture of the oil quenched and tempered (HRC 50 ASTM A229) spring was by stress-corrosion cracking after the crane fell into the sea because fatigue cannot account for the fractures observed. The crane failure was caused by an overload created by the operator catching a free-falling load.

A 208 cm (82 in.) ID steel aqueduct (ASTM A572, grade 42, type 2 steel) fractured circumferentially at two points 152 m (500 ft) apart in a section above ground. A year later, another fracture occurred in a buried section 6.4 km (4 mi) away. Both pipes fractured during Jan at similar temperatures and pressures. The pipe had a 24 mm wall thickness, and the hydrostatic head was 331 m (1085 ft). The air temperature was approximately -13 deg C (9 deg F), the water temperature approximately 0.6 deg C (33 deg F), and the steel temperature approximately -4 deg C (25 deg F). The pipe had been shop-fabricated in 12 m (40 ft) lengths, then shop welded into 24 m (80 ft) lengths. Field assembly was with bell-and-spigot joints. Investigation (visual inspection and Charpy V-notch testing) supported the conclusion that brittle fracture of the aqueduct pipe was attributed to a combination of stress concentrations at the toes of the fillet welds due to poor welding technique, including shop welds made without preheat, and a brittle condition of the steel at winter temperatures. Recommendations included revised welding techniques, installation of expansion joints, and the use of steel plate rolled from fully killed ingots.

The fatigue failure of a wire rope used on a skip hoist in an underground mine has been studied as part of the ongoing research by the Bureau of Mines into haulage and materials handling hazards in mines. Macroscopic correlation of individual wire failures with wear patterns, fractography, and microhardness testing were used to gain an understanding of the failure mechanism. Wire failures occurred predominantly at characteristic wear sites between strands. These wear sites are identifiable by a large reduction in diameter; however, reduction in area was not responsible for the location of failure. Fractography revealed multiple crack initiation sites to be located at other less noticeable wear sites or opposite the characteristic wear site. Microhardness testing revealed hardening, and some softening, at wear sites.