A number of AISI 347 stainless steel bellows intended for use in the control rod drive mechanism of a fast breeder reactor were found to be leaking before being placed in service. The bellows, which had been in storage for one year in a seacoast environment, exhibited a leak rate on the order of 1 x 10−7 cu cm/s (6 x 10−8 cu in./s). Optical metallography revealed numerous pits and cracks on the surfaces of the bellow convolutes, which had been welded to one another using an autogenous gas tungsten arc welding process. Microhardness measurements indicated that the bellows had not been adequately stress relieved. It was recommended that a complete stress-relieving treatment be applied to the formed bellows. Improvement of storage conditions to avoid direct and prolonged contact of the bellows with the humid, chloride-containing environment was also recommended.

Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, SEM, x-ray diffraction, microhardness measurement, and chemical and microstructural analysis. It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the corrosion problem.

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that. Although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. SEM observations of the roller fracture surface revealed striations indicative of fatigue, and EDS analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.

The right front spring hanger on a dual rear axle of the tractor of a tractor-trailer combination failed, causing the vehicle to roll-over. The hanger was made from malleable cast iron that had been heat treated to produce a decarburized surface layer and a pearlitic transition layer. It had been repair welded after breaking into two pieces longitudinally in a prior incident, using cast iron as weld metal. The repair weld bead on both surfaces missed the fracture over 15 to 20% of their lengths. The incomplete repair weld and brittleness of the weld metal and heat-affected zones led to the failure.

Music wire springs used in a printer return mechanism failed near the bend in the hook portion of the spring during qualification testing. Samples were examined in a scanning electron microscope equipped with an energy-dispersive x-ray microprobe. Fatigue fractures originated at rub marks on the inside edge of the spring. An investigation of loads encountered in service indicated that the springs had been loaded to a large fraction of the yield strength. Redesign of the spring mechanism was recommended.

A failure analysis case study on railroad rails is presented. The work, performed under the sponsorship of the Department of Transportation, addresses the problem of shell and detail fracture formation in standard rails. Fractographic and metallographic results coupled with hardness and residual stress measurements are presented. These results suggest that the shell fractures form on the plane of maximum residual tensile stresses. The formation of the shells is aided by the presence of defects in the material in these planes of maximum residual stress. The detail fracture forms as a perturbation from the shell crack under cyclic loading and is constrained to develop as an embedded flaw in the early stages of growth because the crack is impeded at the gage side and surface of the rail head by compressive longitudinal stresses.

Failure of AISI type 321 stainless steel internal springs from newly manufactured lip seals on a shaft between a turbine power unit and a pump in a commercial aircraft secondary unit was investigated. Examination of the coils from two failed springs showed that both had failed by fatigue. The springs contained drawing defects that served as the fatigue crack initiation sites. It was recommended that the wire drawing process be investigated for various levels of steel cleanliness to predict the incidence of drawing defects at the wire surface. Stress analysis to determine the minimum tolerable defect size was also recommended.

This case study describes the failure analysis of a steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low-carbon CrMoV alloy steel that was subsequently single-pass butt welded using gas tungsten arc welding. Although no cracks were found when the welds were visually inspected, X-ray radiography showed small discontinuous surface cracks adjacent to the weld bead in the heat affected zone. Further investigation, including optical microscopy, microhardness testing, and residual stress measurements, revealed that the cracks were caused primarily by the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in high transformation stresses. These transformation stresses, working in combination with thermal stresses and constraint conditions, resulted in intergranular brittle fracture.

A compression hip screw is a device designed to hold fractures in the area of the femur in alignment and under compression. A side plate, which is an integral part of the device, is attached by screws to the femur, and it holds the compression screw in position. The device analyzed had broken across the eighth hole (of nine holes) from the end of the plate. The detailed metallurgical failure analysis of the device, including metallography and fractography, is reported here. It was found that the device had adequate metallurgical integrity for the application for which it was intended. It is believed that failure was caused by the lack of a screw in the ninth hole. Evidence is also presented which indicates that the device was bent prior to insertion, and the local plastic deformation may have caused structural changes leading to premature crack initiation.

A pair of steam generators operating at a pressurized water reactor site were found to be leaking near a closure weld. The generators were the vertical U-tube type, constructed from ASTM A302 grade B steel. The shell material exhibited high hardness values prior to confirmatory heat treatment, indicating high residual stresses in the area of the weld. All cracks were transgranular and were associated with pits on the inside surfaces of the vessels. It was concluded that the cracking was caused by a low-cycle corrosion fatigue phenomenon, with cracks initiating at areas of localized corrosion and propagating by fatigue. The cause of the pitting/cracking was related to the unit's copper species in solution.

An 18-MW gas turbine exploded unexpectedly after three hours of normal operation. The catastrophic failure caused extensive damage to the rotor, casing, and nearly all turbo-compressor components. Based on their initial review, investigators believed that the failure originated at the interface between two shaft sections held together by 24 marriage bolts. Visual and SEM examination of several bolts revealed extensive deterioration of the coating layer and the presence of deep corrosion pits. It was also learned that the bolts were nearing the end of their operating life, suggesting that the effects of fatigue-assisted corrosion had advanced to the point where one of the bolts fractured and broke free. The inertial unbalance produced excessive vibration, subjecting the remaining bolts to overload failure.

A type 321 stainless steel bellows expansion joint on a 17-cm (6 in.) OD inlet line (347 stainless) in a gas-turbine test facility cracked during operation. The line carried high-purity nitrogen gas at 1034 kPa (150 psi) with a flow rate of 5.4 to 8.2 kg/s (12 to 18 lb/s). Cracking occurred in welded joints and in unwelded portions of the bellows. The bellows were made by forming the convolution halves from stainless steel sheet, then welding the convolutions together. Evidence from visual examination, liquid penetrant inspection chemical analysis, hardness tests, and metallographic examination of sections etched with Vilella's reagent supports the conclusions that failure of the bellows occurred by intergranular fatigue cracking. Secondary degrading effects on the piping existed as well. Recommendations included the acceptability of Type 321 stainless steel (provided open-cycle testing does not result in surface oxidation and crevices) Although type 347 stainless steel would be better, and Inconel 600 would be an even better choice. Welds would also need modified processing for reheating and annealing. Prevention of oil leakage into the system would minimize carburization of the piping and bellows.

A helicopter main rotor bolt failed in the black-coated region between the threads and the taper section of the shank during assembly. The torque applied was approximately 100 N·m (900 in.·lbf) when the bolt sheared. No other bolts were reported to have failed. The failed bolt material conformed to AISI E4340 steel, as specified. The microstructure was tempered martensite, with hardness ranging from 41 to 45 HRC. Failure was in the shear ductile mode. The crack initiated in the area of slag inclusions. Inspection of other bolts from the same shipment was recommended.

An 1100 aluminum alloy connector of a high-tension aluminum conductor steel-reinforced (ACSR) transmission cable failed after more than 20 years in service, in a region of consider able industrial pollution. The steel core was spliced with a galvanized 1020 carbon steel sheath. Visual examination showed that the connector had undergone considerable plastic deformation and necking before fracture. The steel sheath was severely corroded, and the steel splice was pressed off-center in the axial direction inside the connector. Examination of the fracture surface and micro-structural analysis indicated that the failure was caused by mechanical overload, which occurred because of weakening of the steel support cable by corrosion inside the fitting. The corrosion was ascribed to defective assembly of the connector which allowed moisture penetration.

SAE grade 5 U-bolts were used to fasten auxiliary dual wheels to the axles on a farm tractor. Under typical farm usage, the bolts are expected to have infinite life. However, several U-bolts made of 29 mm diam rod broke after less than 100 h of service. The bolt legs in which the failures occurred were all in the same position relative to the direction of wheel rotation. Visual examination showed the break was a fairly flat transverse fracture in the threaded section between the washer and the nut. The appearance of the fracture surfaces was characteristic of failure by low-cycle fatigue, with a smooth matte fatigue failure region showing beach marks and generally extending over about 40 to 60% of the fracture surface, which indicated severe overload. The point of initiation of fatigue was at the root of the last thread at the edge of the nut on the side toward this washer. The U-bolts fractured in fatigue because the bolt material had poor hardenability relative to the diam of the bolts. The bolt material was changed from 1045 steel to 1527 steel, a warm-finished low-alloy steel. The diameter of the bolts was reduced to 27.2 mm and the threads were rolled rather than cut.

A detailed failure analysis was conducted on an ammonia refrigerant condenser tube component that failed catastrophically during its initial hours of operation. Evidence collected clearly demonstrated that the weld between a pipe and a dished end contained a sharp unfused region at its root (lack of penetration). Component failure had started from this weld defect. The hydrogen absorbed during welding facilitated crack initiation from this weld defect during storage of the component after welding. Poor weld toughness at the low operating temperature facilitated crack growth during startup, culminating in catastrophic failure as soon as the crack exceeded critical length.

Repeated failures of high-pressure ball valves were reported in a chemical plant. The ball valves were made of AFNOR Z30C13 martensitic stainless steel. Initial examination of the valves showed that failure occurred in a weld at the ball/stem junction end of austenitic stainless steel sleeves that had been welded to the valve stem at both ends. Metallographic examination showed that a crack had been introduced into the weld by improper weld heat treatment. Stress concentration at the weld location resulting from an abrupt change in cross section facilitated easy propagation of the crack during operation. Proper weld heat treatment was recommended, along with avoidance of abrupt change in cross section near the weld. Due penetrant testing at the ball stem junction before and after heat treatment was also suggested.

The genesis of failure of 6.1 mm thick electric resistance welded API 5L X-46 pipes during pretesting at a pressure equivalent to 90% of specified minimum yield strength was investigated. Cracks were found to initiate on the outer surface of the pipes in the fusion zone and propagate along the through-thickness direction. The presence of extensive decarburization and formation of a soft ferrite band within the fusion zone may have contributed to the nucleation of the cracks. Crack propagation was aided by the presence of exogenous inclusions entrapped within the fusion zone. Analysis of these inclusions confirmed the presence of Fe, Si, Ca, and O, indicating slag entrapment to be the most probable culprit.