High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.

Coiled tubing used in drilling operations failed at the halfway point of its estimated fatigue life. The failure was found to be transverse to the tubing axis. Visual examination revealed a flat fracture surface extending 13 mm with the rest of the fracture showing shear lips indicative of tensile overload. The flat portion of the fracture surface was typical of fatigue cracking. Fatigue striations were revealed by SEM. Corrosion pitting on the tubing ID from which the fatigue crack had propagated were observed on closer examination. The corrosion pitting was speculated to have occurred when the tubing was idle and fluids accumulated at the bottom of the tubing wraps. The coiled tubing was concluded to have failed prematurely due to low-cycle fatigue initiated at corrosion pitting sites. Corrosive attack on the coiled tubing was recommended to be reduced by completely removing fluids or modifying the fluids in the tubing or purging by flowing dry nitrogen to dry it out.

Molded plastic couplings used in an industrial application exhibited abnormally brittle properties, as compared to previously produced components. The couplings were specified to be molded from a custom-compounded glass-filled nylon 6/12 resin. An inspection of the molding resin used to produce the discrepant parts revealed that the pellets were of two general types, neither of which matched the pellets from a retained resin lot. Investigation included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The thermograms supported the conclusion that the brittle couplings contained a significant level of contamination, polypropylene and nylon 6/6. The source of the polypropylene was likely the purging compound used to clean the compounding extruder. The origin of the nylon 6/6 resin was unknown but may represent a previously compounded resin.

Evidence of destructive pitting on the gear teeth (AMS 6263 steel) in the area of the pitchline was exhibited by an idler gear for the generator drive of an aircraft engine following test-stand engine testing. The case hardness was investigated to be lower than specified and it was suggested that it had resulted from surface defects. A decarburized surface layer and subsurface oxidation in the vicinity of pitting were revealed by metallographic examination of the 2% nital etched gear tooth sample. It was concluded that pitting had resulted as a combination of both the defects. The causes for the defects were reported based on previous investigation of heat treatment facilities. Oxide layer was caused by inadequate purging of air before carburization while decarburization was attributed to defects in the copper plating applied to the gear for its protection during austenitizing in an exothermic atmosphere. It was recommended that steps be taken during heat treatment to ensure neither of the two occurred.

A resistance-welded carbon steel superheater tube made to ASME SA-276 specifications failed by pitting corrosion and subsequent perforation, which caused the tube to leak. The perforation was found to have occurred at a low point in a bend near the superheater outlet header. It was found that the low points of the superheater tubes could not be completely drained during idle periods. Water-level marks were noticed on the inside surface above the area of pitting. It was revealed by microscopic examination that localized pitting had resulted from oxidation. It was concluded that water contained in the tube during shutdowns had accumulated and cumulative damage due to oxygen pitting resulted in perforation of one of the tubes. Filling the system with condensate or with treated boiler water was suggested as a corrective action. Alkalinity was suggested to be maintained at a pH of 9.0 and 200 ppm of sodium sulfite should be added to the water.

Six transformer brackets failed in service, sending a group of three pole-mounted transformers to the ground below. The brackets were made from acrylonitrile-butadiene-styrene (ABS) resin and had been in service for more than 30 years. Remnants of the fractured brackets were analyzed using optical and scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The exterior surfaces of all six brackets were alike and shared similar features, including witness marks, discoloration, mechanical deformation, and secondary cracking, along with crack networks. Both FTIR and TGA analyses indicated that the surface material was in a highly degraded state, likely due to weathering and thermal and ultraviolet exposure. This, in turn, led to the formation of cracks that propagated under the cyclic forces of vibration and wind. As the cracks grew larger, the weight of the transformer eventually overloaded the brackets, resulting in failure.

A nozzle in a wastewater vaporizer began leaking after approximately three years of service with acetic and formic acid wastewaters at 105 deg C (225 deg F) and 414 kPa (60 psig). The shell of the vessel was weld fabricated from 6.4 mm (0.25 in.) E-Brite stainless steel plate and measured 1.5 m (58 in.) in diameter and 8.5 m (28 ft) in length. Investigation (visual inspection, chemical analysis, radiography, dye-penetrant inspection, and hydrostatic testing of all E-Brite welds, 4x images, 100x/200x images electrolytically etched with 10% oxalic acid, and V-notch Charpy testing) supported the conclusion that failure of the nozzle weld was the result of intergranular corrosion caused by the pickup of interstitial elements and subsequent precipitation of chromium carbides and nitrides. Carbon pickup was believed to have been caused by inadequate joint cleaning prior to welding. The increase in the weld nitrogen level was a direct result of inadequate argon gas shielding of the molten weld puddle. Two areas of inadequate shielding were identified: improper gas flow rate for a 19 mm (0.75 in.) diam gas lens nozzle, and contamination of the manifold gas system. Recommendations included changes in the cleaning and welding process.

The self-powered flux detectors used in some nuclear reactors are Pt or V-cored co-axial cables with MgO as an insulator and Inconel 600 as the outer sheath material. The detectors are designed to operate in a He atmosphere; to maximize the conduction of heat (generated from the interaction with gamma radiation) and to prevent corrosion. A number of failures have occurred over the years because of a loss of the He cover gas in the assembly. This has resulted in either acid attack on the Inconel 600 sheath in a wet environment or gaseous corrosion in a dry environment. In the latter case, nitriding and embrittlement occurred at temperatures as low as 300 to 400 deg C (determined from an examination of the oxidation of the Zircaloy-2 carrier rod on which the detectors were mounted). Recent results are described and discussed in terms of the oxidation and nitriding kinetics of Zircaloy-2 and Inconel 600, respectively.

Initially, two vertical double-acting two-stage compressors delivering chlorine gas at a pressure of 100 psi appeared to be running satisfactorily. About six months later the LP piston-rod of the No. 2 compressor failed due to burning, the compressor being worked double-acting at the time. About five months later, the HP piston rod of the No. 1 compressor failed in a similar manner. Specimens for microscopic examination were cut from the rod in the region of the failure and from the extreme end that had been situated above the piston and hence not subjected to an appreciable rise in temperature. The material was a steel in the normalized condition with a 0.35% C content. It appears probable that deficient lubrication of the gland resulted in overheating of the rod due to friction. The presence of a sprayed-metal coating was probably an additional factor in promoting failure, as it would present to the gas a surface area considerably greater than that of a homogeneous material.

A 76 mm (3 in.) type 304 stainless steel tube that was used as a heat shield and water nozzle support in a hydrogen gas plant quench pot failed in a brittle manner. Visual examination of a sample from the failed tube showed that one lip of the section was eroded from service failure, whereas the opposite side exhibited a planar-type fracture. Sections were removed from the eroded area and from the opposite lip for microscopic studies and chemical analysis. The eroded edges exhibited river bed ditching, indicative of thermal fatigue. Microstructural analysis showed massive carbide formations in a martensite matrix and outlining of prior-austenite grains by a network of fine, white lines. These features indicated that the material had been transformed by carburization by the impinging gas. The outer surface exhibited a heavy scale deposit and numerous cracks that originated at the surface of the tube. The cracks were covered with scale, indicating that thermal fatigue (heat cracking) had occurred. Chemical analysis confirmed that the original material was type 304 stainless steel that had been through-carburized by the formation of an endothermic gas mixture. It was recommended that plant startup and shutdown procedures be modified to reduce or eliminate the presence of the carburizing gas mixture.

This article reviews analytical techniques that are most often used in plastic component failure analysis. The description of the techniques is intended to familiarize the reader with the general principles and benefits of the methodologies, namely Fourier transform infrared spectroscopy, energy-dispersive x-ray spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. The article describes the methods for molecular weight assessment and mechanical testing to evaluate plastics and polymers. The descriptions of the analytical techniques are supplemented by a series of case studies to illustrate the significance of each method. The case studies also include pertinent visual examination results and the corresponding images that aided in the characterization of the failures.

Several stainless steel coils cracked during a routine unwinding procedure, prompting an investigation to determine the cause. The analysis included optical and scanning electron microscopy, energy-dispersive x-ray spectrometry, and tensile testing. An examination of the fracture surfaces revealed a brittle intercrystalline mode of fracture with typical manifestations of clear grain facets. Branched and discrete stepwise microcracks were also found along with unusually high levels of residual hydrogen. Mechanical tests revealed a marked loss of tensile ductility in the defective steel with elongations barely approaching 8%, compared to 50% at the time of delivery weeks earlier. Based on the timing interval and the fact that failure occurred at operating stresses well below the yield point of the material, the failure is being attributed to hydrogen-induced damage. Potential sources of hydrogen are considered as are remedial measures for controlling hydrogen content in steels.

A precipitation-hardened stainless steel poppet valve assembly used to shut off the flow of hydrazine fuel to an auxiliary power unit was found to leak. SEM and optical micrographs revealed that the final heat treatment designed for the AM-350 bellows material rendered the AM-355 poppet susceptible to intergranular corrosive attack (IGA) from a decontaminant containing hydroxy-acetic acid. This attack provided pathways for which fluid could leak across the sealing surface in the closed condition. It was concluded that the current design is flight worthy if the poppet valve assembly passes a preflight helium pressure test. However a future design should use the same material for the poppet and bellows so that the final heat treatment will produce an assembly not susceptible to IGA.

A ring-type joint in a reactor pipeline for a hydrocracker unit had failed. Cracks were observed on the flange and the associated ring gasket during an inspection following a periodic shutdown of the unit. The components were manufactured from stabilized grades of austenitic stainless steel; the flange from type 321, and the ring gasket from 347. Examination revealed that the failure occurred by transgranular stress-corrosion cracking, initiated by the presence of polythionic acid. Detailed metallurgical investigation was subsequently conducted to identify what may have caused the formation of polythionic acid in the process gas.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

Corrosion in a closed-loop cooling water system constructed of austenitic stainless steel occurred during an extended lay up of the system with biologically contaminated water. The characteristics of the failure were those of microbiologically influenced corrosion (MIC). The corrosion occurred at welds and consisted of large subsurface void formations with pinhole penetrations of the surfaces. Corrosive attack initiated in the heat affected zones of the welds, usually immediately adjacent to fusion lines. Stepwise grinding, polishing, and etching through the affected areas revealed that voids generally grew in the wrought material by uniform general corrosion. Tunneling or worm-holing was also observed, whereby void extension occurred by initiating daughter voids probably at flaws or other inhomogeneities. Selective attack occurred within the fusion zone, i.e., within the cast two-phase structure of the weld filler itself. The result was a void wall which consisted of a rough and porous ferritic material, a consequence of preferential attack of the austenitic phase and slightly lower rate of corrosive attack of the ferrite phase. The three-dimensional spongy surface was studied optically and with the scanning electron microscope.

This article reviews the analytical techniques most commonly used in plastic component failure analysis. These include the Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and dynamic mechanical analysis. The descriptions of the analytical techniques are supplemented by a series of case studies that include pertinent visual examination results and the corresponding images that aid in the characterization of the failures. The article describes the methods used for determining the molecular weight of a plastic resin. It explains the use of mechanical testing in failure analysis and also describes the considerations in the selection and use of test methods.

A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, energy dispersive x-ray analysis (EDXA) indicated significant segregation of sulfur and chlorine along the grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O 2 that entered the tube during shutdowns and startups. Residual stresses, originating from in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries made the material more susceptible to cracking.

On 21 April 1995, the contents of a large blender (6 cu m) reacted and caused an explosion that killed and injured a number of workers at a plant in Lodi, NJ. A mixture of sodium hydrosulfite and aluminum powder was being mixed at the time of the accident. This report focuses on evaluations of the blender to determine if material or mechanical failures were the cause of the accident. The results indicate that the mixing vessel was metallurgically sound and did not contribute to the initiation of the failure. However, the vessel was not designed for mixing chemicals that must be isolated from water and excessive heat. Water leaking into the vessel through a graphite seal may have initiated the reactions that caused the accident.