A spherical carbon steel fixed-catalyst bed reactor, fabricated from French steel A42C-3S, approximately equivalent to ASTM A201 grade B, failed after 20 years of service while in a standby condition. The unit was found to contain primarily hydrogen at the time of failure. The vessel had a type 304 stainless steel shroud around the catalyst bed as protection against the overheating that was possible if the gas bypassed the bed through the refractory material. The failure was observed to have begun at the toe of the shroud-support ring weld. The ring was found to have a number of small cracks at the root of the weld. The cleavage mode of fracture was confirmed by SEM. The presence of extensive secondary cracking and twinning (Neumann bands) where the fracture followed the line of the shroud-support ring was revealed by metallography. It was revealed by refinery maintenance records that the ring had been removed for hydrotest and welded without any postweld heat treatment. The final cause of failure was concluded to be cracking that developed during the installation of the new shroud ring. Stress-relief heat treatments were recommended to be performed to reduce residual-stress levels after welding.

Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning. The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% Cr ferritic stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the weld metal and heat-affected zones. The hardness in these regions was up to 35 HRC, and the weld had received a postweld heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature control during PWHT was the most likely cause of failure.

A valve stem made of 17-4 PH (AISI type 630) stainless steel, which was used for operating a gate valve in a steam power plant, failed after approximately four months of service, during which it had been exposed to high-purity water at approximately 175 deg C (350 deg F) and 11 MPa (1600 psi). The valve stem was reported to have been solution heat treated at 1040 +/-14 deg C (1900 +/-25 deg F) for 30 min and either air quenched or oil quenched to room temperature. The stem was then reportedly aged at 550 to 595 deg C (1025 to 1100 deg F) for four hours. Investigation (visual inspection, 0.7x/50x images, hardness testing, reheat treatment, and metallographic examination) supported the conclusion that failure was by progressive SCC that originated at a stress concentration. Also, the solution heat treatment had been either omitted or performed at too high of a temperature, and the aging treatment had been at too low of a temperature. Recommendations included the following heat treatments: after forging, solution heat treat at 1040 deg C (1900 deg F) for one hour, then oil quench; to avoid susceptibility to SCC, age at 595 deg C (1100 deg F) for four hours, then air cool.

The bolt in a bolt and thimble assembly used to connect a wire rope to a crane hanger bracket was worn excessively. Two worn bolts, one new bolt, and a new thimble were examined. Specifications required the bolts to be made of 4140 steel heat treated to a hardness of 277 to 321 HRB. Thimbles were to be made of cast 8625 steel, but no heat treatment or hardness were specified. Analysis (visual inspection, hardness testing, and metallographic examination) supported the conclusion that the wear was due to strikingly difference hardness measurements in the bolt and thimble. Recommendations included hardening and tempering the bolts to the hardness range of 375 to 430 HRB. The thimbles should be heat treated to a similar microstructure and the same hardness range as those of the bolt. Molybdenum disulfide lubricant can be liberally applied during the initial installation of the bolts. A maintenance lubrication program was not suggested, but galling could be reduced by periodic application of a solid lubricant.

A pushrod made by inertia welding two rough bored pieces of bar stock installed in a mud pump fractured after two weeks in service. The flange portion was made of 94B17 steel, and the shaft was made of 8620 steel. It was disclosed by visual examination that the fracture occurred in the shaft portion at the intersection of a 1.3 cm thick wall and a tapered surface at the bottom of the hole. The fatigue crack was influenced by one-way bending stresses initiated at the inner surface and progressed around the entire inner circumference. A heavily decarburized layer was detected on the inner surface of the flange portion and sharp corner was found at the intersection of the sidewall and bottom of the hole. It was concluded that the stress raiser due to the abrupt section change was accentuated by decarburized layer. As a corrective measure, the design of the pushrod was changed to a one-piece forging and circulation of atmosphere during heat treatment was permitted through a hole drilled in the flange end of the rod to avoid decarburization.

A section from a stop-block guide fell to the floor on a crane runway after it failed. A brittle crystalline-type break was disclosed by examination of the fracture surface. The point of initiation was in a hardened heat-affected layer that had developed during flame cutting and welding. The metal was identified to be 1020 steel. It was indicated by the coarse as-rolled structure (grain size of ASTM 00 to 4) of the base metal that the weldment (stop block and guide) had not been normalized. The brittle failure was evaluated to have been initiated at a metallurgical and mechanical notch produced by flame cutting and welding. As corrective measures, fully silicon-killed 1020 steel with a maximum grain size of ASTM 5 were used to make new stop-block weldments. The weldments were normalized at 900 deg C after flame cutting and welding to improve microstructure and impact strength. All flame-cut surfaces were ground to remove notches.

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.

A hook on a two-leg chain (each 13 mm diam, included angle 60 deg) failed at the junction of the eye and shank while lifting a 4990 kg load. The diam of the hook at this junction was approximately 22 mm. Light intergranular oxidation at the surface on the side of the hook where cracking started was revealed by visual examination of the fracture region. Almost 50% of the fracture surface was found to contain beach marks (indicative of fatigue failure) while the remainder contained cleavage facets. A medium-coarse acicular as-forged structure was revealed by metallographic examination and the metal was showed by chemical analysis to be semikilled 1015 steel. The fatigue fracture was concluded to have initiated in the intergranular oxidation region and the failure of the hook was contributed by the poor fatigue and impact properties of the forged structure. As a corrective measure, the chain-sling hook was replaced with one made of normalized, fully killed, finegrain 1020 steel.

A hook on a two-leg chain (each 13 mm diam, included angle 60 deg) failed at the junction of the eye and shank while lifting a 4990 kg load. The diam of the hook at this junction was approximately 22 mm. Light intergranular oxidation at the surface on the side of the hook where cracking started was revealed by visual examination of the fracture region. Almost 50% of the fracture surface was found to contain beach marks (indicative of fatigue failure) while the remainder contained cleavage facets. A medium-coarse acicular as-forged structure was revealed by metallographic examination and the metal was showed by chemical analysis to be semikilled 1015 steel. The fatigue fracture was concluded to have initiated in the intergranular oxidation region and the failure of the hook was contributed by the poor fatigue and impact properties of the forged structure. As a corrective measure, the chain-sling hook was replaced with one made of normalized, fully killed, finegrain 1020 steel.

A 10,890-kg coil hook torch cut from 1040 steel plate failed while lifting a load of 13,600 kg after eight years of service. The normal ironing (wear) marks were exhibited by the inner surface of the hook. It was revealed by visual examination that cracking had originated at the inside radius of the hook. Beach marks (typical of fatigue fracture) were found extending over approximately 20% of the fracture surface. Numerous cracks were revealed by macroscopic examination of the torch-cut surfaces. It was revealed by macrograph of an etched specimen that the cracks had initiated in a hardened martensitic zone at the torch-cut surface and had extended up to the coarse pearlite structure beneath the martensitic zone. The fatigue fracture was concluded to have initiated in the brittle martensitic surface while failure was contributed by the 25% overload. As a corrective measure, the coil hooks were flame cut from ASTM A242 fine-grain steel plate, ground to remove the material damaged by flame cutting and stress relieved at 620 deg C.

Following disruption of the austenitic stainless steel basket of a hydro-extractor used for the separation of crystals of salt (sodium chloride) from glycerin, samples of the broken parts were analyzed. Examination revealed that the fish-plates joining the reinforcing hoops had broken, the shell had split from top to bottom adjacent to the weld, the top and bottom cover plates had become loose, all the rivets having pulled out, and the shaft was also found to be bent. Fracture took place in an irregular manner and was of the shear type towards both ends; it occurred immediately adjacent to the weld or a short distance from it and on alternate sides. Microscopical examination did not reveal any intergranular carbide precipitation, such as is well known to result in the weld-decay mode of failure. It was concluded that the primary cause of failure was stress-corrosion cracking arising from the combined effect of residual stresses and the corrosive effect of the material being centrifuged. If the shell had been stress-relieved after fabrication, the failure likely would not have occurred.

A large pressure vessel designed for use in an ammonia plant failed during hydrostatic testing. It was fabricated from ten Mn-Cr-Ni-Mo-V steel plates which were rolled and welded to form ten cylindrical shell sections and three forgings of similar composition. The fracture surfaces were metallographically examined to be typical for brittle steel fracture and associated with the circumferential weld that joined the flange forging to the first shell section. Featureless facets in the HAZ were observed and were revealed to be the fracture-initiation sites. Pronounced banding in the structure of the flange forging was revealed by examination. A greater susceptibility to cracking was interpreted from the higher hardenability found within the bands. Stress relief was concluded to have not been performed at the specified temperature level (by hardness and impact tests) which caused the formation of hard spots. The mode of crack propagation was established by microstructural examination to be transgranular cleavage. It was concluded that failure of the pressure vessel stemmed from the formation of transverse fabrication cracks in the HAZ fostered by the presence of hard spots. It was recommended that normalizing and tempering temperatures be modified and a revised forging practice explored.

A missile detached from a Navy fighter jet during a routine landing on an aircraft carrier deck because of a faulty missile launcher detent spring. Visual inspection of Inconel 718 detent spring assembly revealed that four of the nine spring leafs comprising the assembly were plastically deformed while two of the deformed leafs did not meet minimal hardness or tensile requirements. Liquid penetrant testing revealed no cracks or other surface discontinuities on the leaf springs. Material sectioned from the soft spring leafs was heat-treated according to specifications in the laboratory. The resultant increase in mechanical properties of the re-heat-treated material indicated that the original heat treatment was not performed correctly. The failure was attributed to improper heat treatment. Recommendations focused on more stringent quality control of the heat-treat operations.

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.

Two 25 x 40 mm (1 x 1.5 in.) AISI 4150 hot-rolled steel bars that cracked during heat treatment were examined to determine whether the heat treating procedure had contributed to the failure. Metallographic examination of a cross section taken through the fracture revealed an oxide coating on both sides of the fracture surface. The oxide was also found on the top and bottom sides of the sample. Sawcut sides of the bar did not exhibit the oxide layer The presence of the oxide in the fracture, combined with its absence on all exterior surfaces, indicated that the fracture occurred as a result of an oxide seam in the original material rather than from oxide from heat treating. Nondestructive testing prior to machining and heat treatment was recommended.

Cracks were discovered in the cast 17-4 PH stainless steel outboard leading edge flap support of an aircraft wing during overhaul inspection. Failure analysis focused on an apparently intergranular area of fracture surface. It was determined that the original mode of crack growth was cleavage, probably caused by cast-in hydrogen. The intergranular appearance resulted from heat treatment of the already cracked part, which caused the formation of grain-boundary “growth figures” on the exposed crack surfaces. It was recommended that the castings be more closely inspected for defects before further processing and that foundry practices be reviewed to correct deficiencies leading to excessive hydrogen absorption during melting and casting.

Tube failures occurred in quick succession in two boiler units from a bank of six boilers in a refinery. The failures were confined to the SAE 192 carbon steel horizontal support tubes of the superheater pack. In both cases, the failure was by perforation adjacent to the welded fin on the crown of the top tubes and located in an area near the upward bend of the tube. The inside of all the tubes were covered with a loosely adherent, black, alkaline, powdery deposit comprised mainly of magnetite. The corroded areas, however, had relatively less deposit. The morphology of the corrosion damage was typical of alkaline corrosion and confirmed that the boiler tubes failed as a result of steam blanketing that concentrated phosphate salts. The severe alkaline conditions developed most probably because of the decomposition of trisodium phosphate, which was used as a water treatment chemical for the boiler feed water.

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.

Six wrist pins in a high-performance six-cylinder automotive engine failed after 4800 km (3000 mi) of normal operation. The pins were made of low-carbon steel that had been carburized both inside and outside. Two failed pins were examined. One had fractured into three pieces. The other had not fractured, but exhibited circumferential cracks on the surface of the central zone. Visual surface examination and metallographic and chemical analyses were performed on the specimens. Cracking was attributed primarily to poor heat treatment, resulting in a brittle grain-boundary network of cementite, and to a design that had a raised central section of the inner diameter whose fillets were locations of high stress concentration. Rough machining of the inner diameter and an excessively deep case also contributed to failure. A double type of heat treatment after carburizing and change of the design to eliminate the raised central section were recommended.

Gears in a strip mining dragline failed in service. The material was identified as a low-alloy (NiCrMoV) steel. SEM analysis indicated that the initial fracture and subsequent fractures resulted from impact or a suddenly applied load. Mechanical testing indicated that the gears had low impact strength. Failure was attributed to low toughness caused by the absence of, or improper, heat treatment. Casting defects identified during metallographic examination were determined to be the fracture initiation site, but were considered less significant than the low as-received impact strength of the material. It was recommended that the equipment manufacturer implement an appropriate heat treatment to meet the impact requirements of the application.