The fan used to cool a diesel engine fractured catastrophically after approximately 100 h of operation. The fan failed at a spider, which was resistance spot welded to a shim placed between two circular spiders of 3 mm thickness. The detailed analysis of the fracture indicated that the premature failure of the fan was due to inadequate bonding between the sheets at the weld nugget. The fracture was initiated from the nugget-plate interface. The inadequate penetration and lack of fusion between the steel sheets during resistance spot welding led to poor weld strength and the fracture during operation. The propensity to crack initiation and failure was accentuated by improper cleaning of the surfaces prior to welding and to inadequate nugget-to-sheet edge distance.

A forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

The conveyor chain link failed by fracturing through a fabrication weld after some time in service. The link was made of cast low-alloy steel heat treated to a hardness of 285 HRB. The fabrication weld was made using E7018 electrodes. The fracture surfaces were almost entirely covered with beach marks, indicating fatigue cracking. The beach marks emanated from a 6.4 mm (1/4 in.) wide band of unusual appearance running across the center of the fracture. Saw cuts were made perpendicular to both of the fracture surfaces and across the unusual-looking bands. The cut surfaces were ground and etched with ammonium persulfate solution revealing that complete penetration had not been achieved at the weld root. The band observed in the fracture was apparently part of the unfused weld-preparation surface at the root of the weld. This failure was caused by fatigue that initiated at an incomplete penetration defect at the root of the fabrication weld. As a result of this failure, steps were taken to ensure that good welding practice (back-gouging) would be followed in the future fabrication of these links.

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 falling film black liquor evaporator consisted of flat twin plate heat exchangers and was used to increase black liquor solids content prior to its burning in the recovery boiler. Several plate heat exchangers were fabricated of AISI type 316L stainless steel by electric resistance welding. Cracks initiated at the inside surface of the welded areas and penetrated through the wall thickness. In several locations, the weld fractured and the plates separated with significant spring back, indicative of high residual stresses attributed to fabrication and weld procedures. The cracks had extended radially from the electric resistant weld into the base metal. Metallographic examination revealed the cracks were transgranular and branching, characteristic of SCC in austenitic stainless steels. The fracture surfaces had a brittle cleavage-like appearance, typical of SCC in austenitic stainless steels. Chlorides in the service environment were a contributory factor. The primary factor causing SCC localized at the electric resistant welds was substantial residual stresses as a result of fabrication procedures. It was recommended that the heat exchanger plates be subjected to stress-relief heat treatment following fabrication and welding.

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.

A gas-fired, ASTM A-106 Grade B carbon steel vaporizer failed on three different occasions during attempts to bring the vaporizer on line. Dye penetrant examination indicated the presence of multiple packets of ductile cracks on the inside of the coil radius at the bottom of the horizontal axis coils. Visual examination of the inside of the tubing indicated the presence of a carbonaceous deposit resulting from decomposition of the heat-exchanging fluid. Subsequent metallographic examination and microhardness testing indicated that the steel was heated to a temperature above the allowable operating temperature for the fluid. The probable cause for failure is thermal fatigue due to the localized overheating. Flow conditions inside the tubing should be reexamined to ensure suitable conditions for annular fluid flow.

The horizontal heat-exchanger tubes made of copper alloy C70600, in one of two hydraulic-oil coolers in an electric power plant, leaked after 18 months of service. River water was used as the coolant in the heat-exchanger tubes. Several nodules on the inner surface and holes through the tube wall, which appeared to have formed by pitting under the nodules, were revealed by visual examination. Steep sidewalls, which indicated a high rate of attack, were revealed by microscopic examination of a section through the pit which had penetrated the tube wall. The major constituent of reddish deposit on the inner surfaces of the tubes was revealed to be iron oxide and slight manganese dioxide. Effluent from steel mills upstream was indicated by the presence of these and other constituents to be the source of most of the solids found in the tubes. It was concluded that the tubing failed by crevice corrosion. The tubing in the cooler was replaced, and cooling-water supply was changed from river to city water, which contained no dirt to deposit on the tube surfaces. An alternate solution of installing replacement tubes in the vertical position to make deposition of solids from river water less likely was suggested.

A heat exchanger made of a pipe in which oil was heated from the outside from approximately 90 deg C to 170 deg C, by superheated steam of about 8 to 10 atmospheres had developed a leak at the rolled joint of the pipe and pipe bottom. The pipes were supposed to be made from St 35.29 steel and annealed at the rolled joint to 100 mm length. The outer pipe surface was strongly pitted by corrosion all around the rolled joint. In the vicinity of the steam chamber the pipe wall had oxidized through from the exterior to the interior at one spot. Adjoining this spot, grooves caused by erosion were noticeable. This was a typical case of crevice corrosion. The rolled joint evidently was not entirely tight, so that saturated steam condensate could penetrate into the gap.

A 680,000 kg (750 ton) per day ammonia unit was shut down following a fire near the outlet of the waste heat exchanger. The fire had resulted from leakage of ammonia from the type 316 stainless steel outlet piping. The outlet piping immediately downstream from the waste heat exchanger consisted of a flange made from a casting, and a reducing cone, a short length of pipe, and a 90 deg elbow, all made of 13 mm thick plate. A liner wrapped with insulation was welded to the smaller end of the reducing cone. All of the piping up to the flange was wrapped with insulation. Investigation (visual inspection, 10x unetched images, liquid-penetrant inspection, and chemical analysis of the insulation) supported the conclusion that the failure occurred in the area of the flange-to-cone weld by SCC as the result of aqueous chlorides leached from the insulation around the liner by condensate. Recommendations included eliminating the chlorides from the system, maintaining the temperature of the outlet stream above the dewpoint at all times, or that replacing the type 316 stainless steel with an alloy such as Incoloy 800 that is more resistant to chloride attack.

A fuel-nozzle-support assembly showed transverse indications after fluorescent liquid-penetrant inspection of a repair-welded area at a fillet on the front side of the support neck adjacent to the mounting flange. Visual examination disclosed an irregular crack. The crack through the neck was sectioned; examination showed that the crack had extended through the repair weld. The crack had followed an intergranular path. The crack was opened, and binocular-microscope examination of the fracture surface showed that the surface contained dendrites with discolored oxide films that were typical of exposure to air when very hot. Several additional subsurface cracks, typical of hot tears, were observed in and near the weld. There had been too much local heat input in making the repair weld. The result was localized thermal contraction and hot tearing. The cracking of the repair weld was attributed to unfavorable welding practice that accentuated thermal contraction stresses and caused hot tearing. Recommendations involved use of a small-diameter welding electrode, a lower heat input, and deposition in shallow layers that could be effectively peened between passes to minimize internal stress.

A steel socket pipe conduit NW 150 cracked open during pressure testing next to the weld seam almost along the entire circumference. The crack occurred in part in the penetration notch and in part immediately adjacent to it. While the uncracked pipe showed the light etch shading of a low-carbon steel in which the zone heated during welding was delineated only slightly next to the seam, the other pipe was etched much darker, i.e., higher in carbon, and the heated zone appeared to stand out darkly against the basic material. The overlapping weld was defect-free and dense. The uncracked pipe consisted of soft steel that obviously was made for this purpose, while the cracked pipe consisted of a strongly-hardenable steel which contained not only more carbon and manganese than customary but also a considerable amount of chromium. Therefore, the damage was caused by a mix-up of materials that allowed an unsuitable steel to be used for the weldment.

During 5.7 years of service, dye penetrant inspection of Inconel 800H pigtail connections regularly showed cracks at weld toes. Weld repairs were not able to prevent reoccurrence but often aggravated the condition. Samples containing small, but detectable, reducer-to-pigtail cracks showed intergranular cracks originating at weld toes and filled with oxidation product, which precluded determination of the cracking mechanism. All weldments exhibited high degrees of secondary precipitates, with original fabrication welds exhibiting higher apparent levels than repair welds. SEM/EDS analysis showed base metal grain boundary precipitates to be primarily chromium carbides, but some titanium carbides were also observed. Failure was believed to result from the synergism of thermally driven tube distortion, which resulted in over-stress, and from the intergranular oxidation products and intergranular carbides which contributed to cracking. It was recommended that stresses be reduced and /or that materials and components be changed. Refinements in welding procedures and implementation of preweld/postweld heat treatments were recommended also.

Five cylinders out of a group of nine in a drying machine developed leaks after a few months service in a textile mill. Leakage was reported from locations between the hoop and body and from the circumferential welds. The materials in the affected area were 18/8 Ti and 18/10/3/Mo austenitic stainless steels. Examination of the cracks at high magnification revealed them to be of the stress-corrosion type. The welds were of satisfactory quality. Cracking was also visible at these locations, this again being of the stress corrosion type. The method of cylinder construction introduced a crevice between the outer hoop and the cylinder at the inboard edge so that during washing of the rolls, water could penetrate the crevice and subsequent heating would lead to the concentration of chlorides within the crevice. Redesign of the cylinder to eliminate the crevice was recommended.

An E-Brite /Ferralium explosively bonded tube sheet in a nitric acid condenser was removed from service because of corrosion. Visual and metallographic examination of tube sheet samples revealed severe cracking in the heat-affected zone between the outer tubes and the weld joining the tube sheet to the floating skirt. Cracks penetrated deep into the tube sheet, and occasionally into the tube walls. The microstructures of both alloys and of the weld appeared normal. Intergranular corrosion characteristic of end-grain attack was apparent. A low dead spot at the skirt / tube sheet joint allowed the Nox to condense and subsequently reboil. This, coupled with repeated repair welding in the area, reduced resistance to acid attack. Intergranular corrosion continued until failure. Recommendations included changing operating parameter inlet to prevent HNO3 condensation outside the inlet and replacement of the floating skirt with virgin material (i.e., material unaffected by weld repairs).

Liquid penetrant inspection of an ASTM A296 grade CA-15 residual heat removal pump impeller from a nuclear plant revealed a crack like indication that approximated the outer contour of the wear ring. Examination of a section containing the crack and three sections from near the main crack indication revealed that the failure was caused by hot cracking related to original weld repairs performed on the impeller casting.

Several pressurized air containers (i.e., diving tanks) made of non-heat-treatable Al-5Mg aluminum alloy failed catastrophically. Catastrophic failure occurred when a subcritical stress corrosion crack reached a critical size. Critical crack size for unstable propagation was reached prior to wall penetration, which could have led to subsequent loss of pressure, resulting in explosion of the cylinder. It was recommended that more stress corrosion resistant alloys be used for sea diving applications. Furthermore, cylinders should have a reduced wall thickness that can be determined employing the “leak-before-break” design philosophy, developed using fracture mechanics, to eliminate the possibility of catastrophic ruptures.

The metallurgical condition of a cylindrical induction melter (CIM) vessel was evaluated after approximately 375 h of operation over a two-year span at temperatures between 1400 to 1500 deg C. Wall thinning and significant grain growth was observed in the lower portion of the conical section and the drain tube. No through-wall penetrations were found in the cylindrical and conical sections of the CIM vessel and only one leak site was identified in the drain tube. Failure of the drain tube was associated with localized overheating and creep. The observed degradation resulted from cumulative service at elevated temperature. A recommendation was made to implement a support for the conical section of the CIM and to increase the wall thickness of the drain tube. Thus, the possibility of drain tube misalignment in the induction coils and localized over heating will be minimized. In addition, the use of grain stabilized Pt/Rh alloy should be evaluated as a method to prevent grain growth.

A riveted 0.25% carbon steel oil-storage tank in Oklahoma was dismantled and reassembled in Minnesota by welding to form a storage tank for soybean oil. An opening was cut in the side of the tank to admit a front-end loader. A frame of heavy angle iron was welded to the tank and drilled for bolting on a heavy steel plate. The tank was filled to a record height. In mid-Jan the temperature dropped to -31 deg C (-23 deg F), with high winds. The tank split open and collapsed. The welding used the shielded metal arc process with E6010 electrodes, which could lead to weld porosity, hydrogen embrittlement, or both. At subzero temperatures, the steel was below its ductile-to-brittle transition temperature. These circumstances suggest a brittle condition. Steps to avoid this type of failure: For cold conditions, the steel plate should have a low carbon content and a high manganese-to-sulfur ratio and be in a normalized condition, low-hydrogen electrodes and welding practices should be used, all corners should be generously radiused, the welds should be inspected and ground or dressed to minimize stress concentrations, postweld heating is advisable, and radiographic and penetrant inspection tests should be performed.

One tube in a watertube boiler developed leakage from a perforation. The external surface was covered with a dark deposit indicative of local fusion. Perforation resulted from the development of a crack from the internal surface. Microscopic examination revealed extensive intergranular penetration by molten copper. Particles of copper were seen in scale deposits on the bore of the tube. The tube in general showed a ferritic structure with partially spheroidized carbide. The fact that fusion of the copper had occurred indicated temperatures of 1100 deg C (2012 deg F) had been experienced locally, and the structural condition suggested that the tube in general had been heated at a lower temperature of the order of 600 deg C (1112 deg F) for some appreciable time. In this instance, overheating of the tube in the absence of the copper deposits may not have led to failure.