Search Results for Fittings

During a routine inspection, cracks were discovered in several aluminum alloy (similar to either 2014 or 2017) coupling nuts on the fuel lines of a missile. The fuel lines had been exposed to a marine atmosphere for six months while the missile stood on an outdoor test stand near the seacoast. A complete check was then made, both visually and with the aid of a low-power magnifying glass, of all coupling nuts of this type on the missile. Investigation (visual inspection, spectrographic and chemical analysis, and metallographic examination) supported the conclusion that the cracking of the aluminum alloy coupling nuts was caused by stress corrosion. Contributing factors included use of a material that is susceptible to this type of failure, sustained tensile stressing in the presence of a marine (chloride-bearing) atmosphere, and an elongated grain structure transverse to the direction of stress. The elongated grain structure transverse to the direction of stress was a consequence of following the generally used procedure of machining this type of nut from bar stock. Recommendations included changing the materials specification for new coupling nuts for this application to permit use of only aluminum alloys 6061-T6 and T651 and 2024-T6, T62, and T851.

A neck fitting (cast equivalent of AISI type 317) exhibited extreme corrosion with large, deeply pitted areas. It had been in service in a sulfite digester at 140 deg C (285 deg F) and 689 kPa (100 psi). The liquor was calcium bisulfite, and chloride content was reported to be low. Investigation (visual inspection, and micrographs of sections with electrolytic etching using 10 N KOH and then again after re-polishing and etching with Murakami's reagent) supported the conclusions that the casting never received a proper solution anneal. Recommendations included possible corrosion-screening tests in accordance with ASTM A 262 to ensure adequate corrosion resistance.

Wet natural gas was dried by being passed through a carbon steel vessel that contained a molecular-sieve drying agent. The drying agent became saturated after several hours in service and was regenerated by a gas that was heated to 290 to 345 deg C in a salt-bath heat exchanger. The tee joint in the piping between the heat exchanger and the sieve bed failed after 12 months. A hole in the tee fitting and a corrosion product on the inner surface of the pitting was revealed by visual examination. Iron sulfide was revealed by chemical analysis of the scale which indicated hydrogen sulfide attack on the carbon steel. The presence of oxygen was indicated by the carbon and sulfur found in the scale on the piping and in the sieves indicated that oxygen combined with moisture produced conditions for attack of hydrogen sulfide on carbon steel. Turbulence with some effect from the coarse grain size was interpreted to have contributed. The piping material was changed from carbon steel to AISI type 316 stainless steel as it is readily weldable and resistant to corrosion by hydrogen sulfide.

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 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.

A series of resistance spot welds joining Z-shape and C-shape members of an aircraft drop-tank structure failed during ejection testing. The members were fabricated of alclad aluminum alloy 2024-T62. The back surface of the C-shape members showed severe electrode-indentation marks off to one side of the spot weld, suggesting improper electrode contact. Visual examination of the weld fractures showed that the weld nuggets varied considerably in size, some being very small and three exhibiting an HAZ but no weld. Of 28 welds, only nine had acceptable nugget diameters and fusion-zone widths. The weld deficiencies were traced to problems in forming and fit-up of the C-shape members and to difficulties in alignment and positioning of the weld tooling. The failure of the resistance spot welds was attributed to poor weld quality caused by unfavorable fit-up and lack of proper weld-tool positioning. The problem could be solved by better forming procedures to provide an accurate fit-up that would not interfere with electrode alignment.

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.

Several failures occurred in 64-mm schedule 80 type 304 stainless steel (ASME SA-312, grade TP304) piping in a steam-plant heat-exchanger system near tee fittings at which cool water returning from the heat exchanger was combined with hot water from a bypass. Various portions of the piping were subjected to temperatures ranging from 29 to 288 deg C. Each of the failures were revealed to consist of transgranular cracking in and/or close to the circumferential butt weld joining the tee fitting to the downstream pipe leg, where the hot bypass water mixed with the cool return water. The transgranular cracks suggested that thermal fatigue was a more likely cause of failure than SCC. It was concluded by temperature measurements that circumferential temperature gradients, in combination with inadequate flexibility in the piping system as a whole, had caused the failures. The tee fitting was redesigned to alleviate the thermal stress pattern.

Resistance spot welds joining aluminum alloy 2024-T8511 stiffeners to the aluminum alloy 6061-T62 skin of an aircraft drop tank failed during slosh and vibration testing. Visual examination of the fracture surfaces showed that the failure was by tensile or bending overload. Measurements of the fractured spot welds established that all welds were below specification size. Review of the assembly procedures revealed that there had been poor fit-up between the stiffeners and the tank skin, which resulted in weak, undersize weld nuggets. The spot welds failed because of undersize nuggets that were the result of shunting caused by poor fit-up. The forming procedures were revised to achieve a precise fit between the stiffener and the tank wall. Also, an increase in welding current was suggested.

After a quick-release fitting of an ejection seat broke, an investigation was performed to determine the manner and cause of crack propagation. Most fractography-based investigations aim to characterize only qualitative characteristics, such as the fracture orientation and origin position, topology, and details of interactions with microstructural features. The aim of this investigation was to use quantitative fractography as a tool to extract information, including striation spacing and size of the stretched zone, in order to make a direct correlation with fracture mechanic concepts. As the crack propagated, striations were created on the fracture surface as a result of service-induced load changes. The size of the striations were measured to estimate crack propagation rate. Remaining lifetime estimates were also made. The dimensions of plastically stretched zones found at the tips of the cracks were evaluated using electron micrograph stereo image pairs to characterize local fracture toughness. To complete the failure analysis, nondestructive evaluation, metallographic examination, and chemical investigations were carried out. No secondary cracks could be found. Most of the broken parts showed that the microstructure, the hardness, and the chemical composition of the Al-alloy were within the specification, but some of the cracked parts were manufactured using a different material than that specified.