Postflight inspection of a gas-turbine aircraft engine that had experienced compressor stall revealed that the engine air-intake bullet assembly had dislodged and was seated against the engine-inlet guide vanes at the 3 o'clock position. The bullet assembly consisted of an outer aerodynamic shell and an inner stiffener shell, both of 1.3 mm (0.050 in.) thick aluminum alloy 6061-T6, and four attachment clips of 1 mm (0.040 in.) thick alclad aluminum alloy 2024-T42. Each clip was joined to the outer shell by 12 spot welds and was also joined to the stiffener. Analysis (visual inspection, dye-penetrant inspection, and 10x/150x micrographs of sections etched with Keller's reagent) supports the conclusion that the outer shell of the bullet assembly separated from the stiffener because the four attachment clips fractured through the shell-to-clip spot welds. Fracture occurred by fatigue that initiated at the notch created by the intersection of the faying surfaces of the clip and shell with the spot weld nuggets. The 6061 aluminum alloy shell and stiffener were in the annealed (O) temper rather than T6, as specified. Recommendations included heat treating the shell and stiffener to the T6 temper after forming.

Linear indications on the outer surface of a cross in a piping system were revealed by dye-penetrant examination. The cross was specified to be SA403 type WP 304 stainless steel. The cross had been subjected to induction-heating stress improvement. The linear indications on the cross were located in wide bands running circumferentially below the cross-to-cap weld and above the cap-to-discharge-pipe weld. The material was found to conform to the requirements both in terms of hardness and strength. Intergranular cracks filled with oxide were observed on metallographic analysis of a sectioned and oxalic acid etched sample. The grain size was found to exceed the ASTM standard. No indications of sensitization were observed during testing with practice A of ASTM A 262. Definitive evidence of contaminants to support SCC as the failure mechanism was not disclosed during analysis. It was concluded that overheating or burning of the forging, which classically results in large grain size, intergranular fractures, and fine oxide particles dispersed throughout the grains was the possible reason for the failure.

A number of machined end frame steel forgings made of Cr-Si-Mn alloy showed tiny cracks during magnetic particle inspection after heat treatment. The cracks were mostly confined to base edges and fillet radius. No significant abnormality was observed in chemical composition and microstructure. SEM, optical microscopy, and gas analysis revealed that the subsurface discontinuous cracks at the bore edges and in the fillet radius of the heat-treated end frame component had occurred due to hydrogen embrittlement, and not because of faulty heat treatment. This conclusion was supported by the presence of cracklike indications in machined bore surface of the annealed part.

Deformation, surface cracking, and spalling on the raceway of the outer ring (made of 4140 steel) of a large bearing caused it to be replaced from a radar antenna. The raceway surfaces were to be flame hardened to 55 HRC minimum and 50 HRC 3.2 mm below the surface, according to specifications. Samples from both the inner and outer rings were examined. A much lower hardness (25.2 to 18.9 HRC) was indicated during a vertical traverse 4.1 cm from the outer surface of the outer ring while slightly lower hardness values (46.8 to 54.8 HRC) were seen on the hardness traverse on the inner ring raceway. The lower hardness values were attributed to improper flame hardening. It was confirmed by metallographic examination of a 3% nital etched sample that the inner ring (tempered martensite and ferrite) and the outer ring (ferrite, scattered patches of pearlite, and martensite) were not properly austenitized. Displacement of metal on the outer raceway was revealed by elongation of grain structure. It was concluded that the failure of the raceway surface was due to incomplete austenitization caused by the improper heat treatment during flame hardening process.

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.