Banding wires of the rotor of an 1800 hp motor were renewed following replacement of the banding rings. After about six months of service, a breakdown occurred due to bursting of the banding wires in several places. The 0.064 in. diam wire was nonmagnetic and of the 18/8 Cr-Ni type of austenitic stainless steel. The fractures were short and partially crystalline, with no evidence of slowly developing cracks of the fatigue type. Microscopical examination of sections taken through the fractures showed the cracking to be of the multiple branching type. Because the material was in the heavily cold-worked condition, it was not possible to determine with certainty if the cracks were of the inter- or trans-granular type. It was concluded that failure was due to stress-corrosion cracking in a chloride environment. Failure of the wires was likely due to the use of a chloride-containing flux during the soldering operation.

Failure analysis was performed on threaded Ti-6Al-4V fasteners that had fractured in the threads during installation. Scanning electron microscopy (SEM) and optical metallography revealed that the fractures initiated in circumferential shear bands present at the thread roots. The fractures propagated by microvoid coalescence typical of that observed in notched tensile specimen fractures of the same material. For comparison, Ti-6Al-4V fasteners from various commercial sources were tested to failure in uniaxial tension and examined in the SEM. In all cases, the fracture appearances were similar to that exhibited by the fasteners that failed during installation. In addition, results of optical microscopy indicated that the geometry and extent of the shear bands appeared to depend on the fabrication process employed by the individual manufacturers. Causes of shear band formation are discussed along with potential methods to eliminate these microstructural in homogeneities.

A bomb retaining ring fabricated from type 302 stainless steel unwrapped during a practice flight, causing the bomb fins to deploy. The retaining ring was able to unwrap itself because it was thinner and softer than required. Hardness testing, metallography, and tensile testing confirmed that the component was in the annealed condition and not in the required work-hardened 1/4-hard condition.

The connecting end of two forged medium-carbon steel rods used in an application in which they were subjected to severe low-frequency loading failed in service. The fractures extended completely through the connecting end. The surface hardness of the rods was found to be lower than specifications. The fractures were revealed to be in areas of the transition regions that had been rough ground to remove flash along the parting line. The presence of beach marks, indicating fatigue failure, was revealed by examination. The fracture origin was confirmed by the location and curvature of beach marks to be the rough ground surface. An incipient crack 9.5 mm along with several other cracks on one of the fractured rods was revealed by liquid penetration examination. Metallographic examination of the fractured rods indicated a banded structure consisting of zones of ferrite and pearlite. It was established that the incipient cracks found in liquid-penetrant inspection had originated at the surface in the banded region, in areas of ferrite where this constituent had been visibly deformed by grinding. Closer control on the microstructure, hardness of the forgings and smooth finish in critical area was recommended.

Two failures of AP15A grade J-55 electric resistance welded (ERW) tubing in as our gas environment were investigated. The first failure occurred after 112 days of service. Replacement pipe failed 2 days later. Surface examination of the failed tubing indicated that fracture initiated at the outside surface. Metallographic analysis showed that the fracture originated in the upturned fibers adjacent to the ERW bond line. Cross sections of the weld were removed from three random locations in the test sample. At each location, the up turned fibers of the weld zone contained bands of hard-appearing microstructure. Hardness measurements confirmed these observations. The cracks followed these bands. It was concluded that the tubing failed from sulfide stress cracking, which resulted from bands of susceptible microstructure in the ERW zone. The banded microstructure in the pipe suggested that chemical segregation contributed to the hard areas. Postweld normalized heat treatment apparently did not sufficiently reduce the hardness of these areas.

A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F). The outer diameter of the tube exhibited a crack (fissure) oriented approximately 45 deg to the longitudinal axis and 3.8 cm (1.5 in.) long. The inner diameter surface showed a fissure in the same location and orientation. Microstructure at the failure near the outer diameter surface exhibited evidence of creep cracking and creep void formation at the fissure. A nearly continuous band of graphite nodules was observed on the surface of the fissure. In addition to the graphite band formation, the microstructure near the failure exhibited carbide spheroidization from long-term overheating in all the tube regions examined. It was concluded that preferential nucleations of graphite nodules in a series of bands weakened the steel locally, producing preferred fracture paths. Formation of these graphite bands probably expedited the creep failure of the tube. Future failures may be avoided by using low-alloy steels with chromium additions such as ASTM A213 Grade T11 or T22, which are resistant to graphitization damage.

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 steel lifting eye, manufactured from grade 1144 steel, failed during service. The eye ring fractured in two places, adjacent to the threaded shank and diametrically opposite to this region. Woody overload features, typical for resulfurized steels were revealed by SEM. The directionality of the features was found to be suggestive of shear overload. It was observed that fracture preferentially followed the nonmetallic inclusions. The fracture was revealed to be parallel to the direction of the manganese sulfide stringer inclusions. The presence of significant banding of the ferrite and pearlite microstructure was revealed by etching. It was also observed that the fracture is primarily along the inclusions and through bands of ferrite. It was concluded that the lifting eye failed as a result of overload. Fracture occurred parallel to the rolling direction, through manganese-sulfide stringers and ferrite bands in the base metal matrix. The material used for this application was very anisotropic, exhibiting substantially poorer long and short transverse mechanical properties than longitudinal properties.