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.

A valve-seat retainer spring (made of 0.23 mm thick 17-7 PH stainless steel) from a fuel control on an aircraft engine was found to be broken after 3980 h of service. The two inner tabs were found to be broken off. The part was revealed to be in relative rotation against its contacting member by the radial wear marks on the convex surface. Beach marks indicating that fatigue fracture had been initiated at the convex surface of the washer and had propagated across to the concave surface were revealed by examination of the fractured surfaces of the washer. The cracks were revealed to have originated in the 0.38-mm radius fillet between the tab and the body of the washer. It was interpreted from the analysis of the compound fracture that it was composed of fatigue fractures caused by the formed tab being loaded so as to compress the spring along the axis of its centerline and produce torsional vibrations. It was concluded that the two inner tabs had broken in fatigue as the result of cyclic loading that compressed and torsionally vibrated the spring. The fillets were replaced with slots to minimize stress concentration at the corners as a corrective measure.

The governor on an aircraft engine failed and upon disassembly of the unit, it was discovered that the retainer for the flyweight pivot pins was broken. The channel-shaped retainer was made of 0.8 mm (0.030 in.) thick 1018 or 1020 steel. The part was plated with copper, which acted as a stop-off during carburizing of the offset, circular thrust-bearing surface surrounding the 16-mm (0.637-in.) diam hole. The bearing surface was case hardened to a depth of 0.05 to 0.1 mm (0.002 to 0.005 in.), then austempered to obtain a minimum hardness of 600 Knoop (1-kg, or 2.2-lb, load). Considerable vibration was created in the installation because of the design of the mechanical device used to transmit power to the governor. The pins were permitted to slide axially a small distance. Analysis (visual inspection, microscopic examination, and ductility measurements) supported the conclusion that failure of the retainer was the result of fatigue caused by vibration in the flyweight assembly. Impact of the pivot pins on the retainer also contributed to failure. Recommendations included redesign of the flyweight assembly, and replacement of the channel-shaped retainer with a spring-clip type of pin retainer.

An impact breaker bar showed signs of rapid wear. The nominal composition of this chromium alloy cast iron was Fe-2.75C-0.75Mn-0.5Si-0.5Ni-19.5Cr-1.1Mo. The measured hardness of this bar was 450 to 500 HRB. The desired hardness for this material after air hardening is 600 to 650 HRB. The microstructure consisted of eutectic chromium carbides (Cr7C3) in a matrix of retained austenite and martensite intermingled with secondary carbides. Analysis (visual inspection and 500x view of sections etched with Marble's reagent) supported the conclusion that the low hardness resulted from an excessive amount of retained austenite. This caused reduced wear resistance and thus rapid wear in service. Recommendations included avoiding an excessive austenitizing temperature and excessive cooling rates from the austenitizing temperature and controlling the chemical composition to avoid excessive hardenability for the section size involved.

A shrunk-fit 18 Mn-5Cr steel retaining ring failed without warning during normal unit operation of a 380 MW electrical generator. The cause of the ring failure was determined to be intergranular stress-corrosion cracking (IGSCC) because of the high strength of the ring material and the presence of moist hydrogen used to cool the ring. Factors which promoted the failure were higher than normal strength levels in the ring material, lower than normal ring operating temperatures, possible moisture in the lubrication oil system, periodic poor performance of the hydrogen dryers, and a ring design which allowed water to become trapped in a relief groove. These factors caused pitting in the ring in an estimated 100 hours of operation. The ring had been inspected previously 18 months prior to the failure and no defects or pitting were found. Calculations showed that a 0.127-cm (0.050-in.) deep pit could grow to a critical size in 3000 to 4000 hours of operation. To prevent further failures, it was recommended that the ring be replaced with an 18 Mn-18Cr alloy with superior resistance to IGSCC. A program of periodic inspection and replacement of other retaining rings in the system was also recommended.

Two clevis-head self-retaining bolts used in the throttle-control linkage of a naval aircraft failed on the aircraft assembly line. Specifications required the bolts to be heat treated to a hardness of 39 to 45 HRC, followed by cleaning, cadmium electroplating, and baking to minimize hydrogen embrittlement. The bolts broke at the junction of the head and shank. The nuts were, theoretically, installed fingertight. The failure was attributed to hydrogen embrittlement that had not been satisfactorily alleviated by subsequent baking. The presence of burrs on the threads prevented assembly to finger-tightness, and the consequent wrench torquing caused the actual fractures. The very small radius of the fillet between the bolt head and the shank undoubtedly accentuated the embrittling effect of the hydrogen. To prevent reoccurrence, the cleaning and cadmium-plating procedures were stipulated to be low-hydrogen in nature, and an adequate post plating baking treatment at 205 deg C (400 deg F), in conformity with ASTM B 242, was specified. A minimum radius for the head-to-shank fillet was specified at 0.25 mm (0.010 in.). All threads were required to be free of burrs. A 10-day sustained-load test was specified for a sample quantity of bolts from each lot.

After less than 30 days in service, several cadmium-plated retaining rings, made of 4140 steel tubing and heat treated to 36 to 40 HRC, broke during operation that included holding components of a segmented fitting in place under a constant load. Photographic and 100x nital-etched micrographic examination showed a microstructure of tempered martensite with low inclusion content as well as a pit or burned spot on the outer area of the ring. The defect was approximately 0.18 mm (0.007 in.) deep and 0.5 mm (0.020 in.) in diam and had a hardness of 58 to 60 HRC. The base metal adjacent to the defect had a hardness of 36 to 40 HRC. Small cracks or fissures were also evident within the defect. Thus, the rings failed in brittle fracture as the result of an arc strike (or burn) on the surface of the ring. At the site of the arc strike, a small region of hard, brittle untempered martensite was formed as the result of an arc strike during the cadmium-plating operation. Fracture occurred readily when the ring was stressed. No recommendations were made.

Two freshwater tanks (0.81 mm (0.032 in) thick, type 321 stainless steel) were removed from aircraft service because of leakage due to pitting and rusting on the bottoms of the tanks. One tank had been in service for 321 h, the other for 10 h. There had been departures from the specified procedure for chemical cleaning of the tanks in preparation for potable water storage. The sodium hypochlorite sterilizing solution used was three times the prescribed strength, and the process exposed the bottom of the tanks to hypochlorite solution that had collected near the outlet. Investigation (visual inspection, 95x unetched images, chemical testing with a 5% salt spray, chemical testing with sodium hypochlorite at three strength levels, samples were also pickled in an aqueous solution containing 15 vol% concentrated nitric acid (HNO3) and 3 vol% concentrated hydrofluoric acid (HF) and were then immersed in the three sodium hypochlorite solutions for several days) supported the conclusion that failure of the stainless steel tanks by chloride-induced pitting resulted from using an overly strong hypochlorite solution for sterilization and neglecting to rinse the tanks promptly afterward. Recommendations included revising directions for sterilization and rinsing.