The gun mount used in two types of self-propelled artillery consists of an oil-filled recoil cylinder and a sand-cast (MIL-I-11466, grade D7003) ductile-iron piston that connects to the gun tube through a threaded rod. The piston contains several orifices through which oil is forced as a means of absorbing recoil energy. During operation, the piston is stressed in tension, pulled by oil pressure on one end and the opposing force of the gun tube on the other. The casting specification stipulated that the graphite be substantially nodular and that metallographic test results be provided for each lot. Investigation (visual inspection, fatigue testing, 0.25x/0.35x/50x magnifications, 2% nital etched 60x/65x magnifications, and SEM views) showed that most of the service fractures occurred in pistons containing vermicular graphite. Recommendations included ultrasonic testing of pistons already in the field to identify and reject those containing vermicular graphite. In addition, metallographic control standards were suggested for future production lots.

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.

Suspension lugs fabricated from AISI 4340 steel used to facilitate loading of bombs onto the underside of military aircraft could not sustain required loads during routine proof load testing. Three failed lugs underwent visual examination, chemical analysis, metallography, hardness testing, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. It was determined that the failures were due to forging defects. Both forging laps and seams acted as stress concentrators when the lugs were loaded during proof testing.

Aircraft missile launcher attachment bolts fabricated from cadmium-coated Hy-tuf steel were found broken. Subsequent analysis of the broken bolts indicated three causes of failure. First, the bolts had been carburized, which was not in conformance with the heat treating requirements. Second, macroetching showed that the bolts has been machined from stock rather than forged, and the threads cut rather than rolled. It was also determined that hydrogen-assisted stress-corrosion cracking also played a part in the failure of the high-strength bolts.

Breech bolt assemblies from the Gatling guns used on fighter aircraft failed during firing tests. Metallography of the failed components revealed considerable decarburization which resulted in a loss of surface hardness. It was also determined that the maraging steel components were not in the nitrided condition as was required. This resulted in lower wear and fatigue resistance. These components also had a silicon content nearly double of that specified. The high silicon content lowered the notch tensile strength and toughness of the components.

Cluster bomb tailcone assemblies each containing two aluminum die-cast components were rejected because of the poor surface condition of the die castings. Numerous heat checks were found on the surfaces of the tailcones and radiographic inspection revealed inclusions, gas holes, and shrinkage defects in the castings. Most of the components failed to meet required mechanical properties because of these casting defects.

Welded low-carbon steel bomb fins were rejected because of poor weld practice. Visual and metallographic examination revealed that the resistance plug welds that attach the outer skin to the inner spar displayed inadequate weld penetration. Recommended changes to the resistance welding parameters resulted in acceptable welds.