A ring clamp (8740 (AMS 6322), steel forged and cadmium plated) used for attaching ducts to an aircraft engine became loose after three hours of service. When the clamp was removed from the engine, the hinge tabs on one clamp half were found to be broken. Analysis (visual inspection and microscopic and metallographic examination) supported the conclusion that both hinge tabs on the clamp half fractured in a brittle manner as the result of gross overheating, or burning, during forging. The mechanical properties of the metal, especially toughness and ductility, were greatly reduced by burning. Evidence that burning was confined to the hinge end of the clamp indicated that the metal was overheated before or during the upset forging operation. Recommendations included notifying the supplier of the burned condition on the end of the clamp. The clamps should be macroetched before cadmium plating to detect overheating. The clamps in stock should be inspected to ensure that the metal had not been weakened by overheating during the upset forging operation.

A heat-treated, cadmium-plated AISI 8740 steel bolt broke through the head-to-shank fillet while being handled during assembly. Fractographic and metallographic examination of the bolt traced the cause of failure to quench cracking, which occurred when the part was water cooled following hot heading and prior to the production run. The process chart for hot heading was changed from water quenching to air cooling following the forming operation.

During an inspection of a structure two weeks after assembly, the heads of several cadmium-plated AISI 8740 steel fasteners were found to be completely separated from their respective shanks. SEM examination of the fracture surfaces revealed a brittle, intergranular fracture mode, indicating hydrogen embrittlement. An investigation was conducted to determine the extent of hydrogen embrittlement in the various lots of cadmium-plated 8740 steel fasteners. It was found that hydrogen embrittlement was caused by the use of a bright, impervious cadmium electroplate that hindered diffusion of mobile hydrogen outward from the surface of the pin. After the cadmium layer was removed, the mobile hydrogen contained on the surface of the steel and in the electroplated deposit was released, and the embrittlement problem was alleviated. To prevent reoccurrence, the bright cadmium layer was stripped from the pins, which were then baked and repeated with a dull, porous cadmium layer that allowed outward diffusion of hydrogen. The pins were baked again after deposition of the porous cadmium layer. This eliminated the problem.

The heads of two AISI 8740 steel bolts severed while being installed into an Army tank recoil mechanism. Both broke into two pieces at the head-to-shank radius and the required torque value had not been attained nor exceeded prior to the failure. A total of 69 bolts from inventory and the field were tested by magnetic particle inspection. One inventory bolt failed because of a transverse crack near the head-to-shank radius. It was deduced that either a 100% magnetic particle inspection had not been conducted during bolt manufacturing, or the crack went undetected during the original inspection. Optical and electron microscopy of the broken bolts revealed topographies and the presence of black oxide consistent with quench cracking. The two bolts failed during installation due to the presence of pre-existing quench cracks. Recommendations to prevent future failures include: ensuring that 100% magnetic particle inspections are conducted after bolts are tempered; using dull cadmium plate or an alternative to the electrode position process, such as vacuum cadmium plate or ion-plate or ion-plated aluminum, to mitigate the potential for delayed failures due to hydrogen embrittlement or stress-corrosion cracking; ensuring that the radius at the shoulder/shank interface conforms to specifications; and replacing all existing bolts with new or reinspected inventory bolts.