Failure analysis of maraging steel fasteners used in nozzle assembly of solid propulsion system

Owing to high strength and better toughness, maraging steels are being widely used for aerospace application. In one such application, maraging steel was used to fabricate fasteners for nozzle assembly of propulsion system. Few fasteners were found broken during second stage of torque application. Failure was at head/shank junction in each of the three failed fasteners. Detailed metallurgical investigation revealed that the fasteners were under assembly load and exposure to severe corrosive environment. Crack originated from the thread root, propagated intergranular mode with tree root morphology. Considering such evidences, it was inferred that the failure occurred due to stress corrosion cracking.