Stress Corrosion Cracking of High Strength 18Ni-8Co-5Mo Maraging Steel Fasteners

Maraging steels have attractive combination of strength and toughness making them ideal choice for a number of structural applications. In one such application, this steel was used to fabricate shear bolts in an interstage of a satellite launch vehicle. Four numbers of these shear bolts were found to fail exactly at the head-shank interface under assembly condition. The failure has occurred when the structure was assembled with bolts to the specified torque of 43N-m. Detailed investigations revealed that the fasteners were under assembly loads and exposed to marine environment. The cracks initiated at the corrosion pits located at the head-shank interface propagated inwards in a transgranular mode. Fractography revealed brittle transgranular features with cleavage facets and corrosion products on the fracture surface. The EDS analysis at the corrosion pits near the fracture edge indicated the presence of chlorine. Based on detailed metallurgical analysis, it was concluded that the shear bolts failed due to the “Stress Corrosion Cracking”.