Hydrogen embrittlement of 3.5Ni–1.5Cr–0.5Mo steel fastener

Cadmium plated 35NCD16 steel threaded fasteners are used for aerospace applications. One such fastener parted into two pieces during application of required torque. The fracture surface of the failed fastener consisted of two distinct regions, a bright region at the center encircled by a relatively dark region. The dark region spread all along the periphery of the fracture surface. Fractography revealed that the central bright region consisted of dimples, while the dark region consisted of intergranular facets. From the evidences it was concluded that the fastener failed due to hydrogen embrittlement. The possible sources for hydrogen entrapment in the material were the uncontrolled furnace atmosphere during long spheroidization treatment and inadequate baking after cadmium plating.