Hydrogen-Induced Cracking (HIC) of Hardened and Tempered Steel Fastener Used in Space Application

Cadmium-plated 0.35C–3.5Ni–1.5Cr–0.5Mo steel threaded fasteners of 1230 MPa properties class are used for aerospace applications. These fasteners were torqued to 13 N·m. Few fasteners parted into two pieces while in use under sustained assembly load for a period of 50 days. The fracture surface of the failed fasteners had two distinct regions when viewed under microscope at higher magnification. Fractography revealed that the larger region consisted of predominantly intergranular features, whereas the smaller region had features of microvoid coalescence. From the metallography evidences it was concluded that the fastener failure was due to hydrogen embrittlement. Electro-deposit of cadmium was identified to be the main source for hydrogen entrapment, which could not be compelled completely by post-plating baking treatment.