Failure of 35NCD16 steel fastener – A metallurgical investigation

Cadmium plated 35NCD16 steel threaded fasteners are being used for aerospace applications. One such fastener, used in retainer assembly of holder interface of solid propellant motor case, parted into two pieces during its removal. The fastener was torqued to 10 Nm and was in assembly for a cumulative period of 45 days. The fracture surface of the failed fastener had a narrow central region consisted of dimples, while its surrounding region exhibited intergranular facets. From the evidence, it was concluded that the fastener failed due to hydrogen embrittlement. The possible sources for the hydrogen entrapment in the material were the furnace environment during heat treatment of the steel or inadequate baking of electroplated cadmium deposits.This paper highlights the details of metallurgical investigations carried out.