Metallurgical Analysis of 35NCD16 Steel M6 Fasteners Failed During ALMA Test.

High strength steel 35NCD16 is used as fasteners in aerospace applications. During ALMA (American Lock nut Manufacturer's Association) test, some of the fasteners failed at the corner of Head-Shank junction. A thorough metallurgical analysis was conducted to analyze the cause of failure. Tensile test and torque test of unfailed fasteners were also carried out. The fracture surface of the failed fasteners revealed fatigue beach marks with brittle features and microcracks. Microcracks were observed at the corner of head-shank junction of the torque tested, tensile tested and unused fasteners. Microstructure consisted of tempered martensite and hardness of the fasteners indicated hardened and tempered condition. Fracture analysis indicated failure is due to 'fatigue', which was initiated through the microcracks present at the head shank region acting as notch/stress raiser. [ABSTRACT FROM AUTHOR]