Failure Analysis of 35NCD16 Steel M12x1.25 Nut in Aerospace Component Structural Qualification Testing: A Metallurgical Perspective

During the structural qualification testing of propellant tanks of a liquid engine, one M12 nut made of 35NCD16 steel used for the testing was found broken. Structural qualification testing was conducted to verify the design adequacy of tanks in human-rated missions. A detailed metallurgical investigation was conducted on the failed nut, bolt, and washers to determine the underlying cause of fracture. This investigation included detailed visual inspections, hardness testing, as well as microstructural characterization using tools such as the optical and scanning electron microscopy. In a comparative study, the nuts adjacent to and diametrically opposite from the failed assembly were examined along with exemplar nuts. Fractographic analysis revealed dimpled rupture characteristics with substantial inclusions in the longitudinal direction of the nut. A detailed investigation of the failed nut revealed long, aligned non-metallic inclusions, primarily mixed oxides of Mn, Si, and Cr. These inclusions were found to be the underlying cause of nut fracture below maximum tensile stresses.