Investigation of Screw Failed During Installation

Material used for screw production was investigated due to its inconvenient properties which caused the screw breaking on threads or in the head during the installation. Chemical composition of analyzed material corresponded with standard STN 17153 according to technological drawing for specific product. The metallographic analysis showed that failure of screws happened due to improper microstructure resulting from unsuitable thermal treatment of material. Fine inclusions based on aluminum nitride (AlN) and chromium carbonitride (Cr(C,N)) were segregated along the ferritic grain boundaries. Coarse aluminum nitride inclusions (AlN) in ferritic matrix affected the character of present fracture surface characterized by cleavage facettes. The fracture was propagated step by step following the planes with the increased concentration of inclusions across the whole cross-section of the screw.