Failure analysis on 20MnTiB steel high-strength bolts in steel structure

•The chemical composition distribution of corrosion products on the surface.•Fracture crack propagation pattern.•Microstructure and structure tests of the failed high-strength bolts.•Fracture mechanism analysis of high-strength bolts taken from the bridge. A total of 57 high-strength bolts of 20MnTiB steel have been found broken off in a bridge in Chongqing since 2009. To ensure the safe operation of the bridge, the most representative samples of three M24 bolts were selected from them. The chemical composition, micro-morphology, metallographic structure, and mechanical properties such as hardness and impact performance were analyzed and compared with new high-strength bolts of the same specification to find out the reasons for the failure of high-strength bolts. The results show that micro-defects are found near the bottom of the thread of three failed high-strength bolts and the cracks are initiated here. At the same time, three bolts are located in the top chord area of the central arch of the bridge, which is subject to large stress and corrosion from the external environment. It changes the metallographic structure and mechanical properties of the bolt, thus accelerating the crack growth. When the external load exceeds the bearing capacity of the bolt, the bolt breaks.