Fracture Cause of SWRCH22A Steel Screw

SWRCH22A steel screws fractured below the design load. The fracture cause was studied by fracture morphology and microstructure observation, hardness and galvanized thickness test, finite element simulation, and so on. The results show that the fracture characteristic of the screw was hydrogen embrittlement fracture; A tempered martensite structure with strong hydrogen embrittlement sensitivity was formed in the screw after heat treatment. The hydrogen atoms entered the screw structure during the galvanizing or service process. Under effects of service loads, stresses were concentrated in the screw head and thread root, and hydrogen atoms gathered at stress concentration, leading to crack initiation and propagation. Finally, the screw fractured at the root of the first thread where stress concentration was the largest, leading to crack initiation and propagation, and finally fracture.