Fracture Reason Analysis of Shelf Line Elbow

The fracture reasons for elbow of the shelf line were analyzed by means of fracture analysis, chemical compositions analysis, metallographic examination and other methods. The results show that during production process of the shelf line, in order to correcting the shape of the elbow the un-normal use of the tools caused a big residual stress at the root of the elbow. In addition, stress concentration formed at the mechanical damage marks remained by the non-normal tool straightening, which sped up the crack initiation. At the same time, the coarse grains of the welding heat affected zone of the elbow reduced the fatigue strength of the material. The above factors eventually led to fatigue fracture of the elbow during the durability and vibration test at the mechanical damaged welding heat affected zone of the elbow.