Fracture toughness—microstructure investigation of hyperbarically welded joints in offshore application

The metallurgical changes have been studied in a typical offshore structure material, BS4360 grade 50E steel at various simulated depths (0–185 m) in the dry underwater welding conditions and the weld metal and heat affected zone (HAZ) fracture toughness have been related with the microstructure and micromechanisms of fracture. The results indicated substantial decrease in weld metal CTOD toughness at 185 m welding depth whereas HAZ toughness remained generally unaffected. High toughness of weld metal was found to be associated with greater proportion of the polygonal and the acicular ferrites and negligible proportion of aligned cerbide. High HAZ toughness was due to fine ferrite (6–8 μm) and absence of martensite in the structure.