Cleavage failure of transformer storage tank under dynamic rates of loading: Influence of base plate and weldment microstructure and toughness

A high capacity transformer storage tank that was in service for more than a decade ruptured due to an explosion and a visual examination showed that the fracture facets were essentially brittle in nature with little evidence of plastic deformation. Fracture occurred mainly along weld seam; however, there were sharp cracks even in the base plate. Present study is a focused effort to find reasons for this brittle behavior of normally ductile steel. This entailed an in-depth metallographic examination of samples, selectively cut out from the tank walls and fractured weldments, supplemented by characterization of hardness, tensile, ductility and toughness properties. Results indicate that base plate and weld metal meet the required specifications for static design but not for the dynamic loading experienced under this failure mode (i.e. cleavage). Under dynamic loading conditions, the weld metal is the weakest link in terms of toughness and resistance to crack propagation. The need for adopting modern strong, tough, weldable steels to the construction of these tanks are pointed out.