Fracture Toughness and Brittle Failure – a Pressure Vessel Case Study

That low fracture toughness can cause brittle failure in steel has been documented in well-known studies. These include failure of Liberty Ships during WWII and, more recently, connection failures in steel buildings during the Northridge Earthquake. In both those cases, the manufacturing and fabrication process introduced stress states and changes to steel microstructure that reduced fracture toughness of critical elements and caused brittle failures.

Control of the manufacturing and fabrication process to maintain high fracture toughness remains a challenge for the Oil & Gas Industry. In this paper, a case study of a pressure vessel failure due to low fracture toughness is examined. The head of the pressure vessel detached about its circumference at a low operating pressure (<50 psi), a pressure much less than the vessel’s maximum operating pressure. A root cause analysis of the event identified factors from the vessel’s service conditions, manufacture, and fabrication that combined to cause the brittle failure.

The causal factors from manufacture and fabrication of the vessel head will be discussed. These include: (a) stress relief or normalization of the mill plate; (b) cut-out of the blank from the mill plate; (c) residual stresses from dishing and flanging the head; (d) residual stresses from welding of the head to the shell; and (e) stress relaxation of the fabricated vessel. Discussion of the factors will include review of guidelines in the ASME Boiler & Pressure Vessel Code (BPVC).

The effect of the manufacturing process on the vessel steel’s mechanical properties will then be examined. These include strength, ductility and fracture toughness. Finally, deficiencies in these properties will be used to explain failure of the vessel head at the low operating pressure.