Examination of the fracture faces found evidence of an initial crack 115mm long at the fillet weld toe of the compensating plate on the external surface of the shell. This defect extended approximately halfway through the plate thickness. Fracture mechanics analyses carried out as part of the failure investigation showed that the applied stress intensity factor at the defect was close to the measured values of fracture toughness obtained at 12°C.
The origin of the initiating defect was thought to be hydrogen cracking in the HAZ of the fillet weld. Regions of martensite with hardnesses in the range 500-515 VPN were found near the toe of the fillet weld. A hardness survey of the area of the toe of the weld measured hardnesses from 393 to 496 VPN. (In comparison, the hardness of the parent plate was 190 VPN). With a carbon content of 0.33%, the plate was considered to be liable to hydrogen cracking in the high hardness regions. Although the manufacturer's recommendations for drying the electrodes were followed (150°C for 1 hour), it was suggested by the investigators that this was insufficient given the hardness levels present in the HAZ.
Tests on the failed plate showed the chemical composition in the initiation region to be susceptible to liquation cracking. It was postulated that the initiating defect may have formed as a liquation crack which extended by hydrogen cracking under the conditions of high restraint and residual stresses present at the fillet weld toe.
An investigation was made into the effects of post-weld heat treatment on the welds in the as-welded failed section of the pressure vessel. It was found that little improvement in toughness properties was obtained in the range of temperatures associated with hydrotesting although the peak welding residual stresses were reduced which would be beneficial.
This failure arose from a material problem: the toughness of the parent plate was low and it was susceptible to liquation cracking and formation of high hardness zones in the HAZ, leading to probable hydrogen cracking. The selection of a fine grained steel with superior toughness properties meeting specified levels would give greater protection against this type of failure during hydrotest or during service should the temperature be reduced below normal operating temperature.