The investigation into the failure found that the tower fractured at the circumferential weld between the replacement ring and the lower course. Four large cracks in the heat affected zone (HAZ) had been present prior to the failure, originating at the inner surface of the tower and extending almost through the wall thickness. About 35% of the vessel circumference was affected. The location of the first leak observed corresponded to one of these HAZ cracks which was approximately 800mm long.
Microhardnesses measured in the HAZ near the surface exceeded 29 HRC and peak hardnesses of 40 to 48 HRC were found near the fusion line. These facts, taken with the in- section appearance of the pre-existing cracks (straight in the HAZ near the surface and then zig-zagging through the base material at the limit of the HAZ), pointed to the cracks initiating by hydrogen cracking and then progressing by hydrogen-induced stepwise cracking (HISC). Tests according to a NACE standard procedure confirmed that the material was susceptible to HISC.
The fracture ran around the HAZ of the circumferential weld at right angles to the axial stress of 35N/mm 2. The fact that this stress level was so low and the crack did not change directions to run in a direction perpendicular to the higher hoop stress, indicated very low toughness material in the HAZ. Charpy V notch tests of the replacement course material and the weld between the replacement course and the upper part of the tower showed the weld metal and HAZ to have superior notch toughness to the base material. (20J transition temperatures: 0°C for parent plate, -51°C for weld metal, -40°C for HAZ). Fracture toughness tests measuring crack tip opening displacement (CTOD) in the HAZ material gave much greater critical CTOD values than the applied CTOD in the tower at the time of failure, estimated ignoring any residual stresses, as 0.064mm. Tests on hydrogen charged specimens did, however, reveal much reduced CTOD fracture toughness values in the range of approximately 0.070-0.080mm at 38°C. A later fracture mechanics assessment of the tower found that when residual stresses were taken into account, failure was predicted at the level of CTOD measured in non-hydrogen charged specimens.
Taking all of these findings into account, it can be concluded that this failure occurred because the welding procedure used when replacing a section of the vessel caused the formation of a hard microstructure in the HAZ of the weld. This hard region was susceptible to hydrogen assisted cracking resulting in growth of large cracks in the vessel. The uncracked material in the vicinity of the existing cracks had low toughness due to hydrogen embrittlement and failed at the applied CTOD in the vessel arising from the operating pressure and residual stresses associated with the weld.