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John Thompson pressure vessel

In December 1965 a large pressure vessel being manufactured by John Thompson (Wolverhampton) Limited for the ICI Immingham plant fractured during a hydraulic test.

Damage to the vessel was extensive with four large pieces being thrown from the vessel. One of these, weighing approximately 2 tonnes, went through the workshop wall and landed some 46m away. One minor casualty was reported.

The vessel consisted of a plain shell of 1.7m inside diameter made from cylindrical strakes of 150mm thick silicon killed Mn-Cr-Mo-V steel. The end closures were forged, one end being a flange with a flat cover. The forging material was similar to the shell steel, however, a higher carbon content of 0.2% was used in the flange forging in order to meet the strength requirements in the thickest section. The forgings were supplied in a normalised and tempered condition. The overall length of the vessel was 18.2m and it weighed 167 tonnes. It had been destined for use as an ammonia converter with a design pressure of 35N/mm2 at 120°C.

The proof test requirement was for 48N/mm2 gauge pressure at ambient temperature (not less than 7°C) but the testing of the vessel was troubled by leaks from the bolted flange joint and several re-pressurisations were required. At the first attainment of 34N/mm2 pressure, the vessel failed accompanied by 'a kind of dull thud'. No one present noticed anything unusual before the failure. The ambient and water temperatures at the time were determined to be less than10°C.

The failure occurred at the flange end of the vessel. The flange forging was cracked through in two locations, the first two shell strakes broke into several pieces and cracking extended into the third strake.

Investigation revealed two fracture initiation sites. These were small pre-existing cracks in the heat affected zone (HAZ) of the submerged arc weld joining the flange end forging to the vessel shell. The cracks were located on the forging side of the weld about 15mm below the outer surface in regions of segregation where the carbon and alloying element contents were locally increased. This segregation would have increased the susceptibility of the material to hydrogen cracking, the probable cause of the original crack formation. The welding procedure was such that the preheat was discontinued immediately on completion of the weld, thus not allowing the reduction of hydrogen levels in the faster cooling surface regions.

The failure occurred by the extension of these pre-existing cracks into the adjacent weld metal which had poor toughness properties due to inadequate heat treatment. The toughness of the forging and shell plate, although meeting the requirements, was not sufficient to arrest a running crack of size equal to the weld cross section. The forged flange and first strake sub-assembly had been furnace post-weld heat treated. The specified conditions were 620-660°C for six hours, however high hardnesses measured on the casualty material indicated that the temperature of the sub-assembly had not reached this level. The furnace temperature had been monitored by pyrometers lowered from the roof. From subsequent temperature measurements made on similar components in the furnace, it was estimated that the actual temperatures achieved in the circumferential weld between the forging and the first strake were between 520-610°C, depending on the position around the weld.

The Charpy V notch requirements for the forging, plate and weld metal were 38J absorbed energy at +20°C. The weld metal did not meet these requirements and the absorbed energies measured at +7°C were in the range 12-25J which was considered the lower shelf for this material. Re-heat treating the casualty material at 650°C for six hours considerably improved the Charpy properties of the weld metal but only at temperatures of 20°C and above.

Residual stresses were also considered a contributory factor to crack initiation at the relatively low applied stress level as the heat treatment conditions had not been sufficient for full relief of the residual stresses.

The British Welding Research Association report of the investigation into this failure proposed that fracture mechanics principles be used when setting fracture avoidance criteria for thick high strength steels. It also recommended carrying out pressure tests at temperatures above the ductile-brittle transition temperature of the vessel material in order to reduce the risk of failure.

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