Analysis of cracking in LPG Horton spherical vessel

Occurrence of cracking in the Horton spherical vessel made of a 537 class I steel used for containing liquefied petroleum gas has been analysed. The cracks were located using ultrasonic technique and their propagation behaviour was studied through scanning electron microscope examinations. All the cracks were found to be of embedded type, confined in the heat affected zone (HAZ) and located around mid-thickness region of the vessel. Tensile testing, hardness measurement, Charpy test, bend test and microstructural studies were conducted to determine the causes for incidence of cracking. The cracking is apparently hydrogen induced which has been assisted due to unfavourable micro-structure of the HAZ, high residual stresses and an unsuitable composition of the alloy. The remedial action to avoid cracking has been suggested.