Effect of welding process and groove angle on type IV cracking behaviour of weld joints of a ferritic steel

Creep tests were carried out at 923 K and various stress levels on weld joints of 11CrMoWVNb ( P122) steel prepared via gas tungsten arc, electron beam, and laser welding processes. All the weld joints exhibited typical type IV cracking. However, the rupture life of the weld joints was found to increase with decreases in the width of the heat affected zone (HAZ) and the groove angle. A comparison of the results with those for the simulated fine grained HAZ (FGHAZ) revealed that the creep damage and the type of fracture of the weld joint differ considerably from those of the simulated FGHAZ specimen. In the weld joints fracture was characterised by low ductility and formation of creep cavities, whereas the FGHAZ specimens exhibited good ductility and hardly any creep cavities were observed in them. These differences are attributed to the presence of a triaxial stress state in the weld joints, due to the existence of zones such as parent metal, weld metal, FGHAZ, and coarse grained HAZ, which differ considerably in their creep properties. Changes in the width of the HAZ or the groove angle of the joints alter the stress state in the weld joint, leading in turn to corresponding changes in the creep behaviour of the weld joints. Finite element analysis of the stress - strain distribution in the weld joint specimens during creep also confirmed these influences.