Creep behaviour of weld joints and simulated HAZs of a high Cr ferritic steel

Creep tests were carried out on I11Cr-0.5Mo-2WCuVNb steel weld joints and simulated HAZs produced by heat treatment and using a weld simulator. For weld joints, the fracture occurred in the fine grained heat affected zone (FGHAZ); with rupture life considerably lower than that of the base metal. Rupture life was found to depend on width of HAZ and groove angle of the weld joint. HAZ specimens simulated by heat treatment had uniform microstructure. Their creep properties varied widely with temperature of simulation and creep strength was minimum for simulation temperatures corresponding to fine grained HAZ (FGHAZ) in a weld joint. In general, the rupture life of the simulated FGHAZ was found to be lower than that of the weld joint. For HAZ simulation by weld simulator, simulation temperature was uniform only for short length at the center of the specimen and hence, the microstructure of the specimen varied along its length and fracture of the specimen during creep testing occurred at the fine grained zone formed in the specimen during simulation. Rupture life did not vary much with the simulation temperature; it was close to that obtained for FGHAZ specimen simulated by heat treatment. Multiaxial stress state formed in the weld joint during testing due to existence of different HAZs, base metal and weld metal, which vary widely in their properties appears to affect the fracture behaviour of the weld joints in addition to the low creep strength of the FGHAZ. As the stress state is altered by the HAZ width and groove angle, these parameters also influence the rupture life of the weld joint.