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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.

Fig. 14. Simulated HAZ specimens (produced by weld simulator) after creep test at 923 K and at a stress level of 100 MPa.

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600°C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550°C, 600°C, and 650°C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600°C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.