The relation between creep rupture properties and microstructure of weld metal on 2.25Cr-1Mo steel was investigated. Small round bar specimens were prepared from the weldment performed by multi-layer welding. The creep rupture tests of the weld metal specimen were conducted at 550°C and the stress ranged from 69 to 159MPa under the constant load condition. The fracture mode of weld metal specimen was examined and related to the microstructure. The major conclusions are:
(1) Three kinds of microstructure were observed in the weld metal performed by the multi-layer welding: columnar structure due to rapid cooling, microstructure due to reheating by subsequent passing, and a similar bainitic microstructure to that of the base metal arising from cyclic heating. The difference was not observed in the creep rupture properties of the weld metal specimens sampled from different locations in the plate.
(2) Creep rupture lives become larger in the order of the base metal, the weld metal and the welded joint at the stresses higher than 98MPa, however the difference becomes smaller at the lower stresses and the long times.
(3) The decrease ratio of the hardness of the grip head in the specimen ruptured after 77845h to that of the aswelded specimen reached only 10%, while the ratio of the hardness of the gauge portion in the same specimen was 30%. The decrease strongly depended on the applied stress and the cause of the decrease would be ascribed to agglomeration and coarsening of carbides or recovery of the dislocation substructure.
(4) As the applied stress increased, fracture mode of the weld metal specimens varied from ductile fracture, via the fracture at columnar interface, to the mixed fracture at the columnar interface and at prior austenitic grain boundaries.
