Abstract
This paper reports the investigation of creep-rupture properties of welded joints of 2.25Cr-1Mo steel. Both small round bar and full-thickness (50mm) specimens were prepared from the joints fabricated by multi-layer welding. The creep-rupture testing was conducted at 550°C, using stresses in the range of 78-159MPa. Detailed observation was made to identify the location and mode of fracture in the specimens with different geometries of the welded joint. The major conclusions are:
(1) For both small and large welded joint specimens, the location of fracture is at the fine grained region toward the base metal in the HAZ. In the small specimens fracture is transgranular with local necking, but in the large specimens fracture is intergranular without necking.
(2) The fracture location in the small joint specimen that failed after 40980h was in the weld metal.This is because softening is more predominant in the weld metal than in the HAZ.
(3) The HAZ structure of the multi-layer welded plate contains a special region with fine grains (6μm) where face and back beads meet. The crack initiates in the fine grained region rather than in the coarse (40μm) and/or mixed grains as traditionally reported.
(4) It is necessary to evaluate the creep damage mechanism of welded joints by use of large specimens so that the location of cracking can be identified, whether at the surface or in the interior of the joint.
