Experimental study on creep damage evolution process of Type IV cracking in 9Cr–0.5Mo–1.8W–VNb steel welded joint

The microstructure degradation and creep damage evolution process of Type IV cracking in a 9Cr–0.5Mo–1.8 W–VNb steel (ASME P92) welded joint during creep were investigated. Creep tests were carried out at 923 K and 70 MPa, and terminated at specific times in order to analyze the microstructure change and the creep voids formation in the welded joint. The welded joint finally ruptured in the fine grained heat affected zone adjacent to the base metal, identified as Type IV cracking. It revealed that the creep strength in the fine grained heat affected zone was lower than other zones of the welded joint. Creep voids were likely to nucleate and grow in this zone. In addition, carbides played an important role as nucleating sites for creep voids. As the creep time increased, creep voids linked to each other and finally formed micro-cracks resulting in the fracture in the fine grained heat affected zone. On the basis of microstructure degradation and creep voids propagation, the creep damage evolution process was proposed.