Creep Fracture Mechanism in the Welded Joints of P91 Steel

The creep rupture strength of the welded joint of P91 steel (9% Cr) is limited by that of the fine-grained region of its heat affected zone (HAZ). The fine-grained HAZ region has the lowest creep rupture strength and the creep voids are mostly concentrated in this region in compassion with the coarse-grained HAZ, the weld metal and the base metal of the welded joint. Type IV fracture occurred in the fine-grained HAZ of the cross weld specimens after the long-term creep tests at low stresses at 600 and 650 degree C. It occurred by creep cavitation. There are two types of creep cavitation observed in the fine-grained HAZ depending on the test temperature and the applied stress: wedge type cracking observed at intermediate stresses and round type cracking observed at low stresses. The creep voids form mostly at the interfaces between matrix and the coarse Laves phase and Cr-rich carbide particles. (author)