Long term ageing effect on fracture toughness of the GTAW welded joints for nuclear power main pipelines

Gas tungsten arc welding (GTAW) joints of nuclear power main pipelines were subjected on accelerated thermal ageing tests until 30000 h to investigate long-term ageing effect on fracture property. The results indicate that fracture toughness values J(IC) and J(SZW) of both base metals and welded metals decreases clearly and fracture mode transits subsequently from typical ductile to brittle fracture with increasing ageing time, but J-resistance (J-R) curves still lie above the recommend low-bound lines given by NUREG/CR after 30000 h thermal ageing. Moreover, base metals have better fracture toughness than welded metals regardless of thermal ageing time, and thus welded metals are still the weakest part for the whole welded joints. In addition, thermal ageing has no significantly influence on the location of crack initiation at the crack tip and crack growth mode because the thermal ageing could enhance the hardness and strength of ferrite phases and thus the austenite phases still have lower strength.