The Long-term Thermal Aging Effect On The Fracture Properties Of Welded Joints Of Primary Coolant Pipes

As an advanced welding technology,narrow gap Tungsten-Inert Gas(TIG)welding method is widely applied for the primary coolant pipes of CPR1000+ nuclear power plants owing to its excellent welding forming property,stable welding quality and high welding efficiency.Considering that base metals and welded metals of TIG welded joints are both two-phase structures which contains austenite and ferrite phases,thermal aging embrittlement will occur during long-term operation.In this paper,an accelerated thermal aging tests have been carried out on TIG welded joints at 400 ? up to 30000 h to investigate the resulting fracture properties.The results show that:(1)TIG welded joints of primary coolant pipes have overmatch character.The strength increases but toughness decreases in all areas of TIG welded joints during thermal aging process.Meanwhile,different thermal aging driving forces result in different tendencies of strength and toughness in different areas of TIG welded joints.With the increasing thermal aging time up to 30000 h,the thermal aging saturation is observed in base metals but welded metals still exhibit further thermal aging embrittlement continuously.However,no instability brittle fracture is obtained in any area of TIG welded joints.(2)Base metals have a higher fracture property than other areas in TIG welded joints of primary coolant pipes.Moreover,the fusion line is the weakest area with lowest fracture property and relevant JIC throughout 30000 h thermal aging process.More important,JSZW is much lower based on stretch zone width(SZW)method,representing a lower fracture property.(3)Spinodal decomposition and G phase precipitation are successively observed in ferrite phases both of base metals and welded metals for TIG welded joints during long term thermal aging process.However,similar phenomenon is never obtained in austenite phases both of base metals and welded metals.In addition,more spinodal decomposition and G phase precipitation are significantly observed in ferrite phases of welded metals than that of base metals,but toughness still decreases more in base metals owing to higher ferrite content.According to fracture toughness testing results,the concept of failure probability is introduced and then a probabilistic fracture prediction equation is established for casting austenitic stainless steels(CASS)based on Argonne National Lab(ANL)model.Meanwhile,Leak-Before-Break(LBB)analysis technology is used to evaluate the structural integrity of primary coolant pipes.The results show that long term thermal aging decreases critical crack size by 40-50% in base metals but by 30% in fusion line at a certain ductile tearing modulus during typical loading process.However,thermal aging has a low influence on critical crack size of welded metals.Thermal recovery is applied on TIG welded joints for performance regeneration.The results show that thermal recovery at 550? weakens the spinodal decomposition phenomenon,decreases G phases in ferrite phases and thus improves the toughness of TIG welded joints.In addition,further thermal aging is performed on TIG welded joints after thermal recovery to investigate the resulting fracture toughness,which demonstrates that thermal recovery has no influence on subsequent service performance.Therefore,it is feasible to recover performance of TIG welded joints by thermal recovery process.