The Effect Of Thermal Aging On The Mechanical Properties Of Austenite-ferrite Duplex Stainless Steels

Austenite-ferrite duplex stainless steels are primary materials for the coolant circuit in the second generation nuclear power plant.Their service temperature is 327?,under which the ferrite phase is susceptible to occur spinodal decomposition resulting in the variation of the mechanical properties.Therefore,a series of accelerated thermal aging tests were performed on the CF8 and Z3CN20.09M duplex stainless steel to investigate the effect of thermal aging on their impact toughness,low cycle fatigue and uniaxial ratcheting behavior.The research results are valuable to the failure and life assessment of the primary coolant circuit.Impact machine was employed to investigate the influence of thermal aging on the CF8 duplex stainless steel.Test results showed that the impact crack initiation energy and crack propagation energy decreased with the extension of thermal aging duration under various impact temperatures.On the contrary,the ductile-brittle transition temperature increased with the prolongation of the thermal aging duration.The fracture profile of the specimens with different thermal aging time were observed using scanning eclectron microscope.It was found that there were a large mount of dimples in the fracture profile of the virgin specimen,which was the typical characteristic of ductile fracture.The quasi-cleavage fracture characteristic could be seen in the specimen aged for 1000 h.When the specimens were aged up to 3000 h,the specimens turned into complete cleavage fracture.It means that CF8 duplex stainless steel was embrittled by the long time thermal aging.In the low cycle fatigue test on Z3CN20.09M duplex stainless steel,it was found that the stress response and the fatigue life of the material increased with the extension of thermal aging duration.SEM images showed that the fatigue crack always initiated on the boundary of the ferrite and austenite phases in both virgin and aged material.After initiation,micro cracks grew to macro cracks by the coalescence of the adjacent micro cracks,and then propagated trans granularly.Thick fatigue striations are found on the fracture surface of fatigued specimen subjected to longer thermal aging duration.In addition,amount of slip bands arrested in the ferrite were found in the vicinity of the fracture surface in the aged specimens,which released the stress concerntration on the phase boundary.It was deemed as the cause of the fatigue life prolongation in the aged material.A series of ratcheting tests was conducted on Z3CN20.09M duplex stainless steel in virgin condition and conditions aged for 1000 h,3000 h and 6000 h.The results show that ratcheting strain increased with the increase of the mean stress and stress amplitude in both the virgin and aged material,whereas it decreased with the increasing of the stress rate.Loading history also exerted a great influence on ratcheting behavior.Moreover,it was unexpected that ratcheting strain was reduced with the extension of the thermal aging time,which can be ascribed to the strengthening of ferrite by thermal aging.Finally,Ohno-Wang ? constitutive model was employed to simulate the ratcheting strain of Z3CN20.09M duplex stainless steel,and the predited result was good agreement with the experimental results.Base on Ohno-Wang ? model,a modified model was used to simulate the ratcheting behavior of the aged materials.and a satisfactory result was achieved.