Preparation And Microstructure Studies Of High-chromium Ferritic/martensitic Steels For Advanced Nuclear Power Plant

High-chromium ferritic/martensitic steels are considered as the primary candidate structural materials for fuel cladding and core in Generation IV fusion and blanket in fission nuclear reactors because of their low thermal expansion coefficient,high thermal conductivity,neutron radiation resistance,high-temperature strength and corrosion resistance.In this study,a 10Cr2.5WVTa reduced activation ferritic/martensitic steel(lOCr steel)was designed according to the thermodynamic equilibrium phase diagram calculated by JMatPro software.Then its microstructure and properties was investigated by means of FESEM,TEM,high-temperature tensile and creep tests,SCW corrosion and ion radiation.Besides,the evolution of microstructure and ?-ferrite in an 11Cr3W3Co ferrittic/martensitic steel(11Cr steel)and its influence on high-temperature mechanical properties were studied by means of annealing at 1100?,tensile and creep tests.The microstructure of the lOCr steel after standard heat treatment(normalized at 980? for 0.5 h and tempered at 760?for 1 h)consisted of tempered lath martensite and a few amount of ?-ferrite,and the Cr-rich M23C6 phase and Ti-Ta-V-rich MX phase with a complicated structure were precipitated on the lath boundaries and within the lath,respectively.For a relatively high value in strain rate sensitivity m at high temperature,the steel obtained an excellent plasticity without a serious loss in tensile strength.The short-term creep test indicated that the values of stress exponent n were 8.89 and 10.92 at 600 and 650?,respectively.Both of them corresponded to the creep mechanism controlled by the pinning effects of precipitates,grain boundaries and phase interfaces on mobile dislocations.And dislocations climbing was considered as the creep mechanism at 600?.The surface oxide after SCW corrosion for 168 h was composed of Fe304 and FeCr204 polyhedron particles.As the corrosion time increased,the oxide particles grew gradually and a porous oxide structure was observed,and the weight gain exhibited a parabolic growth kinetics.After Fe13+ ion radiation at room temperature,400 and 700? for 2 h,the radiation induced defects included point vacancy/vacancy clusters,micro cavities and coarse precipitates,respectively.The microstructure of the 11Cr steel after standard heat treatment(normalized at 1050? for 0.5 h and tempered at 780? for 1.5 h)consisted of tempered lath martensite and large amounts of continuous?-ferrite parallel to the rolling direction.A high dislocation density and Nb,V,Ta-rich dispersive precipitates were observed in the lath,and Cr,W-rich precipitates with a large size were distributed on the prior austenite grain boundaries,lath boundaries and interface of ?-ferrite and martensite.As the annealing time increased,the amount of ?-ferrite decrease to its minimum,and then increased again.The morphology experienced changes from continuous bamboo shape to small granular shape and then a coarse block shape.After annealing at 1100? for 4 h,Fe-W-Cr-rich precipitate was discovered.Its amount exhibited an ascending tendency and its distribution tended to become continuous with the annealing time.Additionally,the prior austenite grain also grew obviously.The declining in amount of ?-ferrite benefited the tensile strength at 300 and 650?.But it seriously worsened the short-term creep properties as the annealing time increased to 10 h,which depended on the morphology of ?-ferrite and its relatively distribution to the loading axis direction.When the annealing time exceeded 10 h,the precipitation of Fe-W-Cr-rich phase and grain growth might further worsen the creep properties of the steel.