Study On Properties And Microestructure Of11Cr2W0.15Ta Reduced Activation Ferritic-martensitic Steel

High chromium (9-12wt.%) ferritic-martensitic steels are the key materialto increase the thermal efficiency of fossil-fired steam power plants,because of their good creep rupture strength, adequate corrosion/oxidationresistance, lower thermal expansion coefficient and high thermalconductivity. Some of these steels with modified composition are nowbeing considered for blanket module of nuclear fusion reactors because oftheir mechanical properties and lower shift in ductile brittle transitiontemperature on neutron irradiation. Elements such as molybdenum,niobium and nickel have long activation periods after irradiated underfusion neutron spectra. This causes the handling and disposal problem offusion reactor structural materials. Taking this factor into consideration,many alloys with elements that have shorter term residual radioactivitywere produced known as low activation materials.T122steel is one of the conventional ferritic-martensitic steels used infusion reactor because of its excellent mechanical properties. The T122 steel has better oxidation and corrosion resistance compared withconventional9Cr-1Mo steel, because of the higher Cr content.Nevertheless, it contains long activation periods elements such asmolybdenum, niobium, nickel and cobalt. Therefore, in this paper, thechemical composition of T122steel was adjusted by substituting theelements having long activation periods with relatively lower activationcounterparts such as tungsten, tantalum, manganese and vanadium, named11Cr-2W-0.15Ta RAFM steel.The optimal austenitizing and temper temperature were comfirmed as heatpreservation at1100℃for30mins followed by air-cooling torutemperature then at760℃for1.5hours followed by air-cooling torutemperature through studying the influence of heat treatment on tensilestrength at650℃and microstructure.Using the empirical chromium and nickel equivalent calculation andorthogonal experimental design method involving three factors and threelevels, the effect of vanadium, manganese and copper addition on themechanical properties of this steel was studied. By the orthogonal rangeand variance analysis, the optimized composition was determined. Thebest creep property can be obtained when vanadium and manganesecontent are0.1%and1.5%, respectively, without copper addition. The creep life of the optimal specimen was368hours at650℃and140M Pa.The results are interpreted based on the microstructure analysis throughoptical microscope, scanning electron microscope and transmissionelectron microscope.