Study Of Microstucture And Mechanical Property On Ferritic/Martensitic Steel For Advanced Reactors

Reactors of Generation Ⅳ have extreme working conditions of hightemperature、tense radiation and strong corrosion, in such conditionsmechanical property、anti-irradiation property and chemical property ofthe existing fuel cladding material Zr alloy of water cooled reacter can’tmeet the need of reactors of Generation Ⅳ. The first serious challengecome from fuel cladding and core structural materials for thedevelopment of reactors of Generation Ⅳ.9%-12%Cr ferrite/martensitesteel possesses excellent comprehensive properties: good thermalconductivity; small coefficient of thermal expansion; excellentanti-irradiation property in high temperature（400-550oC）; goodresistance to high temperature corrosion; good high temperature strengthand creep strength; good economy. Therefore,9%-12%Crferrite/martensite steel has become one candidate material of reactor fuelcladding and core structural materials.Microstructure of11Cr ferritic/martensitic steel before and after high temperature creep was investigated using optical microscope、scanning electron microscope and transmission electron microscope, andmeanwhile mechanical property and microcosmic mechanism of serratedflow of11Cr ferritic/martensitic steel were investigated using hightemperature tensile testing machine with relevant dislocation theory.Experimental results of microstructure show that11Crferritic/martensitic steel before and after creep consists of martensite anda little δ-ferrite.11Cr ferritic/martensitic steel before creep contains twokinds of precipitate phases, which are Nb-rich MX phase and Cr-richM₂₃C₆phase. Some of MX phase appear three obvious changes aftercreep: Nb decreases slightly and other metallic elements increase slightly;Nb decreases obviously，while Ta and W increase obviously, changinginto Ta-rich MX phase; Nb decreases obviously，while Cr and V increaseobviously. Chemical composition of M23C6phase does not on the wholechange, meanwhile the amount and the size of the M₂₃C₆phase increaseafter creep. Laves phase or Fe3W3C phase is precipitated in δ-ferrite afterhigh temperature creep. A kind of long flaky precipitated phase whichmay be Fe-Cr-rich M₅C₂phase is found in normalized and tempered highCr ferritic/martensitic steel for the first time, and the phase disappearsafter high temperature creep.Tensile test results show that the serrated flow becomes moreobvious when the strain rate becomes lower when11Cr ferritic/martensitic steel is stretched at600℃、625℃and650℃, but at700℃the serrated flow don t change basically when the strain ratebecomes lower. The serrated flow in tensile of11Cr ferritic/martensiticsteel belongs to abnormal serrated flow and effective activation energyfor its generation is about43KJ/mol. The serrated flow is generated dueto the dynamic strain aging which may be caused by the interactionbetween interstitial atom C or N diffusing by dislocation pipe anddislocations.