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

Reactors of Generation â…£ 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 â…£. The first serious challengecome from fuel cladding and core structural materials for thedevelopment of reactors of Generation â…£.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.