Study On Evolution Of Microstructure And Mechanical Properties At Elevated Temperature For Clam Steel

To optimize the structure of power resources, and promote the coordinated andsustainable development of economy and society, many countries in the world vigorouslysupport the nuclear power generation. Compared with the nuclear fission reaction, thenuclear fusion is more efficient and safer. Low activation ferritic/martensitic (RAFM) steelhas been considered as the primary candidate structural material to be applied to thefirst-wall cladding in the future nuclear fusion reactor systems. China low activation(CLAM) steel as one of the RAFM steels, has got great progress in the last decade. WhileCLAM steel preparation has been matured by far, the precise heat treatment, long-termaging and its creep behavior and mechanism are still lack research and data. In this study,the heat treatment as well as the microstructural evolution of CLAM steel during theprocess of aging and creep was investigated, and a connection between microstructuralevolution and mechanical properties was set up.The investigation on normalizing and tempering to control the changes ofmicrostructure for the CLAM steel, such as grain size, martensite lath and precipitatedphase, was carried out. The effect of heat treatment on microstructure and mechanicalproperties was studied. An optimal heat treatment, normalizing at980oC for30min andtempering at760oC for90min, was determined. By comparing the changes of themechanical properties, it can be clearly identified the effect of tempering on mechanicalproperties of CLAM steel was displayed to be stronger.This thesis studied the influence of long-term aging on the microstructure andmechanical properties of CLAM steel at600oC and650oC, respectively, and analyzed theeffects of both aging temperature and aging time on microstructure. Under the synergisticaction of aging temperature and aging time, the microstructural evolution of CLAM steeltook place. The martensitic laths widened and even evolved into subgrains, the M23C6carbides coarsened, and the Laves phase precipitated along with the M23C6carbides andMX carbonitrides. The precipitation behavior of Laves phase was closely related to thetemperature and time. Although the stability of microstructure of CLAM steel after aging at600oC was higher than that at650oC, Laves phase precipitated at600oC more quicklythan that at650oC. In addition, the microstructural evolution would directly affect themechanical properties of the steel, especially the ductile brittle transition temperature(DBTT).There existed a threshold stress between130MPa and140MPa for CLAM steelduring the creeping process at600oC and the creep mechanism was different under thehigh stress (above the threshold stress) and the low stress (under the threshold stress). Thecreep rupture life of CLAM steel increased significantly with the stress reduced to belowthe threshold stress in the range of130MPa to200MPa. The creep rupture life of CLAMsteel was obviously longer than9Cr-1W-0.2V-0.06Ta steel at600oC under130MPa. Inaddition, the creep rupture mechanism was not the same under different stress. The creeprupture mode of CLAM steel was mainly ductile fracture under higher stress and shifted tobrittle fracture under lower stress by observation on the creep fracture morphology of thesamples. The threshold stress would be decreased by the coarsening of precipitates duringthe creep process.Comparing the microstructure during the process of aging with that during theprocess of creep, it was found that the microstructural evolution during creep was higherthan that during aging, and the analyses of microstructural evolution and mechanicalproperties indicated that the stress promoted the microstructural evolution and thenucleation of the Laves phases.