| The upper limited temperature of the reduce activation ferritic/martensitic(RAFM)steel used in test blanket module in ITER is proposed to be 550?.The microstructure of RAFM steel will evolve during long-term exposure at the operating temperature.This results in degradation of mechanical properties of RAFM steel and ultimately affects its safety in fusion application.As one of RAFM steels,China low activation martensitic(CLAM)steel has been chosen as the primary structural material for the China Test Blanket Modules in the International Thermonuclear Experimental Reactor(ITER TBM).Thermal stability of CLAM steel is not only an important property to ensure its operation safety,but also an important index to realize its engineering application.In addition,in order to optimize the microstucture and to improve the thermal stability of CLAM steel,it is important to investigate the degradation mechanism of microstucture and mechanical properties of CLAM steel during long-term thermal aging.Thus,this study was focused on the microstucture evolution and its effects on mechanical properties change of CLAM steel at different thermal aging conditions.Firstly,the microstucture evolution and its effects on degradation of tensile and impact properties of CLAM steel after aging at 550? was investigated.The results showed that with increasing of aging time,the number density of precipitates increased firstly and then decreased,but the size of precipitates increased gradually.The coarsening rate of Laves phase was about 70 times as fast as M23C6 and 130 times as fast as TaC after aging for the same time.It was found that the prior austenite grains coarsened gradually and the martensitic lath evolved into subgrain with,increase of aging time.The results of mechanical properties tests showed that the tensile properties were not seriously affected by aging.However,the ductile-to-brittle transition temperature(DBTT)increased significantly from-90.2? to-37.5?,and the grain size was the main factor to influence the DBTT of CLAM steel after aging at 550? for less than 20,000 h.Hence,grain refinement was conducive to lower DBTT after long-term thermal aging.Secondly,the effect of thermal aging on high temperature mechanical properties including high temperature tensile properties and creep deformation of CLAM steels were investigated in this study.The results showed that tensile properties at high temperature were not seriously affected by aging,but the creep property decreased gradually with increasing of aging time.After aging for 4,000 h,the creep-rupture time decreased from 202 h to 111 h and the minimum creep rate increased from 3.61×10-4 s-1 to 6.84×10-4 s-1.The reason was that the number of solute atom decreased due to second phase precipitated and coarsened after thermal aging.Finally,thermal aging processes at different temperatures were investigated to build a model for accelerating thermal aging of CLAM steel.Precipitation and grain coarsening was observed during thermal aging and raising temperature could accelerate the coarsening process.The results of fracture model analysis showed that the main reason for DBTT increasing was grain coarsening after aging for less than 8,000 h at 550?,600? and 6508.Then a model for accelerating DBTT increasing was built based on Arrhenius formula(DBTT=DBTT0+70D-223).However,due to the limited data,the formula has certain limitations and cannot be extrapolated to the whole aging process of CLAM steel.More experiments and data are needed to establish a reliable model for accelerating thermal aging of CLAM steel.This work is aimed at building a accelerating thermal aging model of CLAM steel to save time and cost for aging experiment. |
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