Neutron Irradiation Hardening And Embrittlement Of CLAM Steel At Near ITER Dose Level And Its Thermal Effect

Reduced Activation Ferritic/Martensitic(RAFM)steel is the primary candidate structural material for the Test Blanket Module(TBM)of International Thermonuclear Experiment Reactor(ITER).The total irradiation dose of RAFMs serviced in ITER is no more than 3dpa under temperature of 300～500℃.The long-term accumulation of neutron irradiation damage will produce many micro-defects inside the nuclear materials during the service time,such as point defects,cavity,dislocation loops and segregation etc,resulting in series of irradiation damage effects,such as hardening and embrittlement,which are not only correlated with irradiation dose,but also influenced by irradiation temperature and synergistic effect of them.To ensure the safety and reliability of RAFM steel in ITER,neutron irradiation experiments near the ITER dose at service temperature should be carried out.And the service performance of materials was comprehensively evaluated by microstructure characterization and mechanical property tests of materials before and after neutron irradiation,combined with the irradiation theories.CLAM steel is one kind of RAFMs developed in China.In this thesis,the thermal effect in neutron irradiation damage of CLAM steel was investigated.On the one hand,neutron irradiation induced hardening and embrittlement was investigated by testing of the mechanical properties of CLAM steel before and after neutron irradiation to 1.61dpa at 300℃ and to 1.73dpa at 480℃.On the other hand,thermal ageing experiments corresponding to the neutron irradiation conditions of CLAM steel were conducted at 300℃ for 2264 hours and at 480℃ for 2424 hours,respectively.And thermal effect in neutron irradiation damage was analyzed by mechanical property tests and microstructure characterization of CLAM steel before and after thermal ageing.The main results are as follows;(1)The irradiation embrittlement effect of CLAM steel irradiated to 1.61dpa at 300℃ was a little strong.And the Ductile to Brittle Transition Temperature(DBTT)shift of CLAM steel was about 56℃,The yield and ultimate strengthes of post-irradiated CLAM steel tested at room temperature increased about 56MPa and 29MPa,respectively.They increased 10.37%and 4.37%compared with those of the unirradiated ones and exhibited some irradiation hardening.(2)The irradiation embrittlement effect of CLAM steel irradiated to 1.73dpa at 480℃was slight,and the DBTT shift of CLAM steel was about 10℃.However,irradiation hardening effect was relatively stronger.The yield and ultimate strength of post-irradiated CLAM steel tested at room temperature increased about 164MPa and 137MPa respectively,increased by 36.50%and 22.30%.(3)Compared with those of other RAFM steels tests at similar experiment condition,CLAM steel exhibited comparable,even relatively superior neutron irradiation hardening and embrittlement resistance capacity.(4)Effect of thermal ageing with equal time at 300℃ and 480℃ on the hardening and embrittlement of CLAM steel was not significant during neutron irradiation process.Both the yield and ultimate strengthes of CLAM steel tested at 300℃ after thermal ageing decreased,which demonstrated that thermal effect may conversely weaken the comprehensive hardening effect of CLAM steel.And thermal ageing had very slight effect on irradiation embrittlement of CLAM steel,the DBTT shift of both ageing experiments was 2℃,taking up 3.57%and 20.00%in neutron irradiation embrittlement when ageing at 300℃ and 480℃,respectively.Therefore,the dose effect and synergistic effect of dose and temperature are the main factors leading to the neutron irradiation hardening and embrittlement of CLAM steel under these experimental conditions.(5)Coarsening rate of original austenite grains was relatively fast at earlier ageing time.The grain size of CLAM steel aged at 300℃ for 2264 hours grew up to 8.92μm,increased by 14.07%,and grew up to 9.39μm after ageing until 5000 hours,increased by 20.07%when compared with that of non-aged CLAM steel.After ageing at 480℃for 2424 hours,the grain size grew up to 9.62μm,increased by 23.02%and then kept stable until 5000 hours.And coarsening of grain would eventually lead to decreasing ratio of grain boundaries with large angle.(6)During the ageing process,width of martensite lathincreased and inititative relatively straight martensite lath became crooked and even degenerated to sub-grains,resulting in the increasing ratio of grain boundaries with small angle.And the number density and size of second phase inside CLAM steel increased in the ageing process.The largest increase in width of martensite lath and size of M23C6 occurred in 480℃for 5000 hours,increasing by 32.66%and 23.02%respectively.These research results not only evaluated the service performances of CLAM steel under neutron irradiation near ITER dose level and revealed the mechanism of thermal effect in neutron irradiation damage,but also could enrich the database of irradiation-resistant materials and provide data reference for the optimization of structural materials in the future.