The Effect Of High Temperature And Long-term Heat Aging On Precipitation Behavior Of CLAM Steel

With the current energy crisis and the increasing demand for energy,nuclear energy emerges as the times require,in which the nuclear fusion can be exploited for the advantages of rich resources,great energy release and environmental friendliness.The Tokamak device,which uses nuclear fusion energy,is one of the most studied nuclear fusion reactors.It is strictly required to maintain good tissue stability and use safety for the first wall materials under adverse conditions such as plasma irradiation,high temperature and high load.While Reduced-activation ferritic-martensitic（RAFM）steel has good geometric dimension stability,better thermal conductivity and mechanical properties,lower radiation swelling rate and thermal expansion coefficient when used as structural component under high temperature irradiation condition.It is considered to be the first choice to develop the cladding structure material of future fusion demonstration reactor and poly variable force reactor.China low activation martensitic（CLAM）steel is selected as the preferred structural material for nuclear fusion reactors for China’s design and research as a kind of RAFM steel developed independently,composition and performance optimization in China.The phase change behavior of CLAM steel was studied by B?hr DIL-805A/D phase change meter and HV-1000 microhardness tester.At the same time,leica-DMI5000M optical microscope,Cai Sigama resolution thermal field emission scanning electron microscope（Zeiss Super 40）and Tecnai G2 F20 S-TWIN（200KV）transmission electron microscope are used to study the evolution of martensite substructures and precipitates of CLAM steel at different temperatures（500℃,550℃,600℃and 650℃）and different aging times（5000h,10000h,15000h and 20000h）.And the tensile test,impact test and hardness of MTS 810 electro-hydraulic servo universal testing machine,Charpy impact tester and HV-1000 microhardness tester were used in the period of high temperature and long time aging process.Tests were carried out to study the changes of strength,plasticity,toughness and hardness during the long term aging of CLAM steel.The main contents of the study are as follows:1.CCT curves of CLAM steel are drawn by using B?hr DIL-805 A/D phase change instrument and HV.1000 microhardness tester.Temperature of Ac1 is measured at 846.2℃and Ac3 at 902.3℃.When the cooling rate is 0.3 to 2℃/min,the ferrite structure with low hardness is obtained.The average microhardness is 169 HV0.1,and the total martensitic microstructure is obtained when the cooling rate is 10 C/min,and the average microhardness is up to 354 HV0.1.2.In the initial state,the number of M₂₃C₆ phase is less and the distribution is more scattered,the number of MX phase is more and the distribution is uniform.As the aging temperature and time increase,the number of M₂₃C₆ increases and the distribution is more dense,while the number of MX phase decreases somewhat,and the distribution of Laves is not obviously changed;the Laves phase precipitates at from 550 to 600℃,and some may grow but the quantity is not much.M₂₃C₆ increased with aging time,and the average size increased from about 160 nm in the initial state to about 280 nm.At 500℃,the size increased as the aging time increased,and the size decreased after 10000h.At 600℃,the size decreased after 5000h,and the size increased gradually at 550℃and 650℃.The size of MX and Laves varies little with aging time,the average size of MX phase is about 60 nm,and the average size of Laves phase is about 350 nm.3.By observing and analyzing the gold phase diagram,scanning diagram and transmission diagram,it was found that the original austenite grain of CLAM steel grew slightly,and the average size of the grain was less than 50 um under the time of 20000 h aging;the structure of the aging process was tempered martensite,and martensitic strip grew with the long time aging process of high temperature.The growth rate of martensitic lath is larger during aging temperature of 500℃,but the growth rate of the strip is slowed down after aging time 10000h,and the apparent softening phenomenon appears.After 20000h aging,the average size of martensitic lath is above 600nm,and the maximum size is close to 1um.4.In the early period of high temperature aging at 500℃,the strength and hardness are improved.After aging,the properties of 5000h are all descending.But after aging,the properties tend to be stable.In the long time aging process of other aging temperatures,the strength,plasticity,toughness and hardness are all decreasing in varying degrees.5.At the early stage of 5000h,the structures changes are obvious.The lath roughing and recovery and the phases of M₂₃C₆ and Laves coarsening and segregation are obvious.The strength and the plasticity of the martensitic slate are obvious,but the structure changes tend to be stable at the later stage of 10000h,but the properties decrease obviously,But even at the age of 650℃and 20000 h,The mechanical properties of CLAM steel can also be maintained at a relatively high level.