Study On Microstructures And Properties Of Ultra-low Carborn RAFM Steel

Fusion energy will be the optimal new energy for the sustainable development of human beings. As the structural materials, good mechanical properties, resistance to radiation, high temperature resistant and corrosion resistantperformance are necessary for the internal environment requires of fusion reactor. One of the key issues faced is which kind of structural material to choose. By contrast, RAFM steel has good comprehensive performance and mature industrial fabricationbasis, which is one of the fusion reactor candidate structural materials.Optimization of the RAFM steel composition and fabrication of RAFM steel were done in the paper. The processing and heat treatment process were groped, and the fundamental research was made by observation of microstructure, properties test and relative mechanism analysis. Ultra-low carbon RAFM steel was designed and fabricated in order to avoid the reducing of fracture toughness and the increasing of ductile-brittle transition temperature caused by the nuclear transmutation after neutron radiation. Compared with traditional RAFM steel fabricated in our laboratory, the ultra-low carbon RAFM steel has more excellent performances, including good strength and plasticity, superiortoughness, and lowerductile-brittle transition temperature of-80℃. which is reduced40℃.The reason is that ultra-low carbon composition design greatly improves the steel plastic and toughness, and the smaller TaC precipitationsimprove comprehensive mechanical properties in the form of precipitation strengthening and refining austenite grain.The continuous cooling transformation curves of two different kinds of RAFM steel were drawn using thermal dilation method and metallographic observation. When the cooling rate was fast, the microstructures were all martensite whose morphology was mainly narrow and slender lath and lots of slats in groups and connected with one another in parallel.By hot rolling with step test, the dynamic recrystallization behavior of austenite of RAFM steel was studied. Analysis of the effect of different pass reduction and rolling temperature on the dynamic recrystallization behaviorwas made, and the region graph ofthe dynamic recrystallization was drawn. It will provide the basis for formulating the practical production process of the RAFM steel.The static recrystallization behaviour of RAFM steel was studied on Gleeble-3500hot simulator by double-pass hot compressive test. Influences of different time intervals, deformation temperatures, strain rates, deformations on the static recrystallization behaviour were discussed and analyzed. The fractional softening of the static recrystallization under different deformation conditions was calculated with the stress compensation method of2%. According to the experimental data, the activation energy of the static recrystallization was calculated and the dynamic model of the static recrystallization was established. The dynamic model was verified by experiment result.The microstructures of two kinds of RAFM steel after quencging and tempering were observed by SEM and TEM. It was found thatmicrostructuregradually became tempered sorbite from the initial tempered martensite as the increasing of tempering temperature.Recovery and recrystallization would occuras the increasing of tempering temperature. By comparison, the mechanical properties of ultra-low carbon RAFM steel was the most excellent. The best thermal refining was consist of quenching process whichkept at980℃for1hour then quenched to room temperature, and tempering process which kept at750℃for1hour then air cooling to room temperature. According to this process, the mechanical properties at room temperature are that the yield strength is541MPa, tensile strength of668MPa, elongation of25%, and the toughnessimpact energy of200J, meanwhile, at600℃, yield strength of294MPa, tensile strength of321MPa, elongation of29%.There are two kinds of precipitations in RAFM steel which are M23C6and MC having the face-centered cubic structure. Due to the different components, the precipitations of RAFM steel are mainly M23C6of150nm and MC (TaC or VC) of about30～50nm. While the precipitations of the ultra-low carbon RAFM steel are mostly M23C6of100nm and MC (TaC) of about15nm.In situ tensile experiment, the inclusions in the RAFM steel were the source of micropore nucleation. The first type of inclusions cracked to form micropore under the larger stress which combined with the other cracks to form "micropore nucleation-pore grown-micropore polymerize". The second kind of inclusion was relatively hard that was not easy to crack and break under the action of external force. In the situ tensile process, the stress concentration caused by dislocation pileup led to inharmoniously deform and produce micropores.The toughness of the ultra-low carbon RAFM steel was excellent. The stress concentration easily produced at notch and inclusion during impact. At the low temperature, the toughness of the RAFM steel is very poor because of the cleavage fracture. The organization near the main crack had a strong internal stress which made the stress concentration in the surrounding microstructure, then the secondary cracks formed. Almost all the secondary cracks formed at the main crack tip of the cleavage fracture area. The cracks expanded through the connection of these micro-cracks.