Addition Forging Mechanisms Of Large-size ODS Steels Applied To Advanced Nuclear Equipments

Nuclear energy is a kind of low-carbon,high-efficiency,and sustainable clean energy,and has attracted international attention.Compared with the second and third generation nuclear power systems,the fourth-generation nuclear systems and fusion reactor have higher operating temperatures and irradiation doses,and harsher chemical corrosion environments.Since traditional nuclear power materials cannot meet these requirements,the development of high-performance nuclear power materials is one of the key factors to promote the development of nuclear energy.Oxide dispersion strengthened(ODS)steel contains a high number density of dispersed nano-oxides and defect sinks,exhibiting excellent high-temperature creep performance and irradiation swelling resistance.Therefore,ODS steel has been considered as one of the most promising materials for the fourth-generation nuclear fission reactor cladding tubes and nuclear fusion reactor blankets.Due to the unique microstructure,ODS steel cannot be prepared by the traditional smelting method,and powder metallurgy is the best method at present.While homogenized large-size ODS steel cannot be prepared by this method,which greatly limits the engineering application of ODS steel.Metal additive forging(MAF)technology,a solid-state deformation bonding technology of "by making greatly small",is expected to solve the problem of preparing homogeneous large-size ODS steel.In this paper,the key issues of MAF were systematically studied with 9Cr ODS steel as the research object.On the one hand,the effects of the MAF processing parameters and initial microstructures of material were investigated to reveal the healing mechanism of the MAF.Furthermore,the feasibility of MAF process was verified by engineering practices.On the other,the oxidation behavior of the bonding interface and the comprehensive evolution law of interfacial oxides were studied,and the interaction mechanism between matrix nano-oxides and interfacial oxides was elucidated.Moreover,the high-temperature stability of the matrix microstructures and nano-oxides were also clarified.All these results together verified the applicability of MAF on ODS steel.The main contents and conclusions in this thesis are as follows:(1)The effect of MAF processing parameters on interfacial microstructure and mechanical property.The bonding interface healing could be promoted by increasing deformation temperature and strain rate,reducing strain rate,and soaking treatment.The critical processing parameters for MAF were determined by the analysis of mechanical properties of joints:deformation temperature≥1000℃,strain≥30%,strain rate ≤0.01s-1,soaking temperature≥1000℃ and soaking time≥4 h.Based on the above critical processing parameters,the engineering practice of four pieces of kilogram-scale 9Cr ODS steel was carried out,and defect-free bar with 26 mm diameter and 600 mm length was obtained,which verified the feasibility of MAF on ODS steel.(2)The effect of initial microstructures on interfacial healing and the interaction mechanism between the matrix nano-oxides and the interfacial oxides.The interfacial recrystallization behavior of the coarse-grained 9Cr steel was classified as discontinuous dynamic recrystallization,while the fine-grained HIP 9Cr steel and 9Cr ODS steel were rotation dynamic recrystallization.Based on the interfacial bonding ratio,the ranking of interface bonding ability was 9Cr steel<9Cr ODS steel<HIP 9Cr steel.According to the characterization results of the interfacial oxides for the joint of 9Cr ODS steel and HIP 9Cr steel,the interfacial oxides changed from initial CrO to Y-Ti-O type oxides and then migrated to HIP 9Cr steel side.Therefore,the healing mechanism of the 9Cr ODS steel was as follows:the interfacial grain structure reached the matrix level by rotational dynamic recrystallization and grain growth,and the interfacial oxides transformed from the initial Cr-rich oxides to Y-TiO nano-oxides and diffused to both sides of the interface.Finally,the bonding interface achieved metallurgical bonding and interfacial microstructures became homogeneous.(3)The high-temperature oxidation behavior of the bonding initial interface under closed limited oxygen conditions.The oxidized surface consisted of inner Fe2O3 scale and outer Cr2O3 particles at 800℃.Reoxidation at 1000℃,Cr2O3 particles were decomposed and converted into Cr(Ti)O2.With the increase of oxidation time,Fe2O3 and Cr(Ti)O2 were decomposed and Y2O3 pegs formed.Furthermore,TiO2,Y2O3 and Y-Ti-O nanoparticles formed along grain boundaries.In conclusion,the oxidation products evolution process could be summarized as Fe2O3→Cr2O3→Cr(Ti)O2→TiO2+YaO3→Y-Ti-O nanoparticles.(4)The comprehensive evolution of interfacial oxides under thermodynamic coupling conditions was investigated.Under low temperatures and strains,the interfacial oxides were mainly Cr2O3 and TiCrO3.With improving the MAF processing parameters and soaking treatment,there was higher cumulative storage energy at the bonding interface,and the interfacial oxides transformed into TiO2,Y2O3 and Y-Ti-O type oxides.According to the detailed characterization,the main evolution process of 9Cr ODS steel interface oxides:Cr2O3→TiCrO3→TiO2→Y2O3→YTiO3→Y2TiO5→Y2Ti2O7/non-stoichiometric Y-Ti-O.Hence,the interfacial oxides could be further transformed into stable Y-Ti-O nanoparticles with sizes comparable to the matrix nano-oxides,which resulted in a continuous and uniform distribution of nano-oxide particles around the bonding interface and verified the applicability of the MAF process.(5)Microstructural stability and quantitative evaluation of nano-oxides under thermal aging at high temperatures.The microstructure was relatively stable during thermal aging at 1000℃ up to 500 h.Within 50 h of aging at 1200℃,δ-ferrite appeared,and its content increased with aging time.However,δ-ferrite completely disappeared and hexagonal TiO2 particles with the size of 1-3 μm were uniformly distributed in the matrix after aging for 200 h.The nano-oxide particles coarsened slightly at 1000℃,and the average size was～4.3 nm after aging for 500 h.Whereas significant coarsening took place at 1200℃ after aging for more than 20 h,and the particle size increased from 4.37 nm at 20 h to 5.88 nm at 50 h.These results indicate that the MAF process of 9Cr ODS steel should be less than 20 h at 1200℃or kept at lower temperatures to maintain the stability of nano-oxide particles and the overall microstructures.