固溶热处理对新型核用FeCrAl合金包壳管组织和力学性能的影响

通过新型核用锻态FeCrAl合金包壳管在不同温度及时间下的固溶处理实验,利用光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射(XRD)、维氏硬度仪(HV)和电子万能试验机(EUTM)分析了固溶处理对FeCrAl合金包壳管显微组织以及力学性能的影响规律。结果表明,当固溶热处理温度低于1200℃时,保温60 min后FeCrAl合金组织中仍然存在一定数量的σ-FeCr相、富Cr相和Fe₂(Mo/Nb)Laves析出相。随着温度的升高,析出相逐渐溶解到基体之中,当温度升高到1200℃时,保温60 min后组织内的析出相基本回溶,但晶粒发生了异常长大。随着保温时间的不同,析出相的数量、形态、分布和成分都会发生变化。随着温度的升高,FeCrAl合金的抗拉强度逐渐降低,延伸率先升高再降低,硬度先减小再增大。在1150℃加热、60 min保温和水冷的固溶热处理制度下,核用FeCrAl合金包壳管的伸长率可达20.1%,抗拉强度R_m为675 MPa,屈服强度R_p0.2为560 MPa,硬度为HV 261,塑性提高,加工性得到显著改善,可满足管材后续轧制的要求。

Microstructure and Mechanical Properties of a New Nuclear FeCrAl Alloy Cladding Tubes with Solution Heat Treatment

Nuclear fuel cladding tube is the first line for nuclear power plant safety.It is an important component to ensure the safe andnormal operation of nuclear power plants and prevent nuclear fuel leakage under accidents.In recent years,it has been found that Fe-CrAl alloy has excellent resistance to high temperature oxidation,radiation,corrosion and excellent comprehensive mechanical proper-ties,and is expected to become the next generation of accident-resistant fuel cladding substitute materials.However,the addition ofmultiple elements produced more insoluble precipitated compounds in the FeCrAl alloy,resulting in high strength and low plasticity ofthe forged FeCrAl alloy during cold rolling.By studying the effect of solution heat treatment on the microstructure and mechanical prop-erties of forged tube blanks,a suitable solution heat treatment was finally determined to improve the workability of FeCrAl alloy.First-ly,the FeCrAl alloy samples were processed with different solution heat treatment using box-type resistance furnace.Then the sampleswere mechanically polished and corroded with a mixed solution of HCl(25%)+HNO₃(25%)+H₂O(50%).The microstructure was ob-served by metallography(OM)and scanning electron microscope(SEM).The microhardness was measured using a Vickers hardnesstester(HV).Tensile performance was tested using electronic universal testing machine(EUTM).The mainly types of the precipitateswere observed using SEM and X-ray diffraction(XRD).When the solution heat treatment temperature was below 1200℃,a certainamount ofσ-Fe Cr phase,Cr-rich phase and Fe₂(Mo/Nb)Laves phase still existed in the FeCrAl alloy.The precipitates gradually dis-solved into the matrix with the increasing temperature.The precipitated phase dissolved into matrix at 1200℃,but the grains grew ab-normally.The types of precipitates in FeCrAl alloys changed with the solution heat treatment temperature.The precipitated phases inthe forged FeCrAl alloy and solution heat treatmented at 1050℃were mainly Fe₂M(M=Mo,Nb)(hP12-Mg Zn₂ type)andσ-Fe Cr phas-es.Theσ-FeCr phase was gradually dissolved with the temperature increased to 1150℃,while Fe₂(Nb/Mo)Laves was partially re-tained.At 1200℃,the precipitated phase transformed into a small amount of Nb-rich phase.The holding time had a significant effecton the number,size,shape,distribution and composition of the precipitated phases.When the solid solution temperature was lowerthan 1200℃,the length of holding time had little effect on the precipitated phase.At 1200℃,a short time of holding could not re-dis-solve all the precipitated phases.When the holding time was extended from 30 to 60 min,the precipitated phases basically re-dis-solved,leaving only a small amount of insoluble ellipsoidal precipitated phases.When the time was extended to 120 min,the secondphase precipitated from the ferrite matrix.The precipitation behavior of Fe₂(Nb/Mo)Laves precipitated phase could be controlled bythe length of holding time.With the increase of the solution temperature,the FeCrAl alloy grains showed a trend of homogenization,and the grainsize showed a trend of first decreasing and then increasing.When the solution heat treatment temperature was lower than 1150℃,FeCrAl alloy had not completely recrystallized,and there were still some deformed grains,and the grain size was not uniform.The FeCrAl alloy structure was completely recrystallized with the heat treatment temperature increased to 1150℃,the grains were uni-form and fine.At 1200℃,most of the precipitated phase re-dissolved into the matrix,which reduced the resistance at the grain boundaryleading to abnormal growth of crystal grains.With the increase of temperature,the tensile strength of FeCrAl alloy gradually decreased,the elongation increased first and then decreased,the hardness decreased first and then increased.When the solution heat treatment tem-perature was lower than 1200℃,the strength of FeCrAl alloy was gradually decreased from 1023 to 650 MPa with the increasing tempera-ture,and the elongation was firstly increased from 6.0%to 20.1%and then decreased to 18.4%,the hardness first decreased from HV368 to HV 244 and then increased to HV 264.The solution heat treatment system was finally selected as heating at 1150℃,holding timefor 60 min and cooling with water temperature.Compared with the forged alloy,the elongation of the core FeCrAl alloy cladding tubecould be increased from 4%to 20.1%,and it was also resistant to tension.The strength(R_m)reduced from 1023 to 675 MPa,the yieldstrength(R_p0.2)reduced to 560 MPa,the hardness reduced to HV 261,the plasticity was significantly improved,and the workability wassignificantly improved,which could meet the requirements of subsequent rolling of tubes.The solution heat treatment temperature andholding time had a great influence on the grain size of FeCrAl alloy and the number,morphology,distribution and composition of precipi-tated phases.The optimized solution heat treatment process could increase the elongation of the FeCrAl alloy cladding tubes,the plastici-ty and the workability were improved significantly,which could meet the requirements of subsequent cold rolling.All the above researchresults could provide a theoretical basis for the rolling production of FeCrAl alloy cladding tubes.