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
来源期刊:稀有金属2021年第1期
论文作者:靳舜尧 黄重国 李升
文章页码:10 - 18
关键词:核用FeCrAl合金;包壳管;固溶热处理;显微组织;力学性能;
摘 要: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Rm为675 MPa,屈服强度Rp0.2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。
网络首发时间: 2019-04-04 10:15
稀有金属 2021,45(01),10-18 DOI:10.13373/j.cnki.cjrm.xy19010006
靳舜尧 黄重国 李升
北京科技大学机械工程学院
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Rm为675 MPa,屈服强度Rp0.2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。
核用FeCrAl合金;包壳管;固溶热处理;显微组织;力学性能;
中图分类号: TM623;TG156.94;TG132
作者简介:靳舜尧(1984-),男,北京人,博士,工程师,研究方向:轻量化制造技术、航空航天材料特性及成形工艺研究、车辆结构设计与制造技术、新能源关键材料及装备研究,E-mail:jsy_white@126.com;;*黄重国,副教授,电话:13810596811,E-mail:jinshunyao@ustb.edu.cn;
收稿日期:2019-01-03
基金:中央高校基本科研业务费专项资金项目(FRF-TP-19-013A2)资助;
Jin Shunyao Huang Zhongguo Li Sheng
School of Mechanical Engineering,University of Science and Technology Beijing
Abstract:
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 of σ-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)and σ-Fe Cr phas-es.The σ-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(Rm)reduced from 1023 to 675 MPa,the yieldstrength(Rp0.2)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.
Keyword:
nuclear FeCrAl alloy; cladding tubes; solution treatment; microstructure; mechanical property;
Received: 2019-01-03
核燃料包壳管是核电站安全的第一道安全防线,是保证核电站安全正常运行和在意外事故下防止核燃料泄露的重要部件
核用Fe Cr Al合金中主要是以Fe为基础,在其中添加了Cr,Al和其他微量元素Nb,Y,Zr,Mo。Cr可以提高合金的高温下耐腐蚀能力、抗中子辐照能力,但是过高的Cr会使合金有脆化的风险。Al能增强合金的高温抗氧化能力,但是添加过多时容易影响室温的成形能力
国内外学者(以美国橡树岭国家实验室为主)已对核用Fe Cr Al合金进行了相关研究,对其进行了各合金元素的最佳配比实验研究,以及对一系列的高温下氧化、腐蚀性以及辐照性能进行了系统研究
本文根据研究的最佳成分配比熔炼核用FeCr Al合金,研究经过锻造、打孔的管坯的固溶热处理制度对其显微组织和力学性能的影响,最终确定合适的固溶热处理制度,以改善Fe Cr Al合金的加工性,解决冷轧阶段难变形的问题。
1实验
1.1材料
实验材料为Φ35 mm×4.5 mm锻态Fe Cr Al合金打孔管坯,化学成分如表1所示,显微组织如图1所示,由图1可知锻态Fe Cr Al合金基体显微组织尺寸不均匀,其中仍存在畸变的晶粒,合金基体中储存着较大的畸变能。
1.2方法
在管坯上切割取样,采用KSW-12-12A型箱式电阻炉进行不同温度的固溶热处理试验,具体工艺如表2所示。将固溶处理后的样品经初磨、细磨和机械抛光后,采用盐酸25%+硝酸25%+水50%混合溶液擦拭腐蚀30~60 s,进行金相(OM)、扫描电镜(SEM)的组织观察。将试样用水砂纸逐道次磨至2000#后,采用HV-1000型维氏硬度计进行测定,负荷为0.49 N,加载时间为30 s,在每个试样上打5个压痕,计算出各个点的材料硬度。拉伸实验使用INSTRON5569电子万能材料拉伸试验机,额定载荷为5 t,拉伸速率为1.00 mm·min-1。析出相扫描和能谱(EDS)分析采用Phenom飞纳-台式扫描电镜,放大倍数150000倍,分辨率8 nm。用Bruker D8 Focus X射线衍射仪(XRD)(Cu Kα,λ=0.15406nm)检测不同固溶热处理后系列合金的析出相主要类型。
表1 核用Fe Cr Al合金化学成分 下载原图
Table 1 Nuclear Fe Cr Al alloy chemical composition(%,mass fraction)
图1 锻态Fe Cr Al合金管材组织
Fig.1 OM image showing microstructure of forged Fe Cr Al al-loy tube
表2 固溶热处理实验方案 下载原图
Table 2 Experimental of solution heat treatment
2结果与讨论
2.1固溶热处理对合金析出相的影响
2.1.1固溶热处理温度的影响
析出相的数量、大小和形状都会对材料的塑性产生影响,减少沉淀的产生一定程度上能够增加塑性,改善室温下的可加工性
图2 不同固溶温度保温60 min后水冷的Fe Cr Al合金SEM图像
Fig.2 SEM images of Fe Cr Al alloy solution treated at different temperatures for 60 min and water cooling
(a)Forged;(b)1050℃;(c)1100℃;(d)1150℃;(e)1200℃
图3 锻态Fe Cr Al合金的XRD图谱
Fig.3 XRD pattern of forged Fe Cr Al alloy
锻态Fe Cr Al合金组织中弥散分布着数量较多、尺寸较大的球形析出相,其尺寸在3~6μm,如图2(a)所示,利用SEM分析A处析出相的能谱,如图5所示,根据表3的能谱扫描分析和图3的XRD结果,可以判断A处析出相主要类型为Fe2(Nb/Mo)Laves析出相,它可以在较高温度下存在,并抑制铁素体基体的长大,虽然它的存在可以提高其高温强度,但对于轧制阶段是不利的。根据表3,从化学成分分析B处可能主要包含σ-Fe Cr及富Cr脆性相,这种类型的析出相容易使合金发出脆断,对轧制十分不利,应该利用固溶热处理使其回溶。当温度低于1200℃时,根据表3数据,可以知道析出相的主要类型没有发生变化。当固溶热处理温度上升到1200℃时,大部分析出相基本回溶到基体之中,只留下很少的椭球形的析出相,如图2(e)所示,根据表3能谱扫描分析,J处大尺寸析出相其主要为富Nb相,这种析出相属于难溶相,可以在高温下存在,但因Nb属于添加的微量元素,所以没有形成大量的析出相。
图4为锻态及不同固溶热处理温度下的FeCr Al合金析出相尺寸变化图,结合图2可知,FeCr Al合金经过加热1050℃,60 min保温和水冷的固溶热处理之后,与锻态组织相比,析出相在数量上变化不大,平均尺寸有所降低,如图2(b)所示,这说明固溶热处理温度较低时,第二相无法向基体中回溶。随着固溶热处理温度升高到1100℃和1150℃,析出相数量开始减少,平均尺寸也在减小,如图2(b,c),说明第二相在不断地向铁素体基体中回溶。另外从图2中也可以发现,小尺寸析出相首先发生回溶,大尺寸析出相后发生回溶,这是因为σ-Fe Cr及富Cr相熔点较低,而Fe2(Nb/Mo)Laves析出相熔点较高,熔点高的析出相高温下存在的可能性更大。而当固溶热处理温度为1200℃时,析出相数量下降到很低,椭球状析出相保留了下来。
表3 不同固溶热处理温度下的Fe Cr Al合金沉淀能谱统计表 下载原图
Table 3 Precipitation statistics of Fe Cr Al alloy at different solutiontreatmenttemperatures(%,massfraction)
图4 锻态及不同固溶热处理温度下的Fe Cr Al合金析出相尺寸变化图
Fig.4 Dimensional change of precipitated phase of Fe Cr Al al-loy of forged and solution treatment at different tempera-tures
2.1.2固溶热处理时间的影响
实验中还发现,在1050,1100和1050℃加热,水冷和不同保温时间下(30,60,120 min),析出相区别不大,但在1200℃加热,水冷的情况下保温30,60和120min的析出相从数量、大小、形态、分布以及成分上都发生了显著变化,如图6所示。保温30 min时,析出相依然以一定数量存在,如图6(a)所示,大尺寸球形析出相成分(原子分数)为Fe54.42%,Cr7.79%,Al4.03%,Mo6.00%,Nb25.51%,Y2.15%,判断其依然是Fe2(Nb/Mo)Laves析出相,小尺寸球形析出相成分为Fe67.81%,Cr8.96%,Al8.25%,Mo3.22%,Nb8.96%,Y1.03%(原子分数),可以知道该析出相依然为上文的σ-Fe Cr及富Cr相,由此可知,短时间的保温无法使全部的析出相回溶。当保温60 min时,析出相基本回溶,只留下少量难溶的椭球形析出相,如图6(b)所示,定量分析表明,该析出相成分(原子分数)为Fe4.43%,Cr2.14%,Al2.04%,Mo2.42%,Nb86.52%,Y2.43%,为富Nb相。而当保温时间延长到120 min时,第二相重新从铁素体基体中析出,如图6(c)所示,析出相主要沿着晶界析出,呈现长条状,成分为Fe54.76%,Cr7.68%,Al4.25%,Mo6.51%,Nb25.22%,Y1.58%,可以判断为Fe2(Nb/Mo)Laves析出相,由此可知,Fe2(Nb/Mo)Laves析出相的析出行为是可控的,利用保温时间的长短可以控制其析出,以达到在不同工艺阶段对析出相的控制要求。
图5 锻态Fe Cr Al合金点A析出相能谱分析
Fig.5 EDS analysis of precipitated phase at Point A of forged Fe Cr Al alloy
图6 1200℃固溶不同时间水冷后的Fe Cr Al合金SEM图像
Fig.6 SEM images of Fe Cr Al alloy after water cooling at 1200℃for different time
(a)30 min;(b)60 min;(c)120 min
2.2固溶热处理对合金组织晶粒的影响
影响冷轧管加工性能的因素众多,其中晶粒的均匀性和尺寸大小是很重要的影响因素,固溶热处理对此具有有效的调控作用,合适的固溶热处理制度能够改善晶粒均匀性和尺寸大小
图7是Fe Cr Al合金在不同固溶热处理温度下的显微组织,图8为锻态及不同固溶热处理温度下的Fe Cr Al合金晶粒尺寸变化图,从图中可以看出,随着固溶温度的提高,Fe Cr Al合金晶粒组织呈现均匀化趋势,尺寸呈现先减小再增大的趋势。当固溶热处理温度为1050℃和1100℃时,Fe Cr Al合金组织没有发生完全再结晶,如图7(a,b)所示,组织中仍有部分锻造时的变形晶粒,尺寸尚不均匀。当固溶热处理温度提高到1150℃时,Fe Cr Al合金组织发生了完全再结晶,如图7(c)所示,原有锻态变形组织在固溶热处理下重新开始生长,新生长出的组织晶粒均匀且细小,尺寸在50~70μm。其中1200℃时,如图7(d)所示,晶粒尺寸较1150℃时粗大,尺寸在400~700μm,这是由于1200℃时,析出相大部分回溶到基体之中,使晶界处的阻力减小,高温下晶粒发生了异常长大,这可能会导致合金力学性能的下降。
2.3固溶热处理对合金力学性能的影响
如图9所示为不同固溶温度下Fe Cr Al合金硬度变化图,从图中可以看出,Fe Cr Al合金硬度随着温度的升高先从HV 368减小到HV 244再增大到HV 264。锻态Fe Cr Al合金的维氏硬度为HV 368,经过固溶热处理后硬度均下降到HV 270以下,说明固溶热处理能够显著降低Fe Cr Al合金的硬度。1100℃后硬度呈现一定程度的回升,可能是因为析出相逐渐回溶,产生溶质强化作用,使基体硬度得到提高[27]。
图7 不同固溶温度保温60 min后水冷的Fe Cr Al合金显微组织(OM)
Fig.7 OM images of Fe Cr Al alloy after water cooling at different temperatures for 60 min
(a)1050℃;(b)1100℃;(c)1150℃;(d)1200℃
图8 锻态及不同固溶热处理温度下的Fe Cr Al合金晶粒尺寸变化图
Fig.8 Grain size change diagram of Fe Cr Al alloy in forging and at different solution treatment temperatures
图9 锻态及不同固溶温度保温60 min后水冷的Fe Cr Al合金硬度变化图
Fig.9 Change of hardness of Fe Cr Al alloy after water cooling in forging and at different solution temperatures for 60 min
锻态Fe Cr Al合金经过固溶热处理后,组织晶粒和析出相都发生了改变,其力学性能也会发生变化,如图10为不同固溶温度下Fe Cr Al合金的力学性能变化图,由图10可知,随着温度的升高,伸长率(A)增加再减小,最高点为20.1%,相比锻态伸长率5%,热处理后伸长率得到明显改善,抗拉强度(Rm)和屈服强度(Rp0.2)随着温度的升高而降低,这是因为,随着固溶热处理温度的提高,析出相不断向基体中回溶,晶粒之间的阻力和固溶强化作用不断被削弱,导致合金强度降低。值得注意的现象是,在1200℃时,强度下降的幅度放缓,伸长率(A)开始降低,其原因是:一方面,随着固溶热处理温度的提高,合金中的Laves析出相不断回溶到基体中,到达1200℃时,原有的σ-Fe Cr相、富Cr相和Fe2(Nb/Mo)Laves析出相大部分回溶到基体中,导致晶粒在高温下发生了异常长大,晶粒尺寸较大且不均匀。另一方面,不断回溶的析出相也会对基体产生一定的溶质强化作用。
图1 0 不同固溶温度下Fe Cr Al合金力学性能变化图
Fig.10 Change diagram of mechanical properties of Fe Cr Al al-loy at different solution temperatures
2.4固溶热处理工艺优化
锻造后的核用Fe Cr Al合金包壳管存在数量较多、尺寸较大的析出相,组织晶粒存在畸变,尺寸不均匀,导致其管材硬度高、强度大、塑性差。欲采用冷轧工艺,必须通过合适的固溶热处理工艺使析出相回溶到基体中,降低其硬度和强度,改善加工性能。根据实验的结果讨论,Fe Cr Al合金在低温(1050℃、1100℃)和短时间保温(30 min)的固溶热处理制度下都无法使σ-Fe Cr相、富Cr相和Fe2(Nb/Mo)Laves析出相完全回溶,长时间保温(120min)会使本来回溶的析出相又重新析出,高温(1200℃)下析出相回溶情况较好但易发生晶粒的异常长大,力学性能实验也表明1200℃时延伸率反而有所降低,强度下降的幅度放缓。而在加热1150℃、保温60 min和水温冷却的固溶热处理制度下,析出相回溶较多,晶粒尺寸也没有发生异常变大,硬度较低,延伸率达到最高,塑性较好,可作为最终Fe Cr Al合金包壳管的固溶热处理制度。
3结论
1.当固溶热处理温度较低时,保温60 min后Fe Cr Al合金组织中仍然存在一定数量的、尺寸大小在3~6μm的σ-Fe Cr相、富Cr相和Fe2(Mo,Nb)Laves析出相,随着温度逐渐升高,析出相开始向基体中回溶,当温度升高到1200℃时,保温60 min后析出相基本回溶,但是组织晶粒发生了异常长大。
2.固溶热处理温度低于1200℃时,随着温度的升高,Fe Cr Al合金的强度逐渐从1023 MPa降低到650 MPa,延伸率从6.0%先升高到20.1%再降低到18.4%,硬度先从HV 368减小到HV 244再增大到HV 264。
3.最终选定固溶热处理制度为加热1150℃、保温60 min和水温冷却,相比锻态合金,核用FeCr Al合金包壳管的伸长率(A)从原来的4%提高到20.1%,同时抗拉强度(Rm)由1023 MPa降低到675MPa,屈服强度(Rp0.2)降低到560 MPa,硬度降低到HV 261,塑性得到显著提高,加工性得到显著改善,可满足管材后续轧制的要求。
4.核用Fe Cr Al合金中析出相的种类会随着固溶热处理温度的变化而变化。锻态和1050℃固溶热处理下析出相主要为Fe2M(M=Mo,Nb)(h P12-Mg Zn2型)和σ-Fe Cr相,经过加热1150℃,析出相,随着温度的提升,σ-Fe Cr相逐渐溶解,而Fe2(Nb/Mo)Laves被部分保留,到了1200℃,析出相转变为少量的富Nb相。
参考文献