【作者】 王颖；

【导师】 徐德生；

【作者基本信息】 吉林大学 ， 材料加工工程， 2020， 硕士

【摘要】 随着人类生活水平不断提高,人们对能源的需求大幅度增加,传统能源的大量消耗对环境压力持续增大,为此大力发展清洁能源-核能成为我国能源战略之一。为实现技术革新,我国引进第三代核电站-AP1000核电站,所使用的是屏蔽式电机。由于核电站多建在沿海附近,海洋大气中的微小的氯化物颗粒及盐雾对金属零件的表面会产生腐蚀作用。为保证核电的安全,核电系统必须具有很高的耐蚀性能,因此,在核电关键零部件制造中大量采用哈氏合金与不锈钢等耐蚀性好的材料。AP1000核主泵定子屏蔽套采用的是哈氏合金C-276,与其相连的端盖部分材质则采用304不锈钢。在电机主泵零部件生产中会大量采用焊接方法,对C-276合金及其与304不锈钢使用传统的熔化焊接方法如TIG焊,由于焊接热输入量高,焊接接头热影响区较宽,焊缝晶粒粗大,且易生成有害相,使得焊缝耐蚀性差,成为影响材料的使用寿命最薄弱区域,为此需要寻求更先进的焊接方法。激光焊是一种新兴焊接方法,由于能量集中且密度高、热输入量低等特点,在对镍基合金、不锈钢等材料的焊接中表现出很大的优势,受到国内外学者广泛关注,出现对镍基合金及其不锈钢的激光焊接接头的微观组织及其力学性能大量研究,但接头处的耐蚀性很少提及,缺乏系统研究。为此本文将采用脉冲激光焊接方法制备成型良好的C-276合金/C-276合金和C-276合金/304不锈钢焊接接头,采用金相观察、扫描电镜观察(SEM)及能谱分析(EDS)等方法观察激光焊接接头的微观组织和元素分布,分析其微观组织形态及元素分布规律;分别从晶间腐蚀、电化学腐蚀和应力腐蚀三个方面对焊接接头的耐蚀性进行研究,结合微观组织分析其腐蚀机理,研究结果表明:(1)焊接参数为78A、7.5ms、9Hz和190mm/min的C-276/C-276脉冲激光焊接头平整,成型良好,无明显热影响区。激光焊接作用下焊缝组织包括柱状晶、胞晶和等轴晶,晶粒尺寸明显细化,对比20-30μm的母材晶粒尺寸,焊缝最小晶粒尺寸为2-3μm。对焊缝进行能谱扫描,发现有轻微元素偏析,晶界处的Mo元素含量略有降低。(2)晶间腐蚀试验结果表明,焊缝处有明显的晶间腐蚀趋势,主要是焊缝晶界处有贫Mo现象。电化学腐蚀试验表明,中性、酸性和碱性溶液中的C-276合金/C-276合金焊缝耐蚀性均优于母材,主要与焊缝晶粒细化有关。在中性溶液中,C-276合金与C-276合金/C-276合金焊缝发生选择性腐蚀,划伤处腐蚀明显,其余部分腐蚀较浅;在酸性溶液中,C-276合金与焊缝均发生晶间腐蚀,是Mo元素偏析导致;在碱性溶液中,C-276合金与焊缝腐蚀类型均为点蚀,与晶格缺陷有关。C-276合金/C-276合金激光焊接接头慢应变速率拉伸的结果表明,在空气与腐蚀溶液中试样均没发生断裂,接头处的力学性能良好,但腐蚀溶液中接头处的延伸率与收缩率明显下降,说明腐蚀溶液对材料的力学性能有影响。(3)C-276合金与304不锈钢在脉冲激光焊接参数为75A、7ms、8Hz及180mm/min下进行焊接,焊接接头成型良好,焊缝呈现旋涡状,组织包括柱状晶、等轴晶,晶粒尺寸较母材明显细化。在焊接过程中,合金元素重新分配,有元素偏析现象,但其焊缝处未形成析出相。(4)C-276合金/304不锈钢焊缝晶间腐蚀试验结果表明,C-276合金耐蚀性最好,其次为焊缝,最差为304不锈钢。C-276合金、304不锈钢及其焊缝电化学腐蚀结果表明,中性和酸性溶液中的耐蚀性大小为C-276合金>焊缝>304不锈钢,碱性溶液为焊缝>304不锈钢>C-276合金。在中性溶液中,304不锈钢发生局部腐蚀,焊缝发生均匀腐蚀;在酸性溶液中,304不锈钢与焊缝均属于晶间腐蚀,这与元素偏析有关,焊缝由于晶粒细小,整体腐蚀均匀;在碱性溶液中,304不锈钢及焊缝表面有较大且深的腐蚀坑,这与钝化膜缺陷有关,其中焊缝的腐蚀坑主要集中在下表面,与元素分布有关。在慢应变应力拉伸试验中,空气中与腐蚀溶液中的C-276合金/304不锈钢接头试样断裂所用时间分别是64h与50h,腐蚀溶液中试样的延伸率及断面收缩率均较空气中下降,溶液对接头的应力腐蚀敏感性有一定影响,断裂类型均属于韧窝断裂与沿晶断裂混合断裂。更多还原

【Abstract】 With the continuous improvement of human living standards,people’s demand for energy has increased substantially,and the huge consumption of traditional energy has continued to increase the pressure on the environment.Therefore,to vigorously develop clean energy-nuclear energy has become one of China’s energy strategies.China has introduced the third generation of nuclear power plant-AP1000 nuclear power plant in order to achieve technical innovation,which uses shielded motor.While nuclear power plants are mostly built near the coast,the metal parts would be corroded by small chloride particles and salt mist in the atmosphere.Nuclear power system must have high corrosion resistance to ensure the safety of nuclear power.Therefore,a large number of materials with good corrosion resistance,such as Hastelloy and stainless steel,are used in the manufacturing of nuclear power key parts.The stator shield sleeve of AP1000 nuclear main pump is made of Hastelloy C-276,and the end cover connected to it is made of 304 stainless steel.Welding would be used to product the motor main pump parts.Traditional fusion welding methods such as TIG welding with high heat input are used for C-276 alloy and 304 stainless steel,and wide heat affected zone and coarse grain of weld were got,and harmful phase is formed to make the corrosion resistance of welding joint poor,which has become the weakest area affecting the service life of materials.Therefore,it is necessary to seek for more advanced welding methods.Laser welding is a new welding method due to the characteristics of high energy concentration,high energy density and low heat input,it has shown great advantages in the welding of nickel base alloy,stainless steel and other materials,and has attracted more and more attention of researchers,and there are appearance of a lot of researches on the microstructure and mechanical properties of laser welded joints of nickel base alloy and stainless steel,but systematic research of the corrosion resistance of the joints is lacking.In this paper,the well formed C-276 / C-276 and C-276 / 304 pulse laser welded joints were prepared.The microstructure and element distribution of laser welded joints were observed and analyzed by metallographic observation,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The corrosion resistance of the welded joints was studied in three aspects,and the corrosion mechanism was analyzed.The results showed that:(1)The C-276 / C-276 pulsed laser welded joint with welding parameters of 78 A,7.5ms,9Hz and 190 mm / min is well formed and has no obvious heat affected zone.The weld structure includes columnar crystal,cellular crystal and equiaxed crystal,and the grain size is obviously refined.Compared with the base metal grain size of 20-30 μm,the minimum grain size of the weld is 2-3 μm.It is found that there is slight element segregation and the content of Mo element in the grain boundary in the weld is slightly reduced.(2)The results of intergranular corrosion test show that there is an obvious tendency of intergranular corrosion of the weld,mainly due to the phenomenon of Mo deficiency at the weld boundary.The electrochemical corrosion test shows that the corrosion resistance of C-276 / C-276 in neutral,acid and alkaline solution is better than that of the base metal,which is mainly related to the refinement of weld grain.In the neutral solution,the weld of C-276 alloy and C-276 / C-276 has selective corrosion,with obvious scratch corrosion and shallow corrosion in the rest part;in the acid solution,intergranular corrosion occurs between C-276 alloy and the weld,which is caused by Mo segregation;in the alkaline solution,C-276 alloy and weld happened pitting corrosion,which is related to lattice defects.The results of slow strain rate tensile test of C-276 / C-276 laser welded joint show that there is no fracture in the air and corrosion solution,and the mechanical properties of the joint are good.The elongation and shrinkage of the joint in the corrosion solution decrease obviously in the solution,which shows that there is an impact on the mechanical properties of the material.(3)C-276 alloy and 304 stainless steel were welded under the parameters of 75 A,7ms,8Hz and 180mm/min by pulse laser welding.The welding joint was well formed,and the weld showed vortex shape.The alloy elements are redistributed,and there is element segregation,but no precipitate is formed in the weld.(4)The results of intergranular corrosion test of C-276 / 304 showed that the corrosion resistance of C-276 alloy is the best,followed by weld,and the worst is 304 stainless steel.The results of electrochemical corrosion showed that the order corrosion resistance of C-276 alloy,304 stainless steel and the welds in neutral and acid solutions is C-276 alloy > weld > 304 stainless steel,and that is weld > C-276 alloy > 304 stainless steel in alkaline solutions.304 stainless steel happened local corrosion,and the welds were uniformly corroded in the neutral solution;304 stainless steel and the weld occurred intergranular corrosion in the acid solution,which is related to the element segregation;and 304 stainless steel and the welds surface have large and deep corrosion pits in the alkaline solution,and the corrosion pits of the weld are mainly concentrated in the bottom side,which related to element distribution.The results of slow rate stress test showed that the fracture time of C-276 / 304 joints in air and solution is 64 h and 50 h respectively.The elongation and area shrinkage of the sample in solution are lower than those in air.The solution has a certain influence on the stress corrosion sensitivity of the joint.The fracture types are mixed fracture of dimple fracture and intergranular fracture.更多还原