铁/镍基合金过渡接头的制造及微观组织表征  

Fabrication and microstructure characterization of graded transition joints between Fe-and Ni-based alloys

作　　者：刘洁钰 李克俭[1,2] 韩潮宇 蔡志鹏[1,2] LIU Jieyu;LI Kejian;HAN Chaoyu;CAI Zhipeng(Department of Mechanical Engineering,Tsinghua University,Beijing 100084,China;Key Laboratory for Advanced Materials Processing Technology of Ministry of Education,Department of Mechanical Engineering,Tsinghua University,Beijing 100084,China)

机构地区：[1]清华大学机械工程系,北京100084 [2]清华大学机械工程系,先进成形制造教育部重点实验室,北京100084

出　　处：《清华大学学报（自然科学版）》2023年第12期1974-1983,共10页Journal of Tsinghua University(Science and Technology)

基　　金：国家自然科学基金青年科学基金资助项目(51901113)。

摘　　要：使用镍(Ni)基焊材连接马氏体耐热钢与镍基合金形成的异种金属焊接接头(dissimilar metal welds,DMWs)在高超超临界火电机组中有着广泛的应用。由于镍基焊缝与马氏体耐热钢的化学成分差异较大,在镍基焊缝与马氏体耐热钢的界面附近存在较大的成分梯度,导致界面两侧的微观组织和力学性能差异较大,界面往往成为该类接头蠕变并早期失效的位置。本研究以COST E马氏体耐热钢和617B镍基合金作为母材,采用双丝电弧增材制造技术制造了一种新的梯度过渡接头(graded transition joint,GTJ)。对该GTJ进行了化学成分、微观组织和硬度分析,结果表明:相较于常规DMWs,GTJ的化学成分梯度大幅降低;从马氏体耐热钢侧到镍基合金侧,过渡区域的基体组织类型从淬火马氏体转变为马氏体+奥氏体两相共存,最后全部转变为γ镍基组织,硬度分布表现为在淬火马氏体区最高(最高约500HV0.2),进入两相共存区后急剧下降(低于200HV0.2),在γ镍基组织区域又缓慢增加(不高于250HV0.2)。使用动力学计算方法考查了两相共存区在凝固过程中的元素分配行为,发现Ni、铬(Cr)和钼(Mo)元素倾向于在枝晶界处偏析,从而降低枝晶界处马氏体的转变起始温度(martensite start,Ms),使枝晶界处在室温时依旧为奥氏体组织,而枝晶芯部转变为马氏体组织。[Objective] Dissimilar metal welds(DMWs) between martensitic heat-resistant steels and nickel-based alloys filled with nickel-based filler metals are widely used in advanced ultra-supercritical power plants. A large composition gradient is present at the interface between weld metals and martensitic heat-resistant steels due to the obvious difference in chemical composition between these two materials, resulting in an abrupt change in microstructure and mechanical properties across the interface. For DMWs exposed to creep conditions, interfacial failure is a commonly seen failure mode that reduces the creep life to less than half of the expected life. To eliminate the interface with a large composition gradient, a graded transition joint(GTJ) between the martensitic heat-resistant steel(named COST E) and 617B nickel-based alloys is fabricated using a dual-wire tungsten inert gas(TIG) welding technique in the present study. [Methods] The key part of the GTJ is a functionally graded material(FGM) in the middle, of which the chemical composition varied gradually from martensitic heat-resistant steels to 617B nickel-based alloys over a distance of 14 mm or less. During fabrication, the feeding rates of the two wires are varied in a controlled manner to obtain the desired dilution rates. After FGM fabrication is completed, the two ends of the FGM are joined by similar welds with corresponding base metals, thus fabricating a GTJ between COST E steels and 617B nickel-based alloys. Optical microscopy, scanning electron microscopy, energy dispersive spectrometry, electron probe microanalysis(EPMA), electron back-scattered diffraction, and a microhardness tester are used to investigate the chemical compositions, and the microstructure and hardness of the GTJ in as-weld condition are characterized. The dynamic kinetics module of Thermo-Calc, DICTRA, is used to investigate why mixed austenite(A)+martensite(M) formed based on the Scheil solidification equation. [Results] The results showed that the chemical composition gradie

关 键 词：异种金属焊接接头 梯度过渡接头 化学成分 微观组织 硬度 

分 类 号：TG401[金属学及工艺—焊接]