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COSTE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500 HV0.2),进入两相共存区后急剧下降(低于200 HV0.2),在7镍基组织区域又缓慢增加(不高于250 HV0.2).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.
Abstract
[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 gradient was greatly reduced compared with conventional DMWs,as expected,and the microstructure from the steel side to the nickel-based side varied from quenched martensite,mixed A+M,and finally,full γ nickel-based microstructure.A hardness peak as high as 500 HV0.2 was found in the quenched martensite region,and hardness decreased sharply to lower than 200 HV0.2 once entering the martensite and austenite dual-phase region,followed by a gradual increase to 250 HV0.2 or less in the full y nickel-based microstructure region.The dynamic kinetics module revealed that Ni,Cr,and Mo tended to segregate into interdendritic regions during solidification.[Conclusions]Ni,Cr,and Mo segregation are due to the reason that the equilibrium partition coefficients of the three elements are less than 1,meaning that these elements are higher in concentration in the liquid than in the solid at the liquid/solid interface.Therefore,the concentrations of these elements in the newly formed solid are higher than those in the previously formed solid.The segregation of these elements in interdendritic regions lowers the martensite transformation starting temperature Ms below the ambient temperature,and thus,the austenite in interdendritic regions is stable even at room temperature,while the austenite in the dendrite core region transforms into martensite in the following cooling process,thus forming the A+M dual-phase region.
基金项目
国家自然科学基金青年科学基金资助项目(51901113)
国家科技期刊平台
NSTL
维普
万方数据