| MECHANICAL ENGINEERING |
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Fabrication and microstructure characterization of graded transition joints between Fe- and Ni- based alloys |
| LIU Jieyu1, LI Kejian1,2, HAN Chaoyu1, CAI Zhipeng1,2 |
1. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China;
2. Key Laboratory for Advanced Materials Processing Technology of Ministry of Education, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China |
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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 γ 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.
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| Keywords
dissimilar metal welds
graded transition joints
chemical composition
microstructure
hardness
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Issue Date: 06 November 2023
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2023,
Vol. 63
