提高燃煤发电机组的热效率，增加煤电转化率是减少燃煤电厂碳排放的必然要求。发展更高参数、更大容量的700 ℃先进超超临界(A-USC)燃煤发电技术可将发电效率可提高约50％，煤炭消耗量和CO₂排放量降低24％。HT700(GH4070P)镍铁基高温合金具有优异的机械性能和高温组织稳定性，被认为是700 ℃超超临界机组主蒸汽管道的备选材料。燃煤发电机组主蒸汽管道是典型的焊接结构部件，要求基体材料具有良好的焊接性能，并对其焊接接头的高温力学性能提出了更高要求。本文首先对HT700高温合金进行TIG焊点状拘束裂纹试验和横向拘束裂纹实验，分析了不同焊接条件下焊接接头的微观组织、性能和近缝区液化裂纹敏感性，结合热力学模拟分析了焊缝凝固裂纹敏感性，然后对管道焊接接头进行焊后热处理、长时热暴露以及持久性能实验，分析了焊接接头显微组织和性能，并采用Larson-Miller参数法预测了不同热处理制度下的焊接接头蠕变寿命，实现了焊后热处理制度的优化。全文获得以下主要结论：

（1）点状拘束裂纹实验表明，HT700合金自熔焊缝区存在凝固裂纹，焊接热影响区的近缝区由于Mo、Nb、W元素向γ基体晶界扩散形成合金碳化物MC，并与γ发生共晶反应而产生液化裂纹。随着焊接热输入的增大，近缝区的液化裂纹敏感性降低，当热输入达到29.2 kJ时，最大裂纹长度MCL最小。

（2）热力学稳态相图计算表明，Inconel 617焊丝和Haynes 282焊丝在凝固过程中均无杂质相产生，Inconel 617焊丝填充的焊缝金属具有更小的凝固温度区间(78.0 ℃)和更小的热膨胀系数(27.73×10^-6 K^-1~19.48×10^-6 K^-1)；Haynes 282焊丝熔覆金属在凝固温度区间具有比Inconel 617更低的液相粘度。

（3）横向拘束裂纹实验表明，在348.9 J/mm、384.0 J/mm的热输入条件下，HT700合金填充Haynes 282焊丝形成的焊缝与基体熔合效果良好，但有凝固裂纹产生；填充Inconel 617焊丝形成的焊缝成型良好，未发现凝固裂纹。随着热输入的增大，Inconel 617的枝晶间偏析系数较小，组织更为均匀，MC碳化物也更细小。热力学模拟结果显示，HT700合金填充Haynes 282焊丝与Inconel 617焊丝的焊缝相组成接近，填充Inconel 617形成的焊缝金属具有更小的液相粘度、凝固温度区间和热膨胀系数。

（4）HT700合金管道焊接焊缝成型良好，焊缝区组织由γ枝晶和二次碳化物M₆C组成；热影响区晶粒无明显长大现象，但晶界发生弯曲变形。两种焊后热处理的焊接接头热影响区和基体均析出了球状γ'强化相。经过675 ℃/4000 h热暴露后，两种焊后热处理的HT700合金焊接接头热影响区和基体沉淀强化相γ'形态始终保持球状，长大规律符合LSW理论。

（5）持久实验表明，700 ℃和980 ℃焊后热处理的焊接接头在210 MPa/700 ℃、230 MPa/700 ℃、260 MPa/675 ℃、280 MPa/675 ℃的温度/载荷条件下的持久断裂时间分别为2765.4 h、683.3 h、2472.5 h、2009.6 h和2188.1 h、1098.4 h、2564.6 h、1293.2 h。利用Larson-Miller参数法建立了焊接接头服役温度、载荷与蠕变寿命的定量关系，在675 ℃/30 MPa的服役条件下，700 ℃ PWHT焊接接头的蠕变寿命(982692.4 h)大于980 ℃ PWHT的蠕变寿命(751704.4 h)。

Improving the thermal efficiency of coal-fired generating units and increasing the conversion rate of coal-fired power is an inevitable requirement for reducing carbon emissions from coal-fired power plants. The development of 700 °C advanced ultra-supercritical (A-USC) coal-fired power generation technology with higher parameters and larger capacity can increase power generation efficiency by about 50%, and reduce coal consumption and CO₂ emissions by 24%. HT700 (GH4070P) nickel-iron-based superalloy has excellent mechanical properties and high temperature microstructure stability, and is considered as an alternative material for the main steam pipeline of 700 °C ultra-supercritical units. The main steam pipe of coal-fired generating units is a typical welded structural component, which requires the matrix material to have good welding performance, and puts forward higher requirements for the high temperature mechanical properties of its welded joints. In this paper, firstly, TIG spot-like constraint crack test and transverse constraint crack test were carried out on HT700 superalloy. The microstructure, properties and near-seam liquefaction crack sensitivity of welded joints under different welding conditions were analyzed. The sensitivity of weld solidification crack was analyzed by thermodynamic simulation. Then, post-weld heat treatment, long-term thermal exposure and creep performance experiments were carried out on pipeline welded joints. The microstructure and properties of welded joints were analyzed. The creep life of welded joints under different heat treatment systems was predicted by Larson-Miller parameter method, and the post-weld heat treatment system was optimized. The main conclusions are as follows :

(1) The spot-varestraint constraint crack test shows that there are solidification cracks in the self-fusion weld of HT700 alloy. The near-seam area of the welding heat affected zone is liquefied due to the formation of a eutectic liquid film resulting from the diffusion of Mo, Nb, and W elements to the grain boundary of the γ matrix to form alloy carbide MC and react with the γ phase. With the increase of welding heat input, the liquefaction crack in the near-seam area decreases. When the heat input reaches 29.2 kJ, the maximum crack length MCL is the smallest.

(2) The calculation of thermodynamic steady-state phase diagram shows that there is no impurity phase in the solidification process of Inconel 617 welding wire and Haynes 282 welding wire. The weld metal filled with Inconel 617 welding wire has a smaller solidification temperature range (78.0 °C) and smaller thermal expansion coefficient (27.73×10^-6 K^-1~19.48×10^-6 K^-1). The Haynes 282 wire cladding metal has a lower liquid viscosity than Inconel 617 in the solidification temperature range.

(3) The trans-varerestraint crack test shows that under the heat input conditions of 348.9J/mm and 384.0J/mm, the weld formed by HT700 alloy filled with Haynes 282 welding wire has a good fusion effect with the base metal, but there are solidification cracks. The weld formed by filling Inconel 617 welding wire is well formed and no solidification crack is found. With the increase of heat input, the interdendritic segregation coefficient of Inconel 617 is smaller, the microstructure is more uniform, and the MC carbides are finer. The thermodynamic simulation results show that the weld phase composition of HT700 alloy filled with Haynes 282 wire is close to that of Inconel 617 wire, and the weld metal formed by filling Inconel 617 has smaller liquid viscosity, solidification temperature range and thermal expansion coefficient.

(4) The weld of HT700 alloy pipeline is well formed, and the microstructure of weld zone is composed of γ dendrite and secondary carbide M₆C. There is no obvious grain growth in the heat affected zone, but the grain boundary is bent. The spherical γ' strengthening phase was precipitated in the heat affected zone and the base metal of the welded joints of the two post-weld heat treatment. After 675 °C/4000 h thermal exposure, the γ ' morphology of the heat affected zone and the base metal precipitation strengthening phase of the HT700 alloy welded joint of the two post-weld heat treatments always remains spherical, and the growth law conforms to the LSW theory.

(5) The creep rupture experiments show that the creep life of the welded joints treated at 700 °C and 980 °C under the temperature/load conditions of 210 MPa/700 °C, 230 MPa/700 °C, 260 MPa/675 °C and 280 MPa/675 °C is 2765.4 h, 683.3 h, 2472.5 h, 2009.6 h and 2188.1 h, 1098.4 h, 2564.6 h, 1293.2 h. The quantitative relationship between service temperature, load and rupture life of welded joints was established by Larson-Miller parameter method. Under the service condition of 675 °C/30 MPa, the creep life of 700 °C PWHT welded joints (982692.4 h) was greater than that of 980 °C PWHT (751704.4 h).

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