High strength, ductile braze repairs for stationary gas turbine components - Part II
High strength, ductile braze repairs for stationary gas turbine components - Part II
复制标题
固定式燃气轮机部件的高强度、延展性钎焊修复 - 第 II 部分
DOI：
10.1115/1.4000149
复制
发表时间：
2010
影响因子：
1.5
通讯作者：
M. Toit
中科院分区：
文献类型：
--
作者：
W. Miglietti;M. Toit
航空和陆基涡轮机部件（例如叶片/喷嘴、燃烧室、衬套和过渡件）在使用中经常会退化和破裂。创新维修可以帮助降低检修和维护成本，而不是更换新部件。这些部件由 FSX-414 等钴基固溶体高温合金或 IN738 等镍基伽玛沉淀强化高温合金铸造而成。 FSX-414和IN738的标称成分分别为Co-29.5Cr-10.5Ni-7W-2Fe [max]-0.25C-0.012B和Ni-0.001B-0.17C-8.5Co-16Cr-1.7Mo-3.4A1-2.6W-1.7Ta-2Nb-3.4Ti-0.1Zr。扩散钎焊用于修复此类部件的裂纹和退化已有 40 多年的历史。通常，用于部件修复的钎焊材料是含有 B 和/或 Si 作为熔点抑制剂的镍基和钴基钎焊填料。特别是在修复工业燃气轮机部件上常见的宽裂纹时，这些熔点抑制剂会形成脆性金属间硼化物和硅化物相，从而影响低循环和热疲劳等机械性能。这项工作的目的是研究和评估过共晶 Ni-Cr-Hf 和 Ni-Cr-Zr 钎焊填充金属的使用，其中熔点抑制剂不再是 B，而是 Hf 和/或 Zr。通常，当接头间隙或裂纹宽度小于 0.15 毫米时，可以单独使用钎焊填充金属。对于大于 0.15 毫米的裂纹，将高温合金粉末与钎焊填充金属混合，可以成功地钎焊修复宽裂纹。作为鉴定扩散钎焊修复的一种手段，进行了冶金和机械性能评估。金相评估包括光学和扫描电子显微镜以及微探针分析。扩散钎焊区域由细晶粒等轴结构组成，其中碳化物相、γ (γ) 枝晶、花状/玫瑰花状γ-γ') 共晶相以及分散在晶间和晶内的 Ni 7 Hf 2 、Ni 5 HF 或 Ni 5 Zr 金属间相。硬度测试表明，Ni-Hf和Ni-Zr金属间相的硬度范围仅为250-400 HV，而典型的Cr-硼化物相的硬度范围为800 HV至1000 HV。因此，Ni-Hf和Ni-Zr金属间相的硬度值比Cr-硼化物金属间相软2.5-3.2倍。因此，宽间隙 Ni-Cr-Hf 和 Ni-Cr-Zr 钎焊接头的低周疲劳 (LCF) 性能优于 Ni-Cr-B 钎料。机械性能评估包括室温和高温下的拉伸测试、760°C 至 1093°C 的应力断裂测试，最后是 LCF 测试，后者是最重要和最严格的测试之一，因为修复的裂纹是由热疲劳驱动的。在最佳钎焊热循环下，获得的机械测试结果至少为 80%，有时与母材性能相当。
Both aviation and land based turbine components such as vanes/nozzles, combustion chambers, liners, and transition pieces often degrade and crack in service. Rather than replacing with new components, innovative repairs can help reduce overhaul and maintenance costs. These components are cast from either Co-based solid solution superalloys such as FSX-414 or Ni-based gamma prime precipitation strengthened superalloys such as IN738. The nominal compositions of FSX-414 and IN738 are Co-29.5Cr-10.5Ni-7W-2Fe [max]-0.25C-0.012B and Ni-0.001B-0.17C-8.5Co-16Cr-1.7Mo-3.4A1-2.6W-1.7Ta-2Nb-3.4Ti-0.1Zr, respectively. Diffusion brazing has been used for over 4 decades to repair cracks and degradation on these types of components. Typically, braze materials utilized for component repairs are Ni- and Co-based braze fillers containing B and/or Si as melting point depressants. Especially when repairing wide cracks typically found on industrial gas turbine components, these melting point depressants can form brittle intermetallic boride and silicide phases that affect mechanical properties such as low cycle and thermal fatigue. The objective of this work is to investigate and evaluate the use of hypereutectic Ni-Cr-Hf and Ni-Cr-Zr braze filler metals, where the melting point depressant is no longer B, but Hf and/or Zr. Typically, with joint gaps or crack widths less than 0.15 mm, the braze filler metal alone can be utilized. For cracks greater than 0.15 mm, a superalloy powder is mixed with the braze filler metal to enable wide cracks to be successfully brazed repaired. As a means of qualifying the diffusion braze repair, both metallurgical and mechanical property evaluations were carried out. The metallurgical evaluation consisted of optical and scanning electron microscopies, and microprobe analysis. The diffusion brazed area consisted of a fine-grained equiaxed structure with carbide phases, gamma (γ) dendrites, flower shaped/rosette gamma-gamma prime (y-y') eutectic phases, and Ni 7 Hf 2 , Ni 5 HF, or Ni 5 Zr intermetallic phases dispersed both intergranularly and intragranularly. Hardness tests showed that the Ni-Hf and Ni-Zr intermetallic phase only has a hardness range of 250-400 HV, whereas, the typical Cr-boride phases have hardness ranges from 800 HV to 1000 HV. Therefore the hardness values of the Ni-Hf and Ni-Zr intermetallic phases are 2.5-3.2 times softer than the Cr-boride intermetallic phases. As a result the low cycle fatigue (LCF) properties of the wide gap Ni-Cr-Hf and Ni-Cr-Zr brazed joints are superior to those of the Ni-Cr-B braze filler metals. The mechanical property evaluations were tensile tests at both room temperature and elevated temperature, stress rupture test from 760°C to 1093°C, and finally LCF tests, the latter being one of the most important and severe tests to conduct since the cracks being repaired are thermal fatigue driven. At the optimum braze thermal cycle, the mechanical test results achieved were a minimum of 80% and sometimes equivalent to that of the base metal properties.