High strength, ductile braze repairs for stationary gas turbine components - Part I
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-3.4Al-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 dendrites, flower shaped/rosette gamma-gamma prime - eutectic phases, and Ni7Hf2, Ni5HF, or Ni5Zr 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.