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 (γ) dendrites, flower shaped/rosette gamma-gamma prime (γ-γ′) 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.

High Strength, Ductile Braze Repairs for Stationary Gas Turbine Components—Part II

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 effect 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 braze 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 microscopy, and microprobe analysis. The diffusion brazed area consisted of a fine-grained equiaxed structure, with carbide phases, γ (gamma) dendrites, flower shaped/rosette γ-γ′ (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 tests from 760°C to 1093°C and finally LCF, 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 metals properties.

High Strength, Ductile Braze Repairs for Stationary Gas Turbine Components: Part 2

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 four 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 effect mechanical properties such as low cycle and thermal fatigue. The objective of this work is to investigate and evaluate the use of hyper-eutectic 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 0.15mm, the braze filler metal alone can be utilized. For cracks greater than 0.15mm, a superalloy powder is mixed with the braze filler metal to enable wide cracks to be successfully braze 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 microscopy, and microprobe analysis. The diffusion brazed area consisted of a fine-grained equiaxed structure, with carbide phases, γ [gamma] dendrites, flower shaped/rosette γ-γ′ [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 250Hv to 400Hv; whereas, the typical Cr-boride phases have hardness ranges from 800Hv to 1000Hv. 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 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 tests from 760°C—1093°C and finally low cycle fatigue [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 metals properties.

High Strength, Ductile Braze Repairs for Stationary Gas Turbine Components: Part 1

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 four 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 effect 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 0.15mm, the braze filler metal alone can be utilized. For cracks greater than 0.15mm, 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 microscopy, and microprobe analysis. The diffusion brazed area consisted of a fine-grained equiaxed structure, with carbide phases, γ [gamma] dendrites, flower shaped/rosette γ-γ′ [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 250Hv to 400Hv; whereas, the typical Cr-boride phases have hardness ranges from 800Hv to 1000Hv. 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 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 tests from 760°C–1093°C and finally low cycle fatigue [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 metals properties.

Silver-copper oxide based reactive air braze for joining yttria-stabilized zirconia

We investigated a new method of ceramic-to-metal joining, referred to as reactive air brazing, as a potential method of sealing ceramic components in high-temperature electrochemical devices. Sessile drop wetting experiments and joint strength testing were conducted using yttria stabilized zirconia (YSZ) substrates and CuO–Ag-based air brazes. Results from our studies indicate that the wettability of the braze improves substantially with increasing CuO content, over a compositional range of 1–8 mol% CuO, which is accompanied by an increase in the bend strength of the corresponding brazed YSZ joint. The addition of a small amount of TiO2 (0.5 mol%) to the CuO–Ag braze further improves wettability due to the formation of a titanium zirconate reaction product along the braze/substrate interface. However, with one notable exception, the bend strength of these ternary braze joints remained nearly identical to those measured in comparable binary braze joints. Scanning electron microscopy analysis conducted on the corresponding fracture surfaces indicated that in the binary braze joints, failure occurs primarily at the braze/YSZ interface. Similarly in the case of the ternary, TiO2-doped brazes joint failure occurs predominantly along the interface between the braze filler metal and the underlying titanium zirconate reaction layer.

NICROBRAZ 31

Nicrobraz 31 is a high-chromium, oxidation-resistant nickel braze filler metal with enhanced flow properties. This datasheet provides information on composition. It also includes information on corrosion resistance as well as joining. Filing Code: Ni-614. Producer or source: Wall Colmonoy Corporation.

NICROBRAZ 170

NICROBRAZ 170 is an extra high strength at high temperature braze filler metal. It is good for brazing nickel-base materials containing cobalt, tungsten, and molybdenum. This datasheet provides information on composition, physical properties, and hardness. It also includes information on corrosion resistance as well as joining. Filing Code: Ni-476. Producer or source: Wall Colmonoy Corporation.