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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

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

[+] Author and Article Information
Warren Miglietti, Madeleine Du Toit

Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa

J. Eng. Gas Turbines Power 132(8), 082103 (May 11, 2010) (10 pages) doi:10.1115/1.4000149 History: Received May 07, 2009; Revised May 07, 2009; Published May 11, 2010; Online May 11, 2010

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.

Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Binary Ni–Hf phase diagram (21)

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Figure 2

Binary Ni–Zr phase diagram (22)

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Figure 3

Configuration of tensile and stress rupture test specimens

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Figure 4

Plate of 247.7 mm length with 1.5 mm groove machined in the center

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Figure 5

1.5 mm groove machined down the center/middle of the plate

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Figure 6

Configuration of tensile and stress rupture test specimens

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Figure 7

1.5 mm wide cracks on the sidewalls of a typically industrial gas turbine nozzle segment

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Figure 8

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Hf braze, produced at 1238°C for 4 h

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Figure 9

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Hf braze, produced at 1238°C for 12 h+1232°C for 4 h (SHT)

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Figure 10

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Hf braze, produced at 1238°C for 12 h+1232°C for 4hrs SHT and a HIP cycle at 1080°C for 4 h

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Figure 11

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze, produced at 1238°C for 4 h

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Figure 12

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze, produced at 1238°C/12 h+1232°C/4 h (SHT)

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Figure 13

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze, produced at 1238°C/12 h+1232°C/4 h (SHT) and a HIP cycle at 1080°C for 4 h

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Figure 14

SEM micrograph of the Ni–Cr–Hf braze, showing the intergranular Ni7Hf2 intermetallic phase

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Figure 15

SEM micrograph of the Ni–Cr–Hf braze, showing cuboidal γ′ precipitates within the MarM247 powder particles

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Figure 16

SEM micrograph of the Ni–Cr–Hf braze, showing the intergranular Ni5Zr intermetallic phase

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Figure 17

Secondary electron image of the Ni–Cr–Hf braze, highlighting the location of three spot chemical analyses of the intermetallic phases within the braze

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Figure 18

Secondary electron image of the Ni–Cr–Zr braze, highlighting the location of four spot chemical analyses of the intermetallic phases within the braze

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Figure 19

Larson–Miller plot of the two brazed joints (produced at 1238°C for 12 h followed by a 1232°C for 4 h solution heat treatment) tested at 900°C at various stresses/loads

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Figure 20

Larson–Miller plot for the IN738 base metal and the MarM247/Ni–Cr–Hf and MarM247/Ni–Cr–Zr joints after brazing for 12 h at 1238°C, followed by solution heat treatment (SHT) of 1232°C/4 h of 1232 and a HIP cycle

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Figure 21

LCF Properties at 870°C of IN738 Base Metal versus the MarM247/Ni–Cr–Zr & MarM247/Ni–Cr–Hf wide gap brazed joints

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