0
Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

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

[+] 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), 082102 (May 11, 2010) (12 pages) doi:10.1115/1.3155397 History: Received April 10, 2008; Revised May 04, 2008; 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 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.

Copyright © 2010 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 1

Binary Ni–Hf phase diagram

Grahic Jump Location
Figure 2

Binary Ni–Zr phase diagram

Grahic Jump Location
Figure 3

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

Grahic Jump Location
Figure 4

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

Grahic Jump Location
Figure 5

Configuration of tensile and creep rupture test specimens

Grahic Jump Location
Figure 6

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

Grahic Jump Location
Figure 7

Microstructure of a Ni–Cr–Hf narrow gap brazed joint produced at 1238°C/18 h (magnification of 200×)

Grahic Jump Location
Figure 8

Scanning electron micrograph of the Ni–Cr–Hf brazed joint

Grahic Jump Location
Figure 9

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Hf braze produced at 1238°C for 40 min (magnification of 200×)

Grahic Jump Location
Figure 10

Scanning electron micrograph of the wide gap brazed joint produced at 1238°C/40 min by mixing MarM247 powder and a Ni–Cr–Hf braze filler metal

Grahic Jump Location
Figure 11

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Hf braze produced at 1238°C for 4 h (magnification of 200×)

Grahic Jump Location
Figure 12

Scanning electron micrograph of the wide gap brazed joint produced at 1238°C/4 h by mixing MarM247 powder and a Ni–Cr–Hf braze filler metal

Grahic Jump Location
Figure 13

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 (magnification of 200×)

Grahic Jump Location
Figure 14

Scanning electron micrograph of the wide gap brazed joint produced at 1238°C for 12 h+1232°C for 4 h SHT by mixing MarM247 powder and a Ni–Cr–Hf braze filler metal

Grahic Jump Location
Figure 15

Microstructure of a Ni–Cr–Zr narrow gap brazed joint produced at 1238°C/18 h (magnification of 200×)

Grahic Jump Location
Figure 16

Scanning electron micrograph of the Ni–Cr–Zr brazed joint

Grahic Jump Location
Figure 17

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze produced at 1238°C for 40 min (magnification of 200×)

Grahic Jump Location
Figure 18

Scanning electron micrograph of the wide gap brazed joint produced by mixing MarM247 superalloy powder and a Ni–Cr–Zr braze filler metal

Grahic Jump Location
Figure 19

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze produced at 1238°C for 4 h (magnification of 200×)

Grahic Jump Location
Figure 20

Scanning electron micrograph of the wide gap brazed joint produced at 1238°C/4 h by mixing MarM247 powder and a Ni–Cr–Zr braze filler metal

Grahic Jump Location
Figure 21

Microstructure of MarM247 superalloy powder mixed with the Ni–Cr–Zr braze produced at 1238°C/12 h+1232°C/4 h SHT (magnification of 200×)

Grahic Jump Location
Figure 22

Scanning electron micrograph of the wide gap brazed joint produced at 1238°C for 12 h+1232°C for 4 h SHT by mixing MarM247 powder and a Ni–Cr–Zr braze filler metal

Grahic Jump Location
Figure 23

Larson–Miller plot of the two brazed joints tested at 843°C at various stresses/loads

Grahic Jump Location
Figure 25

Larson–Miller plot of the IN738 base metal versus the two brazed joints

Grahic Jump Location
Figure 24

Larson–Miller plot of the two brazed joints tested at 982°C at various stresses/loads

Tables

Errata

Discussions

Related

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In