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

Fatigue Properties of Narrow and Wide Gap Braze Repaired Joints

[+] Author and Article Information
Thomas Henhoeffer

 Liburdi Turbine Services Incorporated, Dundas, ON L9H 7K4, Canada

Xiao Huang

Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON K1S 5B6, Canada

Scott Yandt, Peter Au

Institute for Aerospace Research, National Research Council, Ottawa, ON K1A 0R6, Canada

J. Eng. Gas Turbines Power 133(9), 092101 (Apr 15, 2011) (7 pages) doi:10.1115/1.4002824 History: Received July 15, 2010; Revised July 15, 2010; Published April 15, 2011; Online April 15, 2011

With the increasing utilization of braze repair in the gas turbine industry, the properties of braze joints under simulated service conditions become vital in selecting braze repair over other processes. While braze repair has often been claimed to deliver mechanical properties equivalent to that of the parent material, this is largely based on the results of tensile or accelerated creep tests for most gas turbine hot section components failure occurs as a result of thermal fatigue or thermomechanical fatigue. The damage that occurs under such conditions cannot be assessed from tensile or creep testing. This study was undertaken to characterize the fatigue properties of narrow and wide gap brazed X-40 cobalt-based superalloy and compare these properties to that of the X-40 parent material. Butt joint narrow gap and wide gap specimens were vacuum brazed using BNi-9 braze alloy. X-40 and IN-738 were used as additive materials in wide gap braze joints. To characterize the fatigue properties of the braze joints and parent material, isothermal fatigue tests were conducted at 950°C and under load control using a fully reversed sinusoidal wave form having stress amplitude of 75% of the yield strength of the parent material. The braze specimens were fatigue tested in the as-brazed condition. The fatigue test results showed that the fatigue lives of the brazed specimens were lower than that of the parent material, particularly for the narrow gap samples and wide gap samples containing IN-738 additive alloy. All fatigue failures in the brazed samples occurred in the braze joints. An analysis of the fracture surfaces using a scanning electron microscope revealed that porosity was the major contributing factor to fatigue failures in the wide gap braze joints. The testing life debit observed in the narrow gap braze samples can be attributed to the presence of brittle boride phases in the braze joint. This study also included examination of techniques for reducing the aforementioned porosity and presence of brittle intermetallic phases.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Gap width for WGB specimen

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Figure 2

Mechanical testing coupon geometry

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Figure 3

Fatigue test load waveform

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Figure 4

Narrow gap braze joint microstructure

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Figure 5

Porosity in WGB joints with (a) X-40 and (b) IN-738 additive alloys

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Figure 6

WGB and NGB fatigue test results

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Figure 7

Features of baseline X-40 fatigue coupon fracture surface: (a) possible initiation site, (b) fatigue striations in high crack growth region, and (c) cracking between eutectic arms in overload region of fracture surface

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Figure 8

Features of an as-brazed narrow gap braze fatigue coupon fracture surface: (a) possible initiation site and ((b) and (c)) exposed eutectic phases on fracture surface.

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Figure 9

WGB joint with X-40 additive alloy fatigue coupon (a) fracture surface and (b) initiation site

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Figure 10

Features of a WGB joint with IN-738 additive alloy fatigue coupon fracture surface: (a) possible initiation site showing fatigue river pattern and (b) fatigue striations near high crack growth rate zone

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Figure 11

Fatigue path along the brazed region. (a) Crack initiation site at surface. (b) Center of the fracture surface. Note that there is lack of voids in the narrow gap brazed region.

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Figure 12

Secondary cracks associated with boride particles (light arrows). Oxidation along cracked surfaces (dark arrows).

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Figure 13

Overview of fracture surface of WGB joint with X-40 and BNi-9

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Figure 14

(a) Fracture surface near fatigue crack initiation, (b) secondary crack developed adjacent to the fracture surface, and (c) secondary cracks crack propagation path and intergranular separation for WGB sample with X-40 and BNi-9

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Figure 15

Overview of fracture surface of WGB joint with IN-738 and BNi-9

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Figure 16

(a) Crack initiation site linked to interfacial voids (20 deg tilt). (b) Secondary cracks radiated from the fracture surface (20 deg tilt, steady crack propagation region) and (c) cracked but isolated Cr/W rich borides adjacent to the crack initiation site. Secondary cracks from voids are being linked near main fracture path. WGB joint with IN-738 +BNi-9.

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