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6006 |
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Tuesday, May 21, 2019, Saal Brüssel 2:40 PM Brazing for turbine applications |
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Brazing process, enabler for lifetime extension of gas turbines |
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Sabrina Michelle Puidokas / General Electric (Switzerland) GmbH, Switzerland Kornelia Weidemann* / General Electric, Switzerland Sabrina Michelle Puidokas/ General Electric (Switzerland), Switzerland Banu Berme/ General Electric, Turkey |
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Brazing is a thermal process, which allows challenging repairs of gas turbine components where other processes like welding are limited in terms of minimum localized heat input and minimum wall thicknesses. Noteworthy wear out modes are cracking and degraded sacrificial wear surfaces. Wear modes, amount and exact location strongly vary depending on the specific engine type and its operation mode (firing temperature, cycling, fuel, etc.). These considerations are well known by servicers and operators of combustion turbine equipment. The intent of this work is to share some interesting cases wherein these types of damage have been healed by high temperature brazing allowing a successful and efficient repair of a gas turbine component. Since 2000 General Electric has repaired post service hot gas path components made from nickel-base alloys. Of particular interest in this study is repair of cracking, which has occurred during operation. This includes a multi-step cleaning procedure to achieve an oxide free crack and a high temperature diffusion braze heat treatment to restore the part function. This contribution demonstrates that high temperature diffusion brazing can lead to an excellent joining quality. Cases of various static and rotating hardware are given for consideration. Field experience of crack braze repair was collected on stationary components made from single crystal (SX) material MK-4HC. Such crack brazed gas turbine components were investigated after service and have shown high integrity of the crack brazing. New cracks developed in the base material, but not within the brazed zones. The resistance of the brazed crack to thermomechanical loads is discussed, including the enabling relationship between the braze joint and the crystallographic orientation, microstructure types and sizes of the base material. Crack brazing repair with nickel-base fillers is successfully used for conventional cast nickel-base superalloys as well. In 2017 a set of IN738LC blades, typically considered unrepairable due to substantial cracking in critical leading edge cooling circuits, have been repaired by brazing. Customization of the brazing repair process considering specifically adapted preparation steps is discussed. The newly developed process chain enables reaching the required quality and thus the salvage of future components with similar wear. Aeroderivative Gas Turbines embody various honeycomb (HC) structures in order to reduce air leakage. In that function, the honeycombs have also to act as mechanical protective shields of the base metal parts. During severe service conditions, they are aimed to wear instead of the base metal parts. Replacement of the worn honeycomb is easier and cheaper than the repair of the base metal parts. The replacement of brazed HC is thus a very common and high volume repair process. GE LM2500 and LM6000 aeroderivative engines repair is an exemplary case of honeycomb braze repairs. In these examples many steps are required to repair the honeycomb including removal, cleaning and brazing. Nickel based braze alloy powders are the most used powders for the brazing processes and are included in this analysis. Effects of surface finish preparation and surface cleaning on final brazing quality are discussed in this study. |