In modern industrial gas turbines (IGT), investment-cast parts are made of expensive superalloys to withstand the extreme thermal, mechanical and chemical loads experienced by these hot-gas-path components. However, to maximize efficiency and reduce the risk of unscheduled outages, these components must be periodically refurbished using a sophisticated brazing repair process or be replaced at great cost.
IGT parts with hundreds of thousands of service hours become severely oxidized. To maximize the effectiveness of the brazing repair process, all existing oxidation, sulphidation and hot corrosion must first be thoroughly removed from component surfaces, cooling passages and cracks that are often very narrow and deep.
Oxide Scale in Component CracksWhile an IGT component is in service, oxide scale typically forms on the mating faces of cracks that occur in hot-gas-path areas. These cracks become packed full of scale all the way to the tips. It is the goal of the service shop to repair these hot-gas-path components by filling the cracks with a braze alloy, but braze alloy cannot flow into cracks that are full of oxide scale.
To complicate matters, the alloys used to make turbine hot-gas-path components are nickel- and cobalt-based superalloys that usually contain aluminum and titanium to improve strength. The presence of these elements causes the resulting scale to contain complex spinels that are extremely difficult to remove.
“At the narrow tip of a crack, scale forms during service,” said Donald Bell, chief engineer at a prominent gas-turbine repair facility. “The scale occupies a larger volume than the metal from which it formed. This results in the narrow spaces at the tips of cracks being totally packed with scale. You cannot fill the crack with braze alloy if it is already filled with oxide scale.”
Traditionally, fluoride-ion cleaning has been performed at atmospheric pressure to remove oxidants from components, but metallurgical studies have shown it only works well when cleaning wide cracks. In addition, it can add extra steps to the oxide cleaning process that result from chromium-fluoride or chromium-carbide buildup during the process.
More recently, however, an innovative Dynamic Fluoride Ion Cleaning (DFIC) process has offered turbine refurbishment professionals the ability to clean deep, narrow cracks of oxides by cycling between negative, atmospheric and positive pressure for more ideal surface preparation prior to brazing.
Beyond DFICThe DFIC process, also known as hydrogen-fluoride (HF) ion cleaning, results from the reaction of fluorine with various oxides. HF gas can be toxic if it escapes into the atmosphere. However, improvements in gas-monitoring sensors and digital electronics (resulting from its widespread use in the semiconductor industry) have made it safe and reliable for parts cleaning.
At temperatures greater than 1750°F (950°C), the fluoride ion reacts with oxides that have formed on the crack faces in turbine hot-gas-path components, converting them to gaseous metal fluorides. This allows them to be easily removed. They depart through the off-gas stream of the reactor.
There are significant drawbacks to the early fluoride-ion cleaning processes developed in the 1970s, which utilize fluoride compounds in powdered form and perform the work at normal atmospheric pressure. Besides having difficulty penetrating into deep, narrow cracks, the early processes were less flexible and not continuous. They relied on a single charge of powder to produce their HF gas. This often resulted in parts having to be processed through more than one cleaning cycle.
“When compounds are in powdered form, such as chromium fluoride, aluminum fluoride or PTFE, there is a finite amount of reaction that can occur,” Bell said. “When they’re done, they’re done, and if the parts are not yet clean, the cleaning process often has to be repeated.”
DFICFortunately, the DFIC process has been proven to be more effective, flexible and repeatable. What separates the DFIC process from first-generation fluoride-ion cleaning equipment is that the reaction temperature, fluorine concentration, pressure level and duration are all independently controlled variables.
The sophisticated digital control systems that come with today’s equipment can be programmed with hundreds of “recipes” for cleaning specific alloy types, widths of cracks, and levels of scale and oxidation.
During the cleaning process, HF and H2gas are introduced into the system through precision metering, so time and gas concentrations can be precisely controlled. For example, a typical cleaning cycle may begin as 94-96% hydrogen. But within that cycle, it may be changed to 82:18 H2to HF ratio, depending on the substrate material.
Some DFIC systems, such as those available from Hi-Tech Furnace Systems, are designed to perform the cleaning process at sub-atmospheric pressures from 100-650 torr (133-867 millibar) while at processing temperature.
One of only a few DFIC manufacturers in the world, Hi-Tech Furnace’s customers includes Siemens, TurboCare, General Electric, Chromalloy, Goodrich and others.
By varying the pressure between positive, negative and atmospheric levels, the DFIC system “pulses” HF in and out of cooling channels, deep cracks and small holes to more effectively clean oxidized, hard-to-reach areas. Control of both HF ratio and atmospheric pressure can help efficiently remove even highly embedded oxidation from IGT parts.
“We use Hi-Tech Furnace’s DFIC equipment to modulate pressure from low to high to pneumatically push the fluoride ions down into the tips of the cracks and hold them there for a while,” Bell said. “We can cycle back and forth as needed for the best cleaning results.”
Bell added that by performing the process under vacuum, in addition to the removal of surface oxidation, aluminum and titanium are depleted from the substrate, creating a denuded zone approximately 0.0005 inches deep.
“This gives us a buffer. During furnace brazing, residual oxygen in the vacuum chamber can re-oxidize a clean part,” Bell said. “The denuded zone give us time to get the braze filler to flow and wick into the cracks before re-oxidation occurs.”
As an added benefit, the use of HF at sub-atmospheric pressure often eliminates extra steps in the brazing preparation process.
Cobalt-based alloys used to make IGT hot-gas-path components contain a significant amount of chromium. This can react with fluorine during the process to create a chromium-fluoride film on the surface of the parts. Chromium fluoride is the most refractory (temperature-resistant) compound of all the metal fluorides. As a result, it does not volatize at the usual temperatures used in FIC.
Without the vacuum capability in the cleaning process, the part must then be moved to a vacuum furnace, where the part is subjected to the higher temperature and lower pressure required until the chromium fluoride volatilizes. The resulting fluorides, however, can contaminate the brazing furnace or the vacuum pump, which should be kept very clean and are not designed to handle acidic gases.
According to Bell, chrome fluoride will remain gaseous at pressures of about 150 torr absolute, “so we’re able to clean without depositing a residue on the joint.” If any chromium fluoride is created during the process, the control system can be set to subject the part to the higher temperature and appropriate pressure to remove it.
“With Dynamic FIC equipment, we are able to clean components in one shot instead of the multiple cleanings typically required with more traditional fluoride-ion cleaning,” Bell said.
Another benefit of the dual-vacuum process is that it uses significantly less HF because oxides are volatilized at a lower temperature and concentration of HF when performed sub-atmospherically. Using less HF also reduces the risk of intergranular attack (IGA), which could otherwise chemically alter the microstructure of the metal being cleaned.IH
For more info, call 586-566-0600; fax 586-566-9253; email email@example.com; visit www.hi-techfurnace.com; or write to Hi-Tech Furnace Systems, Inc. at 13179 West Star Drive, Shelby Township, MI 48315.