OR WAIT null SECS
BRAZING TECHNIQUE REMOVES OXIDES IN DEEP, NARROW CRACKS
Investment cast parts used in modern gas turbines are made of expensive superalloys that can withstand extreme thermal, mechanical and chemical loads experienced by hot gas path components. Parts with hundreds of thousands of service hours, however, become severely oxidized.
To improve efficiency and reduce the risk of unscheduled outages, these parts must either be periodically refurbished using a brazing repair process or replaced. To facilitate brazing repair, all oxidation, sulfidation and hot corrosion must be removed from surfaces, cooling passages and deep, narrow cracks.
Oxide scale typically forms on the mating faces of cracks that occur in hot gas path areas. These cracks become packed full of scale. It is the goal of the service shop to repair these components by filling the cracks with a braze alloy. Unfortunately, braze alloy cannot flow into cracks filled with oxide scale.
To complicate matters, nickel- (Ni) and cobalt-based (Co) superalloys usually contain aluminum (Al) and titanium (Ti) to improve strength. The presence of these elements causes the resulting scale to contain complex crystalline structures that are difficult to remove.
“At the narrow tip of a crack, scale forms during service, which occupies a larger volume than the metal from which it formed,” said Donald Bell, Chief Engineer at a gas turbine repair facility. “You cannot fill the crack with braze alloy if it is already filled with oxide scale.”
Traditionally, fluoride ion cleaning is performed at atmospheric pressure to remove oxidants. However, metallurgical studies have shown that it only works well when cleaning wide cracks. What is known as Dynamic Fluoride Ion Cleaning (DFIC), on the other hand, has the ability to clean narrower cracks by cycling between negative, atmospheric, and positive pressure in preparation for brazing.
The 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 have made it safe for parts cleaning.
At temperatures greater than 1,750°F, the fluoride ion reacts with oxides that have formed on the crack faces, converting them to gaseous metal fluorides. This allows them to depart through the off-gas stream of the reactor.
There were, however, drawbacks to fluoride ion cleaning processes developed in the 1970s, which utilized fluoride compounds in powdered form and operated at normal atmospheric pressure. Besides having difficulty penetrating cracks, the early processes relied on a single charge of powder to produce HF gas.
“When compounds in powdered form, such as chromium-fluoride, aluminum- fluoride, or polytetrafluoroethylene are used, the cleaning process often has to be repeated,” said Bell.
With DFIC, reaction temperature, fluorine concentration, pressure level and duration are independently controlled variables. The control system can be programmed to clean specific alloy types, various widths of cracks, levels of scale and oxidation.
During cleaning, HF and H2 gas are gradually introduced. A typical cleaning cycle may begin as 94% to 96% hydrogen, but may be changed to an 82:18 H2 to HF ratio depending on the substrate material. Some systems can clean at sub-atmospheric pressures from 100 to 650 Torr while remaining at processing temperature. By varying pressure between positive, negative and atmospheric levels, the system pulses HF in and out of cooling channels, cracks and small holes (Figure).
“We use 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,” said Bell. “By performing the process under vacuum, aluminum and titanium are depleted from the substrate, creating a denuded zone approximately 0.0005” deep.”
This provides a buffer as residual oxygen in the vacuum chamber that can re-oxidize a clean part during furnace brazing. The denuded zone allows enough time to get the braze filler to flow and wick into the cracks before reoxidation occurs.
Cobalt-based alloys can react with fluorine to create a chromium fluoride film. 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 DFIC.
Without the presence of a vacuum, the part must be moved to a vacuum furnace to be subjected to higher temperature and lower pressure required until the chromium fluoride volatilizes. However, the resulting fluorides can contaminate the brazing furnace or the vacuum pump. According to Bell, at pressures of about 150 Torr absolute, chromium fluoride will remain gaseous, so cleaning can be done without depositing a residue on the joint.
In addition, this dual vacuum process uses less HF because oxides are volatilized at a lower temperature and concentration of HF when performed sub-atmospherically. This cuts the risk of inter granular attack (IGA), which could otherwise chemically alter the microstructure of the metal being cleaned.
Rob Kornfeld is President of Hi-Tech Furnace Systems, Inc. of Shelby Township, MI, a provider of Dynamic Fluoride Ion Cleaning, Chemical Vapor Deposition and Vapor Phase Coating systems. For more information, visit www.hi-techfurnace.com.