A program was undertaken to develop an improved internal cleaning procedure for high pressure turbine blades. HP turbine blades, such as JT8D and JT9D 1st stage blades, experience a build up of Si-Ca rich dirt/debris during engine service that inhibits the flow of cooling air creating localized “hot spots”. These “hot spots” lead to accelerated internal corrosion that may cause through-wall material failure. Autoclave processes, utilizing heated, pressurized caustic, are ineffective at removing large amounts of internal debris. A new process, called the TURBO-CLEAN℠ process (patent pending), effectively removes large deposits of internal debris in as few as 2 hours. This process involves pumping heated caustic at pressures approaching 400psi through the internal cavities of turbine blades. The increased caustic flow from this process, compared to autoclave processes, improves cleaning effectiveness. This new process does not harm the parent material or damage protective aluminide coatings. The development of this process has led to reduced repair costs for JT8D 1st blades by improving part yield during repair.
The goal of all repair processes is to return the hardware to a serviceable condition. Diffusion braze repairs utilize metallurgical processes to achieve economical repairs of expensive gas turbine components, especially in the turbine section. Component repairs often require dimensional restoration and crack repair on the same part. To achieve this goal, a new diffusion braze repair alloy was developed that combines high strength crack repair and dimensional build up into one material. This new material has mechanical property strength approaching that of the base metal. The improved mechanical properties result from a homogenous gamma prime strengthened diffusion braze zone. As part of an FAA approved test plan, the Howmet ESR (Effective Structural Repair) diffusion braze material was evaluated by tensile and stress rupture testing at elevated temperature. The test results showed high tensile strengths and long stress rupture life. In addition, the effect of the diffusion braze thermal cycle was evaluated on the base metal. A comparison was made between the gamma prime size and shape of engine run JT8D LPT vane clusters before and after the thermal cycle. The thermal cycle was shown to have a beneficial effect on the gamma prime size and shape relative to overaged engine run nozzles. The low cycle fatigue (LCF) life of MarM247 was also shown to improve with the ESR thermal cycle relative to a typical LPT nozzle heat treatment.
An engine test program was undertaken to evaluate a novel component repair process. First stage, high pressure turbine airfoils with either a simple aluminide (Codep on Rene’80) or a high activity Pt aluminide (RT-22 on Rene’142) were studied. Components refurbished with a partial strip, repair and CVD recoat process were installed in rainbow rotors which included new airfoils and/or components with full repairs (complete strip, weld repair and recoat). Following engine service on commercial aircraft, airfoils from the rainbow rotors were examined. The results of the evaluation suggest the desirable qualities of the CVD low activity Pt aluminide coat/overcoat process observed in the laboratory study could be achieved on engine hardware. Hence, a novel repair process is now available to turbine users, which may extend the service life of expensive turbine hardware.
The possibility of extending the service life of the high pressure hardware for gas turbine engines using a novel repair procedure was evaluated in a laboratory study. In the laboratory simulation, both high and low activity platinum aluminide overcoatings of a partially stripped platinum aluminide were evaluated and contrasted. The results indicate that a chemically homogenous, relatively uniform thickness, platinum modified coating can be formed by low activity aluminizing on a substrate with surface areas of both bare alloy and partially stripped platinum aluminide coating. Furthermore, the CVD low activity platinum aluminide overcoat considered had better oxidation resistance than the original coating. The experimental program is outlined, plus the results are presented and discussed.
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