Directionally-solidified (DS) multigrain and single crystal (SC) airfoils have been used in aircraft gas turbines for over ten years and are currently found in aircraft engine-derivative gas turbines used for land-based power generation. However, the adoption of SC technology for large land-based gas turbines is just underway and is not a simple scale-up.
A solicitation was issued in 1994 to extend the capability of single crystal complex-cored airfoil technology to larger sizes so that higher turbine inlet temperatures could be attained in land-based turbines in a cost-effective manner. Two selections were made and contracts have been signed with Howmet Corporation in Whitehall, Michigan, and PCC Airfoils in Cleveland, Ohio.
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The project led by Howmet is supported by a team including ABB, Pratt & Whitney, Solar, Westinghouse, Aracor, and Purdue University. The project is scheduled over a period of 34 months with completion in April 1998. This project includes four technology thrust areas:
The effort on low-sulfur alloys includes producing low-sulfur heats of four alloys, casting single crystal test specimens of each, and conducting cyclic oxidation tests on the specimens to evaluate the effects of sulfur content on oxidation resistance.
Casting process development and understanding encompasses developing the single crystal casting process for two alloys in utility size airfoils and for one alloy in industrial size airfoils, developing improved wax, developing improved molds, and producing improved cores to reduce the occurrence of grain defects and improve dimensional control. Graphic compares single crystal (l) to directionally solidified (center) to equiaxed (r).
Postcast process development and improvement includes investigation of varying heat treatment and hot isostatic pressing treatment on large test specimens, conducting metallographic examinations, performing mechanical property tests to determine effects of varying treatments, and developing inspection techniques for detection of porosity and internal defects.
The area of casting defect tolerance level entails conducting tests to characterize the effects of defects such as freckles, low-angle boundaries, splaying, and of varying size and severity on mechanical properties.
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The project team is led by PCC Airfoils and supported by GE Power Generation. The effort is scheduled over a period of 29 months with completion in July 1997. This project includes four major tasks: alloy melt practice, modification/improvement of single crystal casting process, core materials and design, and grain orientation control.
The task on alloy melt practice involves producing heats of a single-crystal alloy with a very low-sulfur level, casting single crystal test specimens, and conducting cyclic oxidation and hot corrosion tests on the low-sulfur specimens and baseline sulfur-level specimens to determine the effects of sulfur level on oxidation and hot corrosion resistance.
The single crystal casting modification/improvement task entails developing the casting process for one single crystal alloy for utility turbine size airfoils in both cored and solid configurations. This will be done by making a series of castings in which a number of important casting parameters are varied and their effects on the quality of the casting evaluated. To guide the selection of the process parameters, the casting process will also be modeled on a computer program where the casting parameters will be varied. Once the optimum values of the process parameters have been determined, a few additional molds will be made to verify the quality of the airfoils made by the final casting process.
The task on core materials and design will address the issues associated with producing cores that will have the required characteristics to consistently form complex cooling passages and maintain tight wall thickness tolerances in large airfoils. A number of core material compositions will be investigated and evaluated for shrinkage, strength, stability, and porosity. Cores exhibiting superior strength and stability will be selected for the first casting trials. These cores will be evaluated during the casting trials for their performance relative to dimensional control, metal interaction reactions, and core removal characteristics.
The objective of the task on grain orientation control is to evaluate the effects of defects such as grain boundaries and freckles on the fatigue strength of the material. Test specimens will be cut from areas containing specific defects in the airfoils produced in the early casting trials. The specimens will be subjected to low-cycle fatigue testing at typical operating conditions for the airfoil root section. The test results will be evaluated for the effects of each type of defect to help establish criteria for defect tolerance levels in large airfoils.
The Program Plan for the project was completed in April 1995, and the tasks on alloy-melt practice and single-crystal casting are under way.
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