COLUMN // HISTORY
A Singular Sensation
Mechanical engineers and metallurgists sought to improve gas turbine blades by eliminating grain boundaries.
Written by Lee S. Langston
Conventionally cast gas turbine airfoils are polycrystalline, consisting of a three-dimensional mosaic of small, metallic, equiaxed crystals, or “grains,” formed during solidification in the casting mold. Each equiaxed grain has a different orientation of its crystal lattice from its neighbors’. Resulting crystal lattice misalignments form interfaces called grain boundaries.
These grain boundaries are not innocuous. Life-limiting events—such as intergranular cavitation, void formation, increased chemical activity, and slippage under stress loading—happen at grain boundaries. These conditions can lead to creep, shorten cyclic strain life, and decrease overall ductility. Corrosion and cracks also start at grain boundaries. In short, physical activities initiated at superalloy grain boundaries greatly shorten turbine vane and blade life, and lead to lowered turbine temperatures with a concurrent decrease in engine performance.
Over 60 years ago, a small group of mechanical engineers and metallurgists at Pratt & Whitney, one of Connecticut’s jet engine manufacturers, audaciously set out to totally eliminate grain boundaries in superalloy turbine airfoil blades and vanes in their gas turbines. The goal was to develop a casting process that would produce one, single crystal of very large size.
Gas turbine technology history shows that game changers like the single-crystal (SX) program typically entail 30 years or more. The 10-year Pratt & Whitney SX targeted group success is thus worthy of a study in itself.
Turbine blades (first stage, short; second or third stage, taller) are acid etched to show grain structure. Pair at left is single crystal; center, directionally solidified; right, equiaxed.
INCREASING EFFICIENCY
Gas turbine thermal efficiency increases with greater temperatures of the gas flow exiting the combustor and entering the work-producing component: the turbine. Turbine inlet temperatures in the gas path of modern high-performance jet engines can exceed 3,000 °F, while non-aviation gas turbines operate at an upper limit of 2.700 °F.
In high temperature regions of the turbine, special high melting point nickel-base superalloy blades and vanes are used, which retain strength and resist hot corrosion at extreme temperatures. These superalloys, when conventionally vacuum cast, soften and melt at temperatures between 2,200 °F and 2,500 °F. This means blades and vanes closest to the combustor may be operating in gas path temperatures far exceeding their melting point and must be cooled to acceptable service temperature (typically eight-to-nine-tenths of the melting temperature) to maintain integrity.
Thus, turbine airfoils subjected to the hottest gas flows take the form of elaborate superalloy investment castings to accommodate the intricate internal passages and surface hole patterns necessary to channel and direct cooling air (bled from the compressor) within and over exterior surfaces of the superalloy airfoil structure. To eliminate the deleterious effects of impurities, investment casting is carried out in vacuum chambers.
A unique turbine blade/vane casting process was developed, based on maintaining strict control of one-dimension spanwise heat transfer in the ceramic mold to a chill plate, to solidify the molten superalloy. A key part of the process was the invention of a mold helical “pigtail” passage that served to start the single crystal solidification.
Single-crystal blades are made by a process like that shown in the simplified sketch at left. A vacuum furnace is opened after casting a cluster of single-crystal gas-turbine blades in a ceramic mold (right). Note the chill plate beneath the mold.
The making of a SX turbine airfoil starts with carefully controlled mold temperature distributions to ensure transient heat transfer in one dimension only, to a water-cooled chill plate that supports the mold. Columnar crystals form at the knurled chill plate surface in a mold chamber called the “starter.” The upper surface of the starter narrows to the opening of a vertically mounted helical channel called the “pigtail,” which ends at the blade root. The pigtail admits only a few columnar crystals from the starter.
Crystal orientations grow at different rates into the liquid metal in the pigtail, with one orientation growing the fastest. Thus, with ample coils, only one crystal emerges from the pigtail into the blade root, to start the SX structure of the airfoil itself.
Starting in the late 1960s with a very focused single-crystal (SX) turbine blade program, Pratt & Whitney was able to successfully put them into a production jet engine, going from concept to manufacture product within 10 years. The effort was led by former MIT faculty member, Maurice “Bud” Shank, and directional solidification metallurgy expert, Frank VerSynder.
In gas turbine use, SX turbine airfoils have proven to have as much as nine times more relative life in terms of creep strength and thermal fatigue resistance, and over three times more relative life for corrosion resistance, when compared to equiaxed crystal counterparts. By eliminating grain boundaries, SX airfoils have longer thermal and fatigue life, are more corrosion resistant, can be cast with thinner wall—meaning less material and less weight—and have a higher melting point temperature.
These improvements all contribute to higher gas turbine thermal efficiencies in jet engines. In larger electric power and land-based gas turbines (with some airfoil lengths up to 45 centimeters, with each finish casting weighing as much as 15 kilograms) SX airfoils offer corrosion resistance, as well as higher thermal efficiencies.
Lee S. Langston is professor emeritus in the mechanical engineering department at the University of Connecticut at Storrs.
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