Turbine blades: Good, better, best

About 60 years ago, a small group of industrial researchers specializing in gas turbines set out to eliminate grain boundaries in superalloy turbine blades. The result: A class of single-crystal blades that has increased both gas-turbine thermal efficiency and component service life, while providing unmatched resistance to high-temperature creep and fatigue.

Below, Lee S Langston, professor emeritus of mechanical engineering, UConn, traces the road taken. The ASME Fellow’s first job after receiving his PhD from Stanford University in 1964 was as a research engineer at Pratt & Whitney. He is known by many GT owner/operators for his time as editor of the Journal of Engineering for Gas Turbines & Power and as a director of the ASME International Gas Turbine Institute.

Gas turbine (GT) thermal efficiency increases with higher 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 3000F, while non-aviation gas turbines typically operate at 2700F or lower.

In the highest-temperature regions of the turbine, special high-melting-point nickel-base alloy blades and vanes are used because of their ability to retain strength and resist hot corrosion at extreme temperatures. These so-called superalloys, when conventionally vacuum cast, soften and melt at temperatures between 2200F and 2500F.

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. After casting, the working surfaces of these cooled turbine airfoils are coated with ceramic thermal-barrier coatings to increase life and act as an insulator—allowing inlet temperatures 100 to 300 deg F higher.

Grain-boundary problems. Conventionally cast 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.

Life-limiting events happen at grain boundaries—such as intergranular cavitation, void formation, increased chemical activity, and slippage under stress loading. These conditions can lead to creep, shorten cyclic strain life, and decrease overall ductility. Recall that creep, an insidious life-limiter, is the tendency of blade material to deform at a temperature-dependent rate under stresses well below the material’s yield strength.

Corrosion and cracks also start at grain boundaries. In short, physical activities initiated at superalloy grain boundaries greatly shorten the lives of turbine vanes and blades, and dictate lower-than-optimal turbine temperatures with a concurrent decrease in engine performance.

One can try to gain sufficient understanding of grain-boundary phenomena so as to control them. However, in the early 1960s, researchers at Pratt & Whitney Aircraft (now Pratt & Whitney, P&W, owned by United Technologies Corp) set out to deal with the problem through elimination of grain boundaries from turbine airfoils, by inventing techniques to cast single-crystal (SX) turbine blades and vanes.

One-dimensional crystals. P&W’s first step in the development of SX blades was directional solidification (DS). Carried out in vacuum furnace, DS is accomplished by pouring molten superalloy metal into a vertically mounted, ceramic mold heated to metal melt temperatures, and filling the turbine airfoil mold cavity from root to tip (Fig 1).

Blades Fig 1

The bottom of the mold is formed by a water-cooled copper chill plate having a knurled surface exposed to the molten metal. At the knurled chill plate surface, crystals form from the liquid superalloy and the solid interface advances, from root to tip. As the solidification front advances, the mold is slowly lowered out of the temperature-controlled enclosure.

The final result is a turbine airfoil composed of columnar crystals or grains running in a span-wise direction. For the case of a rotating turbine blade, where span-wise centrifugal forces set up along the blade are on the order of 20,000 g, the columnar grains are now aligned along the major stress axis. Their alignment strengthens the blade and effectively eliminates destructive intergranular crack initiation in directions normal to blade span.

In gas-turbine operation, DS turbine blades and vanes have much better ductility and thermal fatigue life than equiaxed-crystal airfoils. They also provide a greater tolerance to localized strains (such as at blade roots), and have allowed small increases in turbine temperature (and, hence, performance). Their first use by P&W in a commercial aircraft engine was in 1974.

One crystal, one turbine blade. The making of a single-crystal turbine airfoil starts in the same manner as a directional solidification airfoil, with carefully controlled mold temperature distributions to ensure transient heat transfer in one dimension only, to a water-cooled chill plate (Figs 2, 3). 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.

Blades Fig 2,3

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 single crystal structure of the airfoil itself.

The first use of SX turbine airfoils in land-based GTs was for corrosion resistance in a 163-MW machine, the Siemens V94.3A (now SGT5-4000F), introduced to market in 1995. In recent years, electric-power GT inlet temperatures have increased to aviation levels, so the SX airfoils with higher temperature capacity are now needed for long life. The SX turbine blades and vanes in GE and Siemens H-class machines are huge, with lengths of from about 1 to 1.5 ft, with each finished casting weighing more than 30 lb.

The result. In gas-turbine use, single-crystal 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 walls—meaning less material and less weight—and have a higher melting-point temperature. These improvements all contribute to higher thermal efficiency.

Posted in 7F Users Group |

Comments are closed.