"Crown Jewels," Feature Article, February 2006

crown jewels

These crystals are the gems of turbine efficiency.

by Lee S. Langston

The recent best seller by Tom Friedman, The World Is Flat, examines how companies are coping with the continuing evolution of globalization. The gas turbine manufacturer Rolls-Royce, for instance, outsources and offshores about 75 percent of its components to its global supply chain. But what of that remaining quarter?

Friedman quotes Sir John Rose, Roll-Royce's CEO, on the components the company still makes: "The 25 percent that we make are differentiating elements. These are the hot end of the engine, the turbines, the compressors and fans and the alloys, and the aerodynamics of how they are made. A turbine blade is grown from a single crystal in a vacuum furnace from a proprietary alloy, with a very complex cooling system. This very high-value-added manufacturing is one of our core competencies."

As Sir John Rose points out, single-crystal turbine blades are valuable to the companies that make them, as a high-technology—and profitable—product. Since they were invented and introduced, their unmatched resistance to high-temperature creep and fatigue have helped to advance the performance and durability of modern gas turbines. Were it not for the cost, they would replace thousands of conventionally manufactured turbine blades in higher-temperature applications.

A number of years ago, I worked in the engineering organization in which these "gas turbine crown jewels" were invented. It is surprising, given their importance, that even today much of the story behind their development is not well-known.

The gas turbine is one of the most technologically advanced energy conversion devices. The first working models were introduced in 1939 in both aviation and in electric power production. Generations of designers, engineers, and researchers have worked to increase gas turbine thermal efficiencies from an inaugural value of 18 percent to an unmatched 60 percent, in modern combined-cycle operation.

Vanes and Blades

Decades of research and development brought about this threefold increase in thermodynamic efficiency. As an example of this sustained activity, ASME's International Gas Turbine Institute over the last 50 years has had 14,000 reviewed technical papers presented at its gas turbine conferences worldwide.

Most gas turbines are axial flow machines, converting the heat from combusted fuel into thrust power (as in a jet engine) or shaft power (to turn, say, an electric generator) by means of momentum changes of gas flow through passages formed by stationary and rotating airfoils. Gas turbine vanes and blades—how they are designed, what they are made of—are key to gas turbine performance.

The beige single crystal blades on this GE 9H turbine—the world's largest—are approximately 18 inches long and weigh more than 30 pounds apiece.

Gas turbine thermal efficiency increases with greater temperature 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 nonaviation gas turbines operate at 2,700°F or lower. 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 and 2,500°F. That 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 temperatures (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 high-temperature cooled turbine airfoils are coated with ceramic thermal barrier coatings to increase life and act as a thermal insulator (allowing inlet temperatures 100 to 300 degrees higher).

Grain Boundary Phenomena

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'. The resulting crystal lattice misalignments form interfaces called grain boundaries.

Untoward 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. Corrosion and creaks 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.

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

"Single-crystal airfoils have proved to have more relative life in terms of creep strength, thermal fatigue resistance, and corrosion resistance."

Single-crystal turbine airfoil development took place in the company's Advanced Materials Research and Development Laboratory, under the direction of Bud Shank. The first important development was the directionally solidified columnar-grained turbine blade, invented by Frank VerSnyder and patented in 1966.

Direction solidification, carried out in a vacuum chamber, 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. 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.

The mold is surrounded by a temperature-controlled enclosure, which maintains a temperature distribution on the lateral surfaces of the mold so that the latent heat of solidification is removed by one-dimensional transient heat conduction through the solidified superalloy to the chill plate. As the solidification front advances from root to tip, 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 spanwise direction. For the case of a rotating turbine blade, where spanwise 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, directionally solidified turbine blades have much improved ductility and thermal fatigue life. 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).

Building upon direction solidification, Pratt & Whitney reached the goal of liquidating turbine airfoil grain boundaries in the late 1960s.

Barry Piearcey patented Pratt & Whitney's first single-crystal turbine blade, which was followed by a patent on an improvement by Bernard Kear. Maury Gell patented single-crystal blade alloy composition improvements that raised the incipient melting temperature by 150 to 200 degrees, which provided for a direct potential increase in engine performance.

One Airfoil, One Crystal

The making of a single-crystal turbine airfoil starts the same as a direction solidification airfoil, with carefully controlled mold temperature distributions to ensure transient heat transfer in one dimension only, to a water-cooled chill plate. 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. As solidification proceeds up the helix, crystal elimination takes place so that only one crystal emerges from the pigtail into the blade root, to start the single crystal structure of the airfoil itself.

Again, one-dimensional transient heat conduction must be maintained as the mold is withdrawn from the temperature-controlled enclosure. Any heat conducted to mold lateral surfaces can cause localized crystallization, which disrupts the single-crystal structure, with secondary grains.

In the 1970s, Pratt & Whitney developed techniques to manufacture single-crystal turbine airfoils, and to overcome casting defects (such as secondary grains, recrystallized regions, and freckle chains). This early pioneering work has been taken over by other manufacturers and improved upon over the past 30 years. Yields greater than 95 percent are now commonly achieved in the casting of single-crystal turbine airfoils for aviation gas turbines, which minimizes the higher cost of SX casting compared to equiaxed casting.

Early on, Pratt & Whitney investigated single-crystal turbine airfoil use in various jet engines. (One of the first was the J58, which powered the Lockheed SR-71 Blackbird.) The very first actual engine use was in Pratt & Whitney's JT9D-7R4 which received jet engine flight certification in 1982. This first single-crystal bladed engine powers the Boeing 767 and the Airbus A310.

Strength and Resistance

In jet engine 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. Modern high turbine inlet temperature jet engines with long life (that is, 25,000 hours of operation between overhauls) would not be possible without the use of single-crystal turbine airfoils. By eliminating grain boundaries, single-crystal 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 efficiencies.

The newest chapter of the story is their recent introduction in new, large land-based gas turbines, where they hold promise of minimum life cycle cost for increased turbine temperatures. Gas turbines used to produce electric power in the 200 to 400 MW range have turbine airfoils that can be 10 times larger than jet engine turbine airfoils. These large SX castings have had production problems in the industry, causing casting yields to go down, driving costs up.

As an example, one 1999 study done for the U.S. Department of Energy found that for a nominal $6,000, 13.6 kg single-crystal blade, a 90 percent yield would raise the cost to $7,000, while a 20 percent yield would shoot unit costs up to $30,000 each. Much work has been going on in the casting industry to increase yields for these large turbine blades and vanes, using liquid aluminum or tin cooling, or inert gas jet cooling, to increase the efficiency of the critical one-dimensional transient heat transfer process that controls single crystal solidification.

General Electric's 9H, a 50 Hz combined-cycle gas turbine, is the world's largest. The first model went into service in 2003 at Baglan Bay on the south coast of Wales, feeding as much as 530 MW into the United Kingdom's electric grid at a combined-cycle thermal efficiency just under 60 percent. The 9 H, at 367,900 kg, has a first-stage single-crystal turbine vane with a characteristic length of 30 cm and first-stage single-crystal blade of 45 cm (the blade lengths in the PW JT9D-7R4 are about 8 cm). Both vane and blade are cooled by steam (from the unit's combined-cycle operation) rather than by air. Each finished casting weighs about 15 kg and each is a single crystal airfoil.

Some crystal. Some crown jewel.

Lee S. Langston, professor emeritus of mechanical engineering at the University of Connecticut, is the editor of ASME's Journal of
Engineering for Gas Turbines and Power.