November/December 1996
Superalloys are the supermen of metals, finding service in the harshest, most demanding applications. Here’s an introduction to how they’re produced and recycled, with a glimpse at their market prospects.
By Rebecca Porter
Rebecca Porter is an associate editor of Scrap.
When you need metals that can hold up under extreme heat, shrug off corrosive environments, offer astounding performance characteristics, and more, who ya gonna call? Superalloys, that’s who.
These metals, considered the elite of metallic engineering materials, aren’t called super-alloys for nothing. Their unique physical properties make them perfectly suited to fill the bill in demanding situations and harsh conditions that would turn other metals into scrap.
Though mostly associated with aerospace transportation and exploration uses—that is, planes and rockets—superalloys have branched into other industrial and consumer niches over the decades, some of which hold promising growth opportunities for these superhero metals in the future.
Superalloy ABCs
Superalloy is a catchall term that encompasses a wide variety of often complex alloys that are generally based on three consecutive elements on the periodic table: iron, cobalt, and nickel. While most have a high nickel content ranging from 25 to 100 percent, some are cobalt- or iron-based and virtually all include other, sometimes exotic, elements such as molybdenum, tungsten, tantalum, chromium, aluminum, titanium, niobium, and more.
As it stands today, there are several hundred superalloys and new ones continue to be developed. Some formulas are patented, but even so there may be subtle variations within certain alloys, depending on the specifications for a customer’s particular use.
Superalloys were developed in the late 1930s and early 1940s to answer the call for metals that could withstand high-temperature applications such as superchargers for piston engines in World War II aircraft. Nickel-based superalloys were the first to hit the scene, though cobalt-based ones were developed around the same time for use in gas-turbine blades. Iron-based superalloys evolved later along with stainless steel technology.
Superalloys generally have the ability to perform under extremely demanding service conditions, or they possess special properties—such as electrical, thermal, or high-creep strength—designed to suit specific applications.
In the former instance—their service characteristics—superalloys can be broken down into three groups:
Corrosion-resistant alloys. Though all superalloys offer significant corrosion resistance thanks to their composition, this category encompasses those that have been specifically designed to withstand the most corrosive conditions. These specialized metals, for example, are used in pulp and paper applications, geothermal energy recovery systems, food processing, and oil and gas recovery, as well as for handling caustic soda. Marine engineering also takes advantage of the brine-resistant superalloys for parts of aircraft carrier decks and other applications on naval surface and submarine vessels. The metal content of these superalloys covers the gamut from commercially pure nickel to blends of nickel-copper, nickel-chromium, and—notably—nickel-molybdenum, a combination that offers some of the most corrosion-resistant nickel alloys.
Heat-resistant alloys. This category covers high-temperature alloys that find use in general purpose industrial thermal-processing applications, such as furnace components used for heat treatment of other metals. The defining characteristics of these metals are their ability to maintain their strength at significant temperatures—above 540ºC/1,000ºF, for instance—and resist oxidation, carburization, and degradation by hot gases or other environmental elements. A combination of nickel, chromium, and iron gives these superalloys much of their heat-resistant properties, though other elements such as aluminum and silicon are sometimes used to boost their abilities.
High-performance alloys. These metals, considered the crème de la crème of superalloys, combine the dual properties of resisting surface degradation at high temperatures and offering extremely high strength at elevated temperatures. These characteristics, coupled with a high strength-to-weight ratio, have made these metals a logical favorite for use in both air- and land-based gas turbine engines, appearing in parts ranging from compressor blades to turbine blades to afterburners to thrust reversers. The majority of these metals are complex nickel-based alloys that are strengthened through heat treatment.
The other general group of superalloys—those with special physical properties—are used for measurement standards, glass-to-metal sealing (as in lamp bulbs, thermostats, and semiconductor lead frames), and heating elements in applications from domestic appliances to industrial furnaces. The metallurgical composition of these metals varies widely and includes such combinations as nickel-iron and nickel-chromium.
How Superalloys Are Made
Because so much depends on superalloys’ metallurgical composition and performance, they must be produced with almost infinite precision in specialized facilities, using high-tech engineering and statistical process controls to ensure quality. The manufacturing process used varies according to the “recipe” required by the end consumer.
As complex alloys, the alloying stage is one of the most critical steps in superalloy production. The compositional and microstructural control can’t be overemphasized—a superalloy may require more than 15 chemical elements added in amounts as precise as 0.05 percent or exclusion of others to the ten-thousandths of a percent.
While some superalloys, primarily cobalt- and iron-based, are melted by air induction, most are produced through vacuum induction melting. This method reportedly reduces gas content of the metals, provides closer control of aluminum, titanium, and other reactive elements, and minimizes contamination. The general rule in superalloy production is: the cleaner the mix, the greater the strength and ductility. Vacuum induction can produce a basic ingot shape or an investment cast of near-net shape.
For wrought processing that requires hot working, such as for engine parts, a second melting is needed. After an alloy is mixed using vacuum induction melting, there are several high-tech options—including vacuum arc remelting, electroslag remelting, electron-beam remelting, and double electrode remelting—that can be used to structurally refine ingots so superalloys can be hot-worked from ingot into shapes by extrusion, forging, or flat rolling.
Extrusion or rolling, followed by annealing, are cost-effective processes and produce a geometric shape. Forging—usually by hammer or press—is another option, and isothermal forging, which keeps the part and forging dies at the same constant temperature, is particularly suited for making jet-engine disks from high-temperature alloys. More advanced thermomechanical processing provides microstructural control of grain size.
These various manufacturing and finishing approaches can create a variety of superalloy products, including plate, sheet, strip, billet, rod, bar, wire, tube, and pipe.
From a quantitative perspective, the 30 to 40 superalloy producers in the Western World manufacture around 100 million pounds of metal in a strong year, with predictions calling for a production level of about 95 million pounds this year. The United States is the world’s largest market for superalloys, and U.S. producers supply most of the demand.
Super Recycling
As with most metal producers, superalloy manufacturers generate scrap and, like most, they try to reuse as much of their home scrap as possible to maximize their raw material use and minimize their raw material costs. Unless the feedstock gets contaminated—by low-melting materials such as lead, tin, bismuth, or antimony, for instance—virtually all superalloy scrap is recyclable. “We get the squeal out of the pig,” says Don Muzyka, president of Special Metals Corp. (New Hartford, N.Y.) and chairman of the superalloy committee of the Specialty Steel Industry of North America (Washington, D.C.). “We recycle everything we can. It’s valuable stuff and it pays to handle it right.”
What isn’t recycled by superalloy producers as home scrap is generally sold to and processed by recyclers who specialize in handling these special metal blends. Superalloy scrap is also generated by forging operations, machine shops, and some foundries and casting houses.
Compared with other types of scrap metal processing, superalloy recycling is a painstaking process that requires advanced equipment and fastidious quality control. At root, the recycler’s major goals are determining composition and guaranteeing purity.
To achieve these ends, superalloy recyclers use a variety of means, from steps as simple as noting color or weight and conducting magnetic or spark tests to the use of thermal conductivity meters and emission spectrographic and X-ray energy dispersive devices. Superalloy scrap sorters often have to attend two years of classes to achieve Grade 1 certification and develop the ability to visually identify more than 100 metals. In addition, superalloy recyclers have to meet not only ISO standards, but they also must comply with rigid consumer specifications and often undergo audits by their consumers. It’s common, in fact, for superalloy melters to certify their scrap suppliers and require maintenance of detailed records of scrap loads. (These records are designed to create a trail of responsibility leading from the end user to the raw material supplier. If an engine blade breaks up, for instance, investigators should be able to trace records back to the engine maker, back to the blade maker, back to the origin of the metal, back to the melter, and back to the recycler, who is often required to keep precise written specifications and even actual melt slices of material that was tested.)
It’s also common for superalloy users such as jet engine makers to tell superalloy melters who they can and can’t buy scrap from and even approve the melting process themselves, directing what scrap can and cannot go into the mix. Reportedly, about a dozen U.S. superalloy scrap recyclers are certified by engine makers to process superalloy solids, while only four are certified to process turnings.
When materials arrive at a scrap operation for recycling, they can be—depending on the operation—first identified by spectroscope, tagged, and sorted piece by piece, as each one must be qualified to mill specifications or alloy designation. Other sorting is done in the lab where an atomic absorption unit detects trace elements such as lead, bismuth, silver, and tin down to parts per million. Small vacuum furnaces enable on-site chemists to do test melts that allow better analyses and sampling. Then, if necessary, the scrap is cut to size, cleaned—usually by shot blasting—then packaged and sent to the melter.
A large volume of the superalloy recycling business now involves turnings, which require particularly acute quality control due to the potential presence of cutting oils and other contaminants. To address these concerns, superalloy processors use turnings processing systems that may feature a crusher, magnetic separator, solids removal equipment, and solvent degreaser to remove carbon-based oils. Samples may be collected every minute or so and tested to determine the material’s composition and check for trace elements in amounts as low as one-half part per million.
At the end of the processing cycle, superalloy scrap may not look much different than when it came in, but it now possesses the required “chemical integrity.” That’s essential as everything sent from the processor to the melter must usually be 100-percent guaranteed by the processor. If a heat is out of composition, in fact, the processor is responsible.
A Future of Market Options
Since most demand for superalloys comes from the aerospace sector, the market fate of most superalloys has tended to rise and fall along with that industry. In the late 1980s and early 1990s, superalloys fell on pretty hard times due to the lean years of the economic recession, Defense Department cuts made under the Bush administration, and sluggish commercial and military aircraft jet engine production.
In the past few years, however, the economy and the aerospace market have recovered and forged ahead, giving superalloys a much-needed boost. “The superalloy business has been growing strong for about the past year—it’s really started to pick up steam,” says a manager with the Vac-Air division (Frewsburg, N.Y.) of Keywell L.L.C. (Baltimore), which recycles the spectrum of superalloy scrap. “You can relate that to the tremendous pickup in the aircraft industry after the past five years of weak business.”
In August, for instance, Boeing received $6.3-billion worth of orders for 68 airplanes from a number of airlines, while McDonnell Douglas logged orders totaling around $700 million.
Military production of the stealth F-22 plane will use an unspecified amount of superalloys. More superalloys will also be needed if full-scale production is approved for the Lockheed Martin X-33, a reusable space launch vehicle. In addition, the Gulf War revealed a need for lighter armor for tanks and military vehicles, and superalloys are being considered for the job.
But superalloys’ market prospects aren’t limited to aerospace and military applications. Operations that must comply with Clean Air Act requirements are sucking up the superalloy stock to use in various types of filters. Coal-burning power plants, for instance, are a key group of nickel-based superalloy users, employing the metals in their steam generators and flue gas desulfurization units.
These market prospects, coupled with relatively tight scrap supplies, diminished stockpiles, and greater control on foreign exports (particularly material coming from the Commonwealth of Independent States), give superalloys a bright outlook—at least in the near term. As the Vac Air manager remarks, “The market future, we’ve been told, looks incredibly good, incredibly strong.” •
Superalloys are the supermen of metals, finding service in the harshest, most demanding applications. Here’s an introduction to how they’re produced and recycled, with a glimpse at their market prospects.