Superalloys are normally nickel-based, although cobalt- and iron-based superalloys have also been developed. They are called “super” because their service conditions in critical applications generally require very high strengths at high temperatures – frequently exceeding 1000˚F. Superalloy systems are very complex, often existing with a variety of second-phase particles in the base-metal matrix. There is a significant amount of other alloy elements added to the base metal. In final applications, forged superalloy components have good creep-rupture strength and good oxidation resistance, so they are quite suitable for high-temperature applications. Their corrosion resistance in harsh environments makes them suitable in, for example, petrochemical components.
Chemistry and Grades
Nickel-based superalloys are comprised of 50-78% nickel. Other major alloying elements in these complex systems include chromium from 14 to 23%, molybdenum from 0 to 18% and tungsten from 0 to 5%. These elements provide solid-solution strengthening and form carbides. Additionally, iron may be added to the alloy 3 to 20% for strengthening; cobalt from 0 to 5% for solid-solution strengthening and to raise the melting point; and titanium from 0 to 6% to form precipitates. As you can see, the superalloy metallurgist has used a large number of the metals in the periodic table in creating these alloys.
The alloy grade designation is not systematic as it is with iron or aluminum alloys. The numbering scheme does have a small amount of information that can be extracted from it. For example, the 6XX (where X stands for another number) alloys are composed of nickel plus chromium plus other elements. The 7XX alloys are composed of the same nickel plus chromium plus other elements but are strengthened by precipitation hardening. The C-XXX alloys are composed of nickel, chromium, molybdenum plus other elements.
The most common superalloy seems to be Alloy 718, often called Inconel 718 or simply Inconel. The latter names occur because the alloy was first developed by International Nickel, and the Inconel designation is a registered trademark.
The 7XX series of superalloys supplement strength by very fine precipitates called gamma prime (). The Greek letter (gamma) is the designation that is often given to high-temperature phases that are face center cubic (FCC). The origin of this designation is the use of the letter gamma for austenite in steels, which are FCC. Since the base nickel is FCC, the structure is often referred to as gamma or, sometimes, as austenitic. The structure consists of the very fine precipitates that form in the alloys that provide high strength to the metal.
Figure 1 shows a light optical microscope image of as-forged 718. Note that at this low magnification the very fine precipitates are not observed, and it appears to be a single-phase material. Figure 2 shows the microstructure of 718 that has received insufficient deformation. Note the bimodal grain size. During deformation, recrystallization occurred at the prior grain boundaries, but there was not enough deformation to cause the entire component to recrystallize. This causes the duplex grain structure. Sometimes there will only be recrystallized grains outlining the prior grains. This outlined structure is called a necklace microstructure because the new small grains surrounding the larger prior grains look like a necklace. Although pretty to observe, the properties of such structures are not often useful.
The 6XX series forms essentially a single-phase FCC phase that gets its strength from the other elements by solid-solution strengthening and by deformation at temperatures below the alloy’s recrystallization temperature. A microstructure of these alloys would consist of a single-phase material with grain boundaries.
It should be noted that other alloying elements in these superalloys can form other precipitates, such as a variety of different carbides or intermetallic compounds. These precipitates can be beneficial at times but are often somewhat detrimental by decreasing the creep resistance during use.
Forged superalloys are used in components that are exposed to temperatures of 1200-1800˚F. Components such as turbine disks, cases, shafts and blades require superalloys because their service at higher temperatures is required to increase aircraft performance and fuel efficiency. Superalloy fittings, pipes and valves are used at high temperatures in corrosive environments. Because of their corrosion resistance, superalloys are also used as pins and replacement joints in medical applications. Since a number of people have an allergic reaction to nickel, the superalloys for medical components are often cobalt-based rather than nickel-based. Figure 3 shows various components made from superalloys.
The temperature range for using superalloys is shown in Figure 3. Note that the operating range is much higher as compared to any of the other alloy systems that we have covered. It is also noteworthy that the high end of the operating range begins to impinge on the low end of the hot-forging region.
Forging of Superalloys
In most alloy systems, the forge shop’s responsibility is to produce a shape, with mechanical properties obtained by heat treatment. Conversely, superalloys obtain strength from their base chemistry, secondary particles (controlled by heat treatment) and grain size. Fine grains are required to meet most commercial and military specifications. Unfortunately, the grains are always growing when in a hot environment, such as preheating for forging or heat treatment. Thus, temperature control during forming is critical to ensure that the proper amount of deformation is imparted at a temperature at which recrystallization can occur but where grain growth is limited.
Typical flow-stress values are shown in Figure 4. Note how high these are relative to any other common forged metal. The cold forming of fasteners and pins is common in alloy and stainless steels. When deploying precision cold forming to superalloys, however, preheating to a few hundred degrees Fahrenheit is almost always performed prior to deformation because the work hardening results in high flow stress with modest deformation.
The forging of superalloys is very challenging. There is usually a very narrow temperature range to forge a given alloy. Their high flow strength makes superalloys very resistant to movement. Consequently, it is challenging to fill detailed die cavities in a closed die without extreme forging pressure. The sizing presses and hammers used in the production of steel parts are grossly inadequate to forge superalloys. Forging on undersized equipment can pose an insurmountable challenge. Smaller equipment results in more hits on a hammer or inadequate deformation in the final forging operation on a press. This, in turn, generally causes incomplete recrystallization and, thus, inadequate strength. Raising the temperature helps with die fill and recrystallization, but grain growth in the heating furnace can more than offset any gains.
Because of their high flow strength at high temperatures, the forger should anticipate very poor tool life when forging superalloys. Numerous examples have been reported of catastrophic die fracture after a handful of forging cycles. Even when the tooling is strong enough to avoid a low-cycle fatigue failure, tool wear is extreme relative to the forging of other metals. In many applications, superalloys are used as die material. It is also common for companies to coat the workpiece to supplement existing lubrication processes, which can easily break down at the required forging pressures.
A three-step, post-forging heat treatment is applied for the precipitation-hardenable superalloys. The first step is a high-temperature solution treatment followed by a quench; the second is a precipitation treatment at a temperature below the solution temperature, which produces the gamma-prime precipitates; and the third is a second precipitation treatment at a temperature lower than the first precipitation step. Additional finer precipitates are formed during this third step. As you can see, not only is the alloy composition complex, but the post-forging thermal treatment is more complicated than those used for other alloy systems.
It should be noted that superalloys are not only challenging to forge, but they are often difficult to machine and to weld. Hence, post-forging operations require careful adherence to proper procedures.
Like some stainless steels, a number of superalloy systems are susceptible to sensitization. Sensitization occurs when some grain-boundary precipitates form upon cooling, robbing the adjacent regions of alloy content (primarily chromium). These alloy-depleted regions next to the grain boundaries are very prone to corrosion attack or oxidation. To avoid these undesirable properties in the component, the alloy needs to be quickly cooled through the sensitization temperature range.
Non-precipitation-hardenable alloys’ strength is obtained by deformation. It is imperative to strictly follow the heating and deformation schedules during forging for these systems.
Superalloys are complex alloys that offer very high strength at high temperatures. It is this property that makes them so difficult to forge. Forging practice must adhere very closely to the specifications of temperature and the amount of deformation imparted. Because of their high strength at forging temperatures, die fill is difficult and die life poor. In spite of these difficulties and challenges, forged superalloys are very desirable in a number of critical applications.
As one might imagine, these materials are very expensive. It is not uncommon to see a tenfold or more price penalty over carbon steel. In addition to the cost of the constituents, the process controls required to melt and produce billet are every bit as challenging as those to forge the material. The high cost and process challenges limit superalloy applications to those with the most service-critical requirements, where failure is simply not an option.
We will continue our discussion of nonferrous alloys by examining copper-based forged alloys in our next article.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.