AGE-HARDENABLE SUPERALLOYS To select the most suitable wrought age-hardenable superalloy for a specific application, engineers must know the basic mechanical properties as well as other characteristics such as resistance to fatigue, crack growth, and corrosion. Richard B. Frank* Carpenter Technology Corporation Reading, Pennsylvania Superalloys are high-performance materials designed to provide high mechanical strength and resistance to surface degradation at high temperatures of 1200 F (650 C) or above. They combine high tensile, creep-rupture, and fatigue strength; good ductility and toughness; and excellent resistance to oxidation and hot corrosion. Furthermore, superalloys are designed to retain these properties during long-term exposures at elevated temperatures. This article focuses on the wrought age-hardenable alloys, which are the most common superalloys. Wrought materials can be formed by hotworking and cold-working operations. Not discussed here are the cast, powder metallurgy, and oxide dispersion-strengthened superalloys that can also offer enhanced properties. Superalloy applications The first age-hardenable, high-temperature alloy *Member of ASM International dates back to about 1929, when various developers added titanium and aluminum to the standard 80% nickel/20% chromium resistance-wire alloy. This was a precursor to the nickel-base superalloy, developed during 1940 to 1944 but still in use today. Little was done to advance the original age-hardenable alloys until the time period of 1935 to 1944, when World War II spurred demand for improved alloys suitable for the early aircraft gas turbine engines. Alloy development activity exploded in the 1950s and 1960s to keep pace with the demands of the gas turbine engine industry. Progress in superalloy development not only made the jet engine possible, but also allowed for constantly increasing thrust-to-weight ratios over the last 60 years. The primary application for superalloys is still in hot sections of aircraft gas turbine engines, accounting for over 50% of the weight of advanced engines. However, the excellent performance of these materials at elevated temperatures has expanded their application far beyond this industry. In addition to aerospace components, these alloys are specified for turbine engines for marine, industrial, and land-based power generation, as well as vehicular applications. Specific engine parts include turbine disks, blades, compressor wheels, shafts, combustor cans, afterburner parts, and engine bolts. Beyond the gas turbine engine industries, superalloys are chosen for applications in rocket engines, space, petrochemical/energy production, internal combustion engines, metal forming (hotworking tools and dies), heat-treating equipment, nuclear power reactors, and coal conversion. Although these alloys are primarily for service Table 1 Nominal compositions of wrought age-hardenable superalloys* Alloy Cr Ni Co Mo Ti Al Nb Zr Fe Other Pyromet 14.5 25 1.25 2 0.2 0.05 max Bal 0.3 V NCF 3015 () 14.5 31 0.7 2.7 1.9 0.7 Bal Pyromet 16 42 1.7 0.2 3 Bal Pyromet 13.5 43 6 3 0.2 Bal Pyromet 18.5 53 3 1 0.5 5.3 19 max Pyromet 19 54 11 10 3.2 1.7 0.04 2 max Pyromet 16 57 15 3 5 2.5 0.04 0.5 max 1.25 W Pyromet 23 57 2 2.3 1.3 0.9 0.05 14 19 58 13 4.25 3 1.4 0.05 2 max Pyromet 15.5 71 2.4 1.3 1 0.05 9 max Pyromet 15 72 2.6 0.75 0.9 0.05 9 max Pyromet 20 75 2.4 1.4 0.05 2 max Low-expansion superalloys (low chromium) Pyromet CTX-909 0.5 max 37 14 1.6 0.15 max 5 Bal Thermo-Span 5.5 25 29 0.9 0.5 5 Bal *These alloys also contain small amounts of C, Mn, S, P, S, B, and Zr. ADVANCED MATERIALS & PROCESSES/JUNE 2005 37
Superalloys are classified into three main groups, according to whether they are based on nickel, iron, or cobalt. at elevated temperatures above 1000 F (540 C), the characteristics of high strength and excellent environmental resistance have made some superalloys an ideal choice for lower-temperature applications. Examples are prosthetic devices and components for deep sour-gas wells in oil/gas exploration. Chemical composition Table 1 shows the nominal compositions of the most common wrought age-hardenable superalloys. These alloys contain various combinations of nickel, iron, cobalt, and chromium, with lesser amounts of other elements such as molybdenum, niobium, titanium, and aluminum. With minor amounts of beneficial elements such as boron and zirconium, these alloys may contain up to 12 intentional additions that help to impart and maintain critical properties at elevated temperatures. Many other elements such as silicon, phosphorus, sulfur, oxygen, nitrogen, and a larger number of tramp elements (such as lead, bismuth, selenium) must be tightly controlled in superalloys to avoid detrimental effects on high-temperature properties. These minor and tramp elements are controlled during raw material selection prior to melting, as well as during the melting/remelting processes. Superalloys are classified into three main groups, according to whether they are based on nickel, iron, or cobalt. Nickel-base superalloys (>50% Ni) are the most common group. About half of the alloys in Table 1 are considered nickel-base alloys, and the others contain large additions of nickel. The nickel base has a high tolerance for alloy additions that might otherwise cause phase instability leading to loss of strength, ductility, and/or environmental resistance. Iron-base superalloys are less costly, but are also less tolerant of alloying additions and typically have lower mechanical properties and lower maximum temperatures. Examples are Pyromet alloy and NCF 3015() alloy. These contain an austenitic stainless steel base with additions of nickel, titanium, and aluminum to promote age hardening. Pyromet alloys and have similar amounts of nickel and iron, and can be considered nickel-iron base superalloys. The higher nickel levels of and alloys allow for larger additions of strengthening elements without deleterious effects. Cobalt-base superalloys are fewer in number than nickel- and iron-base superalloys. They are significantly higher in cost and typically cannot be age-hardened to high strength levels. However, cobalt is an important alloying addition to nickelbase alloys because it extends the maximum service temperature by reducing the solubility of the age-hardening phase. and Pyromet alloys and are nickel-base alloys with 10 to 15% cobalt. These alloys have the highest temperature capability of the common wrought agehardenable superalloys. Chromium, usually in the range of 14 to 23 wt%, is a critical alloying addition to nearly all superalloys. As in stainless steels, chromium forms a tightly adherent, protective oxide film (Cr 2 O 3 ) on the surface to resist oxidation and corrosion at high temperatures, as well as corrosion at lower temperatures. This surface layer protects the alloy from the harmful effects of the elements oxygen, nitrogen, and sulfur. Although most superalloys contain at least 14% chromium, in some applications it is critical to minimize thermal expansion. Pyromet CTX-909 and Thermo-Span alloys are considered low-expansion superalloys that have low chromium contents to minimize expansion of the nickel-cobalt-iron base. However, these lower amounts of chromium mean that resistance to oxidation and hot corrosion is reduced; therefore, high-temperature coatings are often applied prior to service. Of the two alloys, 909 alloy provides the lowest expansion coefficient, while Thermo-Span alloy (5.5% chromium) provides improved environmental resistance. Other elemental additions Refractory elements such as molybdenum, tungsten, and niobium, with their large atomic diameters, raise high-temperature strength and stiffness by straining the nickel-iron base matrix. Alloys and contain larger additions of molybdenum to increase this solid solution strengthening effect. Other alloying additions such as chromium and aluminum also contribute to solid solution strengthening, but to a lesser extent. The elements titanium, aluminum, and niobium are added to the nickel or nickel-iron matrix to form an intermetallic phase Ni 3 (Al, Ti, Nb) during age-hardening heat treatments. The resultant gamma prime or gamma double-prime phases are the primary strengthening agents in superalloys. This will be discussed in more detail in the next section on age-hardening. Although elements such as boron, zirconium, and magnesium may be added at levels less than 0.1 wt%, the beneficial effects can be very potent. These elements segregate to and stabilize grain boundaries, which significantly improves hot workability, high-temperature strength, and ductility. Small additions of carbon also may be added to form carbides that restrict grain growth and grain boundary sliding during high-temperature operation. Age hardening The major strengthening mechanism in superalloys is age hardening. Yield strength of nickel alloys is typically increased by a factor of two or three by precipitation of the gamma prime and/or gamma double-prime, Ni 3 (Al, Ti, Nb) hardening phase. Although the phase is based on the nickel aluminide (Ni 3 Al) intermetallic, up to 60% of the aluminum can be replaced by titanium or niobium, which actually increases strength of the alloy. The gamma prime phase is rather unique in that its strength actually increases with temperature up to 1200 F (650 C), and it is relatively ductile 38 ADVANCED MATERIALS & PROCESSES/JUNE 2005
Table 2 Heat treatment and yield strength at various temperatures for selected alloys Approximate temperature 75 F 1200 F 1300 F 1400 F 1500 F 1600 F Alloy limit, F ( C) (24 C) (650 C) (705 C) (760 C) (815 C) (870 C) Heat treatment 0.2% offset yield strength, ksi (MPa) Pyromet B 1400 93 82 77 65 45 B-2100/2h/AC+1550 (760 ) (640) (565) (530) (450) (310) /24h/AC+1300 /20h/AC Pyromet A 1100 126 110 103 A-1625 /2h/AC+1300 (595 ) (870) (760) (710) /20h/AC Pyromet 1500 99 88 85 79 63 1975/8h/AC+1300 (815 ) (680) (605) (585) (545) (435) (285) /16h/AC Pyromet 1300 100 88 82 62 33 1800/1h/OQ+1325 (705 ) (690) (605) (565) (425) (230) /16h/AC NCF 3015 () 1400 100 98 94 82 59 1922/0.5h/AC+1382 (760 ) (690) (675) (650) (465) (405) /4h/AC Pyromet 1500 109 103 100 97 88 2 2050/1h/AC+1575 (815 ) (750) (710) (690) (670) (605) /4h/AC+1350/4h/AC Pyromet 1500 115 110 105 100 88 2050/1h/AC+1575 (815) (795) (760) (725) (685) (605) /4h/AC+1350/4h/AC B 1600 115 102 100 96 87 75 B-1975/4h/OQ+1550 (870 ) (795) (705) (690) (660) (600) (515) /4h/AC+1400 /16h/AC A 1600 132 115 114 109 96 76 A-1850 /4h/OQ+1550 (870 ) (910) (795) (785) (750) (660) (525) /4h/AC+1400 /16h/AC Pyromet 1400 130 115 111 98 75 2000/2h/WQ+1450 (760 ) (895) (795) (765) (675) (515) /2h/AC+1325 /24h/AC Pyromet 1300 146 125 116 96 1800/1h/AC+1550 (705 ) (1005) (860) (800) (660) /3h/AC+1325 /8h/FC to 1150 /8h/AC Pyromet 1600 150 140 136 132 118 78 1950/4h/AC+1400 (870 ) (1035) (965) (940) (910) (815) (540) /16h/AC Pyromet 1300 168 144 133 111 1800/1h/AC+1325/8h/FC (705 ) (1160) (995) (915) (765) to 1150 /8h/AC Pyromet 1600 173 164 161 152 135 109 2035/2h/AC+1975/4h/ (870 ) (1195) (1130) (1110) (1050) (930) (750) OQ+1200 /24h/AC+1400 /8h/AC Low-expansion superalloys (low chromium) Pyromet CTX-909 1200 148 126 1800 /1h/AC+1325 (650) (1020) (970) /8h/FC to 1150 /8h/AC Thermo-Span 1250 130 120 98 2000 /1h/AC+132 (675 ) (895) (825) (675) 5 /8h/FC to 1150 /8h/AC and resistant to oxidation. Gamma prime precipitates as very fine spheroidal or cuboidal particles in the nickel-iron matrix during aging. Although most of the superalloys are age-hardened by the titanium-rich gamma prime phase, a niobium-rich variant called gamma double-prime is the primary strengthening phase in some superalloys such as Pyromet alloys and. The niobium-rich phase provides higher strength up to 1200 F (650 C), but is unstable above 1200 F (650 C). Thus, and alloys have a lower temperature limit than the alloys strengthened with the titanium-rich gamma prime phase. Because the gamma double-prime reaction is more sluggish, these alloys also tend to have better hot workability and weldability. Heat treatment Proper heat treatment is critical to achieving the necessary level of properties in age-hardenable superalloys. Typical heat treatments for these alloys are listed in the mechanical property Tables 2 and 3. The initial solution heat treatment typically dissolves all precipitated phases except for some primary carbide and nitride phases. The typical range for the wrought age-hardenable superalloys is 1650 to 2100 F (900 to 1150 C) for one to four hours, followed by a rapid air cool or a quench in water, polymer, or oil. The selection of solution treatment time and temperature varies with the alloy and its phase solvus temperatures, and also depends on the specific properties that are most important for the intended application. Alloys with higher hardener contents (Ti, Al, Nb) require higher temperatures to solution any hardener phase that may have precipitated during hot working or cooling. Best tensile and fatigue properties are typically developed with lower solution temperatures that result in a finer grain size. In contrast, better longterm stress-rupture and creep properties are generally achieved with higher-temperature solution treatments that result in coarser grain size and lower tensile yield strength. For these reasons, it ADVANCED MATERIALS & PROCESSES/JUNE 2005 39
Table 3 Stress-rupture properties of wrought age-hardenable superalloys Approximate temperature 1200 F 1300 F 1400 F 1500 F 1600 F Alloy limit, F ( C) (650 C) (705 C) (760 C) (815 C) (870 C) Heat treatment Stress for 1000-hour life, ksi (MPa) Pyromet 1300 46 29 15 1800 F/1h/OQ+1325 F/16h/AC (705) (315) (200) (105) Pyromet 1500 65 42 27 13 1975 F/8h/AC+1300 F/16h/AC (815) (450) (290) (185) (85) Pyromet 1500 68 45 30 18 2050 F/1h/AC+1575 F/4h/ (815) (470) (310) (205) (125) AC+1350/4h/AC Pyromet 1500 68 45 30 18 2050 F/1h/AC+1575 F/4h/ (815) (470) (310) (205) (125) AC+1350/4h/AC Pyromet 1400 67 43 27 15 2100 F/2h/AC+1550 F/24h/ (760) (460) (295) (185) (105) AC+1300 F/20h/AC Pyromet 1400 74 52 30 14 2000 F/2h/WQ+1450 F/2h/ (760) (510) (360) (205) (95) AC+1325 F/24h/AC Pyromet 1300 84 53 25 1800 F/1h/AC+1550 F/3h/ (705) (580) (365) (170) AC+1325 F/8h/FC to 1150 F/8h/AC Pyromet 1300 89 56 28 1800 F/1h/AC+1325 F/8h/FC to (705) (615) (385) (195) 1150 F/8h/AC 1600 89 64 42 26 16 1975 F/4h/OQ+1550 F/4h/ (870) (615) (440) (290) (180) (110) AC+1400 F/16h/AC Pyromet 1600 101 77 49 29 17 1950 F/4h/AC+1400 F/16h/AC (870) (695) (530) (340) (200) (115) Pyromet 1600 110 93 67 45 31 2035 F/2h/AC+1975 F/ (870) (760) (640) (460) (310) (210) 4h/OQ+1200 F/24h/ AC+1400 F/8h/AC NCF 3015 () 1400 64 42 25 1922 F/0.5h/AC+1382 F/4h/AC (760) (440) (290) (170) Low-expansion superalloys (low chromium) Pyromet CTX-909 1200 48 1800 F/1h/AC+1325 F/8h/FC to (650) (330) 1150 F/8h/AC Thermo-Span 1250 63 2000 F/1h/AC+1325 F/8h/FC to (675) (435) 1150 F/8h/AC is common to specify two or more preferred heat treatments for superalloys. In some cases, another objective of the solution treatment is to form a more beneficial distribution of a second phase such as carbide in Pyromet alloy and delta phase (Ni 3 Nb) in Pyromet alloy. After solution treatment, one or more aging treatments are applied to precipitate the hardening phase and possibly other phases in the suitable amount and distribution. As with solution treatment, the selection of aging temperatures depends on the alloy and the combination of properties needed. The aging range for age-hardenable superalloys is 1150 to 1600 F (620 to 870 C). Aging times range from four hours to 24 hours. Double-aging treatments are quite common to maximize strength and to develop the best combination of short-term tensile and long-term creep-rupture properties. The primary aging treatment precipitates a coarser distribution of the hardener phase, and may also improve the type and distribution of carbides on grain boundaries. The secondary age is typically about 200 F (110 C) below the primary aging temperature, precipitating a finer dispersion of the gamma-prime phase. For some higher-strength applications, the alloy is direct-aged after hot, warm, or cold working, without an intermediate solution treatment. The strain from working serves to further enhance tensile and fatigue properties, although with some sacrifice in creep-rupture properties. Mechanical properties For the design engineer or materials specifier, a review of terms defining applicable mechanical properties may be helpful: Tensile properties: The design of load-bearing structures is often based on yield strength, or in some cases, the ultimate tensile strength of the material. Yield strength is a measure of the maximum stress a material can withstand before it permanently deforms. Tensile strength is a measure of the maximum stress a material can withstand before it fractures. Elevated-temperature tensile properties are most applicable to short-term exposures at higher temperatures. Creep and stress-rupture properties are more applicable for longer-term exposures. Creep and rupture properties: Creep and rupture strengths become important when the ma- 40 ADVANCED MATERIALS & PROCESSES/JUNE 2005
terial must withstand the combined effects of high temperature and stress for long periods of time. At elevated temperatures, metals stretch or creep at stresses well below the yield strength. Superalloys are more resistant to creep than low-alloy or stainless steels, but creep still develops at temperatures above about 1000 F (540 C). Creep properties are a measure of the alloy s resistance to stretching under a constant load. Stress-rupture or creep-rupture properties are a measure of resistance to fracture under a constant load (creep test taken to fracture). Both properties are expressed as stress or strength values that will cause a given amount of creep (0.1% to 1%) or rupture in a given amount of time (100 to 100,000 hours). Tables 2 and 3 list typical tensile (yield) and stress-rupture strength properties of the age-hardenable superalloys at temperatures of 1200 to 1600 F (650 to 870 C). Yield strengths at room temperature are also listed in Table 2. It should be noted that the data represents approximate nominal strength values for specific heat treatments. Actual values can vary by up to 35% due to differences in composition, hot/cold working practices, and heat treatment. For example, superalloys such as Pyromet and may contain several different aim compositions within the broader industry ranges to optimize properties for specific applications. Higher levels of the age-hardening elements titanium, aluminum, and niobium result in higher strength. Hot or cold working an alloy to provide a finer grain size typically increases tensile yield strength, but decreases stress-rupture strength. As discussed previously, properties of all agehardenable superalloys depend on heat treatment. Alloys such as Pyromet and have two or more preferred heat treatments (see Table 2) depending on whether the application requires better short-term tensile and fatigue properties or long-term creep and stress-rupture properties. Examples of alternative heat treatments have been shown for and alloys, but the reader should refer to manufacturer datasheets for a more complete listing of alternative heat treatments for the other superalloys. Other properties: Although tensile and creeprupture are the most basic mechanical properties considered for high-temperature applications, design criteria may also include resistance to fatigue (low- and high-cycle), crack growth, and wear/erosion. Hardness and hot-hardness tests are sometimes used as a rough measure of yield strength and resistance to wear/erosion. Alloy selection A simplified method known as the Carpenter Selectaloy system can help designers and engineers select the most suitable superalloy based on strength and maximum temperature requirements. Figures 1 and 2 contain Selectaloy diagrams for the 14 superalloys discussed in this article. Yield strength (Fig. 1) or stress-rupture strength (Fig. 2) increases vertically on the Selectaloy diagram, and Tensile yield strength 909 // Thermo-span / / 909/ Thermo- Span / / / 75 1200 1300 1400 1500 1600 (24) (650) (705) (760) (815) (870) Temperature, F ( C) Fig. 1 Age-hardenable superalloy Selectaloy diagram showing yield strength. Stress-rupture strength / / Thermo- Span 909 1200 (650) / /X- 750/ 1300 (705) / Fig. 2 Age-hardenable superalloy Selectaloy diagram showing stressrupture strength. temperature increases from left to right. The alloys are shown multiple times on the diagrams because the alloys are useful over a range of temperatures. The diagram can be used to estimate not only how the strength of an alloy decreases with temperature, but also how the strengths of different alloys compare at different temperatures. It should be noted that the alloys were positioned on the Selectaloy diagrams based on average strength values that are representative of compositions and heat treatments typical for each alloy. An alloy s relative position could move up or down, left or right, with relatively minor modifications of composition, processing, and heat treatment. Temperature limits should be considered approximate. Therefore, while the Selectaloy diagrams are useful tools to screen candidate alloys, they are not a substitute for a more detailed evaluation of the critical properties required for an intended application. Pyromet alloy is the most basic age-hardenable superalloy in terms of properties and cost. provides the lowest strength levels, but still higher by a factor of two than other non-agehardenable stainless alloys. When increased / 1400 (760) Temperature, F ( C) 1500 (815) ADVANCED MATERIALS & PROCESSES/JUNE 2005 1600 (870)
Thermo-Span 909 Alloy selection must be based on expected cost effectiveness. 0 1 2 3 4 5 Relative raw material cost, ten-year average Fig. 3 Relative raw material cost of age-hardenable superalloys. strength or temperature resistance is required, higher nickel alloys are typically preferred. Alloys with the highest levels of strength and temperature resistance typically contain the highest alloy contents and significant levels of cobalt. Relative cost of these alloys will be discussed in the next section. The Selectaloy diagrams presented in this article provide a method to compare basic strength properties and temperature limitations of common wrought age-hardenable superalloys. However, alloy selection will undoubtedly depend on many other considerations, including other physical and mechanical properties, as well as environmental resistance and cost. For example, Thermo-Span and Pyromet CTX-909 alloys provide a benefit of much lower expansion during heating, but at the expense of oxidation and corrosion resistance in the uncoated condition. Pyromet and alloys provide similar strength and temperature resistance, but the higher chromium content of alloy results in much improved resistance to sulfidation and other forms of hot corrosion. Alloy cost From the user s standpoint, alloy selection must be based on expected cost effectiveness. In today s competitive global environment, overdesign is less common than in the past. The trend is to select the lowest-cost material to meet design requirements for the application. However, a higher-cost alloy may be justified to minimize overall lifecycle cost, or for longer service of certain components in a system that is critical or too expensive to be shut down for maintenance. Surely, knowledge of alloy capabilities is critical in making the best decision. As temperature and strength requirements increase, so does the necessary alloy content. Figure 3 compares the relative alloying costs of the 14 alloys, with Pyromet alloy as a base (cost factor of 1.0). The cost factors are based on ten-year averages of the intrinsic alloying element costs at market prices. Higher-temperature strength and resistance typically require higher nickel and cobalt contents. Nickel and cobalt prices have historically been volatile, with high and low prices varying by a factor of four to five. More recently, the price of molybdenum, a potent solid solution strengthener, has increased in price by a factor of nearly ten over the last two years. As discussed above, the cost factors in Fig. 3 are based only on raw material elemental costs (tenyear averages) that fluctuate significantly with time. Differences in melting, working, and other processing costs, which can be substantial, are not included in these factors. Processing yields and specific end user requirements (grain size, ultrasonic testing, etc.) significantly impact product cost. However, the cost comparisons are useful because alloying costs typically represent a large portion of superalloy product cost. Since superalloys are designed for high-temperature strength and resistance to deformation, processing difficulty and cost also increase with hot strength and maximum temperature capability. It is apparent that the alloys that provide higher levels of strength, temperature resistance, and/or specialized properties also cost more, which reinforces the importance of the alloy selection process. For more information: Richard B. Frank is Staff Specialist, High Temperature R&D, Carpenter Technology Corporation, P.O. Box 14662, Reading, PA 19612-4662; tel: 610/208-2696; fax: 610/736-7148; e-mail: rfrank@ cartech.com; Web site: www.cartech.com. 42 ADVANCED MATERIALS & PROCESSES/JUNE 2005