Gas Turbine Superalloy

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GAS TURBINE SUPERALLOY WITH IMPROVED FABRICABILITY A wrought gamma-prime strengthened superalloy combines high-temperature strength with excellent formability for gas turbine engine components.

Lee M. Pike* H. Lee Flower* Haynes International Kokomo, Indiana

new wrought nickel-chromium-cobalt- molybdenum superalloy, designated Haynes 282 alloy, possesses a unique combination The Haynes 282 alloy is designed for applications in engines for aircraft such of high-temperature strength and fabri- as the Lockheed Martin F-35 Joint Strike Fighter. cability that gives it a distinct advantage Aover other alloys. The attributes of the alloy are cracking. On the other hand, higher levels of well suited for a wide variety of high-tempera- gamma-prime are beneficial for high-temperature applications, particularly those in aero and ture strength. Therefore, the gamma-prime con- land-based gas turbine engines. tent is balanced so that 282 alloy is easy to fabri- With 57% nickel, 20% chromium, 10% cobalt, cate, yet still possesses excellent creep strength. and 8.5% molybdenum (Table 1), the alloy falls The features of 282 alloy make it suitable for into a subcategory within the larger category of critical applications such as sheet fabrications and the familiar gamma-prime-strengthened super- seamless and flash butt-welded rings found in alloys. This subcategory includes such alloys as compressor, combustor, and turbine sections. In Waspaloy, René-41 (or R-41), and 263, which are augmented aircraft gas turbines, the new alloy distinguished by their ability to be produced as will be useful for exhaust and nozzle components. flat products; that is, in the form of hot-rolled plate In land-based gas turbines, 282 alloy is a good op- and/or cold-rolled sheet. tion for transition sections and other hot-gas-path To be produced economically in flat form, a con- components. siderably lower gamma-prime content is needed than is acceptable for alloys produced by other Processing and heat treatment methods, such as casting, powder metallurgy, Several production scale heats of 282 have been or isothermal forging. Moreover, too much successfully processed. The 20,000 lb (9080 kg) gamma-prime can lead to considerable difficulty heats were primary-melted via vacuum induc- in welding, particularly with regard to strain-age tion melting (VIM), and secondary-melted by elec- *Member of ASM International troslag remelting (ESR). The resulting ingots have Table 1 — Nominal composition of several wrought gamma-prime superalloys Alloy Ni Cr Co Mo Ti Al Fe Mn Si C B Other 282 57a 20 10 8.5 2.1 1.5 1.5* 0.3* 0.15* 0.06 0.005 — Waspaloy 58a 19 13.5 4.3 3 1.5 2* 0.1* 0.15* 0.08 0.006 Zr-0.05 R-41 52a 19 11 10 3.1 1.5 5* 0.1* 0.5* 0.09 0.006 — 263 52a 20 20 6 2.4* 0.6* 0.7* 0.4 0.2 0.06 0.005 Al+Ti-2.6 *Maximum a, nickel as balance ADVANCED MATERIALS & PROCESSES/JUNE 2006 39 Test temperature, °C that are caused by shrinkage due the solidification 760 780 800 820 840 860 40 of the weld metal and to the formation of gamma- prime during the post-weld heat treatment. 263 alloy, one heat Critical to the development of the 282 alloy was 282 alloy, two heats Waspaloy, two heats a test that could readily determine the resistance R-41 alloy, of an alloy to strain-age cracking. One test that 30 three heats met the requirements of the development program (inexpensive, reproducible, and quantita- tive) was the controlled heating rate tensile (CHRT) test developed at Rocketdyne in the late 20 1960s. The purpose of this test was to identify heats of R-41 alloy that were more susceptible to Elongation, % strain-age cracking In the CHRT test, the sample is heated to the 10 appropriate temperature at a rate chosen to sim- ulate the heat-up during the first heat treatment after welding. The sample is originally in the solution annealed condition (little to no gamma- prime present), so that gamma-prime precipitates 1450 1500 1550 1600 Test temperature, °F continuously during the test. Typical temperatures are between 1400 and 1600ºF (760 to 871ºC), Fig. 1 — Resistance to strain-age cracking as measured by the controlled and the strain-age cracking resistance of a given heating-rate tensile (CHRT) test. alloy is taken to be related to the minimum elon- Larson-Miller parameter, K x 10-3, C = 20 gation observed in that temperature range. 24 25 26 27 28 30 Two different production scale heats of 282 alloy were evaluated via CHRT tests. The results are 20 shown in Fig. 1 along with similar data on several 100 other alloys as determined in a recent program at 10 Haynes International. For consistency, all of the test samples were taken from 0.063 in. (1.6 mm) sheet. 5 4 In the plot, it is evident that the R-41 and Was- Stress, ksi

Stress, MPa paloy alloys (both of which are well known to 3 263 alloy Waspaloy be susceptible to strain-age cracking) have min- 2 R-41 alloy imum elongations less than 5%. In contrast, 282 282 alloy 10 has a minimum elongation of around 14%, sug- 1 gesting that the alloy has significantly higher re- 42 44 46 48 50 52 sistance to strain-age cracking. The 263 alloy had -3 Larson-Miller parameter, °R x 10 , C = 20 the best performance of the four alloys in the Fig. 2 — The Larson-Miller parameter indicates creep strength of several CHRT test, as might be expected from its reputa- superalloys (sheet) at 1500 to 1700°F (816 to 927°C). tion as being essentially immune to strain-age been processed by several methods, including hot cracking. forging and rolling, as well as cold-rolling and The susceptibility of alloys to strain-age drawing. cracking depends strongly on the amount of Product forms have included hot-rolled plate gamma-prime forming elements, such as alu- and bar, cold-rolled sheet, reforge billet, and wire. minum and titanium. This relationship was re- Haynes 282 is provided in the solution-annealed flected in the CHRT data, with the more suscep- condition. The typical solution annealing tem- tible R-41 and Waspaloy alloys having higher perature is in the range of 2025 to 2100°F (1107 levels of Al + Ti than does 282 alloy , which in turn to 1149°C). After annealing, the alloy is in a soft has more than the 263 alloy. condition and is readily formable. A two-step age hardening treatment is required to put the alloy High-temperature strength into the high-strength condition. The treatment One of the key features of 282 is its excellent includes 1850°F (1010°C)/2 hours/AC (air cool) high-temperature strength. The stress values re- + 1450°F (788°C)/8 hours/AC. quired to produce 1% creep, as well as rupture in 1000 hours at 1200 to 1700ºF (649 to 927ºC), are Strain-age cracking resistance given in Table 2. Of the four alloys, 263 has the Haynes 282 alloy was designed to have supe- lowest creep and rupture strength across the en- rior resistance to strain-age cracking compared to tire temperature range. The rupture strengths other high-strength gamma-prime alloys. Strain- of 282 and Waspaloy alloys are about the same at age cracking is a problem with certain age-hard- lower temperatures, but at higher temperatures, enable alloys and typically occurs during heat up 282 alloy has a distinct advantage. in the first post-weld heat treatment (usually a so- In terms of 1% creep, the 282 alloy is signifi- lution anneal) of a welded component. The cantly stronger than Waspaloy over the entire tem- cracking has been associated with residual stresses perature range. When compared to R-41 alloy, 40 ADVANCED MATERIALS & PROCESSES/JUNE 2006 Table 2 — Stress to produce 1% creep and rupture in 1000 hours Test temperature Haynes °F °C 263 alloy Waspaloy alloy R-41 alloy 282 alloy 282 was Stress to produce 1% creep in 1000 hours, ksi (MPa) specifically 1200 649 58 (400) 67 (462) 84 (579) 80 (552) 1300 704 41 (283) 46 (317) 59 (407) 53 (365) designed 1400 760 25 (172) 28 (193) 34 (234) 35 (241) to have 1500 816 12 (83) 16 (110) 18 (124) 21 (145) 1600 871 6 (41) 7 (48) 9 (62) 10 (69) outstanding 1700 927 3 (21) 3 (21) 5 (34) 5 (34) formability Stress to produce rupture in 1000 hours, ksi (MPa) properties. 1200 649 64 (441) 80 (552) 90 (621) 80 (552) 1300 704 45 (310) 58 (400) 68 (469) 57 (393) 1400 760 28 (193) 36 (248) 43 (296) 38 (262) 1500 816 15 (103) 20 (138) 24 (165) 23 (159) 1600 871 7 (48) 7 (48) 13 (90) 13 (90) 1700 927 4 (28) 3 (21) 7 (48) 7 (48) the 282 alloy has a lower rupture strength up to 1400ºF (760ºC), but the two alloys have essentially 150 the same rupture strengths at temperatures of 1000 1500ºF (816ºC) or greater. Similarly, the R-41 alloy Waspaloy has slightly higher 1% creep strength than 282 R-41 alloy 800 alloy at the lower temperatures, but the distinc- tion the two alloys disappears at higher temper- 100 282 alloy 263 alloy atures, with the 1% creep strength of the two al- R-41 alloy 600 loys being virtually the same at temperatures of 1500ºF (816ºC) or greater. 282 alloy This can be seen clearly in Fig. 2, in which the Waspaloy 400 Yield strength, ksi Yield 1% creep life of 282, R-41, Waspaloy, and 263 al- 50 strength, MPa Yield loys are shown in a Larson-Miller plot. The plot 263 alloy shows experimental data points for samples tested 200 between 1500 and 1700ºF (816 and 927ºC). In this plot, it is clear that 282 and R-41 alloys have virtually the same creep strength, while both alloys RT 1600°F (871°C) are stronger than Waspaloy and 263 alloys. Fig. 3 — Tensile yield strength of several superalloys (sheet) This level of creep strength is remarkable con- in the age-hardened condition sidering the fact that 282 has a significantly lower gamma-prime content than either the R-41 or Was- 110 700 paloy alloys. Its high creep strength derives, in 100 part, from the presence of a relatively high level of the solid-solution strengthening element 90 molybdenum. The high creep strength can also R-41 alloy 600 be attributed to the alloy’s good thermal stability. 80 Waspaloy Waspaloy The yield strengths of 282 at room tempera- 500 ture and at 1600°F (871°C) are shown in Fig. 3, 70 along with the other comparative alloys. At

60 282 alloy room temperature, the yield strengths of the 282 400 Yield strength, ksi Yield and 263 alloys are similar and are lower than the strength, MPa Yield R-41 and Waspaloy alloys, an indication of the 50 263 alloy 300 higher gamma-prime content of the latter two 40 alloys. However, at 1600°F (871°C), the 282 alloy can be seen to have considerably greater yield 30 strength than 263, and is even stronger than Waspaloy. Fig. 4 — Room temperature tensile yield strength of several superalloys (sheet) in the mill-annealed condition. Formability properties An age-hardenable alloy should be soft when temperature to maintain sufficient formability. in the annealed condition so that it can be readily Haynes 282 was specifically designed to have out- formed, and should develop high strength only standing formability characteristics, particularly after a deliberate age-hardening heat treatment. compared with other gamma-prime-containing However, rapid precipitation kinetics prevent alloys. Typical room temperature yield strength many gamma-prime strengthened alloys from values for solution annealed 282, R-41, Waspaloy, being cooled quickly enough from their annealing and 263 alloy mid-gage sheet with thickness 0.060 ADVANCED MATERIALS & PROCESSES/JUNE 2006 41 Waspaloy. This is noteworthy in light of the fact that both 282 and 263 are gas-cooled from the annealing temperature, resulting in a much slower cooling rate than is standard for R-41 and Was- paloy, which are water-quenched. The slower age-hardening kinetics of the 282 and 263 alloys can be attributed the lower content of gamma- prime forming elements (aluminum and titanium) relative to the R-41 and Waspaloy alloys. Haynes 282 alloy billet has been forged into seamless ring sections for a prototype gas turbine engine. The forger reports that 282 alloy forges more easily than Waspaloy alloy. Also, in a sep- arate trial, 282 alloy bar has been flash butt-welded with good success.

Fig. 5 — The outstanding formability of 282 alloy sheet is shown in this gas For more information: turbine engine part, which illustrates a trial aircraft gas turbine fabrication re- Lee Pike is Chief Physical Met- quiring two hydroforming steps. The ductile 282 alloy sheet was formed without allurgist at Haynes International, Kokomo, IN 46904; an intermediate anneal. The material resisted cracking and rippling, and spring- e-mail: [email protected]. Lee Flower is Market back was minimal. The part is approximately 12 inches in diameter and 2 inches Manager at Haynes International; tel: 765/456-6031; deep. fax: 765/456-6175; e-mail: [email protected]; Web site: www.haynesintl.com. to 0.125 in. (1.5 to 3.2 mm) produced by Haynes International are shown in Fig. 4. Haynes and 282 are trademarks of Haynes International. From this data, it is clear that both 282 and 263 have significantly lower yield strength in the so- References Fawley, R. W. and Prager, M., 1970, “Evaluating the Re- lution annealed condition than the R-41 and Was- sistance of Rene 41 to Strain-Age Cracking”, WRC Bul- paloy alloys, indicating greater formability as letin, 150, pp. 1-12. shown in Fig. 5. This suggests that in the solution- Rowe, M. D., 2006, “Ranking the Resistance of Wrought annealed condition, little to no gamma-prime is Superalloys to Strain-Age Cracking,” Welding Journal, present in 282 and 263 alloys relative to R-41 and 85(2), pp. 27-s to 34-s.

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