GB2205109A - Castable, weldable nickel base alloy - Google Patents

Description

1 11 2t r s'. - A f 4uli-"-ug CASTABLE, WELDABLE NICXEL-BASE SUPERALLOY is

The present invention is directed to an alloy composition which is readily castable and which can-be used to provide cast parts of intricate design which are readily weldable without cracking, have high strength at room temperature and at elevated temperatures and have high low-pycle fatigue and improved stress rupture properties.

U.S. Patent No. 3,046,108 was granted to H.L. Eiselstein in 1962 for an alloy popularly known as Alloy 718. According to the Metals Handbook, 9th Edition, Vol. 3, page 748, Alloy 718 nominally contains, by weight percent, 18.6t chromium, I.S.St iron, 0.04% carbon, 0.4% aluminum, 0.9% titanium, St niobium, 3.11 molybdenum, with the balance essentially nickel.

Alloy 718 is the major intermediate temperature investment cast superalloy in use today in the aerospace industry. This alloy ACCOUDtS for approximately 1800 tons of investment cast parts a year. it is an alloy originally designed to he used in the wrought condition and was adopted to casting with virtually no change in alloy composition. Alloy 718 contains relatively large amounts of fe, Nb, and up to 0.351 Si, all of which contribute to Laves phase formation. The Laves phase is a low melting phase which forms at the interdendritic locations in castings and has frequently been cited as causing poor weldability.

is Investment castings require weld repair in order to produce a product due to the state of development of today's casting processes. Without weld repair capability, few investment castings of any size could be produced in a cost effective manner. Therefore, the weldability of an investment cast alloy is critical to its utilization. Alloy 718 has provided sufficient weldability to function adequately for the entire aerospace industry to date. But, the industry is pushing to higher and higher temperatures and requires greater thrust to weight ratios which leads to higher stresses on components. Thus the strength of Alloy 718, particularly in stress rupture and low cycle fatigue, is being challenged beyond the alloy's capability.

With Alloy 718 being pushed to its limit of strength and endurance, issues such as microcracking in weldments become wore important. During welding many of the grain boundaries in the Heat Affected Zone (HAZ) near the fusion zone of a weld rupture either during the initial thermal shock of heat up or during solidification. Due to the composition of Alloy 718, the fusion zone material that backfills the open grain boundaries has the low melting Laves phase as its terminal solidification phase. The backfilled materials are thus the last to freeze. This frequently results in the rupture of these same grain boundaries toward the end of the weld cycle, which then become microcracks.

The new alloy of the instant invention has been developed for use in investment castings in the aerospace industry. This inventive alloy is capable of a wore than 100OF increase in operating temperature over Alloy 718 and in certain cast forms has greater mechanical properties than wrought 718. Its castability is rated as equal to or.better than Alloy 718 and 1 V_ p r is its weldability is much better than cast Alloy 718. The chemical composition of the new alloy was designed to provide Improved high temperature strength, good castability, and minimum susceptibility to fusion welding and post weld heat treatment flaws.

Compared to Alloy 718, the alloy of the present invention has improved weldability both as measured by the Varestraint test and by field trials. With little or no Fe, the nickel-base superalloy of this invention does not form large amounts of Laves phase on solidification either during casting or welding. Since no Laves phase forms during the weld solidification cycle, there is little propensity for microcrack formation. Moreover, this alloy forms a more stable delta phase due to the addition of Ta. This also improves the weldability of this new alloy as the delta phase remains as a stable phase during welding, even into the fusion zone of the weld, as opposed to delta in Alloy 718 which liquates at a slightly higher temperature than the 71R Laves phase.

The superalloys of the invention contain (in weight percent) about 18% to about 20% chromium, about 11% to about 13% cobalt, about 3% to about 3.4% molybdenum, about 4.8% to about 5.3% columbium, about 2.8% to about 3.3% tantalum, about 0.9% to about 1.1% titanium, about 0.4% to about 0.6% aluminum, about 0.002% to about 0.005% boron, about 0.025% to about 0. 03S% carbon, the balance essentially nickel.

The alloys of the invention contain little or no iron, i.e., no more than about O.S% iron, and preferably no more than about 0.05% manganese, 0.10% silicon, 0.010% phosphorus, O.ODS% sulfur, 0.010% zirconium, 0.004% magnesium, 0.10% copper, 0.003% oxygen and 0.0075t nitrogen.

A preferred embodiment contains about 18.8% chromium, about 3.201 molybdenum, about 51 columbium, about 3.1% tantalum, about 1% titanium, about 0.5% alvminum, about 11.8% cobalt, about 0.0031 boroin, and about 0. 03% carbon, the balance essentially nickel.

Cast specimens of the alloys of the invention having the composition shown in Table 1 were made and heat treated according to the following preferred schedule: stress relief at 1750F for 4 hours (17501F/4 hrs), hot isostatically pressed (HIP1d) at 205OeF at 14.5 ksi for 3 hours, solutioned 20000F/1 hr, post solution annealed 1650F/4 hrs, aged 14250F/4 hrs, furnace cooled to 1300F followed by a second age at 1300OF for 8 hours.

TABLE 1

Composition (weight percent) Cr 18.6 B 0.003 Co 11.9 Fe 0.07 Mo 3.15 Mg 0.001 is Cb 4.93 ti 0.02 Ta 2.98 p 0.005 Ti 1.01 S 0.002 AI 0.48 0 10 ppm c 0.033 N 17 ppm ,c The aforementioned heat treatment cycle is presently preferred and is typical of that anticipated in a production environment wherein the casting would typically be inspected following the stress relief and weld repaired, if necessary, to repair casting induced defects and wherein fabrication welding and the repair, if necessary, of the fabrication welds would be done following the post solution anneal.

The aforementioned specimens (a minimum of three per test) were tested to measure their strengths at room temperature and at 12006F, their stress rupture strengths at test conditions of 1300OF and 90 ksi and their low cycle fatigue endurance with the results reported in Table Il. The Alloy 718 specimens received the optimum heat treatment for castings of this alloy: stress relief 2000'FlI hT; HIP 21250F/14.7 ksi/3 hours; solution 19250F/1 hr; age 14000F/5 hrs; cool 1000F/hr to 1200'F; age 12000F/1 hr and cool to room temperature in still air.

-D, Z1 In Table 11, CTS refers to specimens that are cast to size as specimens and CK refers to specimens that are cut from actual castings. As way be noted from Table 11, CTS specimens typically exhibit room temperature tensile strengths that are about 30 ksi higher than their CK counterparts. This effect is primarily due to differences in grain size and is a less pronounced factor for the other properties listed In Table 11. This effect may also he noted by comparing the properties for the alloys of the invention when measured on cut from casting specimens compared to those measured on both cast to size specimens and the fine grain specimens cut from castings cast by the Microcast-X process.

is As may he noted from Table II, the alloys of this invention exhibit far superior properties, especially the design-critical stress-rupture and low cycle fatigue properties, to that of cast Alloy 718. As way also be noted from Table 11, the alloys of the invention demonstrate no evidence of notch sensitivity with the preferred heat treatment as shown by the stress rupture data from notched bar specimens. Alloy 718 was not tested by notched bar tests since it had no life In the smooth baT tests. The low notched bar lives reported for.specimens cut from castings made by the Microcast-X Process is attributed to the ultra fine grain size (relative to conventional castings) and to the low level of boron (a grain boundary strengthener) in the Table 1 alloy beat.

13OV-8972 TABIF 11 Room Temperature Tensile 120OF Tensile 1300F/90 KSI UF IOOOF/60KSI LU 1000F/50 KSI UTS 0. ZIYS 1 EL FU UTS 00-2tys % EL 1 RA Cycles kN In -m -oo-t- TI- 1 ro-t -c- K C_ d Alloys of this 192.5 145.1 12.7 16.3 154.4 116.7 8.9 15.6 93.3 200 161, 9306 277,000 Inventionl (CTS) Alloys of this 164 132 8 in 136 107 9 20 so 2009 24,000 177,0007 Invention2, 8 (CFC) Alloy M3 151 133 is 29 117 104 11 29 0 15,000 37,000 MC) Alloys of this 190 149 8 is 165 131 9 20 87 1 278,000 No Data Invention4 (Fine Grain)S 1 Heat-Table 1 4 Heat 7: "TP/ZOSOF/14 7 ksi/3h; ZOOOF/lh; 175OF/4h; 142SF/4h; and 130OF/Rh 2 Avg. of Several Heats 5 By Howmet Microcast-XIM Casting Process 3 Typical 6 1100'F/50 ksi 7 "eat 6 Preferred "eat Treatment 8 17SOF/4h; HIP 2125F/14.7 ksl/3h, 200OF/lh; 17SOP/4h; 142SF/4h; and 130OF/Rh 9 Some Heats Treated with Preferred Heat Treatment and Some with Ileat Treatment of Note 8 11 It 1 1 is The weldability of the alloys of the invention was ass-essed using the Varestraint test. In the Varestraint test, a test specimen is clamped securely into a fixed jig as a cantilever beam and a timed gas tungsten arc weld is applied to the specAmen. After a preset weldin$ time, the specimen is forced to conform to the radius of a die block having a radius such that a selected augmented strain level, e.g.,.1%, 2%, 3%, etc. is applied. Timing is selected to achieve a steady thermal state in the RAZ resulting from the weld before augmented strain is applied. After testing, crack lengths are measured at 12 diameters with total crack length (TCL) and maximum crack length (MCL) being used to compare hot cracking susceptibility.

The results of Varestraint weldability testing on CTS specimens of cast Alloy 718 and the alloys of the invention are set forth in Table III. As may be noted from Table III, the alloys of the invention are more weldable (less propensity for crack formation) than Alloy 718.

TABLE III Varestraint Tests (2% Augmented Strain) Material Alloy 718 (13 heats) Alloy of this Invention (5 beats) Total Crack Length 0.61 0.23 Theexcellent weldability of castings of the alloys of the invention produced by the investment casti ng process was also demonstrated by machining 30 grooves of controlled contour therein and filling the grooves with weld metal of watching composition using the gas tungsten arc welding process to simulate repair welds. Microscopic examination of the resulting welds confirmed the excellent weldability noted in the Varestraint tests; the results of the simulated repair welds are reported in Table IV.

TABLE IV Simulated Repair Welds Length of Number of Material Microcracks (mils) Microcracks 718 10 17 11 1 Alloy of 10 2 Thi s 20 0 Invention 30 0 1.

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Claims (8)

1. A castable, weldable nickel-base superalloy consisting essentially of (in weight percent) about 181 to -about 20% chromium, about 11% to about 13% cobalt, about 3% to about 3.4% molybdenum, about 4.6% to about 5.3% columbium, about 2.8% to about 3.3% tantalum, about 0.9% to.about 1.1% titanium, about 0.4% to about 0.6% aluminum, about 0.0024 to about 0.005% boron, about 0.02S% to about 0.035t carbon, the balance essentially nickel.

2. A castable, weldable nickel-base superalloy consisting essentially of (in weight percent) about 18.8% Cr, about 3.20% Mo, about 51 Cb, about 3. 1% Ta, about 11 Ti, about 0.5% AI, about 11.8% Co, about 0.003% B, about 0.03%C, the balance essentially nickel.

3. A structural casting consisting essentially of (in weight percent) about 18% to about 20% chromium, about 11% to about 13 cobalt, about 3% to about 3.4% molybdenum, about 4.8% to about S.3% columbium, about 2.8% to about 3.3% tantalum, about 0.9% to about 1.1% titanium, about 0.4% to about 0.6% aluminum, about 0.002% to about O.OOS% boron, about 0.02S% to about 0.035% carbon, the balance essentially nickel.

4. A structural casting in accordance with claim 3 consisting essentially of (in weight percent) about 3.2% 1 v molybdenum, about 18.8% chromium, about St columbium, about 3.1% tantalum, about 1% titanium, about D.S% aluminum, about 11.8% cobalt, about 0.003% boron, about 0.03% carbon, the balance essentially nickel.

S. A method for heat treating a casting whose chemistry consists essentially of (in weight percent) about lat to about 20% chromium, about 11% to about 13% cobalt, about 3% to about 3.4% molybdenum, about 4.8% to about

5.3% columbium, about 2.8% to about 3.3% tantalum, about 0.9% to about 1.1% titanium, about 0.4% to about 0.6% aluminum, about 0.002% to about 0.005% boron, about 0.02S% to about 0.035% carbon, the balance essentially nickel which comprises stress relieving said casting at a temperature of about 1750F for about four hours, HIPing the stressrelieved casting, solution treating the HIPed casting at about 2000F for about I hour, post solution annealing at about 1650F for about four hours, then aging at about 1425OF for about four hours, furnace cooling to 1300F, aging at about 13000F for about eight hrs.

6. The method in accordance with claim 5 wherein said HIPing is conducted at about 2050OF at about 14.7 KSI for up to about five hours.

7. The method of claim 6 wherein said chemistry is about 18.8% CT, about 3.20% Mo, about S% Cb, about 3.1% Ta, about 1% Ti, about 0.5% AI, about 11.8% Co, about 0.0031 B, about 0.03%C, the balance essentially nickel.

8. A substantially iron-free nickel-base alloy, or method of heat treating such alloy, substantially as hereinbefore described.

Published 1988 at The Patent Office, State House, 66771 High Holborn, London WC1R 4TP. Purther copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87-