FR2615869A1 - MOLDABLE AND WELDABLE SUPERALLOY, MOLDED CONSTRUCTION PARTS MADE OF SUCH ALLOY AND METHOD OF HEAT TREATING SUCH A PART - Google Patents

Abstract

A castable and weldable nickel-based superalloy and consisting essentially of (in weight percentages) 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% niobium, 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 0.035% carbon, the balance being essentially nickel; a process for the heat treatment of a structural molded part made of such an alloy consists of expansion, isostatic compression, dissolving, hyperquenching, desursaturation, furnace cooling and further desursaturation under conditions of temperature, time and pressure determined.

Description

The present invention relates to a moldable and weldable superalloy, a

  molded construction part made of this superalloy

  and a method of heat treatment of such a part.

  More particularly, the invention relates to a superalloy composition which is easy to mold and which can be used to provide molded parts of complicated structure, easy to weld without cracks, having a high mechanical resistance at room temperature and at high temperatures and properties of high resistance to oligocyclic fatigue and better

  resistance to stress rupture.

  US Patent No. 3,046,108 was issued to H.L. Eiselstein for an alloy known as Alloy 718. According to Metals Handbook, 9th edition, Vol. 3, page 748, the nominal content of Alloy 718, expressed in weight percentages, is 18.6% chromium, 18.5% iron, 0.04% carbon, 0.4% aluminum, 0.9% titanium, 5% niobium, 3.1% molybdenum, the balance being

mainly nickel.

  Alloy 718 is the primary superalloy for intermediate temperature precision castings used to date in the aerospace industry. About 1,750 tonnes of parts

  precision castings are produced each year with this alloy.

  It is an alloy initially designed to be used in the worked state and it was adopted for casting, practically without modification of its composition. Alloy 718 contains relatively large amounts of Fe and Nb and up to 0.35% Si which all contribute to the formation of a Laves phase. La2phase de Laves is a low melting point phase which forms at the interdendritic locations of molded parts and which has often been cited as the cause of poor weldability. Precision molded parts must be able to be taken up by welding in the current state of casting processes. Without the possibility of rework by welding, few precision castings could be produced cost-effectively. Therefore, the weldability of an alloy for precision molded parts is essential for its use. Alloy 718 has been found to have sufficient weldability to date to suit the entire aerospace industry. However, the industry is evolving towards increasingly higher temperatures and imposes weight reductions subjecting the components to increased stresses. Therefore, the mechanical resistance of Alloy 718, in particular with regard to stress rupture and oligocyclic fatigue, has

  been subjected to requirements beyond its possibilities.

  As Alloy 718 is brought to its limit of mechanical strength and endurance, problems such as microcracks in the welds become more important. During welding, many grain boundaries in the heating zone near the fusion zone of a weld break, either during the initial heating thermal shock, or during solidification. As a result of the composition of Alloy 718, the material of the melting zone which fills the open grain boundaries includes the low melting point Laves phase as the final solidification phase. These filling materials are therefore the last to solidify. This often causes these grain boundaries to rupture towards the end of the

  welding cycle, which can transform them into microcracks.

  The new alloy of the invention was developed for precision molded parts in the aerospace industry. This alloy

  of the invention provides a rise in the operating temperature

  more than 56C compared to Alloy 718 and, for certain molded shapes, its mechanical properties are superior to those of Alloy 718 worked. Its moldability is equal to or greater than that of Alloy 718 and its weldability is much greater than that of molded Alloy 718. The chemical composition of the new alloy has been designed to ensure an improvement in mechanical resistance at high temperatures, good moldability and minimum sensitivity to crack initiations during fusion welding and processing.

thermal after welding.

  Compared to Alloy 718, the alloy of the invention has improved weldability according to the Varestraint test and the "in situ" tests. As a result of the absence of iron, the nickel-based superalloy of the invention does not form significant amounts of the Laves phase during solidification during molding or welding. As there is no Laves phase during the welding solidification cycle, microcracks are unlikely to form. In addition, this alloy forms a more stable delta phase as a result of the addition of tantalum. This also improves the weldability of this new alloy, since the delta phase remains as a stable phase during welding, even in the weld melting zone, unlike the delta phase of Alloy 718 which has a single-phase liquation. temperature slightly above

  that of the Laves phase of Alloy 718.

  The superalloy of the invention contains (in weight percentages) about 18% to about 20% of chromium, about 11% to about 13% of cobalt, about 3% to about 3.4% of molybdenum, about 4.8% to approximately 5.3% niobium, approximately 2.8% to approximately 3.3% tantalum, approximately 0.9% to approximately 1.1% titanium, approximately 0.4 k to approximately 0.6% aluminum , approximately 0.002% to approximately 0.005% boron, approximately 0.025% to approximately 0.035% carbon, the complement

being essentially nickel.

  The alloys of the invention contain essentially no iron, i.e. no more than about 0.5% iron, and preferably no more than about 0.05% manganese, 0.10 % silicon, 0.010% phosphorus, 0.005% sulfur, 0.010% zirconium, 0.004% magnesium, 0.10% copper, 0.003% oxygen

and 0.0075% nitrogen.

  A preferred embodiment contains about 18.8% chromium, about 3.20% molybdenum, about 5% niobium, about 3.1% tantalum, about 1% titanium, about 0.5% aluminum , approximately 11.8% cobalt, approximately 0.03% boron and approximately 0.03%

  carbon, the balance being essentially nickel.

  Molded samples of the alloys of the invention having the composition indicated in table I were prepared and heat treated according to the following preferred protocol: expansion at 954 ° C. for 4 hours (954 ° C. / 4 h), isostatic compression at hot at 1,121'C and 68.94 MPa for 3 hours, dissolved at 1,093'C / 1 hr, quenching at 899'C / 4 hr, aging treatment at 774'C / 4 hr, oven cooling 704 C then second aging treatment at 704 C

for 8 hours.

TABLE I

  Composition (wt%) Cr 18.6 L 0.003 Co 11.9 te 0.07 no 3.15 mg 0.001 Nb 4.93 Si 0.02 Ta 2.98 P 0.005 - Ti 1.01 S 0.002 Al 0.48 O 10 ppm C 0.033 N 17 ppm The aforementioned heat treatment cycle is currently preferred and constitutes a typical example of that provided under production conditions o typically the molded part is inspected after expansion and if necessary subjected to recovery by welding to correct defects caused by molding and o manufacturing welding and possible resumption of welding

  manufacturing are carried out after hyper quenching.

  The aforementioned samples (a minimum of three per test) were subjected to tests to determine their mechanical resistance at room temperature and at 649C, their resistance to rupture under stress under test conditions of 704C i 427 , 47 MPa and their resistance to oligocyclic fatigue, the results of which are shown in Table II. Alloy 718 samples underwent optimal heat treatment for parts

  castings made of such an alloy: detent at 1,093'C / 1 h; compress-

  hot isostatic sion at. 1163'C / 89.63 MPa / 3 h; dissolution

  at 1,052'C / lh; aging treatment at 760 ° C / 5 h; cool

  lie at 56'C / h up to 649'0; aging treatment at 649 ° C / 1 h

  and cooling to room temperature in resting air.

  In Table II, EN designates the samples which are cast to their dimensions and ED designates the samples which are cut from molded parts. As shown in Table II, the EH samples typically exhibit tensile strengths at room temperature which are about 21 daN! / 2 higher than their ED equivalents. This effect is mainly due to differences in grain size and the factor which affects it is less important than that of the other properties indicated in Table II. This effect can also be noted by comparing the properties of the alloys of the invention when they are measured on samples cut from molded parts compared to those measured both on samples molded to size and of fine grain samples cut out. in molded parts

according to the Microcast-Z process.

  As shown in Table II, the alloys of the invention are much superior to Alloy 718 with regard to the stress rupture and oligocyclic fatigue properties which are very important for the design. As is also shown in Table II, the alloys of the invention do not exhibit sensitivity to notching with the preferred heat treatment, as indicated by the breaking values under stress of notched test pieces. Alloy 718 has not been tested on notched test pieces because it has no longevity on non-notched test pieces. The short longevities on non-notched test pieces indicated for the samples cut from molded parts prepared according to the Hicrocast-X process are attributed to the ultrafine grain size (compared to conventional molded parts) and to the low boron content (an agent reinforcement of grain boundaries) in the alloy batch of table I.

Table II

  Traction & ordinary temperature Traction at 649'C Limit Limit Elastic resistance Elastic resistance to rupture at 0.2% Elongation Striction to rupture at. Elongation Striction MPa MPa% _ MPa MPa _ _ Alloys of the invention1 914.23 689.47 12.7 16.3 733.59 554, 33 8.9 15.6 <DI> Alloys of the invention2'8 ' 779.1 626.73 8 18 646 508.14 9 20 (ID) AiD) 713 717 631.55 15 29 555.71 494.35 il 29 Alloys of the invention4 903.20 710.15 8 15 785.99 622, 59 9 20 (fine grains) 5 0% o0 0% Table IX (Continued) Lonasvity (constraint / ruDture) (h) Oliaocyclic fatigue Oliocyclic fatia 704'C / 620.52 MPa 538'C / 413.68MPa 538'C / .44.73 MPu without with Cycles (NF) Cycles (NF) notch notch Alloys of the invention1 93.3 200+ 161 9306 277,000 (! N) Alloys of the invention2'8 50 200 + 9 24,000 177,0007 (BD) Alli a 7183 (l D) 70 - 15o 000 37 000 Alloys of the invention 87 <1 278 000 no value (fine grains) 5 S 1 Lot from Table I 2 Average of several lots 3 Typical 4 Lot 7 : hot isostatic compression / 1,121 * C / 69.33 MPa / 3 h; 1,093 ° C / 1 h; 954 ° C / 4 h; 774'C / 4 h; and 704'C / 8 h According to the Ricrocast-X and Howmet relieving process 6 593'C / 237.86 MPa 7 Lot 6 Preferred heat treatment 8 954 * C / 4 h; isostatic hot compression 1,163'C / 69.63 MPa / 3 h; 1,093 ° C / 1 h; 954 ° C / 4 h; 774'C / 4 h; and 704'C / 8 h 9 Certain batches are treated according to the preferred heat treatment ot some according to the treatment

note 8 thermal.

2615 8 6 9

  The weldability of the alloys of the invention was evaluated according to the Varestraint test. In the Varestraint test, a test sample is fixed in a frame in the form of a cantilever beam and subjected to a programmed tungsten arc welding under gas. After a predetermined welding time, the sample is forced to adapt to a matrix having a radius such as a determined and progressive deformation, for example of 1%, 2%, 3%, etc., or exercised. A programming is chosen such that a regular thermal regime is obtained in the heated zone resulting from welding before an increase in the deformation is exerted. After the test, the length of the cracks is measured at 12 diameters, the total length of the cracks and the maximum length of the cracks being used to compare the sensitivity to

hot cracking.

  The results of the Varestraint weldability test on the EM samples of alloy 718 molded and of the alloys of the invention are shown in Table III. As shown in Table III, the alloys of the invention are more weldable (less tendency to

  formation of cracks) than Alloy 718.

TABLE III

  Varestraint tests (2% increase in deformation) Material Total length of cracks Alloy 718 (13 lots) 0.61 Alloy of the invention (5 lots) 0.23 Excellent weldability of castings made of alloys of invention produced by the precision molding process also appears when 30 grooves of defined profile are machined and the grooves are filled with a filler metal of corresponding composition according to the gas tungsten arc welding process for simulate rework welds. Microscopic examination of the welds obtained confirms the excellent weldability observed in the Varestraint tests; the results of

  simulated recovery welds are shown in Table IV.

TABLE IV

  Simulated recovery welds Length of Number of Alloys microcracks (mm) microcracks Alloy 718 0.25 17

0.51 11

0.76 1

  0.25 2 alloy 0.51 0 0.76

Claims (7)

  1. Nickel-moldable and weldable superalloy consisting essentially of (in weight percentages) about 18% to about 20% of chromium, about 11% to about 13% of cobalt, about 3% to about 3.4% of molybdenum, approximately 4.8% to approximately 5.3% niobium, approximately 2.8% to approximately 3.3% tantalum, approximately 0.9% to approximately 1.1% titanium, approximately 0.4% to approximately 0 0.6% aluminum, about 0.002% to about 0.005% boron, about 0.025% to about 0.035% carbon, the balance being mainly nickel.   2. Mouldable and weldable nickel-based superalloy consisting essentially of (in weight percentages) about 18.8% chromium, about 3.20% molybdenum, about 5% niobium, about 3.1% tantalum, about 1 % titanium, approximately 0.5% aluminum, approximately 11.8% cobalt, approximately 0.003% boron, approximately 0.03% carbon, the balance being essentially nickel.   3. Molded structural part consisting essentially of (in weight percentages) about 18% to about 20% of chromium, about 11% to about 13% of cobalt, about 3% to about 3.4% of molybdenum, about 4.8 % to approximately 5.3% niobium, approximately 2.8% to approximately 3.3% tantalum, approximately 0.9% to approximately 1.1% titanium, approximately 0.4% to approximately 0.6% d aluminum, about 0.002% to about 0.005% boron, about 0.025% to about 0.035%   of carbon, the remainder being essentially nickel.   4. Molded construction part according to claim 3, consisting essentially of (in weight percentages) about 3.2% of molybdenum, about 18.8% of chromium, about 5% of niobium, about 3.1% of tantalum, about 1% titanium, approximately 0.5% aluminum, approximately 11.8% cobalt, approximately 0.03% boron, approximately 0.03% carbon, the balance being essentially nickel. 5. Process for heat treatment of a molded part, the chemical composition of which consists essentially of (in weight percentages) about 18% to about 20% of chromium, about 11% to about 13% of cobalt, about 3% to about 3 , 4% molybdenum, about 4.8% to about 5.3% niobium, about 2.8% to about 3.3% tantalum, about 0.9% to about 1.1% titanium, about 0 0.4% to about 0.6% aluminum, about 0.002% to about 0.005% boron, about 0.025% i about 0.035 t% carbon, the balance being essentially nickel, which includes an expansion of said part molded at a temperature of approximately 954 ° C. for approximately 4 hours, a hot isostatic compression of the relaxed molded part, a dissolution of the molded part subjected to. hot isostatic compression at around 1,093 ° C for about 1 hour. a heat treatment at approximately 894 ° C. for approximately 4 hours, then an aging treatment at approximately 774 ° C. for approximately 4 hours, cooling in an oven at 704 ° C. and a   aging treatment at around 704 ° C for around 8 hours.   6. The method of claim 5, wherein said isostatic compression is carried out at about 1121 ° C and about   69.63 MPa for up to about 5 hours.   7. The method of claim 6, wherein said chemical composition consists of about 18.8% chromium, about 3.20% molybdenum, about 5% niobium, about 3.1 X tantalum, about 1% titanium, approximately 0.5% aluminum, approximately 11.8% cobalt, approximately 0.03% boron, approximately 0.03%   carbon, the balance being essentially nickel.