Nickel-cobalt base alloys

Abstract

A work hardened nickel-cobalt alloy having high strength and ductility at temperatures of about 1300.degree. F. is provided consisting essentially by weight of about 0.05% max carbon, about 20%-40% cobalt, about 6%-11% molybdenum, about 15%-23% chromium, about 1.0% max iron, about 0.0005%-0.020% boron, about 0%-4% titanium, about 0%-2% columbium and the balance nickel, the alloy having been cold worked at a temperature below the HCP-FCC phase transformation zone to a reduction in cross-section between 5% and 50%.

Description

This invention relates to nickel-cobalt base alloys and particularly nickel-cobalt base alloys having excellent corrosion resistance combined with high strength and ductility at higher service temperatures.

There has been a continuing demand in the metallurgical industry for alloy compositions which have excellent corrosion resistance combined with high strength and ductility at higher and higher service temperatures.

The Smith patent, U.S. Pat. No. 3,356,542, issued Dec. 5, 1967, discloses cobalt-nickel base alloys containing chromium and molybdenum. The alloys of the Smith patent are corrosion resistant and can be work strengthened under certain temperature conditions to have very high ultimate tensile and yield strength. These alloys can exist in one of two crystalline phases, depending on temperature. They are also characterized by a composition-dependent transition zone of temperatures in which transformation between phases occur. At temperatures above the upper transus, the alloy is stable in the face centered cubic (FCC) structure. At temperatures below the lower transus, the alloy is stable in the hexagonal close-packed (HCP) form. By cold working metastable face centered cubic material at a temperature below the lower limit of the transformation zone, some of the alloy is transformed into the hexagonal close-packed phase which is dispersed as platelets through the matrix of face centered cubic material. It is this cold working and phase transformation which appears to be responsible for the excellent ultimate tensile and yield strength of the alloy of the Smith patent. The alloy is further strengthed by precipitation hardening. This alloy, however, has stress rupture properties which make it not suitable for temperatures above about 800.degree. F.

In my earlier U.S. Pat. No. 3,767,385 I provide an alloy which is an improvement on the Smith patent and which has stress rupture properties suitable for service temperatures to about 1100.degree. F. In that patent I disclosed my discovery that modifying the Smith composition by including elements which I believe form compounds resulting in additional precipitation hardening of the alloy, supplementing the hardening effect due to conversion of FCC to HCP phase, made it possible to provide higher tensile strength and ductility with a lower amount of cold work. This in turn raised the tensile strength and ductility level at higher temperatures. However, above 1100.degree. F. neither the alloy of Smith nor the alloy of my earlier patent will provide the thermomechanical properties of the present alloy.

The alloy of the present invention provides an alloy which retains satisfactory tensile and ductility levels and stress rupture properties at temperatures up to about 1300.degree. F. This is a striking improvement in thermomechanical properties and is accomplished by modifying the composition so that the transus is raised to higher temperatures and the precipitation hardening effect is maximized. Thus, the iron and aluminum are reduced to incidental proportions, and titanium or columbium or both are increased to limits described below. Accordingly, as pointed out in my earlier patent, not all alloys whose composition falls wtihin the ranges set out herein are encompassed by the present invention, since many of such compositions would include alloys containing embrittling phases.

The formation of these embrittling phases in the transition elements bears a close relationship to the electron vacancies in their sub bands as was predicted by Linus Pauling many years ago ("The Nature of Interatomic Forces in Metals", Physical Review, vol. 54, Dec. 1, 1938). Paul Beck and his coworkers (S. P. Rideout and P. A. Beck, NASA TN 2683) showed how the formation of pure sigma phase in ternary alloys could be related to the atomic percentages of their constituent elements by a formula of the type:

N.sub.v =0.61Ni+1.71CO+2.66Fe+4.66Cr+5.66Mo where N.sub.v is the average number of electron vacancies per 100 atoms of the alloy and the chemical symbols refer to the atomic fraction of that element in the alloy. There is a critical N.sub.v number above which 100% of sigma can be expected to form. In engineering alloys however, the presence of a small amount of the sigma phase can render an alloy brittle. The first onset of sigma can be predicted at a lower N.sub.v number which varies with different alloys. In my earlier U.S. Pat. No. 3,767,385 I describe this variation with the percentage of iron in the alloy. However, in the present alloy, a limit of only 1% iron is imposed and so only one critical N.sub.v number is specified, namely 2.80.

The calculation of the number uses the above formula except that the chemical symbol refers to the "effective atomic fraction" of the element in the alloy. This concept takes into account the postulated conversion of a portion of the metal atoms present, particularly nickel, into compounds of the type Ni3X, where X is titanium, columbium or aluminum. These compounds precipitate out of solid solution thus altering the composition of the remaining matrix to reduce the amount of nickel and effectively to increase the amount of the other transition elements. Thus, the remaining composition has an "effective atomic fraction" of these elements. Consequently many combinations of all the interacting elements can produce the same N.sub.v number (small effects on the N.sub.v due to carbon and boron are not significant and may be ignored in these calculations) Thus, the maximum of titanium when used without columbium and using the preferred analysis is 6%. Similarly, the maximum for columbium without titanium is 10%. Either titanium or columbium may be used in this alloy, alone or in combination, but must be used so that the resulting N.sub.v number does not exceed 2.80. The alloy of this invention, like those of Smith and my earlier patent is a multiphase alloy forming an HCP-FCC platelet structure.

The alloys of the present invention broadly comprise the following chemical elements in the indicated weight percentage ranges:

______________________________________Carbon 0.05 max Cobalt 20-40Molybdenum 6-11 Chromium 15-23Iron 1.0 max Boron 0.005-0.020Titanium 0-6 Columbium 0-10Nickel Bal.______________________________________

The preferred aim analysis for melting the alloy of the invention is, in weight percent:

______________________________________Carbon 0.01 max Cobalt 36Molybdenum 7.5 Chromium 19.5Iron 1.0 max Boron 0.01Titanium 3.8 Columbium 1.1Nickel Bal.______________________________________

The alloy of this invention is melted by any appropriate technique such as vacuum induction melting and cast into ingots or formed into powder for subsequent formation into articles by any appropriate known powder metals technique. After casting as ingots, the alloy is preferably homogenized and then hot rolled into plates or other forms suitable for subsequent working.

The alloy is preferably finally cold worked at ambient temperature to a reduction of cross section of at least 5% and up to about 40%, although higher levels of cold work may be used but with some loss of thermomechanical properties. It may, however, be cold worked at any temperature below the HCP-FCC transformation zone.

After cold working the alloys are preferably aged at a temperature between 800.degree. F. and 1350.degree. F. for about 4 hours. Following aging the alloys may be air cooled.

The unique properties and advantages of the alloy of this invention can perhaps be best understood by referring to the following examples:

EXAMPLE

An alloy composition according to this invention was prepared having the composition by weight:

__________________________________________________________________________C Co Mo Cr Fe B Ti Cb Ni__________________________________________________________________________0.006% 36.3% 7.35% 19.4% 1.04% 0.008% 3.79% 1.20% BAL__________________________________________________________________________

This alloy was hot rolled and divided into two portions one of which was cold worked to 36% and the other to 48%, aged at 1300.degree. F. and formed into test pieces identified by the terms "specimens" which are plain, cylindrical test specimens and "studs" which are threaded test specimens.

These specimens were subjected to mechanical testing at elevated temperatures as set out in Tables I, II and III hereafter.

TABLE I__________________________________________________________________________Aged 1300.degree.TEST STRESS, AREA STEEL COLD t log P p1Temp. .degree.F. ksi in.sup.2 TEST WORK hrs t T/1000 (C = 20) (C = 25)__________________________________________________________________________1350 105.0 .06397 5/16" Studs 36 11.2 1.0492 1.81 38.0991 47.1491 73.0 105.6 2.0237 39.8628 48.91281300 96.0 79.1 1.0982 1.76 38.5408 47.34081200 150.0 83.0 1.9191 1.66 36.3857 44.6857 141.5 75.9 1.8802 36.3212 44.66121350 105.0 .09506 3/8" Studs 36 15.3 1.1847 1.81 38.3443 47.3943 73.0 103.4 2.0145 39.8463 48.89631300 96.0 98.2 1.9921 1.76 38.7061 47.5061 61.1 1035.7 3.0152 40.5068 49.3068 150.0 2.9 0.4624 36.0138 44.81381200 160.5 22.0 1.3424 1.66 35.4284 43.7284 150.0 62.2 1.7938 36.1777 44.4777 141.5 99.4 1.9974 36.5157 44.81571350 105.0 .06397 48 6.2 0.7924 1.81 37.6342 46.6842 64.0 106.5 2.0273 39.8695 48.91951300 90.0 64.4 1.8089 1.76 38.3836 47.18361200 150.0 41.5 1.6180 1.66 35.8860 44.1860 139.0 72.5 1.8603 36.2882 44.58821350 105.0 .09506 48 11.0 1.0414 1.81 38.0849 47.1349 64.0 169.0 2.2279 40.2325 49.28251300 90.0 115.0 2.0607 1.76 38.8268 47.62681200 160.5 33.5 1.5250 1.66 35.7316 44.0316 150.0 63.1 1.8000 36.1880 44.4880 139.0 112.1 2.0496 36.6023 44.90231350 105.0 .0499 36 26.8 1.4280 1.81 38.7849 47.8349 82.5 .0495 97.3 1.9881 39.7985 48.84851300 106.4 .0495 101.9 2.0082 1.76 38.7344 47.53441200 150.0 131.1 2.1176 1.66 36.7152 45.0152 154.2 114.5 2.0588 36.6176 44.91761350 105.0 48 12.0 1.0792 1.81 38.1553 47.2033 75.6 .0499 123.9 2.0931 39.9885 49.03851300 93.0 .0495 180.5 2.2565 1.76 39.1714 47.97141200 161.6 75.8 1.8797 1.66 36.3203 44.6203 150.0 .0503 159.3 2.2022 36.8557 45.1557__________________________________________________________________________ TABLE II__________________________________________________________________________Stud Tensile StrengthAged 1300.degree. F. - 4 hours36% Cold WorkTEST TEST AREA LOAD STRESSTEMP. .degree.F. STEEL in.sup.2 POUNDS psi__________________________________________________________________________ 70 5/16" studs .06397 16,220 253,556 16,180 .+-. 57 252,930 .+-. 885 16,140 252,3051100 13,720 214,476 13,570 .+-. 212 212,131 .+-. 3316 13.420 209,7861200 13,820 216,039 13,730 .+-. 127 214,632 .+-. 1990 13,640 213,2251350 12,840 200,719 12,670 .+-. 240 198,062 .+-. 3758 12,500 195,404 70 3/8" studs .09506 25,025 263,255 24,762 .+-. 371 260,494 .+-. 3905 24,500 257,7321100 20,050 210,919 19,800 .+-. 354 208,289 .+-. 3719 19,550 205,6591200 20,150 211,971 20,050 .+-. 141 210,919 .+-. 1488 19,950 209,8671350 19,475 204,871 19,462 .+-. 18 204,739 .+-. 186 19,540 204,608__________________________________________________________________________ TABLE III__________________________________________________________________________Specimen Tensile PropertiesAged 1300.degree. F. - 4 hours36% Cold WorkTESTTEMP. .degree.F. UTS .2% YS E RA. UTS .2% YS ELONG. RED. OF AREA__________________________________________________________________________ 70 253,507 242,485 14.0 42.6 242,441 + 29,585 226,625 + 36,044 16.7 + 5.5 47.7 + 5.5 208,918 185,371 23.0 53.5 264,898 252,020 13.0 46.91100 213,131 196,969 12.0 34.0 204,912 + 11,623 188,414 + 12,098 14.5 + 3.5 35.6 + 2.2 196,692 179,860 17.0 37.11200 216,364 197,980 11.0 33.3 212,390 + 5,619 193,679 + 6,082 13.0 + 2.8 37.7 + 6.2 208,417 189,379 15.0 42.01350 194,949 16,192 10.0 20.4 194,769 + 255 170,768 + 2,230 10.5 + 0.7 21.7 + 1.8 194,589 172,345 11.0 23.0__________________________________________________________________________

A comparison of the properties of the alloys of the Smith patent, my earlier patent and the present invention are set out hereafter on the attached table:

TABLE IV__________________________________________________________________________ Smith Slaney PresentTreatment 3,356,542 3,767,385 Invention% Cold Work 51% 48% 36%Age 1050.degree. F. 1225.degree. F. 1300.degree. F.Properties Room Temp. 1200.degree. F. 1300.degree. F. Room Temp. 1200.degree. F. 1300.degree. F. Room Temp. 1200.degree. 1300.degree.__________________________________________________________________________ F.Ultimate Tensile 310 Not Not 275 222 Not 242.4 212.4 194.8Strength (KSI)* Suitable Suitable Suitable0.2 Yield Strength (KSI) 290 Above Above 265 210 Above 226.6 193.7 170.8Elongation 11 800.degree. F. 800.degree. F. 8 7 1100.degree. F. 16.7 13.0 10.5Reduction in Area 52 35 22 47.7 37.7 21.7Stress Not Suitable Not Suitable 106.4 KSI @ 1300.degree. F. 101.9 hrs.Rupture Above 800.degree. F. Above 1100.degree. F. 96.0 KSI @ 1300 .degree. F. 98.2 hrs. 96.0 KSI @ 1300.degree. F. 79.1 hrs.__________________________________________________________________________ *KSI = kilopounds/in.sup.2 = 1,000 psi

From the foregoing data it can be seen that this invention provides unique thermomechanical properties at temperatures in the neighborhood of 1300.degree. F. where presently available alloys are no longer serviceable. This provides service temperatures for jet engine fasteners and other parts for higher temperature service, thus making it possible to construct such engines and other equipment for higher operating temperatures and greater efficiency than heretofore possible.

In the foregoing specification I have set out certain preferred practices and embodiments of this invention, however, it will be understood that this invention may otherwise be embodied within the scope of the following claims.

Claims

1. A nickel-cobalt alloy having high strength and ductility at service temperatures of about 1300.degree. F. consisting essentially of the following elements by weight percent:

______________________________________Carbon about 0.05 maxCobalt about 20-40Molybdenum about 6-11Chromium about 15-23Iron about 1.0 maxBoron about 0.0005-0.020Titanium about 0-6Columbium about 1.1-10Nickel Bal.______________________________________

and having a maximum electron vacancy number (N.sub.v) of 2.80, said alloy having been cold worked at a temperature below the lower temperature limit of the HCP-FCC phase transformation zone to a reduction in cross-section between 5% and 50%.

2. A nickel-cobalt alloy as claimed in claim 1 having been cold worked to a reduction in cross-section between 10% and 40%.

3. A nickel-cobalt alloy as claimed in claim 1 or 2 having been aged at a temperature of about 800.degree. F. to 1350.degree. F. for about 4 hours after cold working.

4. A nickel base alloy as claimed in claim 1 or 2 having the composition by weight percent of:

______________________________________Carbon about 0.01 max.Cobalt about 36Molybdenum about 7.5Chromium about 19.5Iron about 1.0 max.Boron about 0.01Titanium about 3.8Columbium about 1.1Nickel Bal.______________________________________

5. A nickel base alloy as claimed in claim 4 having been aged at a temperature of about 800.degree. F. to 1350.degree. F. for about 4 hours after cold working.

6. A nickel cobalt alloy as claimed in claim 1 or 2 which has been cold worked at ambient temperature.

7. A nickel cobalt alloy as claimed in claim 3 which has been cold worked at ambient temperature.

8. A nickel cobalt alloy as claimed in claim 4 which has been cold worked at ambient temperature.

9. A nickel cobalt alloy as claimed in claim 5 which has been cold worked at ambient temperature.

10. A nickel cobalt alloy as claimed in claim 3 having been aged at about 1350.degree. F. for about 4 hours after cold working.

11. A nickel cobalt alloy as claimed in claim 5 having been aged at 1350.degree. F. for about 4 hours after cold working.

12. A nickel cobalt alloy as claimed in claim 7 having been aged at 1350.degree. F. for about 4 hours after cold working.

13. A nickel cobalt alloy as claimed in claim 9 having been aged at 1350.degree. F. for about 4 hours after cold working.

14. A nickel-cobalt base alloy as claimed in claim 1 or 2 having been cold worked to a reduction in cross-section of about 36%.

15. A nickel-base alloy as claimed in claim 4 having been cold worked to a reduction in cross-section of about 36%.