US3676226A - High strength copper-nickel alloy - Google Patents

Abstract

CORROSION RESISTANT COPPER-BASE ALLOY CONTAINING NICKEL AND CHROMIUM HAS SPECIAL HARDENING CHARACTERISTICS WHICH AFFORD HIGH STRENGTH PROPERTIES UPON COOLING FROM ELEVATED TEMPERATURE OVER A WIDE RANGE OF COOLING RATES.

D R A W I N G

Description

July 11, 1-972 READ mu 3,676,226

HIGH STRENGTH COPPER-NICKEL ALLOY /Filed June 15, 1969- 2 Sheets-Sheet J,-

INVENTORS FRANK ARTHUR BADIA GARY NEIL KIRBY ATTORNEY F. A. B-ADIA ETAL 3,676,226

HIGH STRENGTH COPPER-NICKEL ALLOY July 11, 1972 2 Sheets-Sheet 2 Filed June 13 QM QN [I .ri F

United States Patent 3,676,226 HIGH STRENGTH COPPER-NICKEL ALLOY Frank Arthur Badia, Ringwood, N.J., and Gary Neil Kirby, Ann Arbor, Mich., assignors to The International Nickel Company, Inc., New York, N.Y. Continuation-impart of application Ser. No. 708,973, Feb.

28, 1968, which is a continuation-in-part of application Ser. No. 581,066, Sept. 21, 1966. This application June 13, 1969, Ser. No. 840,106

Int. Cl. C22c 9/06 US. Cl. 148-32 12 Claims ABSTRACT OF THE DISCLOSURE Corrosion resistant copper-base alloy containing nickel and chromium has special hardening characteristics which afford high strength properties upon cooling from elevated temperatures over a wide range of cooling rates.

This is a continuation-in-part of application Ser. No. 708,973, filed Feb. 28, 1968 now abandoned, which in turn is a continuation-in-part of application Ser. No. 581,- 066, filed Sept. 21, 1966, now abandoned.

It is well known that the copper-nickel alloys often referred to as cupronickels, e.g., 70/30 cupronickel containing nominally 30% nickel with the balance substantially copper, effectively resist the corrosive effects of many media, including sea water, alkalis and some dilute aqueous acid solutions. Too, the workability characteristics of such cupronickels (rolling, forging, drawing, etc., either hot or cold) are also known to be very good. In view of these and other desirable properties, e.g., weldability, such alloys have been widely used for the production of corrosion resistant apparatus and articles, including condensers, distiller tubes, evaporators and heat exchanger tubes.

However, the yield strength of 70/30 and other cupronickels in the annealed, hot rolled or similar condition is unsatisfactorily low for various applications, e.g., pressurized salt water piping, being only about 20,000 pounds per square inch (p.s.i.) at room temperature. Although higher strengths are obtainable by cold working, such processing entails certain disadvantages. Construction and maintenance of apparatus for which cupronickel is used, especially apparatus for handling pressurized fluids, often requires welding but the heating and cooling that accompany the welding of cold 'worked cupronickel normally leave the alloy in a soft, low strength, air cooled condition at the heat affected zones. Thus, absent other factors the benefit of cold working is lost. Also, overheating to which apparatus may be subjected during construction, maintenance or use can result in the stress relieving or annealing of cold worked alloys so as to result in a serious loss of strength which it is not practical to regain by additional cold working. Moreover, cold working involves unnecessary expense, production time and, in some instances, introduces production control difficulties.

It has been also proposed to fabricate articles from cupronickel alloys containing precipitation hardening constituents capable of imparting strength upon being subjected to special heat treatments such as by a combination of solution annealing and subsequent aging at elevated temperature. However, in many instances it is difficult or impractical, and very undesirable commercially, to empoly precipitation hardening heat treatments, particularly when the alloys must be welded since heat from welding generally destroys strength benefits conferred by precipitation hardening at the weld-heat affected zone. And where weldments are made in large structures, difificulties in applying post-weld heat treatments often give rise to severe and sometimes prohibitive problems such as to preclude the utilization of precipitation hardening ice as a practical matter. In addition, corrosion resistance is sometimes detrimentally affected by the presence of precipitated second phases.

As will be appreciated from the foregoing, there has been a need for a cupronickel alloy characterized by improved high strength, e.g., a yield strength of at least about twice that of annealed 70/30 cupronickel, when in the air cooled condition, including the hot worked, annealed and welded conditions. Concomitant requirements include attaining such strength levels without significant impairment of such characteristics as weldability, ductility, toughness, workability, especially workability that enables the alloy to be rolled to sheet, and corrosion resistance. It has now been discovered these objectives are attained with a new cupronickel alloy containing special amounts of chromium and other constituents as described herein. In fact, it has been found that not only can the yield strength be increased two-fold or more, but toughness and fatigue strength are simutaneously enhanced.

It is an object of the present invention to provide a cupronickel alloy characterized by improved strength, including tensile and yield strengths, when in the air cooled condition and further characterized by a highly satisfactory combination of weldability, ductility, workability and corrosion resistance.

Another object of the invention is to provide cupronickel alloy wrought products including sheet, strip, plate, rod, piping, tubing, extruded shapes and the like characterized by improved yield strength when in the air cooled condition.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which FIG. 1 shows a reproduction of a transmission electron micrograph of an alloy Within the scope of the invention; and

FIG. 2 depicts a chart illustrating a relationship between yield strength and chromium content of various cupronickel alloys in the air cooled condition.

Generally speaking, the present invention is directed to an air hardening cupronickel alloy containing 2.4% to 3.8% chromium, 24% to 33% or 38% nickel, provided that when the nickel content is 33% to 38% the chromium content is at least equal to the amount determined by the relationship Percent Cr=0. 12 (Percent Ni- 13) up to about 2.5% iron and up to about 2.5% cobalt, provided the sum of the percentages of chromium, iron and cobalt does not exceed about 5%, up to 0.1% carbon, up to 6% zinc, up to 3% manganese, up to 0.8% zirconium, up to about 0.5% silicon and the balance essentially copper. When cobalt is present, e.g., in amounts from 0.2% to 2.5 it can replace part of the chromium provided that the chromium content is at least 2.3% and the sum of the chromium plus /2 the cobalt is 2.4% to 3.8%

The alloy contemplated herein develops a high yield strength of at least 40,000 p.s.i. very rapidly upon cooling to room temperature or thereabouts from such elevated temperatures as annealing, hot working and welding temperatures, e.g., 1650 F. to 1900 F. and higher. Satisfactory strength can be obtained throughout a wide range of cooling rates when the alloy cools through the temperature range of 1400 F. to 1000 F. at rates from 180 F. per second F./sec.) to 005 F./sec. and lower. Thus, the alloy is air hardenable in the sense that the required strength is obtainable by air cooling after annealing, hot working, welding, etc., without special control of cooling rate. To obtain high strength, cooling at extremely high rates such as result from water quenching very thin sections should desirably be avoided.

The surprisingly rapid hardening characteristics of the alloy in accordance herewith have led. to investigations of the metallurgical phenomena involved and these investigations have brought to light the further surprising fact that the alloy undergoes spontaneous spinodal decomposition upon simple cooling from elevated temperatures to room temperature-nothing else is required. In spinodal decomposition a solid solution phase existent at a relatively high temperature decomposes at a lower temperature into two new phases which are alike in crystal structure but which differ somewhat from each other in chemical composition. A point of considerable significance regarding this very unusual decomposition reaction, a feature which distinguishes it from many other phase changes, is the rapid occurrence thereof. It spans a period of but a few seconds; indeed, it is difficult to suppress. In the present invention, control of alloy composition in accordance with the ranges and metallurgical relationships set forth herein enables obtaining spinodal decomposition spontaneously, on a practical basis and without need for special heat treatment, when the alloy is cooled from the single phase condition and also provides for concomitantly obtaining highly enhanced strength and toughness characteristics through spinodal decomposition hardening. These characteristics provide many practical processing advantages and are in marked contrast to hardening characteristics of other alloy that depend upon age hardening mechanisms for strengthening. Thus, in view of the spontaneous nature of the spinodal decomposition phenomenon, the subject alloy develops strength much more rapidly than do alloys that depend upon age hardening, which is a much slower acting phenomenon, and enables achieving high strength without resorting to special heat treatments and without incurring the metallurgical and economic drawbacks of heat treating requirements. Furthermore, the spontaneously developed microstructure is highly stable and not susceptible to overaging diificulties such as are encountered with many precipitation hardened alloys.

In carrying the invention into practice, the alloy must contain at least 24% nickel to avoid microstructures having detrimental banded segregates, low strength and poor hot workability. Nickel in excess of 33% is unfavorable to strength unless the chromium content is at least equal to percent Cr=0.12(percent Ni-13). At least 2.4% chromium, or 2.3% chromium in combination with at least 0.2% cobalt, is required to obtain the yield strength level of 40,000 psi. Advantageously, the alloy contains 28% to 32% nickel and 2.4% to 3.8% chromium e.g., 2.75% to 3.25%, to obtain a yield strength of at least 45,000 psi. For good workability together with the required yield strength, the chromium content should be from 2.4% to 3%. Especially high yield strengths of 50,000 psi. and greater are obtained with 3% to 3.8% chromium. However, chromium exceeding 3.8% is highly detrimental to hot workability. For instance, two coppernickel-chromium alloys, one with 31.6% nickel and 4.2% chromium and another with 30.3% nickel and 4.2% chromium severely cracked during attempts to roll the alloys into plate even though the alloys contained 0.2% and 0.8% zirconium, respectively.

The amount of any carbon in the alloy is not greater than about 0.1% and is advantageously controlled to not exceed 0.02% inasmuch as carbon detrimentally lowers yield strength through the formation of carbides. Historically speaking, carbon has long been used in the production of copper-nickel alloys, primarily for deoxidation purposes, the use of charcoal covers being notable in this regard. Indeed, as a practical matter, it is virtually impossible to avoid the presence thereof even if so desired. For example, carbon can be introduced through crucible pick-up, fuels, the use of scrap, carbon stirring rods, etc. To avoid these means of carbon introduction can be expensive. In any case, it has been found that canbon can be present, but it has to be controlled such that excessive carbide formation is avoided.

Zirconium up to about 0.8% improves hot working characteristics, about 0.05% to 0.5% zirconium, more particularly 0.2% to 0.4%, being beneficial for enlarging the hot working temperature range of the alloy. Cobalt improves yield strength and for this purpose is preferably present in amounts of 0.2% to 1.2%. Excessive cobalt is detrimental to hot workability and should not exceed about 2% or 2.5%.

Although the balance of the alloy of the invention is referred to as being essentially copper, it is to be understood that this does not exclude small amounts of auxiliary elements, e.g., deoxidizers, desulfurizers, malleabilizers, etc., and incidental impurities. Thus, the alloy can also contain up to 0.5% each of titanium, silicon, aluminum, columbium and beryllium and up to 0.1% magnesium. Vanadium detrimentally affects hot workability and is accordingly maintained very low, e.g., less than 0.2%, and at most should not exceed about 0.5% Elements such as lead and bismuth are especially detrimental impurities due to having embrittling efiects and are kept as low as commercially practical. Any iron should be kept below about 2.5% to avoid precipitation of an iron-rich phase and attendant annealing difiiculties. Maintaining the iron content to be not greater than about 1% is deemed beneficial. Manganese and zinc in amounts exceeding 3% and 6%, respectively, lower strength. Where especially high strength is required, manganese should not exceed about 2% and the zinc should not exceed about 3%. An amount of manganese of about 0.5 to 1% is considered advantageous in inhibiting coppering.

It should be also added that the amounts of any optional elements should be controlled so as not to substantially exceed their solubility limits in the alloy of the invention in order to avoid development of detrimental metallurgical phases. From this viewpoint, the residual amounts of any columbium, silicon, titanium, aluminum, magnesium and beryllium should not exceed levels totaling more than 2% and it is preferred that any columbium, silicon, titanium, aluminum and beryllium not exceed 0.3% each and that magnesium not exceed 0.05%.

The alloy of the invention is satsifactorily made by melting the ingredients together in an induction furnace or other melting apparatus known to be suitable for producing high quality copper-nickel alloys. Satisfactory melting procedures which were employed in producing alloys of the invention, e.g., alloys 4 through 9 referred to hereinafter, comprised air induction melting electrolytic copper and nickel together, adding electrolytic iron and manganese (ferromanganese) after melt down and thereafter adding electrolytic chromium with the melt at about 2500 F. to 2650" F. Finishing additions were made by plunging 0.1% titanium and 0.25% zirconium (nic=kel zirconium) which were wrapped in copper foil and wired to nickel rods. About 0.4% zirconium was added to alloy 7.

The subject alloy exhibits very good hot workability characteristics when in the single phase condition obtained at elevated temperatures. Thus, it is soft and easily worked at high temperature and, in view of the rapid nature of its spontaneous spinodal decomposition capability, readily air hardens during cooling and becomes strong and tough at room temperature. Hot working temperatures include from about 1950 F. down to as low as -1550 F. However, if 3% or more of chromium and less than about 0.05% zirconium is present, hot working below about 1800" F. should be avoided.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention the following illustrative examples are given.

Alloys in accordance with the invention and having chemical compositions as set forth in Table I were cast into ingots and hot worked by forging, extruding, hot rolling, and sometimes also cold rolled, to produce wrought products. Workability in both the hot and cold conditions was found very satisfactory. Although cold working is not necessary in order to obtain the required strength, it is sometimes convenient to resort thereto in order to produce the alloy in desired section forms and/ or thickness. Even when cold working is employed for purposes other than obtaining strength, the air hardening characteristics of the alloy provide very substantial advantages of negating any need for controlling the amount thick plate to -inch thick strip of alloys 13, 14, 15 and 6 which were rolled to the thicknesses indicated in the table, then annealed for one hour at 1700" F. and thereafter air cooled to room temperature.

and/ or uniformity of cold work in order to obtain a high TABLE HI ield stren th of 40000 .s.i. Elong., y g p Y.S U.'l.S percent TABLE I Alloy No. Product thickness K s.l K 5.1 in 1 inch Percent 88.6 28 Alloy 86.9 28 NO. Ni Cr Si Mn Fe Zr 87. 1 30 89.4 32 so 2. s7 0. 6 0. 6 86. so 30 2.83 0. 6 0. 6 s9. 9 26 28.6 3.7 0.07 0.64 1.0 0.08 29.8 2. 40 0.12 0. 52 0. 69 0. 21

31?; g: 81 Hg 3%; In the condition obtained with spinodal decomposigoa g 3% 3g 8. 2 1 8.2g tion, the alloy of the invention is characterized by a 2:3 50 spinodal microstructure which consists essentially of very 34.2 finely divided and uniformly dispersed copper-rich and 2: 55 j 10 copper-poor regions that are formed from the decomposiggg 3. 18 8.8g 8.23 8.3% 8-13 tion of the parent solid solution. FIG. 1 of the drawing 1 5 1 (1'14 shows a reproduction of a transmission electron micro- NorE.Bal.=Balance, which also included small amounts of aluminum and/or titanium up to 0.25% and carbon up to 0.02%.

Room temperature tensile characteristics of alloys 1 through 12 in the air cooled condition obtained by air cooling after hot rolling, extruding or annealing are set forth in Table II. Tensile specimens were taken in the direction longitudinal to rolling or extruding. Alloy 6 includes results taken in the direction transverse to rolling. The data illustrate that the wrought products of the invention were characterized by yield strengths of at least 40,000 psi. and higher when in the air cooled condition obtained with section thicknesses of V and greater. The high strength of alloy 7 in the condition obtained by oil quenching, which cooled the alloy from 1750 F. to 300 F. in about 20 seconds, shows that very satisfactory high strength can be obtained with alloys having relatively high chromium contents, e.g., 3.0 to 3.8%, after cooling even more rapidly thin air cooling. It will also be noted that good consistency of results is obtained in a broad range of cross-section sizes.

graph made with a thin film specimen of alloy 7 in the wrought and air cooled condition. X-ray diifraction studies indicated that the light and dark areas of the electron micrograph corresponded to copper-rich and copper-poor regions in the microstructure. In view of the very high magnification at which the micrograph was made, as illustrated by the attached 0.10. (micron) scale, it is apparent that the copper-rich and copper-poor regions are of very fine sizes of the order of only 130 angstroms, or one-half a millionth of an inch. It should also be mentioned that no evidence of chromium precipitation was found in the air cooled alloy, although electron diffraction patterns were also made from alloy 7.

As already emphasized hereinbefore, the alloy must contain at least 2.4% chromium, or at least 2.3% chromium in combination with at least 0.2% cobalt. Importance of TABLE II Alloy I Y.S U.T.S Elong., No. Product Condition K s.l K s.i percent 1 rod H. A.C 70.4 93.7 30 d H.R.+C.R.+1,700 F. Ann.+A.C 59.8 99.8 25 2 4" rod H.R.+A.C 76.3 97.6 26 do. H.R.+C.R.+1,700 F. Ann.+A.C 57.9 90.0 24 1' rod Extruded 1,900 F.+A.C 54.8 88.6 30 rod (10 54.4 88.4 30 3 rod d0 s7. 7 94. 3 2s do Extruded 1,900 F.+A.C.+l,800 F. Ann.+A.O 57.8 95.8 30 1" rod Extruded 1,800 F.+A.C 57. 5 93.4 28 rod .do 60.7 96. 3 25 H.R.+C.R.+l,650 F. Ann.+A.O 48.1 84. 3 34 H.R.+C.R.+l,650 F. Ann +A C 50. 4 86. 0 32 H.R.+A.C 62.7 95.0 22 H.R.+A. 61. 6 97. 2 32 H.R.+A.G. (trans ct 61.0 96.4 28 H.R.+O.R.+1,700 F. Ann.+A.C 56. 0 89. 9 26 H.R.+C.R.+1,750 F. Ann.+A.C 60. 2 96.0 32 H.R.+C.R.+l,750 F. Ann.+A.C. 57. 9 92. 9 26 H.R.+G.R.+1,760 F. Ann.+O.Q 50.6 88. 3 38 H.R.+C.R.+1,650 F. Ann.+A.C-- 54. 0 89. 3 32 H.R.+O.R.+1,750 F. Ann.+A.O 54.0 86. 9 24 H.R.+C.R.+l,725 F. C 48. 9 81. 6 27 H.R.+C.R.+l,700 F. Ann.+A.C 46. 0 85. 4 34 H.R.+C.R.+l,700 F. Ann.+A.C 46.1 84. 9 35 U.I.S., K s.1.=Ulti,mate tensile strength, 1,000 p.s.i. units.

Y.S., K s.i.=Yield strength at 0.2% otlset, 1,000 p.s.i. units. Elong., percent=Percent elongation. H.R.=Hot rolled.

R Cold rolled.

Annealed one hour at indicated temperature.

O.Q. Oil quench.

A.O.=Air cool.

The remarkable consistency and high level of tensile strength characteristics of the new alloy in the air-cooled hardened, and thus spinodally strengthened, condition that is obtained over a broad range of section thicknesses is further illustrated by the room temperature tensile test results set forth in Table III. The results were obtained with specimens taken from products ranging from 2-inch starting at 2.4% and higher, in accordance with the invention, are highly effective in improving yield strength whereas lower amounts somewhat below the herein required 2.4% chromium have only a very small, modest efiect on yield strength. For instance, A -inch sheet of an alloy containing 1.20% chromium, 31.5% nickel, 0.68% iron, 0.5% manganese, 0.11% zirconium, 0.07% silicon, 0.07% titanium, and balance essentially copper (alloy X) had a yield strength of only 29,400 p.s.i. in the air cooled condition. Thus, the margin of strength of alloy X over the strength of conventional 70/30 cupronickel amounts to less than one-half the lowest target of the invention, to wit, doubling the air-cooled yield strength of 70/ 30 cupronickel.

Other alloys referred to in FIG. 2 and not in accordance with the invention include alloy Y, which contained 31.5% nickel, 2.25% chromium, 0.70% iron, 0.45% manganese, 0.13% zirconium, 0.064% silicon, 0.05% titanium, less than 0.1% aluminum, the balance being essentially copper, and alloy Z, which contained 30.5% nickel, 0.75% chromium, 0.61% iron, 0.17% manganese, with the balance being essentially copper (no zirconium, silicon, titanium or aluminum having been added). Room temperature tensile test results obtained with specimens from 4 inch sheet in the air cooled condition were, for alloy Y, a yield strength of 37,500 p.s.i., an ultimate strength of 76,700 p.s.i. and a tensile elongation of 31%; for alloy Z the corresponding properties were 21,000 p.s.i., 57,700 p.s.i. and 44%.

With further regard to chromium content, it should be mentioned that an otherwise effective portion thereof may be lost during processing or be impaired through the formation of carbides. Thus, in respect of alloy Y above described, 2.5% chromium, an amount within the invention, was added to the melt but only 2.25% was recovered, roughly 10% of the original addition being lost. This was sufiicient to bring about a considerable drop in yield strength. As to carbon, though an attribute of the invention is that it can be present, it must be controlled as previously mentioned. This is illustrated by the data reported in Table IV concerning an alloy in which nickel (30.5%) and copper chips were placed in a crucible and a charcoal powder spread thereover. The melt temperature was approximately 2500 F., and 2.5% chromium was added by plunging through the charcoal cover. The alloy was processed and treated in the same manner as the inch sheet specimens of Tables I and II.

* Charcoal cover; Bal.=balance plus impurities.

While the chromium level of 2.38% in Table IV is virtually within the invention, the alloy is nonetheless characterized by a low yield strength of 31,400 p.s.i., the reason being attributed to the 0.056% carbon which formed sufiicient carbides to prevent the chromium from imparting its maximum strengthening effect. X-ray diffraction indicated a pattern of mainly Cr C As stated herein, the carbon content should be maintained below about 0.02%. In any event, the minimum chromium level of about 2.4% must be free chromium, i.e. chromium in uncombined form and effective to confer its strengthening capability.

Further advantages of the new alloy include outstandingly good impact and fatigue strengths. Illustrative Charpy impact energy values are set forth in Table V. Each value is an average of 4 tests of Charpy V-notch specimens taken from extruded, cold Worked and annealed bar made of an alloy containing 29.2% nickel, 2.85% chromium, 0.73% iron, 0.73% manganese, 0.10% zirconium, 0.076% silicon, 0.048% titanium, less than 0.05% aluminum and balance essentially copper. Prior to cold working, the extruded bar, which had been extruded TABLE V Impact (foot-pounds) F.) Test Cold temperature worked Anealed -a20 67 174 105 58 169. 5 R.T 58. 5 166. 5 400 58 136 700 55 112 As to room temperature fatigue strength, specimens of the new alloy in the annealed condition endured over million cycles of reversed bending in rotating beam tests (R. R. Moore type) with maximum fiber stress at 40,000 p.s.i., which compares more than favorably to a 100 million cycle fatigue strength of around 23,000 p.s.i., for the conventional 70/ 30 alloy.

Weldments having high yield strength in the condition obtained by air cooling after welding have been made very satisfactorily by using the alloy of the invention as filler metal to weld wrought products of the alloy. A weldment was made by welding in accordance with an invention of W. A. Petersen, described in US. patent application Ser. No. 707,967, filed Feb. 26, 1968 whereby two pieces of annealed /2" thick plate made of alloy 6 were butt welded by argon-tungsten arc welding using filler metal of a composition within the scope of the present invention which contained 30.0% nickel, 2.70% chromium, 0.04% Zirconium, 0.68% manganese, 0.32% silicon, 0.74% iron, 0.04% titanium and the balance essentially copper. Duplicate tensile specimens were taken across the welded joint and tested in the as-welded air cooled condition. Results of testing the as-welded metal were: 79,300 p.s.i. ultimate tensile strength, 56,400 p.s.i. yield strength, 15% elongation; and 80,400 p.s.i. ultimate tensile strength, 57,800 p.s.i. yield strength, 14% elongation. Elongations pertain to a 1" gage length across the weld.

The alloy in accordance herewith has satisfactory corrosion resistance which in at least some environments is superior to the corrosion resistance of 70/ 30 cupronickel. For instance, in 30 day tests comparing the resistance of an alloy of the invention containing 30% nickel and 3.6% chromium with the resistance of a commercial 70/ 30 cupronickel alloy to a high velocity jet of sea water, a test which has been found indicative of the suitability of cupronickel alloys for piping used to carry high velocity sea water, the weight loss of the commercial cupronickel alloy was 50% greater than the Weight loss of the alloy of the invention.

While an important object of the invention is to provide a new alloy having high yield strength in the air cooled condition, it is to be observed that the new alloy can be hardened and strengthened by cold working. For example, sheet of alloy 1 when cold worked 50% after having been hot worked and air cooled had a room temperature yield strength of 110,700 p.s.i. and an ultimate tensile strength of 119,000 p.s.i. with 13% elongation.

Although the alloy is generally in a relatively low strength condition if water quenched to obtain extremely high cooling rates, a yield strength of 40,000 p.s.i. or higher is sometimes obtained with water quenched alloys of the invention. Very slow cooling is not detrimental to the strength of the alloy. For example, when the cooling rate of a /z" thick plate of alloy 7 was suppressed so that the plate took about seven hours to cool from 1750 F. to 300 F., a 65,700 p.s.i. yield strength, 99,300

p.s.i. ultimate tensile strength and an elongation of 29% were obtained at room temperature.

The present invention is particularly applicable to production of corrosion resistant alloys for pressurized salt water piping, heat exchangers, tube sheets, distillers, condensers, containers resistant to alkali corrosion and structural components. The invention is also applicable to production of high strength, corrosion resistant wrought products including sheets, strip, plate, bar, rod, tubing,-

piping, extruded shapes, wire and the like.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and appended claims.

We claim:

1. A new wrought cupronickel metal characterized by a unique combination of (1) chemical composition, (2) unusual spinodal decomposition microstructure Which is further characterized (3) in that it is free of detrimental carbides, (4) markedly enhanced yield strength in the annealed and air-cooled condition, (5) excellent impact toughness (6) improved fatigue strength, and (7) good workability; said composition, by analysis, consisting of nickel from 28% to 32%, chromium from 2.4% and up to an amount not exceeding 3.8% whereby hot workability is not impaired, carbon present in an amount up to not more than about 0.02%, up to 2% manganese, up to 0.5% each of zirconium, titanium and silicon, up to 1% iron, the balance essentially copper; said microstructure being comprised of finely divided copper-rich and copper-lean regions of two different solid solution products resulting from a spontaneous spinodal decomposition of a copper-nickel-chromium solid solution and being devoid of the presence of detrimental carbides; said yield strength being at least about 40,000 p.s.i., this strength level obtaining in conjunction with an improved capability of absorbing impact energy; and a fatigue strength on the order of about 40,000 p.s.i. over a period of about 100 million cycles duration.

2. Cupronickel metal in accordance with claim 1 containing about 0.5 to 1% manganese.

3. Cupronickel metal in accordance with claim 1 containing from 0.05% to 0.5% zirconium.

4. Cupronickel metal in accordance with claim 1 containing about 3% and up to 3.8% chromium, the yield strength being at least 50,000 p.s.i.

5. Cupronickel metal in accordance with claim 1 containing from about 2.75% to 3.75% chromium.

6. A new Wrought Cupronickel metal characterized by a unique combination of (1) chemical composition, (2) unusual spinodal decomposition microstructurewhich is further characterized (3) in that it is free of detrimental carbides, (4) markedly enhanced yield strength in the annealed and air-cooled condition, (5) excellent impact toughness, (6) improved fatigue strength and (7 good workability; said composition, by analysis, consisting of chromium from 2.3% and up to an amount not exceeding about 3.8% whereby hot workability is not impaired, the

chromium being at least 2.4% in the absence of an amount of cobalt such that the sum of the cobalt plus one-half the chromium is at least 2.4% but not in excess of 3.8% with the cobalt not exceeding 2.5%, nickel from 24% to 38% with the proviso that when the nickel is from 33% to 38% the chromium is at least the amount determined by the relationship Percent Cr=0.2 (percent Ni- 13) up to 2.5% iron provided the total chromium, cobalt and iron does not exceed about 5%, up to 0.1% carbon, up to 6% zinc, up to 3% manganese, up to 0.8% zirconium, up to about 0.5% silicon, up to about 0.5% titanium, the balance essentially copper; said microstructure being comprised of finely divided copper-rich and copper-lean regions of two different solid solution products resulting from a spontaneous spinodal decomposition of a copper-nickel-chromium solid solution and being devoid of the presence of detrimental carbides; said yield strength being at least about 40,000 p.s.i., this strength level obtaining in conjunction with an improved capability of absorbing impact energy; and a fatigue strength of about 40,000 p.s.i. over a period of about 100 million cycles duration.

7. Cupronickel metal in accordance with claim 6 containing about 2.75% to 3.75% chromium.

8. Cupronickel metal in accordance with claim 6 containing about 3% and up to 3.8% chromium.

9. Cupronickel metal in accordance with claim 1 containing manganese in an amount up to 2% and from 0.05% to 0.5% zirconium.

10. Cupronickel metal in accordance with claim 6 containing 0.8% to 1.2% cobalt.

11. Cupronickel metal in accordance with claim 6 containing carbon up to 0.02%.

12. Cupronickel metal in accordance with claim 11 and containing 2.4% to 3% chromium, from 28% to 33% nickel and about 0.5% to 1% manganese.

References Cited UNITED STATES PATENTS 1,557,025 10/1925 Cochrane 159 2,067,306 1/1937 Wilkins 75-159 2,430,306 11/1947 Smith 75159 3,253,911 5/1966 Cairns 75-159 FOREIGN PATENTS 338,676 Great Britain 75-159 OTHER REFERENCES Acta Metallurgica, vol. 9, September 1961, Calm, pp. 795-801.

Journal of The Institute of Metals, vol. 84, 1955-56, Meijering et al., pp. 118-120.

Transactions of the Metallurgical Society of AIME, vol. 242, February 1968, pp. 166-179.

CHARLES N. LOVELL, Primary Examiner US. Cl. XR. 75--1.57.5. 1.59

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