US3359094A - Ferrous alloys of exceptionally high strength - Google Patents

Description

United States Patent Delaware No Drawing. Filed May 20, 1965, Ser. No. 457,494 32 Claims. (Cl. 75123) The present invention relates to ferrous alloys and, more particularly, to ferrous-base alloys characterized by strength levels of a tremendously high order of magnitude, to wit, tensile strengths above about 300,000 pounds per square inch (p.s.i.) to about 500,000 p.s.i. and higher. Of considerable significance, the exceedingly high strengths manifested by alloys contemplated herein are obtained with the simplest of heat treatment. Recourse to costly and/or complicated processing techniques, including quenching, tempering, severe plastic deformation, e.g., ausforming, cold rolling, cold drawing, etc., is not at all necessary.

As is generally known to those skilled in steel metallurgy, important and rather striking advances have been achieved within the last score of years in improving the strength capability of steel. A number of diverse factors have undoubtedly underscored this endeavor but infrequent has been the case where the competitive necessity of developing steels having higher strength-to-weight ratios has not assumed an important, if not major, role. Such was the case in the development of steels having yield strengths up to 100,000 p.s.i. as indicated in Metals Handbook, 8th ed. (1960), p. 87, and also in the development of 200,000 p.s.i. to 300,000 p.s.i. ultimate tensile strength steels.

Notwithstanding that steels of ultimate tensile strengths of up to about 300,000 p.s.i. have become a commercial reality, research eflorts have been intensified in the quest of stronger and, thus, lighter steels (based on strength-to-weight ratios). Extremely versatile steels with yield strength of up to about 300,000 p.s.i. (yield strength as contrasted with ultimate tensile strength being a basic tool of the designer) have recently been attained with a good level of toughness, a mechanical property all too frequently found lacking at the highest strength levels in respect of the so-termed ultra strong quenched and tempered low alloy steels. While various proposals have received prominence, among the most notable are the recently introduced maraging steels and those steels amenable to the ausforming technique.

The aforementioned maraging steels, described in US Patents Nos. 3,093,518 and 3,093,519, have been considered as having a combination of characteristics, including strength and toughness, superior to any steel theretofore known, particularly when compared with steels which did not require work hardening through plastic deformation to develop a high plateau of strength. For example, at the high yield strength level of about 300,000 p.s.i., tensile elongations of about 7% or 8% have been reached together with reductions in area of about 30% or 35% (bar properties) in the maraging steels. Apart from strength and toughness considerations, the maraging steels have been deemed particularly attractive since a quenching operation is not required and, thus, the well documented problems, e.g., distortion, warpage, dimensional change, etc., attendant the high strength, quenched and tempered, carbon-containing, low alloy steels were obviated. In addition, temper embrittlement problems characteristic of the latter steels were also greatly minimized, if not completely eliminated. Such attributes of the maraging steels undoubtedly attest to their rather spontaneous commercial acceptance. However, due regard being given to the many virtues of known maraging steels, we are unaware of any such maraging steel which manifests the capability of providing yield strengths of, say, 400,000 p.s.i. or 450,000 p.s.i.

Relatively high yield strength levels have been reported regarding steels subjected to the application of the ausforming process wherein certain steels (usually, if indeed not always, of high carbon content) are heavily plastically deformed during a heat treating cycle and while in the austenitic condition, whereby the strength of the steel is increased and other properties may be improved. Upon completion of the plastic deformation operation, the steel is quenched and tempered. In respect of one particular steel of which we are aware, the literature indicates that a yield strength of slightly over 400,000 p.s.i. was obtained but this required an inordinately high degree of plastic deformation, to Wit, over and the utilization of a narrow and critical temperature range. When nominally plastically deformed about 20% to 30%, comparatively little benefit is obtained. While the ausforming technique is indeed a significant achievement, the applicability thereof is, at present, quite restricted. The amount of working to achieve high strengths is extremely high, e.g., over 70%, and this consideration apart from other disadvantageous factors tends to limit the size, shape and form of products that can be produced. In addition, high carbon-containing alloys are generally necessary to provide a desired response to ausforming and were ausformed material welded, the hot-cold Work function of ausforming would, as a practical matter, be completely lost. Of further importance is the fact that the process in and of itself is, at best, tedious and costly to carry out because of the very careful operational control that must be exercised.

In accordance with the present invention, tensile strengths well above 300,000 p.s.i. and up to about 500,- 000 p.s.i. not only can be obtained but are achieved with the processing advantages attendant the maraging steels and without the disadvantages of the quenched and tempered or ausformed steels. This is not to say, however, that the present invention is to be construed as excluding the application of any appropriate processing or treating operation, e.g., plastic deformation. Cold working, for example, can be used to advantage; in fact, tensile strengths above 500,000 p.s.i. have been obtained in accordance herewith by cold reducing but a nominal amount, to wit, 20% to 30%. It is perhaps of some interest to note that plastically deforming by 30% is quite far removed from the percentage of plastic deformation reported in connection with the ausforming process wherein highest strength levels have been attained.

The magnitude of improvement in tensile strength (including yield strength) or strength plus ductility as contemplated herein can be illustrated by comparison with known maraging steels. From the consideration of tensile strength only, alloys within the invention boost the strength levels of prior art maraging steels by upwards of 175,000 p.s.i. as to strength plus ductility and using corresponding periods of development, if, for example, a maraging steel affording a yield strength level of about 300,000 p.s.i. is used as a representative standard, the present invention encompasses alloys which boost the yield strength by a factor of at least about 75,000 p.s.i. to over 100,000 p.s.i. with a concomitant increase in tensile ductility of from 50% to Again, however, as will be appreciated by those skilled in the art, the present invention is not intended to be restricted to steels having a high degree of ductility. There are numerous commercial applications where high strength or, for that matter, extreme hardness is of prime significance and a high degree of ductility is not of major importance, e.g., dies, bearings, machine tools, knives, razor blades, etc.

It has now been discovered that exceedingly high ma,,- nitudes of tensile strengths (both yield and ultimate tensile strengths) and tremendously high strength-toweight ratios can be achieved with ferrous alloys containing certain essential constituents, notably, nickel, molybdenum and cobalt, provided that the constituents are correlated in respect of each other and are maintained within special compositional ranges. As will become clearer "herein, the ferrous metallurgist, producer, designer, etc., will also have at his disposal steels which will fulfill the requirements of various specific applications. Where tensile strengths above, for example, about 425,000 p.s.i. and advantageously above about 450,000 p.s.i., and/or high hardness, e.g., Rockwell C (R 60 or 62 or even 65 and above, are of primary importance, certain compositions will provide these properties. Where high strengths on the order of 400,000 p.s.i., e.g., 375,000 to about 425,000 p.s.i. and advantageously up to 450,000 p.s.i., are required together with a good level of ductility, this need is also satisfied in accordance herewith. Higher levels of ductility can be obtained where magnitudes of strength of about 350,000 to 375,000 p.s.i. are satisfactory.

Accordingly, it is an object of the invention to provide a novel ferrous alloy.

It is another object of the present invention to provide a ferrous-base composition having what is believed to be (apart from whiskers, crystals, thin films, very thin wires and the like) the highest strength-to-Weight ratio and highest strength heretofore reached in a steel not subjected to deformation (beyond, of course, the usual hot working processing).

It is a further object of the invention to provide steels having strengths above 300,000 p.s.i., e.g., 350,000 p.s.i. and up to about 500,000 p.s.i. and above.

Another object of the invention is to provide ferrous alloys which manifest exceptionally high strength, e.g., 400,000 to 425,000 or 450,000 p.s.i., and which are duetile at such strength levels.

The invention further contemplates providing ferrous alloys which are extremely hard.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates ferrous alloys containing, by weight, from 5% to 16.5% nickel, e.g., 7% to nickel, about 7% to 16% molybdenum, e.g., 8% to 15% molybdenum, about 8% to 30% cobalt, e.g., 10% to 25% cobalt, the sum of the molybdenum plus cobalt being at least about 20% and advantageously at least about 22%, up to 2.5% titanium, up to 2.5 aluminum with the sum of the titanium plus aluminum not exceeding about 3%, and advantageously not exceeding 2.5%, up to 0.3% carbon, the balance of the alloys being essentially iron. As will be readily understood by those skilled in the art, the term balance or balance essentially when used in referring to the amount of iron in the alloys does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not materially affect the basic characteristics of the alloys. In this connection, elements such as sulfur, phosphorus, hydrogen, oxygen, nitrogen and the like should be maintained at low levels consistent with commercial practice. However, supplementary elements can be present in the alloys as follows: up to 2% columbium, e.g., up to 1.5%; up to 4% tantalum, e.g., up to 3%; up to 0.1% boron, e.g., up to 0.05%; up to 0.25% zirconium, e.g., up to 0.15%; up to 8% chromium, e.g., up to 5%; up to 2% vanadium, e.g., up to 1.5%; up to 0.5% silicon and advantageously not more than 0.25%; up to 0.5% manganese and advantageously not more than 0.25%; up to 0.1% calcium, e.g., up to 0.075%; up to 1% beryllium, e.g., up to 0.5%; and up to 4% copper, e.g., up to 2%. It is preferred that the respective amounts of the aforementioned supplementary elements be as follows: up to 1% columbium, e.g., 0.01% to 0.5%; up to 2% tantalum, e.g., 0.01% to 0.5%; up to 0.01% boron, e.g., 0.0005% to 0.0075%; up to 01% zirconium, e.g., 0.001% to 0.1%; up to 4% chromium, e.g., 0.1% to 3.5%; up to 1% vanadium, e.g., 0.1% to 0.5%; up to 0.15% silicon, e.g., 0.01% to 01%; up to 0.15% manganese, e.g., 0.01% to 0.1%; up to 0.05% calcium, up to 0.1% beryllium, and up to 1% copper. The total sum of the supplementary elements should not exceed 10% and advantageously should not exceed 6%. Tungsten can be used to replace molybdenum in part on an atom for atom basis, two parts of tungsten by weight for one part of molybdenum, in an amount up to 8% by weight. However, it is advantageous that the tungsten not exceed 6% and preferably should not exceed 4% particularly since molybdenum, in contrast to tungsten, importantly contributes to improved forgeability and/or hot workability and also imparts enhanced ductility characteristics.

Although a very substantial number of alloys have been prepared and subjected to desired testing, the exact or complete metallurgical explanation regarding the behavior of the steels and the role of their respective constituents is not yet at hand. The behavior of the subject steels is deemed unusual indeed since while they can be considered as maraging steels they run contra to certain accepted principles of known maraging steels. In any event and as will be demonstrated hereinafter, it has been found that it does not take much departure from the compositional ranges set forth herein to lose the advantages of the invention.

The element nickel contributes, among other things, to achieving ductility, toughness and a desired martensitic structure upon cooling from hot working or, where used, solution treatment. The subject alloys are austenitic at high temperature and undergo transformation during cooling. However, with excessive amounts of nickel, retained austenite in deleterious amounts can ensue and/ or there is danger of excessive austenite reversion upon aging. Austenite reversion can be minimized by using low aging temperatures, e.g., below 700 F., but this, in turn, would significantly impair the strength level of the steels. Further, for a given molybdenum and cobalt content, it is considered that a maximum combination of strength and toughness is attained when the nickel content is at a level as high as is consistent with providing the lowest M temperature together with freedom of substantial amounts of austenite, e.g., amounts of austenite on the order of above about 5%. Other factors being equal, it is thought that with the low M temperatures a greater number of dislocation tangles are formed and, as a result thereof, strength characteristics are enhanced. Further, low nickel contents invite the tendency for formation of ferrite or other undesirable and subversive phases and such phases can wreak havoc with various mechanical characteristics of the steels. Accordingly, it is advantageous that the nickel content be from 7% to 16.5% and where optimum toughness characteristics are desired, it is most preferred that the nickel content be at least 11%, e.g., 11% to 16%.

Molybdenum and cobalt principally confer strengthening and hardening characteristics, the former being the more potent in this regard. Molybdenum, as mentioned above herein, also provides, at high strength levels, good forgeability and ductility characteristics. The combined amounts of molybdenum and cobalt should be at least 20% and for best results the total content of these elements should be at least 22% and preferably at least 23%. Apart from the strengthening and hardness characteristics imparted to the alloys by cobalt, it can be used in some measure in controlling the occurrence of transformation of austenite to ma-rtensite, depending, of course, upon a given nickel and molybdenum content. To obtain optimum results when the molybdenum content is on the high side, to wit, 14.5% to 16%, the cobalt content should not exceed about 22%. Further, provided that the molybdenum content is at least 11% and preferably at least 12%, the requirement that the sum of the molybdenum plus cobalt be at least 20% can be relaxed to the extent that the cobalt content can be lowered to 6%. In addition, it is preferred that the elements nickel, molybdenum and c balt be correlated such that the sum of the nickel plus the molybdenum plus one-tenth of the cobalt is not less than 16 and not greater than 27 and most advantageously not less than about 20 and not greater than 26. This correlation greatly contributes to achieving the formation of a satisfactory martensitic condition upon cooling from hot working (or solution treating) Without the necessity of using additional treatments, such as cold treating. The maximum sum of the nickel plus molybdenum plus onetenth of the cobalt can be as high as about 30 or even higher but, in this connection, a cold treatment as by, for example, refrigeration and/ or cold working, may be necessary prior to aging to induce the desired degree of transformation to martensite. The important or essential point is that the alloys be in the martensitic or substantially martensitic condition prior to aging.

Titanium and/ or aluminum serve to provide good deoxidation and malleabilization characteristics. Titanium, for example, serves to fix elements, such as oxygen, nitrogen and carbon. While the respective amounts of titanium or aluminum generally need not exceed about 1%, the total thereof not exceeding 1.5%, when molybdenum is present within the low side of the molybdenum range, e.g. 7% to 10%, titanium and/ or aluminum in an amount of 1%, e.g., 1.5%, to about 3% markedly serves to enhance the strengthening characteristics (as shown hereinafter), particularly in the presence of nickel at the higher end of the nickel range, e.g., 11% to 16% nickel.

While carbon can be present up to 0.3%, for good toughness characteristics it should not exceed 0.1% except where unconventional working practices are employed, e.g., ausforming. For optimum results, carbon should not be present in amounts greater than 0.05%, e.g., not more than about 0.03%. It should be mentioned that for applications where carbides would be particularly useful, e.g., cutting edges, carbon contents up to 1% can be employed. Further, where good ductility characteristics are of especial importance, manganese and silicon each should not be present in amounts above about 0.25% and preferably not above 0.15%.

In addition to the foregoing and as referred to above herein, certain compositional ranges of alloys afford special properties, e.g., exceedingly high strengths and/or hardness, a high magnitude of strength plus good tensile ductility, etc. Thus, for example, where a strength level of about 425,000 p.s.i. or above would be of utmost importance, it is advantageous that the alloy be of the following composition: about 7% to 13% nickel, about 12% to 15% molybdenum, about 12% to 22% cobalt, up to 1% titanium, up to 1% aluminum, up to 0.1% carbon with the balance essentially iron. Most advantageously, such alloys should contain 7% to about 10.5% nickel, 12.5% to 14.5% molybdenum, about 14% to cobalt, up to 0.5 titanium, up to 0.5% aluminum, the sum of the titanium plus aluminum not exceeding 0.75%, up to 0.05% carbon and the balance essentially iron. In this connection, the molybdenum content can be lowered to about 10%, i.e., 10% to 12%, provided the cobalt content is present in amounts of to Where a good combination of strength (375,000 p.s.i. to about 425,000 p.s.i.) and tensile ductility would be required, the alloy should contain about 11% to 16% nickel, about 7.5% to 12% molybdenum, about 10% to 18% cobalt, the sum of the molybdenum plus cobalt being at least 22% and the balance essentially iron. Advantageously for an optimum combination of strength and ductility, the alloy should contain 12.5% to 15.5% nickel, 8% to 10.5% molybdenum, 12% to 16% cobalt, the sum of the molybdenum plus cobalt being at least 22%, up to 0.05% carbon, up to 0.5% titanium, up to 0.5% aluminum, the total titanium plus aluminum not exceeding 0.75%, with the balance being essentially iron. Should maximum hardness be the property of primary interest, the alloy should contain 5% to 10% nickel,

13% to 16% molybdenum and 16% to 30% cobalt. Another highly satisfactory range for exceptionally hard ferrous-base alloys is 6% to 9% nickel, 13.5% to 15.5% molybdenum and 20% to 30% cobalt, the balance being essentially iron.

In carrying the invention into practice, air or vacuum melting practice can be utilized, preferably followed by consumable electrode melting for optimum efifects. It is advantageous to utilize materials of good purity to thereby minimize the occurrence of inclusions, contaminants, etc. In processing, the initially formed cast ingots should be thoroughly homogenized as, for example, by soaking, at a temperature of about 2200 F. to about 2300" F. for about one hour per inch of cross section. Thereafter, the alloys are hot worked (as by forging, pressing, rolling, etc.) and, if desired, cold worked to desired shape. A plurality of heating and hot working operations can be used and are advantageous to assure thorough homogenization of the cast structure through diffusion and to break up the cast structure. Hot working can be sati factorily carried out over a temperature range of 2300 F. or 2200 F. down to 1400 F., e.g., 2150 F. to 1500 F., with suitable finishing temperatures being about 2000 F. down to about 1500 F. Cooling from hot working is preferably accomplished by air cooling although furnace cooling, quenching, etc., can be employed.

Subsequent to cooling from the hot Working temperature to efiect a transformation to the martensitic condition, the steels can be directly aged (no other processing or heating step being necessary) by heating at a temperature of about 750 F. to 1100 F. for about hours to 0.1 hour, the longer aging periods being used in conjunction with the lower aging temperatures. Aging at 950 F. to 900 F. for about one to four hours has been found quite satisfactory. With aging times above about an hour or so, temperatures above about 1100 F. should not be used since deleterious austenite reversion can occur. On the other hand, temperatures appreciably below 750 F. are not recommended in view of the long aging times required, e.g., more than 100 hours, to obtain maximum strength and hardness. However, where especially hard surfaces, particularly in combination with softer cores, are required, the steels can be heated at a temperature as high as 1400 F., e.g., about 1200 F. to 1375 F. and preferably at 1250 F. to 1350 F., for a period of time of not greater than about 30 minutes, e.g., up to 15 minutes, the longer time being associated with the lower temperature. A period of from a few seconds, e.g., 15 seconds, up to five minutes is satisfactory for the temperature range of 1250 F. to 1350 F.

Where desired or deemed advantageous, the steels can be subjected to a solution annealing treatment prior to aging. In this connection, the temperature extends over a range of about 1400 F. to about 2200" F.; however, the temperature used in dependent upon the molybdenum content to a considerable extent. Thus, in accordance herewith, when the molybdenum content is from 7% to about 8%, a solution treatment temperature (if employed) of at least about 1400 F. should be used. With molybdenum contents from above about 8% to about 10%, from above about 10% to 12%, from above about 12% to 14% and above about 14%, solution treatment temperatures of at least about 1600 F., 1800 F., 1900 F., and 2000 F., respectively, should be employed. With lower temperatures, there is the risk of finishing the steels cold such that best results, notably strength, are not obtained.

For the purpose of giving those skiled in the art a better understanding of the invention and/or a better appreciation of the advantages thereof, the following illustrative description and data are given.

In Table I a substantial number of alloy compositions are given together with strength and hardness characteristics, the alloys being illustrative of those which manifest exceptionally high strength and hardness. The alloying 8 p.s.i. for a specimen of Alloy No. 3 subjected to Heat Treatment A. This further attribute of the invention, to wit, high ratios of yield to ultimate tensile strength, is of significant importance since rare is the occasion that a constituents were melted in a vacuum induction furnace 5 designer is not restricted or limited to using the yield and, after solidification, the cast ingots were homogenized strength of a material (as opposed its higher ultimate (soaked) at about 2200 F. to 2300 F. The steels were tensile strength) as a basic criterion in selection of a then hot worked and thereafter machined to specimens material for a given application. In accordance with the of about 0.135 to 0.14 inch in diameter. The steels were present invention the ratio of yield strength to ultimate then heat treated at about 900 F. for about 4 hours 10 tensile strength is advantageously at least 0.9 and higher, (Heat Treamtent A) or for five hours (Heat Treatment e.g., 0.95 and above. It is Worthy of mention to say that B) or at about 950 F. for about one hour (Heat Treatthe tensile ductility (4 x diameter) of various of the ment C). None of the alloys was given a solution treatalloys in Table I was exceptionally high for such strength ment subsequent to the hot working operation. The ultilevels, the highest, for those tested, being a tensile elonmate tensile strength (U.T.S.) is given in thousands of 15 gation of 4% for Alloy No. 2 (non-cold worked condipounds per square inch and the hardness is given in Rocktion, Heat Treatment C), and 2% for Alloy No. 12. well C, R units. In addition to the constituents reported In Table II further data are given including strength in Table I, not more than about 0.03% carbon nor more (Y.S., 0.2% offset), tensile ductility (EL, percent) and than about 0.15% silicon plus manganese was added to reduction in area (R.A., percent) values, the data being the steels, the balance otherwise being iron plus impuri- 20 illustrative of alloy compositions manifesting a good ties. combination of both strength and ductility. In addition TABLE I Alloy Heat Ni, Mo, Co, Ti, Al, U.'I.S., Hardness,

N 0. Treatment percent percent percent percent percent p.s.i. R c

A s 14 1s 0. 2 0. 2 506. 200

A 8 14 18 0. 2 N.A. 490,100 64. 5

o s 14 13 0. 2 N.A. 476,706

o s 14 1s 0. 2 N.A. 476, 500

A s 14 18 N.A. 0. 2 467, 800 65.

o s 14 1s N.A. 0. 2 468, 000

A s 14 1s N.A. 0. 2 461, 900 65 0 s 14 1s N.A. 0. 2 479, 560

1 Cold worked about before aging. 2 0.5% columbium added. 3 Difierent heat from Alloy N0. 1.

N .A.--Not added.

As is apparent from Table I, ultimate tensile strengths well above 400,000 p.s.i. can be readily obtained in accordance with the invention. As reflected by Alloys Nos. 1 and 2, the half-million p.s.i. strength barrier was passed with the application of a nominal amount of cold working prior to aging such alloys and, as a practical matter, was reached by Alloy No. 3 without cold working. The hardness level for each of the alloys exceeded R 60. The yield strengths of the alloys (not given) were also exceptionally high, being exceedingly close to the corresponding ultimate tensile strengths. For example, the yield strength (0.2% offset) of Alloy No. '1 (non-cold worked condition) was 473,800 p.s.i. and for Alloy No. 18 it was to Heat Treatments A and C heretofore described, on occasion, other heat treatments were used as follows: aging for about 96 hours at about 800 F. (Heat Treatment D); refrigerating before aging at 900 F. for about four hours (Heat Treatment E). The alloys were prepared following the procedure used in connection with the alloys of Table I, except that the specimens were about 0.252 inch in diameter and in a few instances air melting practice was used. In addition to the alloying constituents and amounts thereof set forth in Table II, none of the alloys contained more than about 0.04% carbon nor more than about 0.15% of each of silicon and 441,000 p.s.i. The highest yield strength was 486,700 manganese. A small amount of aluminum and titanium,

0.2% of each, was added to each melt, the balance of the alloys being iron plus impurities.

10 about 10% or 11%, tensile strengths of about 425,000 p.s.i. and above, e.g., 450,000 p.s.i., can be obtained pro- TABLE II Allo Heat N1, M0, Co, U.T.S., Y.S., EL, R.A.,

N0. Treatment percent percent percent p.s.i. p.s.i. percent percent A 12 12 12 415, 000 402, 000 3 16 A 10 12 12 415, 000 404, 000 4 10 A 9 13 13 411, 000 403, 000 3. 5 23 D 13 12 400, 800 388, 300 8 38. 5 A 13 10 12 402, 400 394, 900 6 23 A 12 10 16 402, 000 390, 000 5 27 A 13 9 16 399, 000 86, 000 5 23 A 12 10 14 394, 000 381, 000 4 24 A 13 9 14 393, 400 380, 100 5 30 O 13 9 14 392, 600 384, 100 7 33 E 13 9 14 393, 100 383, 100 6 24 E 14 8 16 383, 200 375, 200 8 28 A 14 8 16 384, 100 375, 000 5 25. 5 D 14 8 16 380, 300 371, 200 9 37. 5

*Air melted.

The data in Table II reflects that a markedly good combination of strength and tensile ductility can be obtained in accordance herewith. For example, at a yield strength of about 388,000 p.s.i., Alloy No. 26 manifested an exceptionally high tensile ductility of about 8% together with a reduction in area of about 38%. The data further illustrates the closeness between the yield and ultimate tensile strengths, the ratio therebetween being not less than 0.95% for any of Alloys Nos. 23 through 33.

As has been indicated hereinbefore, when the molybdenum content of the alloys is maintained at about 10% or below, aluminum and/ or titanium markedly enhance the strength characteristic of the alloys. This is illustrated in Table III.

vided the sum of aluminum plus titanium is at least 1%. In this case, the alloys can contain 5% to 16.5% nickel, about 7% to 11% molybdenum, about 8% to about 30% cobalt, the sum of the molybdenum plus cobalt being at least 20%, at least one metal selected from the group consisting of up to 2.5% titanium and up to 2.5% aluminum, the sum of the titanium plus aluminum being at least 1% and not greater than 3%, up to 0.3% carbon, the balance being essentially iron. Advantageously such alloys contain 7% to 15% nickel, about 7% to 10% molybdenum, about 10% to 25% cobalt, at least one metal selected from the group consisting of up to 2.25% titanium and up to 2.25 aluminum, the sum of the titanium plus aluminum being at least 1.5% and not greater than 2.5%, up

TAB LE III Alloy Heat i, Mo, C0, Ti, Al, U.T.S No. Treatment percent percent percent percent percent p.s.i.

A 12 10 14 0. 2 0. 2 394, 000 A 12 10 16 0. 2 0. 2 402, 000 A 12 10 16 0. 2 1 430, 000 A 12 10 16 1 0. 2 445, 000 A 12 10 16 0. 8 0. 8 450, 000 A 12 10 16 2 0. 2 463, 000

As can be seen from Table III, the increase in strength of Alloy No. 27 over Alloy No. 29 is due to the increase in cobalt. Increasing the aluminum or titanium content of Alloy No. 27 from 0.2% to 1% as in Alloys Nos. 19 and 17, respectively, resulted in a marked increase in strength. The greatest increase occurred in respect of Alloy No. 9 wherein the titanium content was 2%. Equal amounts of aluminum and titanium, i.e., 0.8%, in Alloy No. 15 indicate that an excellent increase in strength can be had over Alloy No. 27 which contained 0.2% of each of titanium and aluminum. The magnitude of this striking improvement does not follow With alloys containing 5 ment A.

TABLE IV Alloy Ni, Mo, 00, Ti, Al, Hardness,

N 0. Percent Percent Percent Percent Percent R0 9 5 5 0.2 32 2O 5 30 0.2 0 20 5 0.2 0 4 1O 0 0.2 27 20 10 10 0.2 0 19 10 15 0.2 0 18 1O 20 0.2 0 10 10 0.2 22 18 10 18 0.2 16 15 9 20 0.2 63.5 14 8 24 0.2 62.5 10 10 30 0.2 7 15 15 0.2 65 5 13 18 0.2 63

1 Air Melted.

l 4% Chromium Added.

much above 10% molybdenum, e.g., 12% or above, particularly in the presence of nickel contents below 14%. Thus, where the molybdenum content does not exceed Each of Alloys Nos. 34 through 42 exhibited extremely low hardness, hardness levels more than quite below that characteristic of the alloys contemplated herein. Microstructure studies of Alloys Nos. 34 through 40 (aged condition) revealed that only Alloy No. 34 was martensitic, Alloys Nos. 35 through 40 being austenitic and Alloy No. 37 being ferritic. All the alloys within the invention were martensitic and suflice to say the Rockwell hardnesses thereof were of a magnitude substantially higher than Alloys Nos. 34 to 42.

In addition to the fact that alloys within the invention afford a high degree of strength and/ or hardness as well as being ductile, alloys contemplated herein are also resistant to stress corrosion cracking. The alloys are useful in the production of such items as bar, rod, plate, castings, wire, etc., and products made therefrom, including fasteners, e.g., bolts; In this connection, for optimum processing characteristics, at least titanium and/or aluminum should be used in an amount of at least 0.05%, e.g., 0.1%. Desired shapes are best obtained prior to aging, i.e., in the hot worked or annealed condition since the alloys are comparatively soft and thus are more amenable to shaping operations such as cold working. Further, to minimize processing time, aging temperatures from 850 F. to 950 F. are deemed the most satisfactory, the aging period not exceeding about ten hours.

As will be readily understood by those skilled in the art, the terms martensite or substantially martensite include the decomposition and/or transformation products of austenite obtained upon cooling from the hot working operation (or, where used, a solution annealing treatment). These terms also include transformation products of austenite resulting from the application of a cold treatment, e.g., refrigeration at a temperature down to minus 300 F. and/ or cold working.

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 martensitic, iron-base alloy manifesting an exceptionally high combination of strength and ductility in the aged condition, said alloy consisting essentially of about 11% to about 16% nickel, about 7.5% to about 12% molybdenum, about to about 18% cobalt, the sum of the molybdenum plus cobalt being at least 20%, up to 0.1% carbon, up to 1% titanium, up to 1% aluminum, the sum of the titanium plus aluminum not exceeding 1.5%, and the balance essentially iron.

2. The alloy as set forth in claim 1 wherein the sum of the molybdenum plus cobalt is at least 22%.

3. A martensitic, iron-base alloy manifesting an exceptionally high combination of strength and ductility in the aged condition, said alloy consisting essentially of 12.5% to 15.5% nickel, 8% to 10.5% molybdenum, 12% to 16% cobalt, up to 0.05% carbon, up to 0.5% titanium, up to 0.5% aluminum, the sum of the titanium plus aluminum not exceeding 0.75%, and the balance essentially iron.

4. The alloy as set forth in claim 3 wherein the sum of the molybdenum plus cobalt is at least 22% 5. A martensitic, ferrous-base alloy characterized by a tensile strength of about 425,000 p.s.i. in the aged condition and being of a composition within the following ranges: about 7% to about 13% nickel, about 12% to about 15% molybdenum, about 12% to about 22% cobalt, up to about 0.1% carbon, up to about 1% titanium, up to about 1% aluminum, the sum of the titanium plus aluminum not exceeding about 1.5%, and the balance essentially iron.

6. A martensitic, ferrous-base alloy characterized by a tensile strength of above about 425,000 p.s.i. in the aged condition and being of a composition within the following ranges: about 7.5% to about 10.5% nickel, about 12.5%

12 to about 14.5% molybdenum, about 14% to about 20% cobalt, up to about 0.05% carbon, up to 0.5% titanium, up to 0.5% aluminum, the sum of the titanium plus aluminum not exceeding 0.75%, and the balance essentially iron.

7. A martensitic, ferrous-base alloy characterized by a tensile strength of above about 425,000 p.s.i. in the aged condition, said alloy containing about 7% to 10% nickel, about 10% to 12% molybdenum, about 25% to 30% cobalt, up to 0.1% carbon, up to 1% titanium, up to 1% aluminum, the sum of the titanium plus aluminum not exceeding 1.5%, and the balance essentially iron.

8. A martensitic, iron-base alloy characterized by a hardness of at least Rockwell C 60 in the aged condition and containing from 5% to about 10% nickel, from 13% to 16% molybdenum, from 16% to 30% cobalt, up to 0.1% carbon, up to 1% titanium, up to 1% aluminum, the sum of the titanium plus aluminum not exceeding 1.5 and the balance essentially iron.

9. A martensitic, iron-base alloy characterized by a hardness of at least Rockwell C 60 in the aged condition and containing from 6% to 9% nickel, from 13.5% to 15.5% molybdenum, from 16% to 30% cobalt, up to 0.05% carbon, up to 0.75% titanium, up to 0.75% aluminum, the sum of the titanium plus aluminum not exceeding 1% and the balance essentially iron.

10. The alloy as set forth in claim 9 wherein the cobalt does not exceed about 22%.

11. An iron-base alloy consisting essentially of from 5% to about 16.5% nickel, about 7% to about 16% molybdenum, about 8% to about 30% cobalt, the sum of the molybdenum plus cobalt being at least about 20%, up to 2.5% titanium, up to 2.5 aluminum, the sum of the titanium plus aluminum not exceeding about 3%, up to 1% carbon, up to 2% columbium, up to 4% tantalum, up to 0.1% boron, up to 0.25% zirconium, up to 8% chromium, up to 2% vanadium, up to 0.5% silicon, up to 0.5% manganese, up to 1% beryllium, up to 4% copper, up to 0.1% calcium, the total amount of columbium, tantalum, boron, zirconium, chromium, vanadium, silicon, manganese, beryllium, copper and calcium being not more than 10%, and the balance essentially iron.

12. The alloy as set forth in claim 11 wherein the molybdenum is partially replaced by an equal atomic percentage of tungsten up to a maximum tungsten content of 8% such that the sum of the molybdenum plus one-half the tungsten plus the cobalt is at least 20%.

13. The alloy as set forth in claim 11 wherein the sum of the nickel plus molybdenum plus one-tenth of the cobalt does not exceed about 30% and is not less than about 16%.

14. The alloy as set forth in claim 11 where the carbon content does not exceed 0.3%.

15. The alloy as set forth in claim 14 wherein the cobalt content does not exceed about 22% and the sum of the nickel plus molybdenum plus one-tenth the cobalt does not exceed about 27%.

16. The alloy as set forth in claim 15 wherein the carbon, silicon and manganese contents do not exceed 0.05%, 0.15% and 0.15%, respectively.

17. The alloy as set forth in claim 16 wherein the sum of the molybdenum plus cobalt is at least 22% and the sum of the nickel plus molybdenum plus one-tenth the cobalt does not exceed 26% and is not less than 20%.

18. A martensitic, iron-base alloy consisting essentially of from about 7% to about 15% nickel, about 8% to about 15% molybdenum, about 10% to about 25% cobalt ,the sum of the molybdenum plus cobalt being at least 22%, up to 2.5% titanium, up to 2.5% aluminum, the sum of the titanium plus aluminum not exceeding about 2.5%, up to 0.1% carbon, and the balance essentially iron.

19. The alloy as set forth in claim 18 wherein the sum of the nickel plus molybdenum plus one-tenth the cobalt does not exceed 26% and is not less than about 20%.

20. The alloy as set forth in claim 19 wherein the cobalt content does not exceed about 22%.

21. The alloy as set forth in claim 20 wherein a total of not more than 6% of the following elements are present: up to 1.5% columbium, up to 3% tantalum, up to 0.05% boron, up to chromium, up to 1.5% vanadium, up to 0.15 zirconium, up to 0.25% silicon, up to 0.25% manganese, up to 0.5% beryllium, up to 2% copper and up to 0.1% calcium.

22. The alloy as set forth in claim 21 wherein the carbon, silicon and manganese contents do not exceed 0.05%, 0.15% and 0.15%, respectively.

23. The alloy as set forth in claim 22 wherein the sum of the molybdenum plus cobalt is at least 23% and carbon is present in an amount up to 0.03%.

24. The alloy as set forth in claim 22 wherein the molybdenum is repla-ced by an equal atomic percentage of tungsten up to a maximum tungsten content of 4% such that the sum of the molybdenum plus one-half the tungsten plus the cobalt is at least 23% and the sum of the nickel plus molybdenum plus one-half the tungsten plus one-tenth the cobalt does not exceed 26% and is not less than about 20%.

25. A martensitic, iron-base alloy consisting essentially of from 5% to 16.5% nickel, from 7% to 11% molybdenum, from 8% to 30% cobalt, the sum of the molybdenum plus cobalt being at least 20%, up to 2.5% titanium, up to 2.5 aluminum, the sum of the titanium plus aluminum being not less than 1% nor greater than 3%, up to 0.3% carbon, up to 0.5% silicon, up to 0.5 manganese, and the balance essentially iron.

26. The alloy as set forth in claim 25 wherein the sum of the nickel plus molybdenum plus one-tenth the cobalt does not exceed 27% and is not less than 16%.

27. A martensitic, iron-base alloy consisting essentially of from about 7% to about 15% nickel, about 7% to 10% molybdenum, about 10% to 25% cobalt, the sum of the molybdenum plus cobalt being at least 22%, up to 2.25% titanium, up to 2.25 aluminum, the sum of the titanium plus aluminum being not less than 1.5% nor greater than 2.5%, up to 0.1% carbon, up to 0.5% silicon, up to 0.5 manganese, and the balance essentially iron.

28. An alloy as set forth in claim 27 wherein the sum of the nickel plus molybdenum plus one-tenth the cobalt does not exceed about 26% and is not less than 20%.

29. The alloy as set forth in claim 28 wherein the carbon, silicon and manganese contents do not exceed 0.05%, 0.25 and 0.25 respectively.

30. The alloy as set forth in claim 29 wherein the carbon, silicon and manganese contents do not exceed 0.03%, 0.15% and 0.15%, respectively.

31. A martensitic, iron-base alloy consisting essentially of from 5% to 16.5% nickel, about 11% to 16% molybdenum, about 6% to 30% cobalt, up to 0.3% carbon, up to 1% titanium, up to 1% aluminum, the sum of the titanium plus aluminum not exceeding 1.5 and the balance essentially iron.

32. The alloy as set forth in claim 31 wherein molybdenum is present in an amount of from 12% to 15 the cobalt content does not exceed 22% and the carbon content does not exceed 0.05%.

References Cited UNITED STATES PATENTS 3,093,519 6/1963 Decker et al. -123 X 3,154,412 10/1964 Kasak et a1. 75-126 3,166,406 1/ 1965 Floreen et a]. 75-124 3,243,285 3/ 1966 Fragetta et al. 75-123 3,251,683 5/1966 Hammond 75-128 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE DECORRECTION Patent No. 3,359,094 December 19, 1967 Clarence G. Bieber et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 38, "eflorts" should read efforts Column 2, line 58 "as" should read As Column 4, lines 4 and 7, "01%", each occurrence, should read 0.1% Column 6, line 58, "in" should read is line 72, "skiled" should read skilled Column 7, line 11, "Treamtent" should read Treatment Column 9, line 19, "reflects" should read reflect line 25, "illustrates" should read illustrate Column 11, line 63, before "about" insert above Signed and sealed this 16th day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, J1. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

Claims (1)

11. AN IRON-BASE ALLOY CONSISTING ESSENTIALLY OF FROM 5% TO ABOUT 16.5% NICKEL, ABOUT 7% TO ABOUT 16% MOLYBDENUM, ABOUT 8% TO ABOUT 60% COBALT, THE SUM OF THE MOLYBDENUM PLUS COBALT BEING AT LEAST ABOUT 20%, UP TO 2.5% TITANIUM, UP TO 2.5% ALUMINU, THE SUM OF THE TITANIUM PLUS ALUMINUM NOT EXCEEDING ABOUT 3%, UP TO 1% CARBON, UP TO 2% COLUMBIUM, UP TO 4% TANTALUM, UP TO 0.1% BORON, UP TO 0.25% ZIRCONIUM, UP TO 8% CHROMIUM, UP TO 2% VANADIUM, UP TO 0.5% SILICON, UP TO 0.5% MANGANESE, UP TO 1% BERYLLIUM, UP TO 4% COPPER, UP TO 0.1% CALCIUM, THE TOTAL AMOUNT OF CLOUMBIUM, TANTALUM, BORON, ZIRCONIU, CHROMIUM, VANADIUM, SILICON, MANGANESE, BERYLLIUM, COPPER AND CALCIUM BEING NOT MORE THAN 10%, AND THE BALANCE ESSENTIALLY IRON.