US3347663A - Precipitation hardenable stainless steel - Google Patents

Description

United States Patent 3,347,663 PRECIPITATION HARDENABLE STAINLESS STEEL Clarence George Bieber, Sufiern, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 20, 1967, Ser. No. 617,070

33 Claims. (Cl. 75-124) ABSTRACT OF THE DISCLOSURE Invention directed to stainless steels of the precipitation hardening type which, in addition to iron, contain chromium, nickel, aluminum, titanium, and/or columbium. Manganese, silicon and carbon can also be present as well as other constituents. Chemical composition balanced to provide steels characterized by a combination of both high strength and toughness.

This is a continuation-in-part of applications Ser. Nos. 398,769, now abandoned, and 398,770 filed Sept. 23, 1964.

The present invention relates to stainless steels, and, more particularly, to precipitation hardenable stainless steels which afford a level of strength and toughness characteristics of such magnitude that the steels are eminently suitable for structural applications, e.g., pressure vessels, components for aircraft and the like.

As is well known to those skilled in the art, stainless steels have been classified, in the broadest sense, as being austenitic, martensitic or ferritic. Of the three types, the austenitic variety, particularly the A151 300 series, enjoys the greatest degree of commercial usage. This is readily understandable in view of, inter alia, the outstanding combination of corrosion resistance and ease of fabri cability inherently characteristic of these steels. Nonetheless, the yield strengths thereof are relatively low and, as a general proposition, they do not undergo any appreciable hardening upon being subjected to heat treatment whereby a strengthening effect would be induced. High yield strengths, e.g., up to 200,000 p.s.i. or above, can be attained with respect to the A151 300 series of stainless steels via the application of cold rolling processing, but such techniques can give rise to attendant difficulties. For example, cold rolling is commonly applied in one direction and as a result thereof the mechanical characteristics of the steels generally become, as is known, undesirably anisotropic. Too, the increase in strength is normally achieved at the expense of concomitantly reducing the modulus of elasticity and the latter is an important factor to a designer coping with structural engineering problems.

In contrast to the A181 300 series above mentioned, the AISI 400 series of martensitic stainless steels, e.g., AISI 440, generally manifest hardening capabilities upon heat treatment (quenching followed by tempering) and thus afford strength levels significantly higher than that characteristic of the noncold rolled austenitic stainless steels. For example, the Metals Handbook, 8th Ed. (1961) indicates at page 414 that A151 440 (A) exhibits a yield strength (0.2% offset) as high as 240,000 p.s.i. in the tempered (hardened) condition. This particular grade of martensitic stainless is of high carbon content and to achieve the effectiveness of this potent element quenching must be utilized to obtain the hard martensitic structure. As is the case with the carbon and low alloy steels, quenching operations are no little source of difiiculty and the literature is well doccniented with the problems of quench cracking, distortion, dimensional change, etc., associated 3,347,663 Patented Oct. 17, 1967 with the quench operation. Such steels also invite weldability problems and, where welding is necessary to form a structural component, a postweld quenching treatment is also necessary. Here, too, the problem is rather well documented. I

To the foregoing should be added the important fact that while high strengths can be achieved with the quenched and tempered martensitic stainless steels of the AISI 400 type, the toughness of such steels is quite low, a factor which is the antihesis of good fabricability. The aforementioned Metals Handbook reflects that AISI 440 (A) has a tensile ductility of but 5% and a reduc tion of area of only 20% at the 240,000 p.s.i. yield strength level. (In accordance with the present invention, a reduction in area value of even 30% is considered brittle.) In addition, the Metals Handbook, page 421, reflects that A181 431, for example, has a Charpy V-notch impact strength of about 50 foot-pounds (ft-lbs.) at room temperature but the Rockwell C hardness thereof is only R 2930, i.e., the yield strength thereof is well below 130,000 p.s.i. The yield strength of AISI 431 can be increased to 155,000 p.s.i. but this would be at the expense of impact strength. The same is apposite regarding the other 400 series of A151 steels, e.g., A181 410.

The above discussion is indicative of the fact that a hiatus exists between the A181 300 and A181 400 series of stainless steels. One type is corrosion resistant and relatively easy'to fabricate but is of low strength. The other is characterized by high strength but is afi licted with poor toughness and is thus difiicult to fabricate. Neither type provides a high magnitude of both strength and toughness and this fact is undoubtedly responsible in large measure for the increasing technological activity with regard to the precipitation-hardening stainless steels.

The precipitation-hardening stainless steels can, for convenience, be categorized as falling within three major areas (although there are others). In the first category are those stainless steels which are both martensitic upon cooling from the solution anneal temperature and upon cooling after aging, i.e., they undergo a substantial phase transformation from the austenitic face-centered cubic (f.c.c.) structure to the martensitic body-centered cubic (b.c.c.) structure upon cooling to about room temperature from the annealing treatment. The second class includes those stainless steels which are substantially austenitic upon cooling from the annealing temperature (no substantial phase transformation) but which become substantially martensitic after cooling from the aging treatment. Many of such steels can be transformed prior to aging by the utilization of a conditioning treatment (a heat treatmerit intermediate that of the solution anneal and aging treatments) prior to the final aging step or by the application of a cold treatment, e.g., refrigeration or cold working, or by both conditioning and cold treating. Precipitation-hardening stainless steels which are austenitic steels which are austenitic in both the annealed and aged conditions comprise the third category and thusdonot undergo any substantial martensitic transformation. Common to all three types is an annealing treatment and an aging treatment, although it is to be understood that sundry complex intermediate treatments have been proposed. The present invention is primarily concerned with the first category of precipitatiomhardening Stainless steels, al,

though the invention also encompasses the second class, as will become clearer herein.

With regard to precipitation-hardening stainless steelspart and at least insofar as I am aware, the toughness characteristics of such steels have been lacking in one o more respects. This deficiency has been sufficiently severe so that over-aging techniques have been employed to achieve a better degree of toughness at the expense of strength. Various prior art data reflect, for example, that to raise the tensile ductility of an alloy about, a few percent, say up to about or 12%, has resulted in a considerable drop in yield strength; .Even precipitation-hardening stainless steels with yield strengths on the order of about 150,000 p.s.i. to 200,000 p.s.i., a strength level quite adequate for a host of commercial applications, do not afford, insofar as I am aware and as contemplated herein, an exceptionally high magnitude of toughness, including impact strength.

The term toughness is rather elusive and defies pr cise definition. As used herein in connection with stainless steels exhibiting yield strengths of about 200,000 p.s.i. and above, toughness connotes and includes more than tensile ductility and reduction of area values. It also includes the ability to manifest high notch tensile strengths coupled with the important barometer of a high ratio of notch tensile strength to ultimate tensile strength. Experience has shown that tensile ductility and reduction of area values arrived at from testing smooth specimens are not always an unqualified indicator as to reliability. This stems from the fact that in commercial usage structural components develop crackswhich vmay be internally or externally induced. Too, it is more than possible that the alloy from which the component was formed contained incipient cracks, notches or other flaws.

Notch toughness is a reflection of the ability of a mate rial to yield by plastic fiow to localized stress. As is known,

a crack, notch, or other flaw is an initiating or focal point of self-propagation; It has been established and is generally accepted that stresses tend to concentrate at such points whereby a localized stress concentration is induced. If a material is sufliciently resistant to the propagation of thefiaw, i.e., if it is sufliciently self-yielding, it

is considered-as being notch-ductile; if not, it is deemedv notch-sensitive, i.e., it is proneto the development of deleterious brittle fracture or brittle failure characteristics.

The above-mentioned brittle failure problem occurs in many materials notwithstanding that the yield strength, tensile ductility, and reduction of area of smooth specimens of the material are otherwise acceptable. The propagation of. the flaw leading to brittle fracture can be induced'by a number of factors, including the. heat treatment applied to the material. In addition and of considerable importance is the strength of the material. It is known that with an increasing magnitude of yield and tensile strengths the smaller becomes the minimum size of a flaw which can cause or promotesubsequent brittle fracture. Thus, even relatively small flaws must be taken into consideration. Put another way, where dealing with yield strengths of, say 100,000 p.s.i. or 150,000 p.s.i., the problem is not nearly as severe as is the case where yield strengths of 200,000 p.s.i. and above are involved. The carrying out of the notch-tensile test is well known and is not dwelt upon herein; however, in accordance with the present invention the steels of highest strength contemplated herein, i.e., steels of 200,000 p.s.i. and above, must manifest a ratio of notch-tensile strength to ultimate tensile strength of at least unity (the notch acuity factor, K being 10 or greater) to be classed as being notch-ductile. Advantageously, the ratio is at least 1.2.

From the foregoing, it is clear that it would be desirable to have at hand steels which afford the above-described combination of corrosion resistance, ease of fabrication and high strength together with good toughness, including notch toughness. In terms of strength and toughness and in accordance with the invention, where the emphasis concerns steels manifesting the highest levels of strength, i.e., yield strengths of over 200,000 p.s.i. (0.2% offset), e.g., a yield strength of about 210,000

4- p.s.i. or 215,000 p.s.i. and. above, this requires a tensile ductility of at least 10% (standard gage measurements), a reduction of area of at least 40% and a high notchtensile strength and a ratio of notch-tensile strength to ultimate strength of at least unity (bar stock) and advantageously of at least 1.1. Where emphasis is in regard to steels characterized by yield strengths of about 140,000 p.s.i. .to 200,000 p.s.i., e.g., a yield strengthof 150,000.

p.s.i. to 190,000-p.s.i, a tensile ductility of at least 10% (preferably at least 12%) is required together with a reduction of area of at least 40% (advantageously at least 50%), and, most importantly, a notch impact strength (bar stock) of at least 50 ft.-lbs. (most desirably at least ft.-lbs.). The present invention is directed to accomplishing these objectives.

It is noteworthy of mention that in attaining the foregoing objectives processing operations should be simple to. minimize cost as Well as to obviate processing difficulties. Thus, the optimum would be the ultilization of a simple solution annealing treatment and aging treatment without the necessity of recourse to various cold treatments and/ or intermediate conditioning treatments as will be discussed hereinafter. Further, a number of recent proposals relatingto achieving high strengthinprecipitation-hardening stainless steels have advanced the necessity of using substantial amounts of elements such as.

cobalt. Such constituents are not inexpensive and, thus, measurably contribute to the overall cost of the final stainless steel product.

It has now been discovered that by a judicious balance of special chemical constituents including chromium, nickel, aluminum, manganese, silicon, carbon and at least oneelement selected from the group consisting of columbium and titanium, precipitation-hardening stainless steels possessing the aforemention combinations of characteristics can be attained with a simple, expedient, and inexpensive heat treatment.

It is a primary object of the present invention to provide-novel precipitation-hardening stainless steels.

Another object of the invention is to provide precipitation-hardening stainless steels which exhibit an improved combination of corrosion resistance, fabrication quality, high yield strength and good toughness;

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

Generally speaking, and in accordance with the invention, precipitation-hardening stainless steelscontemplated herein consist essentially (by weight) of from about 11.5% to 15.5% chromium, about 9% to about 12% nickel with the proviso that the sum. of 0.8 times the chromium content plus the nickel contentis at least about 19% or 19.5%, e.g., 19.75%, anddoes not exceed about 22%, e.g., 21.5%, at. least one metal selected from the group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.05% to 1% and preferably from.

carbon in an amount up to about 0.03%, e.g., about 0.005% to 0.02% carbon, up to about 0.2% manganese,

up to about 0.2% silicon, and the balance essentially iron.

In referring to the iron content as constituting the balance or essentially the balance of the stainless steels, it is to be understood, as will be readily appreciated by those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the steels. In this connection, elements such as sulfur, phosphorus, hydrogen, oxygen, nitrogen and the like should be kept at levels as low as is consistent with commercial steelmaking practice. Other elements which can be present in the steels include the following: up to 0.5% vanadium, up to 1% tantalum, up to 0.5 copper, up to 0.1% beryllium, up to 0.01% boron and up to 0.05% zirconium. As mentioned hereinbefore, cobalt is not essential and can be kept to impurity levels. Of course, it is common to also use elements, such as calcium, cerium, etc., in amounts up to 0.1%, e.g., 0.02% or 0.03% calcium, or 0.05% or 0.06% cerium, for purposes of malleabilization, desulfurization, etc.

In achieving an optimum combination of high strength and good toughness, e.g., a yield strength of about 215,- 000 p.s.i. and above, a tensile ductility of at least a reduction of area of over 45%, e.g., about 50% or more, a notch-tensile to ultimate tensile ratio of 1.1 and above, it is most advantageous that the steels be of a composition falling within the following ranges: about 11.75% to chromium, from 9% to 11% nickel, with the proviso that the sum of 0.8 times the chromium content plus the nickel content be from to 22%, at least one element selected from the group consisting of titanium and columbium, the titanium being from about 0.2% to about 0.35% and the columbium being from about 0.2% to about 0.5 about 1.1% to about 1.5% aluminum with the proviso that the sum of the aluminum plus any titanium does not exceed about 1.8%, up to 0.03% carbon, up to 0.15 manganese, up to 0.15% silicon with the sum of the manganese and silicon not exceeding 0.25 and the balance essentially iron.

Where exceptionally high impact strength is of the essence, a Charpy V-notch impact energy level of about 60 ft.-lbs. or higher, e.g., 100 ft.-lbs. and above, and yield strengths down to 140,000 p.s.i., e.g., 160,000 p.s.i. to 190,000 p.s.i. are satisfactory, it is most advantageous that the steels be of a composition falling within the following ranges: about 11.75 to 15 chromium, from 9% to 11% nickel, with the proviso that the sum of 0.8 times the chromium content plus the nickel content be from 20% to 22%, at least one element selected from the group consisting of titanium and columbium, the titanium being from about 0.2% to 0.35% and the columbium being from about 0.2% to about 0.5 about 0.5% or 0.6% to about 0.9% aluminum with the proviso that the sum of the aluminum plus any titanium does not exceed about 1.3%, up to 0.03% carbon, up to 0.15% manganese, up to 0.15% silicon with the sum of the manganese and silicon not exceeding 0.25%, and the balance essentially iron. Such alloys also atford tensile ductilities of at least 12% and reductions of area of over e.g., about or more. The ability to absorb still greater amounts of impact energy can be rather substantially increased with steels containing lower amounts of aluminum, i.e., aluminum contents down to about 0.2%, an aluminum range of 0.2% to 0.4% or 0.45% being particularly suitable.

The chromium content of the steels should not fall below about 11.5% and preferably should be at least 11.75%; otherwise, the basic purpose of maintaining a high degree of corrosion resistance for which stainless steels are noted would be seriously impaired. On the other hand, with chromium contents much above the maximum specified herein, the property characteristics of the steels are adversely affected and/ or unnecessary heat treatments might be otherwise necessary, e.g., intermediate conditioning treatments. For example, should the total nickel content plus 0.8 times the chromium content exceed about 22%, conditioning treatments (heat treating in the austenitic condition) might well be necessary prior to the final aging treatment and subsequent to the solution annealing treatment in order to effect a full transformation from the face-centered cubic structure to a body-centered cubic structure. That is to say, after solution annealing at a temperature of about 1450 F., e.g., 1500 F. or 1600 F., to 1800" F., it would be necessary to heat treat the steels at some temperature below that of the initial solution anneal temperature, e.g., at about 1100 F. to 1350 F. to precondition the steels such that a transformation to martensite will eventually take place. Even then a cold treatment as by refrigeration at a low temperature or cold working may be necessary to effect a maximum degree of transformation prior to aging. Such additional preconditioning heat treatments, insofar as the present invention is concerned, are unnecessary and add to the overall cost of the steels without conferring any benefit in comparison with steels within the invention. Such heat treatments can also promote the formation of precipitates in the austenite grain boundaries and this detracts from the ductility and corrosion resistance characteristics of the steels.

The amount of nickel should be at least 9% in order to obtain a combination of good strength and toughness. Excessive amounts of nickel promote the retention of austenite on aging and also undesirably narrow the range of aging temperature both from the view of retained austenite and overaging. Thus, it is advantageous that the nickel content not exceed 12%. Further, as indicated above herein, the ratio of nickel to the sum of aluminum plus titanium must be at least 5 to 1 to insure good toughness characteristics.

Aluminum is the element most responsible for the precipitation-hardening effect. If excessive amounts of aluminum are present, e.g., 2% to 3%, notch toughness is adversely affected and other toughness characteristics can be detrimentally impaired. With aluminum contents below about 1%, the highest yield strengths are not obtained; however, as indicated above herein, an aluminum range of about 0.2% to 0.4% or 0.45% imparts maximum resistance to impact and an aluminum range of about 0.5 to 1% affords an excellent combination of mechanical characteristics.

Carbon should be maintained at the lowest possible levels. High amounts thereof, apart from promoting intergranular corrosion, drastically reduce the M M transformation range and impair toughness characteristics. The formation of chromium carbides might effect an increase in the M temperature but it is the undesirable precipitation of chromium carbides during heat treatment which can impair toughness. Further, upon aging, opposed reactions would be involved. That is to say, with'relatively high carbon contents the carbon would react in both a tempering and hardening manner. Unstabilized carbon would per se confer a hardening reaction upon cooling from the solution anneal treatment and this hardening reaction would be tempered (lower strength and softer material) upon aging during the precipitation hardening treatment. Thus, the carbon content should be kept at a low order of magnitude and preferably not above 0.02%. It should be mentioned that carbon essentially has no substantive relation with regard to such elements as nickel and manganese of the steels contemplated herein. Carbon cannot (nor, for that matter, can manganese) be used to replace nickel; carbon simply must be maintained at a low level.

The amounts of silicon and manganese should also be kept to a minimum; otherwise, toughness can be adversely affected. For example, silicon and manganese levels of, say, 0.5 or above, detrimentally atfect the notch ductility of the steels. It is most advantageous that the total amount of these elements does not exceed 0.25%. It is most preferred to keep these elements at a level of not more than about 0.1%, respectively; however, this is difficult to consistently achieve commercially because of pickup of these elements from raw materials, slags, refractories, etc.

The use of columbium is beneficial, particularly where optimum strength combined with optimum toughness characteristics are necessary. While the columbium can be present up to 1%, it is preferred that not more than 0.5 columbium be present. This constituent will preferentially combine with carbon and preclude the precipitation of detrimental chromium carbides in the grain boundaries during aging, thereby enhancing toughness and corrosion characteristics. Columbium is particularly effective in contributing to an optimum combination of toughness and high yield strength levels, e.g., 230,000 psi. and above. In respect of various alloys tested, columbium has been found to markedly improve both strength and toughness. The amount of titanium should not exceed 0.5% and advantageously does not exceed 0.35%; otherwise, segregation and other problemsarise in processing the steels. As between titanium and columbium, the latter is more advantageous.

In carrying the invention into practice, air melting may be utilized, preferably followed by consumable electrode melting for optimum effects. Use of materials of good purity is advantageous. In processing, initially formed cast ingots should be thoroughly homogenized as by soaking at temperatures of about 2200 F. to about 2250 F. for about one hour per inch of crosssection followed by hot working (forging, pressing, rolling, etc.) and, if desired, cold working to desired shape. A plurality of heating and hot working operations are desirable for purposes of assuring thorough homogenization of the cast structure through diffusion. Satisfactory hot working temperatures include 1800 F. to 2000? F. with suitable finishing temperatures being about 1600 F. to 1500 F. Subsequently, the steels are solution annealed at temperatures of, about 1450" F., e.g., 1500 F. or 1600 F., to about 1800 F. for about hour to several hours, depending. upon section size. In the production of sheet or strip, short annealing periods, e.g., 10 minutes, can be employed.

Following the solution annealing treatment the steels are cooled, e.g., air cooled. No liquid quench is necessary and, thus, the attendant difiicultiesasociated therewith are obviated. As a result of solution annealing treatment followed by cooling, the steels transform to the martensitic or substantially martensitic condition or can be readily transformed thereto by subjecting the steels to a cold treatment as'by refrigerating the steels at a low temperature; e.g.,' minus 100 F. or below, or by cold working the steels. Both refrigerating and cold working can be utilized if desired. However, an attribute of the steels of the invention having a combined chromium plus nickel content of not more than about 23% is that refrigeration and/or cold working is unnecessary. This aspect minimizes cost and processing difficulties. In the solution annealed condition, the steels are quite ductile and are characterized by a Rockwell C hardness of above about R 20 or R 25 to about R 35. Thus, the steels can be fabricated to shape before aging.

Subsequent to the solution anneal and cooling treatment, the steels are then aged in the martensitic condition to effect the high levels of strength. The aging treatment comprises heating the steels to a temperature of about 800 F. to 1000 F. for about A to 4 hours, the longer aging periods being used in conjunction with the lower aging temperatures. Aging at 900 F. to 950 F. for about 1 to 4 hours has been found very satisfactory. Tempera-.

tures on the order of about 1100 F. or higher should not be utilized since reversion to austenite and/or overaging can occur with adverse consequential effects, e.g., loss of strength. Further, in accordancewith the invention the Rockwell C hardness levels of the steels must be less than identified in Table 1, Alloys 1 through 19 being within.

the invention and Alloys A through R being outside the scope thereof. The steels were either melted in a vacuum induction furnace or standard air melting practice techniques were employed as indicated at the bottom of Table I.

TABLE I.COMPOSITION 1 Alloy Cr, percent Ni, percent ALpercent Ti,percent Cb, percent Fe, percent 14 10 1.3 0.3 N.A. Bal. 15 9 1. 5 0.3 0. 5 Bal. 14 10 1.3 0. 3 0.5 Bal. 14 10 1. 5 0.3 N.A. B21. 12 11 1. 3 N.A. 0.5 1331. 12 11 1. 3 0.3 N.A. Bal. 14 10 1.5 0. 3 0.5 Bal. 15 9 1.3 0.3 N.A. Bal. 14 9 1.3 0.3 0.5 13211. 14 9 1.5 0.3 0.5 Ba]. 12 12 0.8 0.2 N.A. Del. 12 11 0.8 0.2 N.A. Bal. 12 11 0. 8 N.A. 0.5 Bel. 12 11 0.5 N.A. 0.5 Bal. 11. 5. 10. 5 0.5 N.A. 0. 5 Bal. 11.5 10.5 0.3 N.A. 0.5 Bal. 11.5 10.5 0.3 N.A. 0.5 Ba]. 11. 5 10. 5 0.3 N.A. 0.5 Ba]. 11. 5 10.5 0.3 N.A. 0.5 B31. 14 10 3 N.A. N.A. Bal. 12 12 3 N.A. N.A. Bal. 12 12 3 N.A. N.A. Bal. 12 12 N.A. 3 N.A. Bal. 12 12 1 3 N.A. B21. 14 12 1 3 N.A. Bal. 14 11 1 2. 5 N.A. B211. 12 10 1 2. 5 0.5 Bal. 14 11 1 2 N.A. Bal. 12 12 1 2 N.A. Bal. 16 9 1 2 0.5 B31. 10 14 1 3 N.A. Bill. 10 12 1 3 N.A. Bat. 12 13 1. 3 0. 3 N.A. Bal. 15 12 1.1 0. 3 N.A. Bal. 16 10 1.3 0.3 N.A. Bal. 16 10 1. 3 0.3 0.5 Bat. 16 10 1.5 0.3 0.5 Bal.

3 0.005 Li added.

4 Deoxidized with 0.17 silicon, All others deoxidized with 0.15 calcium-s'li N.A.-None added. a 1 con Upon solidification, the cast ingots, 4 inches'by 4 inches, were soaked at about 2250 F. to effect good homogenization, forged to billets 2 inches by 2 inches square and thereafter reheated to 1800 F. and hot rolled to rods about 78 inch in diameter and about 12 to 15 feet long. The alloys were then machined to tensile specimens having a reduced section of about 0.25 inch diameter. Notch specimens (Alloys 1 through had a V-notch of about 60, the radius being 0.0005 inch or less with the acuity factor, K being 10 or greater. Notch acuity factors of less than 10 are not considered adequate to provide sufficiently severe test conditions.

Subsequent to cooling from the hot working temperature the steels were heat treated prior to being subjected to test. A number of different heat treatments were used and are set forth below. Certain of the heat treatments include a conditioning treatment (heat treating in the austenitic condition), a step which was found necessary to effect a substantial martensitic transformation or which was used to obtain a better understanding of the behavior of the steels to various heat treatments. In a few instances, an overaging treatment was employed (heat treatment C below) to determine if toughness characteristics could be improved to a satisfactory level with regard to otherwise unsatisfactory alloys.

H eat treatment A (3) Aged at about 900 F. for about 1 hour.

Heat treatment B 1) Steps 1 and 2 sameas in A. 1 (2) Aged at about 1000 F. for about 1 hour.

Heat treatment C (1) Steps 1 and 2 same as in A. (2) Aged (overaging) at about 1050 F. for 1 hour.

Heat treatment D (1) Solution treated at about 1800 F. for about 1 hour and air cooled. (2) Aged at about 900 F. for about 4 hours.

Heat treatment E F. (Dry Ice) for Heat treatment F (1) Solution treated at about 1600 F. for about 1 hour and air cooled.

(2) Heated at a temperature of about 1300 F. for about 4 hours and cooled.

(3) Refrigerated at minus F. (Dry Ice) for about 16 hours.

(4) Aged at 900 F. for about 4 hours.

1 Conditioning treatment.

TAB LE II Rockwell Hardness, Ito-Treatments Alloy Conditioning Refrigeration H.T.Heat, Treatment.

N.A.-Not available.

N.D.Not determined.

1 Annealed at 1,900 F. for 4 hours.

1 hour rather than 1,600 F. and aged at 950 F. for 1 hour rather than 900 F.

for

2 Annealed at 1,900 F. for 1 hour rather than 1,600 F.

H eat treatment G (1) Same as step 1 of heat treatment P. (2) Heated at a temperature of about 1200 F. for about 24 hours and cooled.

(3) Steps 3 and 4 same as in heat treatment P.

1 Conditioning treatment.

TABLE III Alloy H.T Y.S U.T.S., 131., 11.11., N.1.S., N.T..S./ C.VN., K 5.1 K s.i. Percent Percent K 5.1. U.T.S. it.-lbs.

A 213 220 10 48 327 1. 53 A 215 225 14 57. 5 313 1.39 A 214 224 12 60 316 1. 41 B 217 225 12 52 312 1. 38 D 220 233 13 53. 5 324 1. 47 D 222 232 13 49 328 1. 48 A 226 235 12 51 260 1.105 A 226 233 13 56 339 1. 5 A 232 236 11 57 331 1.43 A 247 250 11 49 324 1. 3 D 147. 7 162 24 70 .T 98 149 29 71. 5 D 193.8 201. 8 15 62 J 181. 9 192. 6 18 65. 5 D 188, 5 195.6 15 65. 5 J 164. 7 176. 22 66. H 196. 4 199. 4 70 H 186. 9 187. 1 17 64. 5 H 157. 3 157.3 18 75. 5 H 163 163. 6 16 69 H 182. 8 183.4 14 64 H 176. 2 177. 4 16 72 B 233 252 3 6 (J 215 233 16 47 B 241 253 6 17v 5 C 199 220 11 27. 5 E 39. 5 86. 5 47 84 E 120 175 22 61. 5 E 52 117 42 66.5 E 76 147 27 62. 5

H.T.Heat Treatment.

Heat treatment H (1) Solution treated at 1500 F. for about 1 hour and air cooled.

(2) Refrigerated at minus 100 F. (Dry Ice) for 16 hours.

(3) Aged 4 hours at 900 F.

about H eat treatment J (1) Solution treated at 1800 F. for 1 hour and air cooled. (2) Aged 4 hours at 1000 F.

sults are set forth in Table III. It is noteworthy of mention that many of the steels outside the invenion were so lacking in a particular characteristic that furthertesting thereof was not conducted. This is particularly apropos where it was considered that a conditioning treatment was required for the mere purpose of obtaining transformation from the face-centeredcubic phase tothe sub stantially body-centered cubic structure. In the absence of a conditioning treatment, such steels were of very low strength and of no significance. By the same token, the fact that such steels had to be subjected to a conditioning treatment rendered them more costly .and the property levels generally were below those characteristic of steels within the invention. It should also be mentioned that none of the steels was cold worked prior to or after aging. Cold working would enhance the property characteristics of the steels, but would interfere with the attaining of a good analysis of results as a reflection of composition and heat treatment.

In Table III the yield strength (Y.S., 0.2% otfset) is given in thousands of pounds per square inch (K:s.i.) as

The data in Tables I, II and III illustrate the adverse effects to be expected with alloys outside the invention. For example, Alloys A, B and C which nominally contained 3% aluminum and in which the ratio of nickel. to the sum of aluminum plus titanium was less than 5 to 1 manifested extremely poor toughness characteristics as illustrated by Alloy A. With heat treatment B, the yield strength of Alloy A is good but the tensile elongation is very low. Heat treatment C applied to Alloy A indicates that over-aging .(1O50v F.) is not necessarily a panacea to achieving a high levelof toughness as is shown by the poor, ratio of notch tensile to ultimate tensile strength, notwithstanding that tensile ductility was increased. The same is also shown by heat treatments B and C regarding Alloy H which contained about 2.5% titanium, an amount well outside the scope of the invention. In the averaged condition, the yield strengths of Alloys A and H dropped about 18,000.p.s.i. and 40,000 p.s.i., respectively. The Rockwell hardness values of Alloys B and C (Rockwell hardness of R 52 and above after cooling from the aging temperature of 900 F.) indicated that the notch toughness characteristics of these alloys would be unsatisfactory and further testing was not conducted. As indicated above herein, in the aged condition (aging followed by cooling) the Rockwell C hardnesses of the alloys within the invention are less than R 50, e.g., about R 42 to 48.

While Alloys D, E, F and G, as in the case of Alloy H, also contained too much titanium, it can be seen from the low Rockwell hardness data given for each. of these alloys under the Solution Treatment column of Table II that a high amount of austenite was present in the steels upon cooling from the solution treatment, i.e., a good. amount of austenite did not transform to martensite..A conditioning treatment was necessary, as reflected by Alloys D, E and G, simply to obtain a high hardness level. In fact, Alloy F, which nominally contained 14% chromium, 12% nickel and 3% titanium, was substantially (if not completely) austenitic even after both a conditioning and a refrigeration treatment (heat treatment P) were employed as is rather evident from the low Rockwell C hardness of only 1-8 in the aged condition (Table II);

The sum of 0.8 times the chromium content plus, the nickel content of Alloy F equals a factor of about 23.2% which is too high. While the resulting factor is within the invention for Alloys A, B, C, D and E, for example, either the aluminum or titanium is far too high. On the other hand, with aluminum and titanium contents within the invention, as illustrated by Alloys N through R, the factor of 0.8 times the chromium content plus the nickel content was sufiiciently above the maximum specified herein such that very poor results were obtained. While each of these alloys was substantially austenitic after cooling from the solution treatment, it will be noted that the yield strengths of Alloys N, P, Q, and R (Table III) are not only very low but there is a considerable spread between the respective yield strengths and ultimate tensile strengths thereof, e.g., there is a spread of about 50,000 p.s.i. for Alloy P. This is indicative that these alloys contained a large amount of retained austenite after aging, notwithstanding the relatively high hardness obtained in the aged condition. Thus, hardness in and of itself, while important, is not the sole controlling factor. It should be mentioned that alloys contemplated within the invention do not contain deleterious amounts of delta ferrite.

In contrast to the alloys outside the invention, Alloys 1 to 19 formulated in accordance with the invention all manifested a satisfactory level of properties. Alloys 6 and 7 and 11 through 13 show that a high level of properties can be obtained without any cold treatment, e.g., refrigeration and/or cold working, whatever. This aspect is quite beneficial where it is necessary to form large vessels. To form such vessels a welding treatment would be necessary and this, in turn, would require a subsequent solution and aging treatment to restore the properties of the parent metal contiguous to the weld zone. Obviously it would be quite desirable, if possible, to avoid subjecting such vessels to a refrigeration treatment prior to aging in view of the impracticality of so doing. Thus, in accordance with the invention condition treatments are not only unnecessary but utilization of refrigeration techniques can be avoided where desired.

The results in respect of Alloys 11 through 19 illustrate that exceptionally high levels of impact strength, e.g., well over 100 ft.-lbs., can be obtained in accordance with the invention together with yield strengths of well above 150,000 p.s.i. The results of Alloy 11 which had a nominal chromium plus nickel content of 24% indicate that in the absence of employing a cold treatment, e.g., refrigerating, the chromium plus nickel contents should be less than 24% if higher strength is desired, as is reflected by Alloys 12 and 13 wherein the nominal combined chromium plus nickel was 23%. The low yield strength of Alloy 11 (subjected to heat treatment H) is considered to be due to reversion austenite, i.e., with a chromium plus nickel content of about 24%, an aging temperature of 1000 F. was, in essence, an overaging treatment whereby martensite transformed back to austenite. This effect would be greatly diminished by the application of a cold treatment prior to aging; however, even a cold treatment is unnecessary by maintaining the total chromium plus nickel content at a level not greater than 23.5% and advantageously at a level of not greater than about 23%. Alloys 14 through 19 reflect the outstandingly high impact strengths obtainable with aluminum contents of 0.5% and below particularly in combination with columbium. Alloys 16 and 17 offer a comparison of results obtained from vacuum processing and air melting, respectively.

The alloys, as noted before herein, are not only martensitic but are, as a practical matter, devoid of retained austenite. Of course, if deemed expedient for some particular application, the alloys can be overaged and in this case some amount of martensite may revert to austenite; however, the austenite content should not exceed Alloys containing chromium and nickel (the sum of 0.8 times the chromium content plus the nickel content being from at least about 20% to about 21.5%)

in the ranges of 13% to 15% chromium and 9% to 9.75% nickel or 11.75% to 13% chromium and 10% to 11.5% nickel, together with 1.1% to 1.5% aluminum, 0.1% to 0.35% titanium, the sum of the aluminum plus titanium not exceeding 1.8%, 0.2% to about 0.5 columbium, up to 0.03% carbon, up to 0.15% manganese and up to 0.15 silicon afford an extremely good combination of yield strength (220,000 p.s.i. and above), notchtensile strength and a high ratio (at least 1.1) of notchtensile to ultimate strength.

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.

I claim:

'1. A precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 11.5 to 15.5% chr0- mium, about 9% to about 12% nickel with the proviso that the sum of 0.8 times the chromium content plus the nickel content is at least about 19.5% and not more than about 22%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.05% to 1%, about 0.2% to 1.6% aluminum with the proviso that the sum of the aluminum and any copresent titanium does not exceed 1.9% and with the further proviso that'the ratio of nickel to the sum of aluminum and any copresent titanium is at least 5 to 1, carbon in an amount up to 0.03%, up to 0.2% manganese, up to 0.2% silicon, up to 0.5% vanadium, up to 1% tantalum, up to 0.5% copper, up to 0.1% beryllium, up to 0.01% boron, up to 0.05% zirconium and the balance essentially iron.

2. An alloy in accordance with claim 1 in which the aluminum content is at least about 0.5 and up to 1% and the sum of the aluminum and any copresent titanium does not exceed 1.4%.

3. An alloy in accordance with claim 1 in which the aluminum content is at least about 1%.

4. An alloy in accordance with claim 1 in which the aluminum content does not exceed 0.45%. v

5. An alloy in accordance with claim 1 in which columbium is present in an amount of at least 0.1%.

6. An alloy in accordance with claim 1 in which titanium is present in an amount of at least 0.2%

7. An alloy in accordance with claim 1 in which titanium is present in an amount of at least 0.2% and columbium is present in an amount of about 0.1% to 0.5

8. An alloy in accordance with claim 1 in which the chromium plus nickel does not exceed about 23.5%.

9. An alloy in accordance with claim 1 in which the chromium plus nickel does not exceed 23%.

10. An alloy in accordance with claim 3 in which the sum of 0.8 times the chromium content plus the nickel content does not exceed 21.5% and is not less than 19.75%.

11. An alloy in accordance with claim 10 wherein the manganese and silicon contents are not greater than 0.15% each and the sum of the manganese and silicon contents is not greater than about 0.25%.

12. An alloy in accordance with claim 11 wherein the aluminum content is at least 0.6% and the titanium content does not exceed 0.35%.

13. An alloy in accordance with claim 11 wherein the aluminum content is at least 1.1% and the titanium content does not exceed 0.35

14. A precipitation hardenable stainless steel characterized by a yield strength (0.2% offset) of at least about 215,000 p.s.i., a tensile ductility of at least about 10%, a reduction in area of at least about 45% and a ratio 15 of notch tensile strength to ultimate tensile strength of at least unity, said steel consisting essentially of a composition falling within the following ranges: about 11.75% to about 15 chromium, about 9% to 11% nickel with the proviso that the sum of 0.8 times the chromium content plus the nickel content is at least 20% and not greater than 22%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from 0.2% to 0.35% and the columbium being from about 0.1% to about 0.5%, about 0.2% to about 1.5% aluminum with the proviso that the sum of the aluminum and any copresent titanium does not exceed 1.8%, carbon in an amount up to 0.03%, up to 0.15% manganese, up to 0.15% silicon, and the balance essentially iron.

15. An alloy in accordance with claim 14 in which the aluminum content is at least about 0.6% and up to about 0.9%.

16. An alloy in accordance with claim 14 in which the aluminum content is at least 1.1%.

17. An alloy in accordance with claim 14 in which the aluminum content does not exceed 0.4%.

18. An alloy in accordance with claim 14 in which columbium is present in an amount of at least 0.2%.

19. An alloy in accordance with claim 14 in which titanium is present.

20. An alloy in accordance with claim 14 in which titanium and columbium or both are present.

21. An alloy in accordance with claim 14 in which the chromium plus nickel does not exceed about 23%.

22. The alloy as set forth in claim 15 wherein the sum of 0.8 times the chromium content plus the nickel content does not exceed 21.5%..

23. The alloy as set forth in claim 22 wherein the sum of the manganese and silicon contents is not greater than 24. The alloy as set forth in claim 23 wherein the carbon content does not exceed about 0.02%.

25. A precipittaion hardenable stainless steel characterized by a good combination of strength and toughnessand consisting essentially of about 13.5% to about 15% chromium, about 9% to 9.75% nickel, at least one element selected from the group consisting of titanium and columbium, with the titanium content being about 0.1% to 0.35% and the columbium content being about 0.2% to about 0.5%, about 0.2% to 1.5% aluminum with the proviso that the sum of the aluminum plus any copresent titanium being not greater than 1.8%, carbon in an amount up to. about 0.03%, up to 0.15% manganese, up to 0.15 silicon, and the balance essentially iron.

26. The alloy as set forth in claim 25 in which both titanium and columbium are present within the ranges specified.

27. An alloy in accordance withclaim 25 in which the aluminm does not exceed 0.4%.

28. An alloy in accordance with claim 26 in which the aluminum does not exceed 0.4%.

29. A precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 11.75%to about 13% chromium, about 10% to 11.5% nickel, at least one element selected from the group consisting of titanium and columbium, with the titanium content being about 0.1% to 0.35% and the columbium content being about 0.2% to about 0.5%, about 0.2% to 1.5% alumium with the proviso that the sum of the aluminum plus any copresent titanium being not greater than 1.8%, carbon in an amount up to about 0.03%, up to 0.15% manganese, up to 1.15% silicon,and the balance essentially iron.

30. The alloy as set forth in claim 29 in which both titanium and columbium. are present within the ranges- 31. An alloy. in accordance with claim 29 in which the ized by a good combination of strength and toughness and consisting essentially ofabout 11.5% to 15.5% chromium, about 9% to about 12% nickel with the proviso that the sum of 0.8 times the chromium content plus the nickel content is at least about 19% and not more than about 22%, at leastone metal selected from the. group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.05% to 1%, about 0.2% to 1.6% aluminum with the proviso that the sum of the. aluminum and any copresent titanium does not exceed 1.9% and with the further proviso that the ratio of nickel to the sum of aluminum and any copresent titanium is at least 5 to 1, carbon in an amount up to 0.03%, up to 0.2% manganese,

up to 0.2% silicon, up to 0.5% vanadium, up to 1% tantalum, up to 0.5% copper, up to 0.1% beryllium, up to 0.01% boron, up to 0.05% zirconium and the balance essentially iron.

References Cited UNITED STATES PATENTS 1,538,337 5/1925 Kuehn -l28. 2,738,267 3/1956 Pakkala. 75--128 X 2,999,039 9/1961 Lula 75-124 X 3,117,861 1/1964 Linnert et a1. 75-124 3,151,978 10/1964- Perry et a1. 75--l24 3,262,777 7/1966 Sadowski 75128 X 3,262,823 7/1966 Sadowski 75128 X 3,278,298 10/1966 Perry 75128 FOREIGN PATENTS 465,916 5/1937 Great Britain.

DAVID L. RECK, Primary Examiner.

F. WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,347,663 O b 17 1967 Clarence George Bieber It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 71, for "doccmented" read documented column 2, line 10, for "antihesis" read antithesis column 11, line 56, for "invenion" read invention column 15, line 38, for "precipittaion" read precipitation line 54, for "aluminm read aluminum column 16, line 12 for "1 .l5%" read 0 15% Signed and sealed this 11th day of February 1969.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

Claims (1)

1. 33. A PRECIPITATION HARDENABLE STAINLESS STEEL CHARACTERIZED BY A GOOD COMBINATION OF STRENGTH AND TOUGHNESS AND CONSISTING ESSENTIALLY OF ABOUT 11.5% TO 15.5% CHROMIUM, ABOUT 9% TO AOBUT 12% NICKEL WITH THE PROVISO THAT THE SUM OF 0.8 TIMES THE CHROMIUM CONTENT PLUS THE NICKEL CONTENT IS AT LEAST ABOUT 19% AND NOT MORE THAN ABOUT 22%, AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND COLUMBIUM, THE TITANIUM BEING FROM ABOUT 0.1% TO NOT MORE THAN 0.5 AND THE COLUMBIUM BEING FROM 0.05% TO 1%, ABOUT 0.2% TO 1.6% ALUMINUM WITH THE PROVISO THAT THE SUM OF THE ALUMINUM AND ANY COPRESENT TITANIUM DOES NOT EXCEED 1.9% AND WITH THE FURTHER PROVISO THAT THE RATIO OF NICKEL TO THE SUM OF ALUMINUM AND ANY COPRESENT TIATNIUM IS AT LEAST 5 TO 1, CARBON IN AN AMOUNT UP TO 0.03%, UP TO 0.2% MANGANESE, UP TO 0.2% SILICON, UP TO 0.5% VANADIUM, UP TO 1% TANTALUM, UP TO 0.5% COPPER, UP TO 0.1% BERYLLIUM, UP TO 0.01% BORON, UP TO 0.05% ZIRCONIUM AND THE BALANCE ESSENTIALLY IRON.