US3556776A

US3556776A - Stainless steel

Info

Publication number: US3556776A
Application number: US585298A
Authority: US
Inventors: William C Clarke Jr; D Cameron Perry
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1963-08-02
Filing date: 1966-10-10
Publication date: 1971-01-19
Anticipated expiration: 1988-01-19
Also published as: BE651249A; SE316023B; BE697441A; GB1077979A; DE1608097A1; DE1608097B2; DE1608097C3; DE1458330C3; GB1138098A; DE1458330B2; DE1458330A1

Abstract

PRECIPITATION - HARDENABLE CHROMIUM-NICKEL-ALUMINUMMOLYBDENUM STAINLESS STEEL WITH CRITICALLY LIMITED AMOUNTS OF CARBON, SULPHUR AND NITROGEN, OF SUCH COMPOSITION BALANCE AS TO BE MARTENSITIC IN THE SOLUTION-TREATED CONDITION AND PRECIPITATION-HARDENABLE THEREFOROM BOTH BY SINGLE HEAT-TREATMENT AND BY DOUBLE HEAT-TREATMENT, I.E., A SINGLE HEATING AT PRECIPITATION-HARDENING TEMPERATURE (900*F. TO 1150*F) OR A FIRST HEATING AND QUENCHING FROM SUBSTANTIAL TEMPERATURE (1300*F TO 1750*), FOLLOWED BY HEATING AT PRECIPITATION-HARDENING TEMPERATURE AS IN HEATTREATING A FABRICATED COMPOSITE STRUCTURE AS REQUIRED BY THE OTHER METALS IN THE STRUCTURE. MORE ESPECIALLY, THE STEEL ESSENTIALLY CONSISTS OF ABOUT 11.5% TO 13.5% CHROMIUM, ABOUT 7.0% TO 10.0% NICKEL, ABOUT 0.5% TO 1.5% ALUMINUM, ABOUT 1.75% TO 2.50% MOLYBDENUM, CARBON NOT OVER 0.05%, MANGANESE NOT OVER 0.50%, SILICON NOT OVER 0.60%, SULPHUR LESS THAN 0.015%, NITROGEN NOT OVER 0.05%, AND REMAINDER ESSENTIALLY IRON.

Description

United States Patent 3,556,776 STAINLESS STEEL William C. Clarke, Jr., Baltimore, Md., and D Cameron Perry, Middletown, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Continuation-impart of application Ser. No. 376,570, June 19, 1964, which is a continuation-in-part of application Ser. No. 299,484, Aug. 2, 1963. This application Oct. 10, 1966, Ser. No. 585,298

Int. Cl. C22c 37/10 US. Cl. 75-124 15 Claims ABSTRACT OF THE DISCLOSURE Precipitation hardenable chromium-nickel-aluminummolybdenum stainless steel with critically limited amounts of carbon, sulphur and nitrogen, of such composition balance as to be martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heat-treatment, i.e., a single heating at precipitation-hardening temperature (900 F. to 1150 F.) or a first heating and quenching from substantial temperature (1300 F. to 175 0 F.), followed by heating at precipitation-hardening temperature as in heattreating a fabricated composite structure as required by the other metals in the structure. More especially, the steel essentially consists of about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not over 0.05%, manganese not over 0.50%, silicon not over 0.60%, sulphur less than 0.015%, nitrogen not over 0.05%, and remainder essentially iron.

CROSS-REFERENCE TO RELATED APPLICATIONS 7 Our application for patent is a continuation-in-part of our copending application Ser. No. 376,570, filed June 19, 1964 (formally allowed Mar. 17, 1967 but now abandoned in favor of the present application), which in turn is a continuation-in-part of our then copending application Ser. No. 299,484, filed Aug. 2, 1963, now abandoned.

INTRODUCTION The present invention is concerned with the precipitation-hardenable chromium-nickel-aluminum stainless steels and with various products and articles fashioned of the same, in a measure being a specific improved embodiment of the steels generally described in the copending application of D Cameron Perry, coapplicant herein, Ser. No. 227,731, filed Oct. 2, 1962 and entitled Chromium- Nickel-tAluminum Steel and Method, now abandoned in favor of the continuation-in-part application Ser. No. 334,923, filed Dec. 3, 1963 and entitled Chromium-Nickel- Aluminum Steel and Method.

One of the objects of our invention is the provision of a chromium-nickel-aluminum stainless steel which may be formed or fabricated in a solution-treated condition by a variety of mechanical working and forming operations; which readily lends itself to brazing and welding in such condition; which is easily hardened by precipitation-hardening treatment at modest temperatures; and which in hardened condition is strong and yet tough and ductile, with substantial ductility even in large sections.

Another object is the provision of a precipitation-hardenable chromium-nickel-aluminum stainless steel which is martensitic in the annealed or solution-treated condition; which lends itself to the production of bars, rods, wire and the like in that condition; and which, moreover, may then be converted by hot or cold-working into plate, sheet, strip and the like, as Well as drawn rods, wire and other products which are precipitation-hardenable from a solution-treated martensitic condition either by single treatment at modest hardening temperatures or by double heat-treatment as desired, depending upon the precipitation-hardening treatment required by other metals with which our steel is associated.

A further object is the provision of particular articles of manufacture comprising the martensitic steel and products of our invention which, following manufacture by mechanical working, forming, shaping and the like and/ or by brazing, welding or the like, are precipitation-hardenable by heat-treatment by single treatment, that is, by merely heating from the solution-treated martensitic condition, where desired, or by double treatment as by heating at a transformation temperature and then reheating at precipitation-hardening temperatures without adverse effect upon properties, the particular precipitation-hardening treatment employed being dictated by time and equipment, or by the requirements of other metal included in the construction of the articles of manufacture under consideration.

Other objects of our invention in part will be obvious and in part pointed out during the course of the following description.

Our invention, accordingly, consists of the combination of ingredients employed in our steel, in the correlation of these various ingredients, in the temperatures and cycles of heat-treatment employed in connection with our steels, and in the articles and products fashioned thereof, all as more particularly described herein, the scope of the application of which is more fully set forth in the claims at the end of this specification.

BACKGROUND OF THE INVENTION Now in order to gain a better understanding of certain features of our invention, it is to be noted that a variety of precipitation-hardenable grades of stainless steels are known in the art. We refer to the chromium-nickel stainless steels containing one or more of titanium, copper, or aluminum. Some of these steels such as the 17-4 PH (about 17% chromium, 4% nickel, 3% copper, and remainder iron) are hardenable by single heat-treatment from an annealed or solution-treated condition. Others such as the 17-7 PH (17% chromium, 7% nickel, 1% aluminum, and remainder iron), the PHl48Mo (14% chromium, 8% nickel, 2% molybdenum, 1% aluminum, and remainder iron), and the PHl5-7Mo (1.5% chromium, 7% nickel, 2% molybdenum, 1% aluminum, and remainder iron) require double heat-treatment, i.e., transformation by heating and cooling, or by cold-reduction from the annealed or solution-treated condition, followed by heating at precipitation-hardening temperature. None, so far as we are aware, is hardenable by single treatment and at the same time is not adversely affected when subjected to a double treatment.

Moreover, we find that known chromium-nickel-aluminum grades of stainless steels in many instances do not develop sufiicient toughness. Particularly is this true where great stresses are encountered in transverse di rection, either width or thickness, especially in short transverse direction, through the thickness, as for example, in the 3" or 4" directions, respectively, of a 3" x 8" or a 4" x 12" section. While great mechanical strength is achieved in a modified chromium-nickel-aluminum grade, i.e., modified to include the presence of substantial quantities of molybdenum in the composition, for example, the PH15-7Mo grade, this steel lacks the toughness desired for many applications.

In the fabrication of composite structures comprising one or more of the chromium-nickel-titanium, the chromium-nickel-copper or the chromium-nickel-aluminum precipitation-hardenable steels, for example, the choice of a further precipitation-hardenable steel, where greater strength or greater ductility is required of one or more of the component portions, is limited by the hardening characteristics of the principal precipitation-hardening metals employed.

Among the objects of our invention is the provision of a chromium-nickel-aluminum stainless steel which readily lends itself to the production of a variety of rolled or drawn products such as plate, sheet, strip, billets, bars, rods, wire and the like, which products in solution treated or annealed condition may be fabricated into a variety of articles of ultimate use or composite parts thereof, by various mechanical operations, and by brazing, welding and the like; which steel and articles fashioned thereof are precipitation-hardenable from the annealed or solution-treated condition either by single treatment or by double treatment, as desired, or as depending upon the hardening characteristics of other precipitation-hardening metals employed in the construction of said articles, all to achieve great strength toegther with good ductility and toughness.

SUMMARY OF THE INVENTION Referring now more particularly to the practice of our invention, We provide a chromium-nickel-aluminum stainless steel of critical composition in terms of the ingredients chromium, nickel and aluminum, which additionally essentially contains a critical amount of the ingredient molybdenum. The steel of our invention essentially consists of about 11.5% to 13.5% chromium, about 7.0% to 9.0% nickel, or even to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, and the remainder essentially iron. Now the ingredients carbon, manganese, silicon, phosphorus, sulphur and nitrogen, which commonly are present, are maintained in low amount. The carbon is not over 0.05%, the manganese not over 0.50%, the silicon not over 0.60%, the phosphorus 0.040% max., and usually not over 0.015%, the sulphur 0.020% max., and preferably not over 0.010%, and the nitrogen preferably not over 0.05

Actually, as more fully developed below, best results are had where carbon is in excess of about 0.02%, but not in excess of about 0.04%, manganese is not over 0.40%, silicon is not over about 0.50%, and the sulphur does not exceed 0.005%. In this preferred steel nitrogen, Where present, does not exceed 0.01%. In these steels the nickel content may be as much as 10.0% for maximum toughness, the nickel then preferably ranging from about 8.0% to 10.0%.

In the steels of our invention boron additionally may be included where desired, to give improved hot-working properties, but this should not exceed 0.005 Titanium in amounts up to about 0.10% may be added Where desired; more especially, titanium in amounts up to about 0.50% and/or columbium in amounts up to 0.75% may be added.

We feel that in our steels the particular amounts of chromium, nickel, aluminum and molybdenum em loyed, and the correlation between these four ingredients, is most critical. Where either lesser or greater amounts of chromium are employed than the range of about 11.5% to 13.5%, the stuctural balance of the steel is disturbed, with the result that the desired hardening both by single heattreatment and by double heat-treatment is not fully achieved. Moreover, with chromium in an amount less than that prescribed, the desired resistance to corrosion is not bad. So, too, Where the amounts of nickel are either greater or less than the range of about 7.0% to 9.0%, or more broadly 7.0% to 10.0%, there likewise is a disturbance in the structural balance, with loss of hardening characteristics; where less than the prescribed amount, there is a tendency toward the formation of ferrite and, where greater, the metal is inclined to become austenitic, with loss of single heat-treatment hardening. Where the carbon, manganese and nitrogen contents are on the low side the nickel content ranges from about 8.0% to 10.0%. Although there may be some latitude in the aluminum content of -our steel, we find that any substantial departure from the range of about 0.5% to 1.5% results in a disturbance of the structural balance, because of the ferrite-forming tendency of aluminum, with resulting undesired change in hardening characteristics and an undesired change in mechanical properties of the metal. With any significant variation in the molybdenum content of the steel from the range of about 1.75% to 2.50%, the structural balance is also disturbed, greater molybdenum additions giving excess ferrite and lesser additions giving less strength than desired.

Similarly, as to the ingredients carbon, manganese, silicon, sulphur and nitrogen, a carbon content in excess of 0.05% causes a precipitation of carbides in heating with adverse effect upon ductility. A carbon content in excess of 0.02%, however, preferably is employed for we find that with a lesser amount there is a tendency to build up ferrite. Manganese should not exceed 0.50% because of its ferrite-forming tendencies. At least 0.20%, however, is desired where oxides are inclined to be present in the metal, in order to assure cleanliness. When the steel is made by vacuum melting both manganese and silicon may be virtually eliminated. However melted, the sulphur content should not exceed 0.020% and preferably should not exceed 0.010%, as previously indicated, this in order to assure a steel of maximum toughness as pointed out hereinafter. The nitrogen content of the steel should not exceed 0.05 because a greater amount disturbs the structural balance with resultant loss in the ability of the metal to harden by single heat-treatment; preferably the nitrogen content is not over 0.01% as previously mentioned. At present this low value is best obtainable by vacuum melting or by degassing processes.

It is in the steel of particular chromium, nickel, aluminum, molybdenum content set out above that we achieve the full complement of mechanical properties and the surprising flexibility in hardening method, i.e., hardening either by a mere heating at precipitation-hardening temperatures from the solution-treated or annealed condition, or by subjecting the steel to heating at transformation temperatures and cooling, and then hardening by heating at precipitation-hardening temperatures, as more fully discussed below.

A preferred steel according to our invention essentially consists of about 11.5 to 13% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2% to 2.5% molybdenum, and remainder essentially iron. Another preferred steel essentially consists of about 12.5% to 13.5% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2.0% to 2.5% molybdenum, and remainder essentially iron. A further preferred steel, this of restricted ferrite content, essentially consists of about 11.5% to 12.5% chromium, about 7.0% to 9.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.5% molybdenum, and remainder essentially iron. In these steels the carbon content does not exceed about .04%, and preferably is .02% to .04%, the manganese content is up to about .50%, and the silicon content is up to about .50%. The sulphur content amounts to no more than .020% and in a preferred steel does not exceed about .0l5%. The nitrogen content is not over about 0.04%, preferably not over about 0.03%. The phosphorus content may amount to as much as 0.040%, although as previously indicated, it usually does not exceed 0.015%.

The ingredients titanium and/ or columbium preferably are included in the composition of our steel, titanium where used being in amounts up to 0.50%, particularly 0.05% to 0.50%, and columbium where used being employed in amounts up to 0.75%, more particularly 0.10% to 0.50%. With the titanium-bearing and/ or columbiumbearing steels, especially the latter, there is assured a freedom from intercrystalline or intergranular corrosion in the as-welded condition and in the welded and precipitation-hardened condition.

In our steel, in hardened condition, there is achieved an excellent balance of strength in longitudinal direction, in short transverse direction (direction of thickness), and in long transverse direction (direction of width), as more fully pointed out hereinafter.

Now the steel of our invention conveniently may be made in the electric arc furnace or in the induction furnace, these being referred to as air-melting or melting at atmospheric pressure. Where desired, the steel may be vacuum-melted, as by melting in electric induction furnace under vacuum conditions. It also may be melted by Way of a double melting process, i.e., melted first in the electric arc furnace at atmospheric pressure, with the resultant heat of steel being cast in the form of electrodes which are then remelted under vacuum conditions. A further double melting process comprises first melting in the vacuuminduction furnace, with the resulting heat of steel being cast into consumable electrodes, which electrodes are then remelted under vacuum conditions.

Certain advantages are achieved in the electric arc melting operation by employing an ingot iron base, that is, ingot iron as the principal source of iron. These advantages are even more pronounced where electrolytic iron is employed as the principal iron source. Advantages also are had with the double-melting operation which in many instances justifies the additional cost of melting.

The steel had, by whatever melting process employed, is in the form of casting which are cast in the form of, or may be converted into, slabs, blooms and billets, and from these into hot-rolled plate, sheet, strip, bars, rods, wire, and like products. The metal works well in the mill.

Our steel in the form of plate, sheet, strip, bars, rods, wire, or like converted products, is supplied the customerfabricator in the hot-rolled and annealed condition. This contemplates heating at 1500 to 2100 F. and cooling. In this condition the steel is martensitic. The hardness is on the order of Rockwell C27-35.

Where desired, of course, the annealing or solutiontreatment may be performed by the customer-fabricator, with heating at some 1500 to 2000 F. Usually a heating at about 1700 F., with time of heating depending upon thickness, is considered satisfactory. And with cooling to room temperature through quenching in either air, oil or water, the metal is workable and formable.

The steel of our invention, where desired, may be supplied in the form of forging billets or hot-rolled plate. Or it may be supplied in a cold-rolled condition, that is, in the form of cold-rolled and annealed sheet, strip, bars, rods, and the like. Or it may be supplied in the form of cold-drawn wire. Here, too, of course, the steel is in a martensitic condition with hardness of the cold-rolled or colddrawn metal on the order of Rockwell C35-40. The steel may be machined as by cutting, drilling, tapping and threading. And of particular consequence, the steel may be brazed or welded in the fabrication of a variety of articles of use, the steel forming the entire article, or a component part as desired. The metal is particularly suited to the production of supersonic aircraft parts, notably the ribs, stiffeners, stringers and like sections. Likewise, it is suited to the production of the skin sections or casings of planes, missiles, rockets, or the like. And to the production of high pressure vessels and tankage where there are encountered stresses along all three major axes.

Following fabrication, the steel of our invention is subjected to a precipitation-hardening or age-hardening treatment. We find that a mere heating at a temperature of 900 to 1150 F. gives a desired hardening. Ordinarily, we recommend a heating at some 950 to 1050 F. for several hours; particularly, we find heating at 950 F. for 1 hour or more and cooling in air, oil or water gives desired results, the hardness amounting to some Rockwell 040-50.

Alternatively, where desired, as for example where the steel of our invention is a component part of an article, the other metals of which are in austenitic or semiaustenitic condition and are not directly amenable to hardening by a single heating, we find that our steel readily hardens where the article is subjected to a transformation treatment and then a hardening treatment. The transformation treatment comprises a heating at 1300 to 1750 F. and then cooling to a temperature of between about 60 F. and -200 F. The durations of the heating and cooling are not critically important.

Where a brazing operation is employed in fabricating an article this may be viewed as a part of the heat-treatment by choosing a brazing alloy which has a flow point of about 1600 to 2000 F. and performing the brazing, preferably at a temperature of 1800 to 2000 F. Following brazing, the article is cooled to a temperature of about 1700 F. where it is held for about 30 minutes. This assures the greater toughness which results from heating the steel within the upper part of the 1300 to 1750 F. temperature range.

The steel, however fabricated, is brought to final hardness by reheating at a temperature of some 900 to 1150 F. and cooling in air, oil or water. Usually, we find that reheating at 950 F. for an hour or more and quenching gives desired results. Here again, the hardness had is on the order of Rockwell C40-50.

In the precipitation-hardened or age-hardened condition our steel is characterized by a combination of strength and ductility. Moreover, these properties are had in longitudinal direction, in short transverse direction and in long transverse direction. And in the preferred steel, that is, the steel of especially loW carbon, sulphur and nitrogen contents, the values of strength and ductility are about equally balanced along these three axes. The titaniumbearing and the columbium-bearing steels in hardened condition, following a welding or brazing operation, are particularly resistant to corrosion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As specifically illustrative of the steels of our invention, the chemical composition of a typical steel melted in the electric arc furnace, together with the typical mechanical properties in longitudinal direction, short transverse direction and long transverse direction for two different ageing conditions are given below in Tables 1(a) and I(b):

TABLE I(a).-'1YPICAL AGE HARDENED AIR-MELTED STEEL OF THE INVENTION: CHEMICAL ANALYSIS C .040 Mn .40 P (max) .020 S (max .020 Si .30 Cr 12.75 N1 8.10 Al 1.00 Mo 2.15

In the steel noted the remainder of the composition is essentially iron, with the amount of nitrogen typically 0.03%.

The mechanical properties of duplicate samples of our steel: i.e., tensile strength, yield strength, elongation, reduction in area and hardness, are reported in Table I(b) below, these for two different sections (3" x 4" and 1" x 4") under differing conditions of heat-treatment.

TABLE I(b).MECI1ANICAL PROPERTIES OF THE STEEL OF TABLE 1(a) Percent Rock- Percent Reducwell U.T.S., 0.2% Y.S., E1. in tion in hard- Section Condition Test Direction p.s.i. p.s.i. 2 inches area ness 1 11. 3 x 4 1111-050 Longitudinal 838 3 21888 5 2 9 9 3 x 4" RIl-050 l Short transverse. 2 221, 000 200, 800 8. U 1" x 4 Long r n ver 221 000 2047 500 7 1 x 4- Brazc cycle plus do 228, 500 20!), 000 3. 0

1 RH'050=1,750 F. for 1ni11., air cool, 100 F. 8 hrs, air warm; 050 F. for 1 hour, air cool.

2 Subsized tensile tests-1 gage length. 3 Braze Cycle=slow heat to 1.025 F. 10

TABLE II(a) [Three Age-Hardened Air Melted Steels of the Invention: Chemical Composition C Mn P S Si Cr Ni Mo Al No'rn The remainder of the compositions is essentially iron, with the amount of nitrogen typically 0.03%.

ruins, cool to 1,000 F. in 45 mins., and air cool; reheat 000 F. for 1 hour and air cool.

It is noted from the information given. in Tables II(b) and II(c) above that the steels of our invention are not only strong with ultimate strengths on the order of 220,000 p.s.i. to 230,000 p.s.i., but they are ductile, as well, with 20 elongation figures in 2" amounting to some 8% to 11%,

and reduction in area figures amounting to some 29% to 39%. Particularly it is noted that the steel in flatter section, as given in Table II( c) is of somewhat more uniform strength and ductility, with the strength in transverse direction amounting to some 222,000 p.s.i. to 226,000 p.s.i.

and elongation figures of 10% to 11% and reduction in area of some 36% to 39%.

The steel of our invention possesses excellent stresscorrosion properties even in the presence of salt atmosphere. Samples of our steel were exposed directly to sea air at Kure Beach, North Carolina, in comparison with similar precipitation-hardened steel considered to possess TABLE II (b) [h'iechanical Properties of the Steels of Table 11(21) in Medium Section] Percent Percent U.T.S., 0.2% Y.S., El. in reduction Size Direction p.s.i. p.s.i. 2 inches area Heat No.:

039043 6 x 6 Transverse 230, 000 220, 000 9. 5 31, 0

22 10. 2 039099 1 x 4 Lo g rm1svers 3; 288 200 800 8 I 221, 800 200, 300 s. 0 32. 0 059100 1 e 4 Norm conditionz 1,000 F. 1 hr.-W.Q. plus 1,750 F. 30 mins. 11.0. plus (Within 24 hrs.) minus hrs. plus 950 F. 1 hr. A.C.

TABLE 11 (0) [Mechanical Properties of Two of the Steels of Table 11(2) in Smaller Section] Percent Percent U.T.S., 0.2% Y.S., E1. in reduction Size Direction p.s.i. p.s.i. 2 inches area Heat No.1

2 1 11. 030099 X 4 c s 5 2; 888 3 8 238 f 222, 300 201 500 10. O 37. 1 039100 V2 x 4 "1 226,300 203, 500 10.0 6. 3

NOTIl COHdlfiOllI Same as that for Table II(b).

good stress-corrosion properties with the results as given in Tables III(a) and III(b) below:

TABLE III (a) [Stress-Corrosion Comparisons-Air-Melted Steel of Invention vs. Air-Melted PH15-7M0 (15% Chromium, 7% N1clrcl 1% Aluminum, 2% Molybdenum, Balance Iron) Bent Beam Specimens at 90% of Yleld Strength Exposed Directly to Sea Air a-t Kure Beach, North Carolina] Chemical composition 1 Induction furnace heat. 2 Electric are furnace heat. No'rE.-Remainder of all compositions 1s essentially iron, with the amount of nitrogen typically 0.03%.

TAB LE III (b) [Stress-Corrosion Comparisons for Steels of Table 111(2) Averages of Exposures Each] Exposure Average U.T.S., 0.2% Y.S., stress, days to Direction Condition p.s.1. p.s.i. p.s.i. failure Heat No.1

Steel of Invention:

039099 1 Transverse BOHT 900 2 246, 300 221, 800 200,000 517 do BCHT 900 249, 300 223, 700 201, 000 456 Longitudina BCHI 900 233, 500 214, 500 193, 000 354 BCHT 900 236, 800 214, 500 193, 000 428 Transverse RH 950 3 246, 000 221, 550 199, 000 9 247, 000 221, 800 200, 000 12 244, 000 225, 200 203, 000 12 245, 000 228, 500 206, 000 2 238, 200 221, 000 199, 000 151 241, 500 221, 200 199, 000 69 1 .025 thick-all other specimens .050 thick.

1 BCHI 900=Heat to 1,675 F.-hold 15 mins.; cool to 1,000 F. in 30 mins., air cool to room temp.; minus 100 F. for 8 hrs., air warm; 900 F. for 8 hrs., air cool.

3 RH 950=1,750 F. for mins., air cool; minus 100 F. for 8 hrs., air warm; 950 F. 1 hr., air cool.

From the corrosion tests reported in Table III(b) it clearly appears that the stress-corrosion properties of our steel are greatly superior to the known PH7M0 grade generally considered to possess good stress-corrosion properties. While the known steel stressed on the order of some 200 K s.i. (200,000 pounds per square inch) in transverse direction failed in some 2 to 12 days, and when stressed in longitudinal direction in some 61 to 159 days, our steels had a life of some 456 days to 517 days with transverse stressing and some 354 to 428 days with longitudinal stressing. It is to be particularly noted that with the steel of the present invention there also is achieved a greater uniformity in stress-corrosion life for the two directions of applied stress, as compared with the known steel.

We fined that in the steels of our invention vastly superior ductility along with great strength in the precipitation-hardened condition is had by preserving the manganese, silicon, sulphur and nitrogen contents at critically about .015 and preferably not exceeding about .005%, with carbon not exceeding about .04% and nitrogen not exceeding about .01%, preferably not exceeding about 005%, and remainder essentially iron.

The preferred steel is achieved through double vacuummelting, that is, melting to specification in the electric induction furnace under vacuum to form electrodes, and then remelting these electrodes under vacuum to achieve the finished metal. A specific example, and the mechanical properties of duplicate samples taken at intermediate sections of length, width and thickness when precipitationhardened by single treatment and also when precipitationhardened by double treatment, is given in Tables IV(a) and (b) below:

TABLE IV(a).CHEMICAL ANALYSIS OF DOUBLE VACUUM-MELTED STEEL OF INVENTION Heat No. VC 5178 .042 low values; the manganese content not exceedlng about Nil .10% the silicon content not exceeding about .10% the 002 sulphur and nitrogen contents each not exceeding about 003 .005 The carbon content, too, in this improved steel 02 is low, this not exceeding about .04%. 12 52 A preferred steel of critically low manganese, silicon, 8 63 sulphur and nitrogen contents essentially consists of about 2 10 11.5% to 13.5% chromium, about 8.0% to 10.0% nickel, 1 00 about .5% to 1.5% aluminum, about 1.75% to 2.50% 0018 molybdenum, manganese not exceeding about .10% silicon not exceeding about .10% sulphur not exceeding Remainder of the composition is essentially iron.

TABLE 1'V(b) [Mechanical Properties of Steel of Table IV(a) in 3 x 8 Section (Duplicate Samples at Intermediate Position) Percent Percent C Charpy U.T.S. 0.2% Y.S E]. in reduction rockwell V-notch Condition Direction of Test p.s.1. p.s.1. 2 Area hardness Ft.-Lbs

Solution-treatment Longitudinal 157, 500 95,700 15.0 163, 500 106, 000 16. 0 212, 800 189, 300 16. 2 do 215, 400 186,500 15. 6 Single-treatment Longitilidinal Trans- 213,600 188, 000 14. 4

V6183. y 214, 400 190, 400 13. 8 Single-treatment 217, 600 1 16.0 216,700 191, 700 1 14. 0 Single-treatment- 216, 200 201, 000 13. 8 216, 800 198, 400 14. 4 S1ng1e-troatment 197, 000 188, 200 14. 4 y do 103, 500 185, 000 15. 0 Single-treatment 172, 800 161, 600 17. 5 y do 172,800 161, 600 17. 5 Double-treatment- 225, 600 205, 600 13. s do 223, 900 201,700 13.1 Double-treatment. Longitilldinal Trans- 224, 800 203, 200 13. 1

VGISQ. (z) do 224, 400 201, 600 13. 1 Double-treatment Short Transversal. 224, 600 203, 700 I 12.0 (2) o 224, 400 203,100 1 10. 0

1 Subsized tensiles=1" gage length. 2 240 ft. lb. impact tester. N OTE:

x=1,825 F. hr. A.C. to F. y =1,825 F. 5 hr. 4.0. (to 60 F.) plus 950 F. 1 hr. A.C. y =1,825 F. hr. 11.0. (to 60 F.) plus 1,000 F. 1 hr. A.O. y =1,825 F. hr. A.C. (to 60 F.) plus 1,050 F. 1 hr. A.C. y =1,825 F. hr. 14.0. (to 60 F plus 1,100 F. 1 hr. 4.0. z=1,700 F. 1 hr. A.C. plus minus F 8 hrs. plus 950 F. 1 hr. A.C.

1 1 It is noted that the steels of our invention possess excellent mechanical properties in precipitation-hardened the tests made on precracked samples of the steel of Table IV(a) and reported in Table IV(c) below:

Heat Treatment Jrack, inches Impact, W A (in.

Direction and it.-lbs. lbS./in.

Location of Test 1,825 F. /2 hr l AG (to 60 F.) plus 950 F. 1 hr.

A.(). Ag (to F.) plus 950 F. 1 hr. itc' t'o 00 F.) plus 050 F. 1 hr.

A. 1,825 F. hr A.C. (to F.) plus l,050 F. 1 hr.

A.C. (to 60jF.) plus 1,050 F. 1111'.

Aid. to 00 F.) 131118 1,050 F. 1 hr.

1 Sec September 1961, Welding Research Supplement to Welding Journal, pp. 405-5 t0 4l0 s, Omar and Hartbower.

condition, whether the steel is subjected to a single heattreatment or to a double heat-treatment. And that these properties are equally achieved, as noted above, in longitudinal direction, in long transverse direction, and in short transverse direction. Thus, while the single treatment steels hardened at 950 F. have tensile strengths in longitudinal, long transverse and short transverse direc tions on the order of some 212,000 p.s.i. to 220,000 p.s.i., with yield strengths on the order of 186,000 p.s.i. to 198,000 p.s.i., the steels subjected to a double heat-treatment (heating at 1700 F. 1 hour and air cool, followed by treatment at F. 8 hours-{-precipitation-hardening at 950 F. 1 hour and air cool) have tensile strengths in longitudinal, long transverse and short transverse directions of some 222,000 p.s.i. to 225,000 p.s.i., with yield strengths of 199,000 p.s.i. to 205,000 p.s.i. Any difference is in favor of the double heat-treatment. It is noted, however, that with the double heat-treatment there is somewhat closer correspondence between the tensile strengths and longitudinal, long transverse and short transverse directions as compared with the single heat-treatment steel.

Perhaps the most surprising result had With the steel of critically low manganese, silicon, phosphorus, sulphur and nitrogen contents of Table IV(a) is the great ductility achieved, this in combination with great strength. Note here from Table IV(b) that both with single heattreatment of the steel and with double heat-treatment of the steel the ductility in all three directions of stress, as indicated by the elongation, is on the order of some 12% to 16%, and reduction in area on the order of some 41% to 66%. These figures greatly exceed those had in the illustrative steels set out above in Tables 1(a) and I(b) and II(a) and II(b) where elongation amounts to some 8% to 11%, with reduction in area amounting to 29% to 46%.

The mechanical properties of the steel of our invention differ somewhat with differences in temperature of final hardening. As best seen in Table IV(b) above, the tensile strength and the yield strength are inclined to fall off as the precipitation-hardening is increased from 950 F., to 1000 F., to 1050 F., and to 1100 F. As may be expected, there is corresponding increase in ductility. Surprisingly, there is had a great increase in impact strength, this latter increasing from the figure of 13 to 23 ft.-lbs. Charpy V-notch for the longitudinal samples with the steel hardened at 950 F., to some 46 to 53.5 ft.-lbs. for that hardened at 1000 F., to some 102 to ft.-lbs. for that hardened at 1050 F., and finally to a figure exceeding 120 ft.-lbs. on up to 149 ft.-lbs. for that hardened at 1100 F.

The toughness of the steel of critically low manganese, silicon and sulphur contents is perhaps best illustrated in It is to be noted that the steel hardened by heating at 950 F. has an impact strength on the order of some 5 to 6 /2 ft.-lbs. with a W/A Factor of some 574 to 800 in. pounds per square inch, while that hardened at 1050 F. has an impact strength on the order of 57 /2 to 72 /2 ft.- lbs. with a W/A Factor of 7162 to 8600 in. pounds per square inch. The depth of the fatigue crack in all cases amounted to some 0.05 to 0.08 inch.

CONCLUSION It will be seen that We provide in our invention a precipitation-hardenable chromium-nickel-alurninum-molybdenum steel in which objects hereinbefore set forth together with many practical advantages are successfully achieved. The steel is of such critical composition balance that it is martensitic in the annealed or solution-treated condition and readily lends itself to hardening either by single heat-treatment from the annealed condition, i.e., by mere heating at precipitation-hardening temperatures, or by double treatment, i.e., by heating at transformation heating temperatures, which does it no harm, and then heating at precipitation-hardening temperature.

The metal is supplied a customer-fabricator in the form of plate, sheet, strip, bar, Wire, rods and the like usually in the annealed or solution-treated condition. It may be machined as by cutting, drilling, tapping and threading. And it may be fabricated by Welding, brazing or other fabricating operations. The steel and fabricated articles are then hardened, as noted above, either by simple heating, or by transformation treatment and then heating at hardening temperature to give strength along with ductility and toughness. Strength and toughness are had in the direction of working as well as in the transverse directions.

The steel of our invention is particularly suited to a variety of applications where stresses are encountered along all three axes. And it is suited to applications encountering corrosion even under stress; the stress-corrosion properties in severe salt atmosphere being surprisingly superior to one of the better known steels of the prior art.

Inasmuch as several embodiments of our invention very well may occur to those skilled in the art to which the invention relates, and many variations may be made in the several embodiments herein disclosed, it will be understood that all material described herein is to be taken as merely illustrative and not as a limitation.

We claim as our invention:

1. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about Longl 0017 6. 5 780' Intermediate 0677 6. 5 800 Short transverso 0498 5. 0 574 Intermediate 0498 6. 0 680 Longl 0782 57. 2 7. 380 Intermediate 0590 72. 5 8, 600

Short transverse 0657 58. 5 7, 160

Intermediate 0485 68. 5 8,

13 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not over 0.05%, manganese not over 0.50%, silicon not over 0.60%, sulphur less than 0.015%, nitrogen not over 0.05 and remainder essentially iron.

2. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not exceeding 0.05 manganese not exceeding 0.50%, silicon not exceeding 0.60%, phosphorus not exceeding 0.040%, sulphur not exceeding 0.010%, nitrogen not exceeding 0.05%, and remainder essentially Iron.

3. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable there from both by single heat-treatment and by double heattreatment to give strength together with ductility, and essentially consisting of about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50 molybdenum, carbon exceeding 0.02% but not exceeding 0.05%, manganese not exceeding 0.40%, silicon not exceeding 0.50%, phosphorus not exceeding 0.040%, sulphur not exceeding 0.005%, nitrogen not exceeding 0.01%, and remainder essentially iron.

4. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 11.5% to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, up to about .05% carbon, manganese and silicon each not exceeding about .10%, sulphur and nitrogen each not exceeding about .005 and remainder essentially iron.

5. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 11.5% to 13% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2% to 2.5% molybdenum, up to about .05% carbon, up to about .50% manganese, up to about .50% silicon, sulphur less than .015 nitrogen up to about 0.05 and remainder essentially iron.

6. A stainless steel which is martensitic in the solu tion-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 11.5% to 13% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2% to 2.5% molybdenum, about .02% to .04% carbon, up to about .50% manganese, up to about .50% silicon, up to about .010% sulphur, nitrogen up to about 0.04%, and remainder essentially iron.

7. A stainless steel which is martensitic in the solution-treated condition and precipitation-hardenable therefrom both by single heat-treatment and by double heattreatment, and essentially consisting of about 12.5% to 13.5% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2.0% to 2.5 molybdenum, about 0.2% to .05% carbon, up to about .50% manganese, up to about .50% silicon, up to about 010% sulphur, nitrogen up to about 0.05%, and remainder essentially iron.

8. A stainless steel which is martensitic in the solutiontreated condition and equally precipitation-hardenable therefrom both by single heat-treatment and by double heat-treatment, and essentially consisting of about 11.5 to 13.5% chromium, about 7.0% to 10.0% nickel, about .5% to 1.5% aluminum, about 1.75% to 2.5% molybdenum, carbon not exceeding about .05%, manganese not exceeding about .10%, silicon not exceeding about .l%, sulphur not exceeding about .015 nitrogen not exceeding about 010%, and remainder essentially iron.

9. A stainless steel which is martensitic in the solutiontreated condition and equally precipitation-hardenable therefrom both by single heat-treatment and by double heat-treatment, and essentially consisting of about 11.5 to 13.5 chromium, about 7.0% to 10.0% nickel, about .5% to 1.5% aluminum, about 1.75% to 2.5% molybdenurn, carbon not exceeding about .04%, manganese not exceeding about .10%, silicon not exceeding about .10%, sulphur and nitrogen each not exceeding .005 and remainder essentially iron.

10. Stainless steel which is martensitic in the solutiontreated condition and about equally precipitation-hardenable from such condition by single heat-treatment and by double heat-treatment to give great strength, and essentially consists of about 11.5 to 13.5 chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75 to 2.50% molybdenum, carbon not exceeding 0.05%, phosphorus not exceeding 0.040%, sulphur not exceeding 0.010%, nitrogen not exceeding 0.05%, and remainder essentially iron.

11. A precipitation-hardenable composite article comprising stainless steel, martensitic in the solution-treated condition, and essentially consisting of about 11.5

to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenurn, carbon not exceeding 0.05 sulphur not exceeding 0.010%, nitrogen not exceeding 0.05%, and remainder essentially iron.

12. A precipitation-hardenable composite fabricated article comprising stainless steel, martensitic in the solutiontreated condition, and essentially consisting of about 11.5% to 13% chromium, about 7.5% to 9.0% nickel, about 1% aluminum, about 2% to 2.5 molybdenum, up to about .05% carbon, up to about .50% manganese, up to about .50% silicon, sulphur not exceeding about .015%, nitrogen up to about 0.01%, and remainder essentially iron.

13. A stainless steel, martensitic in the solution-treated condition and precipitation-hardenable from such condition, essentially consisting of about 11.5 to 13.5 chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, carbon not exceeding 0.05 sulphur less than 0.015%, nitrogen not exceeding 0.05%, and remainder essentially 1mm.

14. A stainless steel, martensitic in the solution-treated condition and precipitation-hardenable from such condition, essentially consisting of about 11.5 to 13.5% chromium, about 7.0% to 10.0% nickel, about 0.5% to 1.5% aluminum, about 1.75% to 2.50% molybdenum, at least one of the group consisting of titanium in amounts up to 0.50% and columbium in amounts up to 0.75%, carbon not exceeding 0.05 sulphur less than 0.015%, nitrogen not exceeding 0.05 and remainder essentially iron.

15. A stainless steel, martensitic in the solution-treated condition and precipitation-hardenable from such condition, essentially consisting of about 11.5% to 13.5 chromium, about 7.0% to 10.0% nickel, about 0.5 to 1.5% aluminum, about 1.75% to 2.50% molybdenum, about 0.10% to 0.50% columbium, carbon not exceeding 0.05%, sulphur less than 0.015%, nitrogen not exceeding 0.05 and remainder essentially iron.

References Cited UNITED STATES PATENTS 3,342,590 9/1967 Bieber 124 HYLAND BIZOT, Primary Examiner US. Cl. X.R, 75--128