US3658513A

US3658513A - Precipitation-hardenable stainless steel

Info

Publication number: US3658513A
Application number: US805039A
Authority: US
Inventors: William C Clarke Jr
Original assignee: Armco Inc
Current assignee: BALTIMORE SPECIALTY STEELS Corp A CORP OF DE
Priority date: 1969-03-06
Filing date: 1969-03-06
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25

Abstract

Martensitic chromium-nickel stainless steel of great strength in the age-hardened condition, and of good ductility and toughness. The steel contains about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, and remainder substantially iron. The carbon and nitrogen contents are maintained in critically low amount, the former not exceeding about 0.05 percent and the latter not exceeding 0.015 percent. Cobalt up to about 2 percent may be partially substituted for nickel. There may be added columbium up to about 0.3 percent and/or titanium up to about 0.15 percent.

Description

[451 Apr. 25, 1972 [54] PRECIPlTATION-HARDENABLE STAINLESS STEEL William C. Clarke, Jr., Baltimore, Md.

Armco Steel Corporation, Middletown, Ohio [22] Filed: Mar. 6, 1969 [21] Appl.No.: 805,039

[72] Inventor:

[73] Assignee:

2,614,921 10/1952 Tanczyn 75/125 3,083,095 3/1963 Tanczyn .75/125 3,362,813 1/1968 Ziolkowski ..75/124 3,408,178 6/1967 Myers ..75/124 3,152,934 10/1964 Lula ..75/125 3,278,298 10/1966 Perry..... ..75/128 W 3,347,663 10/1967 Bieber ..75/125 Primary Examiner-Hyland Bizot Attorney-John Howard Joynt [57] ABSTRACT Martensitic chromium-nickel stainless steel of great strength in the age-hardened condition, and of good ductility and toughness. The steel contains about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, and remainder substantially iron. The carbon and nitrogen contents are maintained in critically low amount, the former not exceeding about 0.05 percent and the latter not exceeding 0.015 percent. Cobalt up to about 2 percent may be partially substituted for nickel. There may be added columbium up to about 0.3 percent and/or titanium up to about 0.15 percent.

13 Claims, No Drawings PRECIPITATION-HARDENABLE STAINLESS STEEL As a matter of introduction, my invention is concernedwith the chromium-nickel stainless steels, more especially those which are hardenable by simple heat-treatment. More particularly, the concern is with the martensitic chromium-nickel stainless steels which are hardened by simple-aging treatment at comparatively low temperatures.

One of theobjects of my invention is the provision ofa chromium-nickel stainless steel which not only works well in the mill, as by rolling, drawing, and forging, but which in the form of rolled and drawn products readily lends itself to a variety of forming and fabricating operations, such as spinning, upsetting, machining, threading, and the like.

Another object is the provision of a martensitic chromiumnickel stainless steel which readily lendsitself to hardening and strengthening by simple heat-treatment and yet retains good ductility and toughness in the hardened and strengthened condition.

A further object of the invention is the provision of a martensitic chromium-nickel stainless steel and various formed,

and fabricated articles fashioned thereof, such as aircraft landing gear, structural parts, fasteners, andthe like, enjoying a high ratio of strength-to-weight and a good combination of strength with resistance to shock and impact;

Other objects of my invention in part will become. apparent during the course of the description which follows, and in part more particularly pointed to.

My invention, then, resides in the combination of elements, in the mixture of ingredients, and in the relation between the same, all as described herein, the scope of the application of which is more particularly set out in the claims at the end of the specification.

BACKGROUND OF THE INVENTION In order to gain a better understanding of certain features of my invention, it may be well to noteat this point that thenumber of grades of stainless steel now available is legion. Perhaps the best known are the austenitic chromium-nickel grades such, for example, as the AISI Types 301 and 302; the former containing about 17 percent chromium, about'7'percent nickel, with remainder iron; and the latter about 18'percent chromium, about 9 percent nickel, and remainder-iron. But these steels, unfortunately, harden significantly during a cold-working operation. The A181 Type 305, however, containing about 18 percent chromium, about 11 percent nickel, and remainder iron, enjoys a substantially lower work-hardening rate. But none of these steels may be hardened by heattreatment.

There are available, however, more'sophisticated grades of chromium-nickel stainless steel which, while enjoying-mostof' the beneficial corrosion-resisting properties of the chromiumnickel grades, and which readily lend themselves to working,

and forming into a variety of products, may be hardened through aging treatment. I refer to the chromium-nickel stainless steel described in the Tanczyn U.S. Pat. No. 3,376,780 of Apr. 9, 1968. That steel typically containsabout 15 to 18 percent chromium, about 7 to 10 percent nickel, about 2 to percent copper, about 0.75 to 1.50 percent aluminum, and

remainder iron. Where desired, molybdenum may be present in amounts up to 5 percent as a partial substitute for chromium. That steel is semi-austenitic in the solution-treated condition. Andwhile characterized by a combination of many highly desirable properties, the steel in large section, thatis, plate, bar or other products exceeding an inch or two in thickness, does not readily lend itself to hardening, for hardening best is achieved by a combination of substantial, or even drastic, cold-reduction followed by heat-treatment. Bar, plate, forgings and other products of significant dimension, where as a result of size or other consideration, cold-reduction may not be had, are not available, then, in the hardened and strengthened condition.

Also Irefer to the further chromium-nickel stainless steel described in the copending Clarke-Perry application Ser. No.

585,298, filed Oct. 10, 1966. That steel typically contains about 13 percent chromium, 8 percent nickel, 2 percent molybdenum, 1 percent aluminum, with critically low amounts of residual elements, and remainder iron. And while that steel lends itself to hardening by heat-treatment, the

SUMMARY OF THE INVENTlON Turning now more especially to the practice of my invention, I provide a chromium-nickel stainless steel which essentially consists of the five ingredients chromium, nickel, molybdenum, copper and aluminum in particular and critical amount, with critically controlled amounts of the further ingredients carbon and nitrogen which commonly are found in all stainless steels. In my steel, best results are had by including in thecomposition the further ingredient columbium, this in small and'critical amount, with titanium partially substituted forthe columbium. Where desired, the ingredient cobalt may be partially substituted for the ingredient nickel. The steel is martensitic in the solution-treated condition.

More especially, in my steel chromium is present in the amount of about 10.5 or 1 1.5 percent to about 13.25 percent and particularly to about 13 percent, nickel in the amount of 7.5 percent to about 9 percent or even to about 9.5 percent and more especially about 8 percent to about 9 percent (or about 8 percent to about 9.5 percent nickel with about 10.5 percent to about 12.5 percent chromium), molybdenum in the amount'of about 1 percent or about 1.3 percent to about 2.25 percent or about 2.5 percent, copper in the amount of about 1 percent to about 2.5 percent, and aluminum inth'e amount of about 1 percent to about 2 percent andparticularly about 1.1 percent-to about 1.4 percent or even to about 1.8 percent. For a best combination of results, as more particularly described below, these several ingredients are maintained in more limited amount.

While the ingredient columbium is not essential to the steel of my invention, certain benefits are had by its presence in amounts up to about 0.3 percent, particularly in the amount of about 0.1 percent to about 0.3 percent. Where desired, titanium may be partially substituted for columbium, this only up to the amount of 0.15 percent titanium, however; any greater amount of titanium is found to cause a loss in strength; 1 attribute that loss of strength to the apparent difficulty of the steel taking into solution greater amounts of titanium in the face of the high aluminum content present.

As indicated above, cobalt may be partially substituted for some of the nickel, but this only up toabout 2 percent cobalt, for best results about 1 percent to about 2 percent cobalt, the nickel content being decreased with the cobalt addition, but only down to about 6 or 6.5 percent. At least 6 percent nickel, even with cobalt and copper present, is required in the steel of my invention in order to assure substantial freedom from delta-ferrite.

As indicated above, the further and commonly present ingredients carbon and nitrogen are maintained in critically low amount, the carbon not exceeding about 0.05 percent max. and the nitrogen not exceeding 0.015 percent max. For a best combination of properties nitrogen should not exceed 0.01

percent or even:0.0l0 or 0.007 percent. The nitrogen content is especially critical. Ifindthat .withan excessive nitrogenc'on-i tent there is a'loss offracture toughness.

Actually, I find a certain amount of carbon beneficial to the steel of my invention, this as an aid in assuring a desired freedom from any significant amount of delta-ferrite, the delta-ferrite being preserved in amount less than 3 percent by volume, for with higher contents there is a loss of strength, particularly in transverse direction. Carbon, then, for best results is employed in the amount of about 0.025 percent to about 0.045 percent, or more broadly, about 0.02 percent to about 0.05 percent.

The further ingredients manganese, silicon, phosphorus and sulphur also are maintained low. The manganese and silicon are maintained at values each not exceeding 0.10 percent, the phosphorus not exceeding about 0.010 percent, and the sulphur not exceeding about 0.005 percent. The remainder of the composition, of course, is substantially all iron.

My steel preferably is melted in the vacuum furnace in order to assure cleanliness and freedom from oxide inclusions or, indeed, other contaminants. A single induction vacuum melting operation ordinarily is sufficient. 1 find, however, that a superior ingot is had, with sound center and minumum waste, by employing a double vacuum treatment, that is, an initial vacuum melting in the induction furnace followed by consumable electrode vacuum remelting. The ingots had are clean, sound, and free of hydrogen embrittlement.

The metal works well in the hot-mill in converting ingot to bloom, billet and the like. Moreover, it works well in further conversion to hot-rolled and cold-rolled products, such as plate, sheet and strip, bars, rod, wire and special shapes. These several mill products are suited to fabrication, as by machining, threading, cold-heading, and the like, as in the production of threaded fasteners and aircraft parts, particularly landing gear, where a high ratio of strength-to-weight is desired. As noted above, the steel is martensitic in the solution-treated condition.

Hardening of my steel and products fashioned therefrom is had merely by heating at aging temperatures, say 900 to 1,050" F., from the solution-treated condition (heating at some l,400to l,750 F. and quenching), as appears more fully hereinafter. Best and most uniform results are had by solution-treating fabricated products following fabrication and then aging at the desired temperature. In this way there is achieved a uniformity of product not had by aging immediately following fabrication.

1n effecting the solution-treatment, especially after fabrication of the desired products, a rather low solution-treating temperature is desired that is, about 1,400 to 1,500 F., and certainly not over l,750 F., because excessive temperatures promote grain growth, this with resulting loss of toughness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the steel of my invention in broadest aspect essentially consists of about 10.5 percent to about 3.25 percent chromium or about 11.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum or about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper or about 1 percent to about 2 percent copper, about 1 percent to about 2 percent aluminum or about 1.1 percent to about 1.8 percent aluminum, with carbon not exceeding about 0.05 percent, manganese not exceeding about 0.10 percent, silicon not exceeding about 0.10 percent, nitrogen not exceeding 0.015 percent, and remainder iron, a best combination of properties is had in steels of more limited composition. One such steel contains about 1 1.50 percent to about 12.25 percent chromium, about 8.40 percent to about 8.90 percent nickel, about 1.35 percent to about 1.60 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.50 percent to about 1.75 percent aluminum, with carbon up to about 0.05 percent and more particularly about 0.025 percent to about 0.045 percent carbon, up to about 0.10 percent each of manganese and silicon, up to 0.010 percent nitrogen, and remainder iron. Certain particular benefits are had by including in the composition of the steel columbium in the amount of about 0.1 percent to about 0.2 percent, as noted below.

A further steel according to my invention essentially consists of about 1 1.85 percent to about 12.75 percent chromium, about 8.20 percent to about 8.65 percent nickel, about 1.50 percent to about 1.85 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.3 percent to about 1.65 percent aluminum, with carbon not exceeding about 0.05 percent and desirably about 0.025 percent to about 0.045 percent, with manganese and silicon each not exceeding about 0.10 percent and preferably each not exceeding about 0.050 percent, with nitrogen not exceeding 0.010 percent and particularly not exceeding about 0.007 percent, and remainder iron. Here again, columbium advantageously is included in the composition, this in amounts up to about 0.3 percent, especially about 0.1 percent to about 0.2 percent.

Another steel essentially contains about 12.5 percent to about 13 percent chromium, about 8 percent to about 8.4 percent nickel, about 1.75 percent to about 2 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.15 percent to about 1.4 percent aluminum, with carbon up to about 0.050 percent and more especially about 0.025 percent to about 0.045 percent, with manganese and silicon each not exceeding about 0.10 percent particularly each of these two ingredients not exceeding about 0.05 percent, with nitrogen not exceeding 0.010 percent and particularly not exceeding about 0.007 percent, and remainder iron. Again, columbium advantageously is included in the composition, this in the amount of about 0.1 percent to about 0.2 percent.

In my steel, the amounts of the ingredients chromium, nickel, molybdenum, copper and aluminum are in every sense critical, as suggested above; where there is any significant departure from the compositional limits of these several ingredients, with either greater amounts than the upper limits set or lesser amounts than the set lower limits, one or more of the desired properties is lost or adversely affected. For example, with a chromium content less than the lowest permissible limit of about 10.5 percent, corrosion-resistance suffers. So, also, there is a loss of strength. And with a chromium content exceeding about 13.25 percent, the hardenability is adversely affected. With a nickel content less than about 7.5 percent, the steel is inclined to contain an excessive amount of deltaferrite, hot-workability suffers, and strength, especially in direction transverse to working, is lost. Moreover, the steel is inclined to harden prematurely. Where the nickel content exceeds about 9.5 percent, the metal becomes too stable and does not readily lend itself to hardening by heat-treatment.

In my steel molybdenum is an essential ingredient. Not only does this ingredient lend a certain improvement to the corrosion-resistant characteristics of the metal, but it inhibits pitting. Additionally, it lends strength to the steel. And, most importantly, the molybdenum content inhibits overaging, that is, a sacrifice of the desired strength where the hardening heattreatment is inadvertently conducted at somewhat excessive temperatures.

Additionally, strength is assured by the copper content of the steel. Any tendency of overaging is further inhibited by the ingredient columbium where employed.

As particularly illustrative of the steel of my invention, 1 give below in Table 1(a) a series of heat-hardenable chromium-nickel stainless steels, some of a composition according to TABLE 1(a) Chemical composition of a series of chromium-nickel stainless steels Percent Heat No, C Cr N1 M0 Cu Al Cb Tl N 1 Steel according to the invention.

Nora-Manganese less than 0.01%, silicon 0.05%, phosphorus 0.002%, sulphur 0.005%.

my invention in which a desired combination of properties is had, and for comparative purposes, others not according to my invention of such composition that one or more of the desired properties is lost; the mechanical properties of the steels of Table 1(a) in the age-hardened condition are presented hereinafter in Table 1(b).

The mechanical properties of the steels of Table 1(a) are given below in Table I(b). These steels in the form of 30- pound ingots were soaked at 2,150 F. for 4 hours and air cooled, equalized at 2,150 F. and forged to 2 inch by 2 inch sections, hot-rolled to bar size, and cut for tensile, micro and impact samples. The samples were solution-treated at l,700 F. for one-half hour and quenched in oil, then age-hardened by heating at 950 F. for 4 hours, followed by cooling in air. Tests were made of tensile strength in pounds per square inch, 0.2 percent yield strength in pounds per square inch, percent reduction in area, percent elongation in 2 inches, Rockwell hardness on the C-scale, and Charpy V-notch impact strength in foot-pounds.

1 Steel according to the invention.

Study of the test data presented above, this taken with the composition of the steels reported on, clearly shows that those steels in which copper is absent (the Heat Nos. 153, 154 and 156) are of inadequate tensile strength. In each case the tensile strength is about 230,000 psi, in any event significantly below the desired figure of 240,000 psi. With the copper addition, however (the heat No. 159), the desired strength is had. The impact strength developed in this steel is a bit low for many applications. Copper, then, is seen to be an ingredient essential to my steel.

A further series of heat-hardenable chromium-nickel stainless steels of somewhat modified chemical composition is set out below in Table (0), with the properties of these further steels, aged at 900 and 950 F., being presented in Table II(b).

TABLE II(a) Chemical composition of a furthetr series of chromium-nickel stainless s ee s Percent Heat No. C Cr Ni Mo Cu Al Cb N 1 Steels according to the invention. 0 g$a.-Manganese 0.01%, silicon 0.02%, phosphorus 0.003%, sulphur Hard- Impact,

Aging P.s.1 Percent ness, ft.

Hea temp., 0- lbs. No. deg. F. Tensile Yield RA. Elong. Rock.

l Steels according to the invention. Premature failure.

It will be seen from Tables [1(a) and ll(b) that those chromium-nickel stainless steels containing about 13 percent chromi um and about 8 percent nickel, with about 1.5 percent molybdenum and about 1 percent copper (the heat No. 186), or containing about 2 percent molybdenum and 2 percent copper (the beat No. 187) enjoy a good combination of strength and ductility. But, here again, the impact resistance is low for some applications. Whether aged at 900 or 950 F., the tensile strength of both steels is in excess of 240,000 psi and the elongation values exceed 11 percent. In the steel of the 2 percent copper and 2 percent molybdenum (the heat No. 187) there is had an impact strength sufficient for most applications. The steel containing about 2 percent molybdenum and about 3 percent copper (the heat No. 188), however, is deficient in strength, this amounting to only some 220,000 psi when aged at 900 F., and even less when aged at 950 F even though the steel enjoys good impact strength.

The chemical composition of five further chromium nickel stainless steels is given in Table lll(a) below:

TABLE lll(a) Chemical composition of an additional series 01 chromium-nickel stainless stee s Steels according to the invention in which a best combination of properties is had.

I Steels according to the invention.

N0'rE.-Manganese 01%, silicon .01/02%, phosphorus 0.001%, sulphur .002/.004%.

The mechanical properties of the steels of Table lll(a) with samples prepared as before but aged by heat-treatment at 1,000 F. for one set and at 1.050" F. for another, are set out below in Table lll(b).

TAB LE lll(b) Mechanical properties of the steels of Table lll(a) Hard- Aging P.s.1 Percent noss, Impact,

Heat temp., C- it No. deg. F. Tensile Yield R.A. Elong. Rock. lbs. 206 1, 000 246,100 229, 000 47. 2 11.7 49. 5 12 206 1,050 226, 214, 600 54. 1 13. 0 47. 5 35 207 1, 000 257, 700 236, 900 33.0 11. 5 49. 5 10 207 1, 050 239, 200 225,000 50. 8 12.0 41). 5 10 208 1, 000 248, 000 231, 000 44. 0 11.7 49. 5 5 208 1,050 224, 100 213,200 59. 1 13. 5 47. 5 13 209. 1,000 234,300 230, 100 1 40. 5 4 209. 1, 050 225, 500 215, 500 3.2 3. 2 48.0 3 211 1,000 240,200 228,000 39. 4 10. 5 49. 0 5 211 L 1,050 225, 200 212, 400 48. 1 11. 5 47. 0 7 1 Steals according to the invention in hich a best combination of properties is had.

Z Stee s according to the invention.

Now in studying the mechanical properties and chemical compositions of the steels reported in Tables lll(a) and lll(b), it will be seen that the steels containing about 12 percent chromium, about 8 percent nickel, about 1.5 percent molybdenum and about 1 percent copper (the heat Nos. 206, 207 and 208) enjoy a good combination of strength, ductility and impact resistance. Especially good results are had in the steels (heat Nos. 206 and 207) which additionally contain columbium in the amount of 0.15 percent. of these two, the one (beat No. 206) having an aluminum content of about 1.4 percent, while of lower tensile strength than the steel having an aluminum content of about 1.6 percent (heat No. 207), is characterized by significantly higher impact strength and somewhat better ductility.

While the ingredient titanium may be effectively substituted for columbium in these steels, this substitution is accompanied by some loss in impact strength, as quickly may be seen by comparing the impact resistance values of the titanium-bearing steel (the heat 208), having an impact strength of about ft.-lbs., with the columbium-bearing steel (the heat No. 206) having an impact strength of at least 12 ft.-lbs., the tensile strengths of both steels being in excess of 240,000 psi when aged at 1,000 F.

In all three of the steels, best results are achieved when aging is had at 1,000 F.; the higher aging temperature of 1,050 F., although resulting in greater impact strength, gives a lower tensile strength. Particularly is this noted in the steel of the lower aluminum content (the heat No. 206), where the 50 difference in aging temperatures results in a difference of some 20,000 psi in tensile strength, even though the impact strength is just about tripled. A best balance of tensile strength and impact strength is had in the steel of the somewhat higher aluminum content (the heat No. 207), where the tensile strength is consistently high and there is had good impact strength.

In my steel the ingredient molybdenum is considered to be essential, for with molybdenum omitted, and even with copper increased and nickel decreased to maintain the balance of the composition (heat No. 209), both tensile strength and impact strength directly suffer, whether aging is had at 1,000 or 1,050 F. And the ductility of the steel suffers even more than its impact strength.

Although under some circumstances the ingredient cobalt may be substituted for a portion of the nickel content of my steel, this ordinarily is not recommended. As may be seen by comparing the steels of Heat Nos. 21 1 and 186, both containing about 13 percent chromium, about 1.5 percent molybdenum and about 1 percent copper, in which about 2 percent cobalt in the heat No. 21 l is substituted for 2 percent of the nickel in heat No. 186, the cobalt-bearing steel, while of somewhat improved impact strength, is somewhat lower in tensile strength. A best combination of strength and impact resistance is had in the cobalt-bearing steel by aging at 1,000 F.

Some eight additional chromium-nickel stainless steels of modified chemical composition are reported in Table 1V(a) below, these all containing about 12 percent chromium, 9 percent nickel, as well as the ingredient columbium.

TABLE IV(a) Chemical com position of another series of chromium-nickel stainless steels Percent Heat No. C Cr Ni Mo Cu Al Cb N 243 l 035 12. 10 8. 78 2. 02 02 1. 28 .18 000 236 2 .031 12.13 0.17 1. 55 07 1. 50 .18 007 237.... .025 12.23 8.75 .01 1.78 .17 .007 241.... .033 12.23 8. EH 1.00 1.62 .16 .009 238.. .035 12.22 8. 88 .01 1.78 .10 .010 2-10. 032 12. 11) T. 04 1. 51 3. 08 1. 00 18 005 211 .030 12.10 8.50 2.22 1.1-1 1.80 18 .005 24'. .030 12.33 8. 00 1. 50 .04 1. 53 .18 005 steels according to the invention in which a best combination of properties is hnd.

Steels according to the invention.

Norm. Manganese .01/.02%, silicon .08/.14%, phosphorus .002/.003%, sulphur .005/.0025%.

Test samples of the eight steels of Table lV(a) were prepared as before with aging by heat-treatment at 1,000 and at l,050 F. The average mechanical properties of duplicate samples of each for the two differing heat-treatments are reported below in Table lV(b).

TABLE IV(b) Mechanical properties of the steels 01 table IV(a) Hard- Aging P.s.i Percent ness, Impact,

Ileat temp., it No. deg. F. Tensile Yield R.A lbs.

243 1, 000 243, 300 223, 800 43. 0 13 243 1, 050 219, 206, 100 53. 8 46 236 1, 000 240, 000 220, 300 35. 9 0 236. 1, 050 225, 600 212, 600 50. 2 2'.) 237 1 000 257, 000 236, 100 3 237 1, 050 232, 500 223, 000 30. 3 3 244 1 000 "46, 200 230-800 3 244 1 050 227,000 215, 000 30. 0 3 238 1 000 3 238 l 050 241. 300 230. 300 35. 4 3 240 1, 000 30, 000 210, 000 42. 2 l5 2-10 l, 050 .200, 300 107. 700 52. 7 -15 241 l 000 .255, 000 23., 000 41. T 7 -241 1, 050 227, 000 .214, 100 55.0 20 242 1, 000 255, 700 233, 200 31. 0 -1 2 12 1, 050 220, 000 217, 700 51. 8 10 properties is bad.

1 Steels according to the invention.

Even in the steels containing about 12 percent chromium and about 9 percent nickel, as may be seen from the test data presented immediately above, molybdenum is a necessary and essential ingredient. In comparing the steels containing about 12 percent chromium, about 9 percent nickel and about 1 percent copper, with some 1.3 to 1.8 percent aluminum, and free of molybdenum (the heat Nos. 237, 244 and 238) with the molybdenum-bearing steels of like chromium, nickel, copper and aluminum contents (the heat No. 243 with about 2 percent molybdenum and the heat No. 236 with about 1.5 percent molybdenum), it will be seen that all three of the steels which are free of molybdenum have rather poor impact values, this only amounting to about 3 ft.-lbs. Although the tensile strengths of the steels aged at about 1,000 F. appear to be generally satisfactory, it is the shortage in impact strength which makes these steels unacceptable.

The steels containing about 12 percent chromium, about 9 percent nickel, about 1 percent copper, with molybdenum about 1.5 percent (the heat Nos. 236 and 242), molybdenum about 2 percent (the heat No. 243 and molybdenum about 2.25 percent (the heat No. 241), when aged at about 1,000 F., not only enjoy a tensile strength in excess of 240,000 psi, but an acceptable impact strength. The best combination of tensile strength and impact strength is had in the steel containing molybdenum in the amount of about 2 percent (heat No. 243). The somewhat superior tensile strength had in the steel of beat No. 242 I attribute to the slightly higher chromium and aluminum contents and the slightly lower nickel and copper contents (12.33 percent chromium and 1.53 percent aluminum, with 8.96 percent nickel and 0.94 percent copper for the heat No. 242 as compared to 12.13 percent chromium, 1.50 percent aluminum, with 9.17 percent nickel and 0.97 percent copper for the heat No. 236).

The steel having a chromium content of about 12 percent and a nickel content of only about 7.5 percent, with molybdenum about 1.5 percent and copper about 3 percent, this in partial substitution for the lowered nickel content (heat No. 240) is not satisfactory. Although the impact strength is high, the tensile strength is inadequate, falling as it does, significantly below 240,000 psi in the aged condition, even where aging is had at 1,000 F.

In further illustration of a steel enjoying a best combination of strength, ductility and impact resistance, there was melted a steel according to the specification given in Table V(a) below, there also being indicated the actual chemical analysis of the steel.

TABLE V(a) Chemical composition of a best steel according to the invention Percent C Cr N 1 Mo Cu Al Cb N Specification .025/.045 11.50/12. 25 8. 40/800 1.35/l.60 1.00/1.50 1.50/1.75 .12/. 17 .010 Heat No. (60689) .036 12.07 8.81 1.61 1.21 1.04 .20 0.000

1 Maximum Manganese 0.01%, silicon 0.05 phosphorus 0.007%, sulphur 0.003%. 2 Steel according to the invention in which a best combination of properties is had.

L Tensile specimens and impact specimens were cut from the steel in the form of Va inch diameter hot-rolled bar, duplicate samples being solution-treated at 1,500, 1,600 and 1,700 F., oil quenched, machined and aged with particular aging as given in Table V(b) below reporting on tensile strength in pounds per square inch, 0.2 percent yield strength in pounds per square inch, percent reduction in area, percent elongation in four times the diameter, and Charpy V-notch impact strength in foot-pounds.

TABLE V(b) Mechanical Properties of the Steel of Table V(a) for Differing Solution-Treating Temperatures Sample Tensile Yield Impact No. R.A. Elong. psi psi Ft.-Lbs. 1 46.7 9.3 262,300 249,700 7 Samples 1 and 2 l,500 F. for 1% hours; oil quench;

machine; 1,000 F. for 4 hours; air cool.

Samples 3 and 4 1,600 F. for 1% hours; oil quench;

machine; 1,000 F. for 4 hours; air cool.

Samples 5 and 6 l,700 F. for 1% hours; oil quench and cool to 60 F.; machine; 1,000 F. for 2 hours; cool to room temperature; 1,000 F. for 2 hours; water quench.

From the results presented immediately above, it will be seen that there is had an excellent combination of' tensile strength, ductility and impact strength. While somewhat greater strength is achieved with a solution-treating temperature of 1,500 F good strength nevertheless is had where the steel is solution-treated at 1,600 F., or even 1,700 F. In point of fact, somewhat greater ductility and impact strength are had with the l,700 F. solution-treating temperature. In every case, aging is had at 1,000 F. whether for 4 hours continuously or two agings at 2 hours each. The tensile strength in all instances is well above 240,000 psi; indeed, for samples aged following solution-treatments at 1,500 and 1,600 F., even the yield strength exceeds 240,000 psi.

In my steel some improvement is had by increasing the duration of the aging treatment beyond about 4 hours. Duplicate samples of the steel of heat No. 60689, solution-treated at l,500 F. for 1 hour, oil quenched, machined and aged at 1,000 F. for 8 hours, and for 12 hours, and air cooled, were tested, with the results set out below in Table V(c).

Samples 11 and 12 l,500 F. for 1 hour; 011 quench;

machine; 1,000 F. for 8 hours; air cool. Samples 13 and 14 l,500 F. for 1 hour; oil quench;

machine; 1,000 F. for 12 hours; air cool. As indicated, some benefit is had by the prolonged aging. With the prolonged aging, the strength falls off a bit, as may be seen by comparing the mechanical properties of the samples ll-12and 13-14 given above with the samples 1-2 of the Table V(b). But with the loss of strength, that is, from an average of 262,800 psi for samples 1-2 to an average of 254,500 psi for samples 11-12, and down to 248,700 psi for samples 13-14, there is some improvement in ductility, the elongation increasing from an average of 9.6 percent for the samples l-2, to 11.4 percent for the samples 11-12 and 13-14, and the impact strength is increased, thisfrom some 7 or 8 ft.-lbs. to some 13 or 14 ft.-lbs.

In conclusion, it will be seen that I provide in my inventiona precipitation-hardenable chromium-nickel-molybdenumcopper stainless steel in which there are had the various objects of my invention, and the many advantages thereof, as more particularly set out above. In my steelthere is enjoyed a combination of great strength in the age-hardened condition. that is, a strength exceeding 240,000 psi, together with toughness and impact strength.

My steel works well'both in the hot-mill and in the cold-mill, readily lends itself to a variety of forming and fabricating operations, such as machining, drawing, rolling, upsetting, and the like, following which the steel may be hardened by simple heat-treatment, that is, by mere aging at moderate temperatures, to achieve a combination of strength and toughness.

Actually, in the production of machine screws and other fasteners I find that maximum uniformity in properties is achieved, as suggested above, by reheating the fasteners to solution-treating temperature (some 1,400 to l,750 F.) and then aging the same, this at the 900 to 1,050 P. temperature. This treatment conveniently is handled as a batch operation. The solution-treating step alleviates the stresses introduced in manufacture. And with the subsequent aging treatment there are had properties which are uniform throughout the batch.

Inasmuch as there are many embodiments which may be made of my invention, and numerous changes made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not by way of limitation.

Iclaim:

1. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding about 0.05 percent, nitrogen not exceeding 0.015 percent, and remainder substantially all iron.

2. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 10.5 percent to about 12.5 percent chromium, about 8 percent to about 9.5 percent nickel, about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, carbon not exceeding about 0.05 percent, nitrogen not exceeding about 0.01 percent, and remainder substantially all iron.

. 3. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about l.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, with at least one ingredient of the group consisting of columbium up to about 0.3 percent and titanium up to about 0.15 percent, carbon not exceeding about 0.05 percent nitrogen not exceeding .01 percent, and remainder substantially all iron.

4. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.85 percent to about 12.75 percent chromium, about 8.20 percent to about 8.65 percent nickel, about 1.50 percent to about 1.85 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.5 percent to about 1.65 percent aluminum, up to about 0.3 percent columbium, carbon not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.

5. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.50 percent to about 12.25 percent chromium, about 8.40 percent to about 8.90 percent nickel, about 1.35 percent to about 1.60 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.50 percent to about 1.75 percent aluminum, ab0ut0.1 percent to about 0.2, percent columbium, about 0.025 percent to about .045 percent carbon, nitrogen not exceeding 0.010

percent, and remainder substantially all iron.

6. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 12.5 percent to about 13 percent chromium, about 8 percent to about 8.4 percent nickel, about 1.75 percent to about 2 percent molybdenum, about 1.4 percent to about 1.5 percent copper, about 1.1 percent to about 1.4 percent aluminum, about 0.025 percent to about 0.045 percent carbon, manganese and silicon each not exceeding about 0.05 percent, nitrogen not exceeding 0.010 percent, and remainder substantially all iron.

7. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9 percent nickel and cobalt taken together with cobalt being up to about 2 percent and with nickel being at least 6 percent, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding 0.05 percent, nitrogen not exceeding 0.015 percent, and remainder substantially all iron.

8. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 6.5 percent to about 9 percent nickel, about 1 percent to about 2 percent cobalt,

about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding about 0.05 percent nitrogen not exceeding 001 percent, and remainder substantially all iron.

9. Precipitation-hardenable martensitic chromium-nickel stainless steel flat-rolled products essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3 percent columbium, carbon not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.

10. Precipitation-hardenable martensitic chromium-nickel stainless steel forgings essentially consisting of about 1 1.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about 1 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper. about 1.1 percent to about 1.8 percent aluminum, with carbon not exceeding about 0.05 percent, manganese and silicon each not exceeding about 0.10 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.

11. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding about 0.05 percent, manganese and silicon each not exceeding about 0.10 percent, nitrogen not exceeding 0.015 percent, and remainder substantially all iron.

12. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3 percent columbium, with a carbon content not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and

remainder substantially all iron.

13. Precipitation-hardenable martensitic stainless steel essentially consisting of 9 11.5 percent to about 13 percent chromium, about 8 percent to about 9 percent nickel, about 1.3 percent to about 2 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.1 percent to about 1.4 percent aluminum, with columbium up to about 0.3 percent, about 0.02 percent to about 0.05 percent carbon, with manganese and silicon each not exceeding about 0.10 percent,

nitrogen not exceeding 0.007 percent, and remainder substantially all iron.

Claims

2. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 10.5 percent to about 12.5 percent chromium, about 8 percent to about 9.5 percent nickel, about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, carbon not exceeding about 0.05 percent, nitrogen not exceeding about 0.01 percent, and remainder substantially all iron.
3. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 Therefore percent copper, about 1.1 percent to about 1.8 percent aluminum, with at least one ingredient of the group consisting of columbium up to about 0.3 percent and titanium up to about 0.15 percent, carbon not exceeding about 0.05 percent nitrogen not exceeding .01 percent, and remainder substantially all iron.
4. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.85 percent to about 12.75 percent chromium, about 8.20 percent to about 8.65 percent nickel, about 1.50 percent to about 1.85 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.5 percent to about 1.65 percent aluminum, up to about 0.3 percent columbium, carbon not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.
5. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.50 percent to about 12.25 percent chromium, about 8.40 percent to about 8.90 percent nickel, about 1.35 percent to about 1.60 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.50 percent to about 1.75 percent aluminum, about 0.1 percent to about 0.2 percent columbium, about 0.025 percent to about .045 percent carbon, nitrogen not exceeding 0.010 percent, and remainder substantially all iron.
6. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 12.5 percent to about 13 percent chromium, about 8 percent to about 8.4 percent nickel, about 1.75 percent to about 2 percent molybdenum, about 1.4 percent to about 1.5 percent copper, about 1.1 percent to about 1.4 percent aluminum, about 0.025 percent to about 0.045 percent carbon, manganese and silicon each not exceeding about 0.05 percent, nitrogen not exceeding 0.010 percent, and remainder substantially all iron.
7. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9 percent nickel and cobalt taken together with cobalt being up to about 2 percent and with nickel being at least 6 percent, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding 0.05 percent, nitrogen not exceeding 0.015 percent, and remainder substantially all iron.
8. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 6.5 percent to about 9 percent nickel, about 1 percent to about 2 percent cobalt, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 perCent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding about 0.05 percent nitrogen not exceeding 0.01 percent, and remainder substantially all iron.
9. Precipitation-hardenable martensitic chromium-nickel stainless steel flat-rolled products essentially consisting of about 11.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3 percent columbium, carbon not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.
10. Precipitation-hardenable martensitic chromium-nickel stainless steel forgings essentially consisting of about 11.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about 1 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, with carbon not exceeding about 0.05 percent, manganese and silicon each not exceeding about 0.10 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.
11. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, carbon not exceeding about 0.05 percent, manganese and silicon each not exceeding about 0.10 percent, nitrogen not exceeding 0.015 percent, and remainder substantially all iron.
12. Precipitation-hardenable martensitic chromium-nickel stainless steel essentially consisting of about 11.5 percent to about 13 percent chromium, about 7.5 percent to about 9 percent nickel, about 1.3 percent to about 2.25 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3 percent columbium, with a carbon content not exceeding about 0.05 percent, nitrogen not exceeding 0.01 percent, and remainder substantially all iron.
13. Precipitation-hardenable martensitic stainless steel essentially consisting of 9 11.5 percent to about 13 percent chromium, about 8 percent to about 9 percent nickel, about 1.3 percent to about 2 percent molybdenum, about 1 percent to about 1.5 percent copper, about 1.1 percent to about 1.4 percent aluminum, with columbium up to about 0.3 percent, about 0.02 percent to about 0.05 percent carbon, with manganese and silicon each not exceeding about 0.10 percent, nitrogen not exceeding 0.007 percent, and remainder substantially all iron.