US3532491A - Maraging steel suitable for heavy section applications - Google Patents

Description

UITABLE FOR HEAVY SECTION APPLICATIONS FLOREEN ET AL MARAGING STEEL s Filed May F/GJ INVENTORS STEPHEN FLOREEN CHARLES JOSEPH NOVAK BY 'ML ATTORNEY 3,532,491 Patented Get. 6, 1970 3,532,491 MARAGING STEEL SUITABLE FOR HEAVY SECTION APPLICATIONS Stephen Floreen, Suffern, N.Y., and Charles Joseph Novak, Ringwoorl, N.J., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 575,126, Aug. 25, 1966. This application May 14, 1969, Ser. No. 830,909

Int. Cl. C22c 39/10, 41/02 U.S. Cl. 75-123 14 Claims ABSTRACT OF THE DISCLOSURE A maraging steel directed to overcoming the deleterious effects attributable to the occurrence of segregation in the production of heavy sections. The steel contains correlated amounts of nickel, cobalt, molybdenum and titanium, and also most advantageously vanadium and zirconium.

hot and cold workability, (d) ease of machinability, (e)

excellent castability, (f) substantial dimensional stability and freedom from distortion, (g) simple heat treatment, etc. Suffice to say, such an imposing list of characteristics has contributed greatly to the rather spontaneous commercial acceptance accorded these steels in diverse fields of application.

The aforementioned virtues notwithstanding, a persistent and vexatious problem evolved, particularly in the production of heavy sections, e.g., forgings. The basic nature of the problem manifested itself in a hitherto propensity for the steels to develop segregation characteristics which in the absence of special and somewhat expensive and tedious processing technique, contributed to an undesirable loss in tensile ductility and toughness (ability to absorb impact energy), properties for which the steels had become notable. This undesirable effect was particularly prominent in heavy sections and when ductility and toughness were determined in the short transverse direction. In the most pronounced and not infrequent instances, banding occurred often followed by internal lamination formation. Of course, such metallurgical behavior, unless obviated, would continue to be a serious drawback in heavy section applications, landing gears being illustrative. Elimination of deleterious segregation effects, including banding and lamination, represents the specific problem to which the invention is addressed.

Noteworthy to the present invention was ascertaining the apparent cause of segregation and subsequent banding and lamination. During the course of a general investigation in which others also participated, the appearance of intermetallic particles was noted upon solidification of melts, an observation which unto itself may not have been too remarkable, since, from a general metallurgical viewpoint, such particle formations per se are not uncommon. Certainly important, however, was the observation that these particles often seemed to be of a localized nature, i.e., they appeared to be more heavily concentrated in certain regions. Examination revealed these regions or pools to be greatly enriched in molybdenum and titanium. Mention of molybdenum and tita nium focuses attention on the specifics of the problem since it is considered that both of these elements promote formation of austenite in the prior art maraging steels and this factor serves to at least partially explain the nature of the banding. Studies established that the banding generally consisted of soft austenite strips enveloped or backed by hard, brittle martensite-thus, the role of the enriched region or pools of austenite promoters. Left to be resolved was how the austenite formed.

As is now generally known, conventional treatment of the 18% nickel maraging steels consists in solution treating at 1500 F. for one hour (formation of austenite) cooling, whereby transformation from austenite to martensite occurs, aging the martensite at about 900 F. for one hour and again cooling. Lower temperatures, e.g., 800 F., have not been recommended since the steels may be underaged with a consequent loss of strength. On the other hand, as the aging temperature is increased, greater is the tendency for austenite reversion to occur. If present in sufficient amounts, austenite greatly detracts from strength. Accordingly, aging temperatures appreciably above 900 F. have usually not been recommended and have been avoided.

Now, it was during the aging treatment that dark bands were noticed which were associated with the small localized regions or pools aforediscussed. It was subsequently deemed that these dark bands containing the small pools represented the early stages or the onset of austenite reversion. Apparently, because of the aforementioned unusual concentration of austenite formers in the regions of segregation, a localized austenite reversion temperature was set up, a temperature lower than the aging temperature. Thus, the door was opened for reverted austenite to form whence came the austenite bands substantially encompassed by the hard, brittle martensite. Too often as a result of the banding, internal laminations occurred.

Various solutions to the problem suggested themselves, including the exclusion of molybdenum and titanium from the steels. This possibility was hardly attractive since molybdenum confers both strength and toughness and titanium is known to be the most influential supplemental hardener. These considerations are reflected in Table I in which there is representatively set forth the compositions (principal constituents) of the three commercially established 18% nickel maraging steels, to wit, the 18 Ni 200, 18 Ni 250 and 18 Ni 300 steels (200, 250 and 300 denote yield strength in thousands of pounds per square inch at 0.2% offset determined on bar):

It will be noted, whereas nickel and cobalt remain substantially constant, the molybdenum and/or titanium increase as yield strength goes from 200,000 to 300,000 p.s.1.

Proposals involving the utilization of modified melting and hot working techniques, soaking practice, and fast cooling rates contribute to alleviating the severity of the problem, but such procedures too often circumscribe or limit the section sizes that might otherwise be produced. For example, achieving sufficiently rapid cooling rates in respect of thick sections, as a practical matter, is virtually impossible. As a further example, while a long soaking period (24 hours) to develop a more thoroughly homo genized structure assists in minimizing banding, it has the attendant and uneconomical disadvantage of tying up equipment for considerable periods of time. What was really needed was a new maraging steel which by virtue of its composition and without more would result in an end to the problem but without sacrifice in mechanical characteristics, particularly strength and toughness.

It has now been discovered that segregation and attendant problems heretofore associated with heavy section (i.e., at least /2 or /8 inch in thickness, and particularly 1 inch or more) maraging steels of the 18% nickel-cobaltmolybdenum type can be greatly minimized, indeed obviated, provided the steels contain special amounts of various constituents, including nickel, molybdenum, c0- balt, titanium, vanadium, zirconium, chromium, etc. This has been achieved, as will be illustrated herein, without loss in mechanical properties. In fact, the magnitude of the very property, toughness, so drastically affected heretofore is not only regained but has been actually improved in some instances upwards of about 50% or more at corresponding strength levels. Of considerable commercial significance, departure from conventional processing and standard heat treatments is not necessary, although other heat treatments can be employed to advantage.

It is an object of the present invention to provide maraging steels of novel composition.

It is a further object of the invention to provide new and improved maraging steels substantially devoid of detrimental segregation effects.

Other objects will become apparent from the following 3 description taken in conjunction with the accompanying drawing in which FIGS. 1 and 2 are reproductions of photomicrographs of the microstructure of a prior art alloy and an alloy within the invention, respectively.

Generally speaking and in accordance with the present invention, maraging steels contemplated herein contain (in percent by weight) about 14% to about 22% nickel, about 12% to 25% cobalt, from 0.9% to 4% molybdenum, up to 0.4% titanium, up to about 0.1%, e.g., up to 0.05%, zirconium, up to 2% vanadium, up to 0.025%, e.g., up to 0.02%, magnesium, up to 3% chromium, up to 0.4% aluminum, up to 0.15%, e.g., up to 0.05%, carbon, and the balance essentially iron. In achieving a highly satisfactory combination of characteristics it is quite advantageous that the steels contain zirconium. Beneficially the steels contain vanadium and an optimum combination of strength and toughness is obtained with alloys containing both zirconium and vanadium. Elements such as phosphorus, nitrogen, oxygen and particularly sulfur and the like should be kept to low levels as is consistent with good commercial steelmaking practice. However, other constituents can be present as shown in Table II.

In carrying the invention into practice, the nickel content should not exceed about 22%; otherwise, the formation of unwanted austenite is encouraged, the martensitic transformation temperature being lowered. On the other hand, low nickel levels result in an undue loss of strength and toughness. At least 14% nickel should be present, a most suitable range being about 15.5% to 20.5%.

While in accordance with the aforementioned U.S. Pat. No. 3,093,519, up to 10% molybdenum can be present in prior maraging steels and while about 5% or more is utilized commercially to achieve yield strength levels of, say, 275,000 p.s.i. and higher, for heavy section applications as contemplated herein, special molybdenum control must be exercised. Amounts above about 4% can promote austenite reversion which, in turn, could revive the very problem obviated. But the steels must contain molybdenum to attain adequate strength and toughness. Although amounts of 1% can be used, at least 2% and beneficially at least 2.5%, and up to 3.5% should be present in attaining the best combination of strength and toughness. Where maximum strength is of utmost importance the molybdenum may be as high as 4%, e.g., 3.5% to 4%. Titanium is not at all essential even in those steels in Which strength levels of 250,000 to 300,000 p.s.i. are required, a specific objective of the invention. However, titanium in amounts of about 0.05% to 0.2% contributes to good deoxidation and results in improved toughness characteristics.

Cobalt serves many functions. Importantly, it neither contributes nor promotes the cause of segregation. Over the ranges contemplated herein, it raises the martensitic transformation temperature thereby inhibiting austenite occurrence that might otherwise be formed through reversion. Additionally, it has been unexpectedly found to impart markedly enhanced strength, provided at least about 12%, more advantageously at least 13%, is present in the steels. A particularly satisfactory range is from 13% to about 20%. However, when the molybdenum content is below about 2%, it is preferred for maximum strength and reasonably good toughness that the cobalt be from 15% to 20%.

As indicated herein, with'the more advantageous steels, toughness is not only regained but markedly improved. For example, the heretofore standard 18% Ni 250 grade of maraging steel absorbs an impact energy level of about 18 to 19 foot-pounds (ft-lbs.) in unidirectionally rolled Az-inch plate (transverse direction to the rolling direction). In accordance herewith, impact energies of 30 ft.- lbs. can be attained at similar yield strengths. This is accomplished by incorporating zirconium, particularly in combination with vanadium, into the steels contemplated herein. While zirconium in amounts from at least 0.001% or 0.002% and up to 0.1% can be used, levels above about 0.05% are usually not necessary. The fact that zirconium confers enhanced toughness invites a rather striking comparison with other maraging steels, to wit, those popularly referred to as 12-5-3 steels (12% Ni-5% Cr-3% Mo), which are generally described in US. Pat. No. 3,262,777. It was found in these latter steels that zirconium markedly detracted from toughness, a role quite opposite to that performed in accordance herewith.

Zirconium is particularly effective in the presence of vanadium and the best combination of strength and toughness has been obtained in alloys containing both of these constituents. Up to 2% vanadium can be present but it is preferred that the steels contain not more than about 1.5%, advantageously not more than about 0.9%. In this connection, the steels should contain at least 0.2% vanadium.

Chromium also promotes toughness but it should not be present in amounts above about 3%. It has been found that chromium acts somewhat in a manner similar to zirconium. However, zirconium is preferred for several reasons. Should an excessive amount of chromium be present, through inadvertence or otherwise, the tendency of the chromium would be to stabilize austenite and result in inferior properties. Moreover, in casting large ingots, chromium tends to segregate. Accordingly, there would be the possibility of forming a localized segregation region rich in chromium. This could aggrevate the problem. Nonetheless, with reasonable control, chromium up to 3%, e.g., up to 2%, can be beneficial.

A most highly satisfactory maraging steel composition, one which offers exceptional strength and toughness with the absence of adverse segregation effects, is as follows: About 17% to 19% nickel, about 14% to 16% cobalt,

about 2.5% to 3.5% molybdenum, up to 0.2%, e.g., 0.05% to 0.2%, titanium, about 0.005% to 0.04% zirconium, about 0.4% to 0.9% vanadium, up to 0.2%, e.g., 0.05% to 0.2%, aluminum, up to 0.02% or 0.03% carbon, up to 0.1% or 0.15% of each of manganese and silicon, and the balance essentially iron. In consistently achieving yield strengths of above about 290,000 p.s.i., the nickel content should be from 17% to 19%, the cobalt from 19% to 21%, and the molybdenum from 3.5% to not more than about up to 4%, the percentages of the other constituents being as given before herein.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative data are given:

Two 100-pound air melts of maraging steel, one being outside the invention, were cast into sand molds that simulated a 140 square inch ingot. The ingots were initially homogenized at 2300 F. for about two hours, rolled to three inch thick billets and then aged three hours at 900 F. The nominal composition of the steel without the invention, a standard 18 Ni 250 grade, and of the steel within the invention are reproduced in Table III:

characteristic of this type of alloy in heavy section. In contrast thereto, the steel within the invention, FIG. 2, is devoid of austenite.

In addition to the foregoing, a substantial number of steels was prepared by vacuum induction melting and using conventional maraging procedures. Heats were homogenized at about 2300 F. and thereafter unidirectionally hot rolled to %-inch thick plate, the initial hot rolling temperature being about 1900 F. Tensile and Charpy V- notch impact energy specimens taken from the plate were tested in the transverse direction at room temperature. It should be emphasized that conventional maraging procedures were purposely followed in respect of preparation and testing of the steels. To the extent possible, this permitted the resulting data to be assessed from compositional efiects without the introduction of variables that might have been otherwise attributable to improved melting techniques, etc.

Each of the steels was subjected (with a few exceptions) to three heat treatments which involved annealing at 1500 F. for about one hour, cooling, and thereafter aging either for three hours at 800 F. (Heat Treatment A), 24 hours at 800 F. (Heat Treatment B) or for Each of the structures (Frys reagent used as etchant) was examined metallographically (100 magnifications), pho tomicrographs being shown in the accompanying drawing. FIG. 1 depicting the commercial 18 Ni 250 grade,

three hours at 900 F. (Heat Treatment C). The compositions, heat treatments, yield strengths in thousands of pounds per square inch (k.s.i.), percent elongation, and Charpy V-notch (C.V.N.) impact energy in foot-pounds shows marked segregation effects with austenlte pools 35 (ft-lbs.) are g1ven in Table IV.

TABLE IV Ni, Mo, 00, Ti, Al, Zr, V, Y.S., EL, C.V.N., percent percent percent percent percent percent percent percent H11. k.s.i percent ft,-1b

Alloy No.: B 204 15 47 1 17. 3 2.1 12.0 0.12 0.11 0.010 n.a n.a 3 60 1 6. 5 2 17. 4 3.1 12. 0 0.11 0.08 0. 003 11.3 n.a 213 14 58 A 238 11 22. 5 3 17.0 3.0 14.9 0. 26 0.12 0. 018 11.9. n.a B 263 11 17. 0 O 248 11 24.0 A 253 11 22. 5 4 19. 3 2. 9 14. 9 0.11 0. 06 0. 012 na n.a.. B 268 11 13 C 256 1 21 A. 262 12 19. 5 5 17.6 3.0 20.0 0.10 0.10 0.004 mm n.a B 275 10 19,0 C 266 11 19. 5

ZIRCONIUM EFFECT A 238 11 22. 5 3 17.0 3.0 14.9 0.26 0.12 0. 018 n a n a B 263 11 17 C 248 11 24 A 240 11 26. 5 6 17.1 2.9 14.7 0.08 0.10 0.003 001..-" na B 257 11 18.2 C 245 11 24. 2 A 242 12 27. 5 7 17.3 3.0 14.9 0.09 0. 29 0.015 0.0l4 n.a. B 261 11 24, 7 C 247 11 23. 5 A 240 11 22.0 8 17.3 2.9 15.0 0.10 0.18 0.010 0.1 n.a B 263 11 19.2 C 252 11 23. 5

VANADIUM EFFECT A 241 12 22. 5 9 18 3. 0 15. 4 11.21.. 0. 09 0.031 n.a 0.16". B 255 11 19, 0 O 244 12 21. 7 A 273 11 16.2 10 18. 6 3.0 15.6 net. 0. 11 0.015 n.a 0.50. B 283 11 16. 5 C 263 12 19. 5 A 246 10 23. 2 11 18. 0 3. 0 15.4 n.a. 0.12 0. 008 n a 1 0 B 268 11 19.1 C 250 12 24. 5 A 238 11 22. 5 3 17.0 3. 0 14. 9 0.26 0.12 0.018 n a n a B 263 11 17 C 248 11 24 A 254 11 21. 2 12 17. 3 3. 0 14. 8 0. 27 0. 14 0. 009 n a 0 5 B 278 11 15. 5 C 259 12 18. 0 A 248 11 24. 5 13 17. 2 3. 1 14. 7 0. 09 0. 14 0. 018 n a 0. 5 B 271 10 16. 5 G 255 13 21.0 A 239 13 27. 0 14 17. 1 3. 0 14. 9 0. 12 0. 2S 0. 021 n a.. 0.5 B 260 10 15!, 0 C 242 14 28.0

See note at end of table.

7 8 TABLE IV-Continued N1, M0, C0, Ti, Zr. V, Y.S EL, C.\".N., percent percent percent percent percent; percent percent percent H.T. k.s.1 percent tt.-lbs.

ZIRCONIUM PLUS VANADIUM EFFECT Alloy No.:

A 257 11 29 15 17.3 2 9 14.0 0.10 0.10 0.005 0.010.... 7 281 11 23.2 C 268 12 20.0 A 243 11 29.2 16 17.2 2 9 15.0 0.10 0.10 0.003 0.01-.." 0 19.....{13 261 12 27.5 C 251 12 29. p A 248 13 27.5 17 17.3 2 9 15.0 0.10 0.26 0.003 0.012.... 0 40 {B 269 12 23.5 C 260 12 24.0 A 256 12 25.5 18 17.3 3.0 14.8 0.10 0.16 0.006 0.014.... 0 53.....{13 280 11 19.2 C 263 11 24.0 A 248 11 27.2 19 17.6 3 0 14.9 0.11 0.16 0.010 0.030.... 0 50-....{13 271 11 22,0 X 363 12 23.7 2 20 18.3 3 0 10.7 11.11 0.12 0.011 0.01.. 0 5 {B is $13 g 233 12 29.5 21 17.3 3.0 14.7 11.11 0.19 0.011 0.013.... 1 {B 332 g C 262 10 15. 5 A 267 11 20. 2 22 17.3 3.0 14.7 0.12 0.20 0.016 0.015.... 1 04 {B 294 10 141 g 9 16.0 23 17.3 2.9 15.0 0.10 0.14 0.003 0.029 0 52 {B 27; O 267 10 24.0

NOTE.-n.a. =none added.

The data in Table IV confirm that outstanding mechanical characteristics are obtainable with alloys within the invention, alloys concerning which segregation was not a problem. Alloy No. l is on the lean side in respect of both molybdenum and cobalt and the yield strength is comparatively low; however, Alloys Nos. 3, 4 and 5 illustrate that yield strengths of 250,000 psi. and higher are easily obtainable with higher molybdenum and cobalt contents. Alloys Nos. 6, 7 and 8 (compared with Alloy No. 3 for convenience) reflect the general improvement obtained as a result of zirconium. Impact strengths as high as about ft.-lbs. at yield strength levels of 250,000 psi. and with low aging temperatures (800 F.) can be achieved. Vanadium also contributes to improved strength and toughness; however, the most notable combination of characteristics are associated with the steels containing both zirconium and vanadium, impact strengths as high as about 30 ft.-lbs. (Alloys Nos. 15 and 16) being attained at strength levels of about 250,000 p.s.i. Such properties also obtain with aging temperatures of 800 F. This is a noteworthy and significant feature of the invention. Heretofore, with low aging temperatures maraging steels would suffer by way of underaging as herein indicated. It might also be added that apart from lower strengths, as a result of underaging toughness was also often relatively poor. Now, however, the user is afforded the opportunity of selecting a much broader range of strength or toughness which would be best suited for a particular need or application and can heat treat accordingly.

It should also be mentioned that a significant influence in overcoming detrimental segregation is believed to stem from the fact that steels within the invention have M transformation temperatures substantially higher than those characteristic of the conventional maraging steels, the difference in temperature being of the order of about 150 F. to 200 F. For example, the M temperatures of Alloys Nos. 5, 9 and 11 are about 447 F., 485 F. and 480 F., respectively. Thus, it is deemed that such high M temperatures aflord greater freedom from both untransformed and reverted austenite whereby banding and lamination tendencies are greatly circumscribed.

Other features of the invention are illustrated by the data in Table V regarding steels processed in the same manner as the steels described in connection with Table IV. Steels identified by letters, to wit, A through G are outside the invention and serve to emphasize the importance, generally speaking, of observing compositional limits.

TABLE V N1, Me, O Ti, Al, Zr, Cr, Y EL, C.V.N. percent percent percent percent percent percent percent percent H.T. k.s.1. percent tt.-lbs.

Alloy A 253 11 22. 5 4- 19. 3 2. 9 14. 0 0.11 0 06 B 268 11 18 C 256 11 21 A 256 12 25. 5 18 17.3 3. 0 14. S 0. l0 0. 16 0. 014 0. 53 B 280 11 19. 2 C 263 11 24. 0 A 258 10 19. 5 24 17. 2 3.0 14.8 0.11 0.14 0.013 0.49 0 B 280 10 19. 5 C 270 11 21. 0 A 243 13 35. 5 25 17. 2 2. 9 14. 4 0. 09 0. 08 0. 44 B 200 12 29. 5 C 248 11 30. 7 A 275 11 19. 0 26 17.4 2.9 14. 3 0. 10 0. 11 1. 9 B 294 10 14.0 C 283 10 19.0 A 87 25 43.0 A 17. 4 2. 9 13. 8 0. 11 0. 09 3 7 B 30 72. 5 C 84 21 35 1; B 204 15 1 17.3 2.1 12.0 0.12 0 11 14 66 A 212 12 44. 0 27- 17. 6 1. 0 15. 6 0. 11 0 05 B 237 14 28. 7 C 223 12 44. 5 A 232 15 22. 5 28 17. 1 l. 0 20. 0 0. l2 0. 09 B 246 12 13. 5 C 230 13 20. 2 A 260 11 2. 5 B 17.3 0.9 24.8 0. l2 0 07 B 289 9 1, 2 C 270 8 1. 5 A 77 34 124 C 24.4 1. 9 20. 0 0. l2 0. 10 B 76 26 113 C 71 31 112 TABLE V-Cntinued Ni, Mo, C Ti, Al, Zr, V, Cr, Y. EL, C.V.N. percent percent percent percent percent percent percent percent H.I. k.s.1 percent ft.-lbs.

Alloy N 0.:

A 51 43 200+ 1)- 27.3 1.0 22.4 0.13 0 15 B 50 42 200+ C 50 43 200+ A 184 16 E 9.7 2. 2 19.8 0. 08 0 05 {B 207 14 C 186 15 11.2 A 184 14 F 18.1 19.5 0 09 0 05 {B 197 14 C 177 16 18 A 154 18 G 17.6 14.9 0.05 0 05 B 159 18 O 150 19 54. 3

N orE.Carbou content of steels not greater than about 0.03%.

In respect of the data in Table V, while the presence of chromium did not confer an improvement in steels containing both zirconium and vanadium as reflected by a comparison of Alloys Nos. 18 and 24, relatively small amounts thereof substantially enhanced toughness as shown by a comparison of Alloys Nos. and 4, Alloy No. 4 being devoid of zirconium and vanadium. Alloy A, outside the invention, indicates the subversive effect attributable to substantial (3.7%) amounts of chromium. The drastic loss in strength is a result of the presence of excessive austenite. While Alloy No. 26 shows a marked drop in toughness when compared with Alloy No. 25, it should be noted that strength is substantially higher. For maximum toughness, chromium should not exceed about 0.5% or 1%.

Alloys Nos. 1, 28, 29 and B illustrate other aspects of the invention, notably the strengthening effect of cobalt. However, when the molybdenum content is very low, the cobalt should not exceed 20% if a high level of tough ness is to be attained. This is shown by Alloys No. 28 and B. Alloys C through G illustrate the consequences of either excessive (C and D) or insufiicient (E) nickel or molybdenum-free steels (F and G).

In preparation of the steels, air melting practice can be employed although vacuum melting is preferred. Conventional processing can be used. For example, homogenization treatments can be carried out at about 2300 F. which temperature is also suitable for the beginning of forging. A temperature of about 2000 F.1900 F. is satisfactory as a finish forging (or hot rolling) temperature. The steels can be aged at a temperature of 700 F. to 1000 F. for about 1 to 100 hours, the longer periods being used with the lower temperatures. A most satisfactory aging treatment comprises heating within the range of about 775 F. to about 925 F. for about 1 to 24 hours. Where solution annealing is desired prior to aging, annealing temperatures of 1400 F. to 1600 F. for 1 to 3 hours are recommended although other suitable treatments can be employed as will be appreciated by those skilled in the art.

A preferred heat treatment for steels of about 275,000 p.s.i. and above consists in a solution annealing at 1500 F. for 1 hour, air cooling, aging for about 24 hours at 800 F. and then again cooling. For such strength levels annealing temperatures much above 1500 F. might result in a loss of properties. Shorter aging times or lower aging temperatures will result in enhanced toughness but lower strengths. An aging temperature of 800 F. for 3 hours is suitable in achieving yield strengths of about 250,000 p.s.i.

As indicated herein, steels within the invention are particularly suitable for production of heavy sections, e.g., forgings, plate, landing gear, die blocks, machine tool parts, fasteners, and the like. In addition to wrought forms, castings, particularly high strength castings, can also be provided.

As will be understood by those skilled in the art, the term balance or balance essentially used in referring to the iron content of the steel does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith in small amounts which do not adversely affect the novel characteristics of the steels.

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

We claim:

1. A maraging steel in heavy section of at least onehalf inch thickness and consisting of about 14% to 22% nickel, about 12% to 25% cobalt, from 0.9% to about 4% molybdenum, up to 0.4% titanium, up to about 0.1% zirconium, up to 2% vanadium, up to 0.025% magnesium, up to 3% chromium, up to 0.4% aluminum, up to 0.15% carbon, up to about 1% silicon, up to about 1% manganese, up to 2% tungsten, up to 0.01% boron, up to 1% beryllium, up to 6% copper, up to 3% columbium, up to 4% tantalum, up to 0.4% nitrogen, and the balance essentially iron, said steel being in the aged condition and being characterized by a microstructure which is devoid of detrimental segregation effects due to austenite reversion.

2. A steel in accordance with claim 1 and containing about 15.5% to 20.5% nickel, about 13% to 20% cobalt and about 2% to 3.5% molybdenum.

3. A steel in accordance with claim 1 in which titanium is present in an amount not exceeding about 0.2%.

4. A steel in accordance with claim 2 in which titanium is present in an amount not exceeding about 0.2%.

5. A steel in accordance with claim 1 containing about 0.001% to 0.1% zirconium and 0.2% to 1.5% vanadium.

6. A steel in accordance with claim 2 containing about 0.001% to 0.1% zirconium and 0.2% to 1.5% vanadium.

7. A steel in accordance with claim 6 and containing 0.001% to 0.05% zirconium and 0.4% to 0.9% vanadium.

8. A steel in accordance with claim 1 and containing 0.01 to 2% chromium.

9. A steel in accordance with claim 2 and containing 0.01% to 2% chromium.

10. A steel in accordance with claim 1 and containing about 17% to 19% nickel, about 19% to 21% cobalt and about 3.5% to 4% molybdenum.

11. A steel in accordance with claim 2 and containing about 17% to 19% nickel, about 14% to 16% cobalt, about 2.5% to 3.5% molybdenum, up to about 0.2% titanium, about 0.005% to 0.04% zirconium, about 0.4% to 0.9% vanadium, up to about 0.5% chromium, up to about 0.2% aluminum, up to about 0.03% carbon, up to about 0.25% silicon, up to about 0.25% manganese, up to about 0.5% tungsten, up to about 0.05% beryllium, up to 4% copper, up to about 2% each of columbium and tantalum, up to about 0.25% nitrogen and the balance essentially iron.

12. A steel in accordance with claim 11 and containing about 0.05% to 0.2% titanium, about 0.05% to 0.2% aluminum, up to about 0.02% carbon, up to 0.15% silicon and up to 0.15 manganese.

13. A maraging steel consisting of about 14% to 22% nickel, about 12% to 25% cobalt, from 0.9% to about 4% molybdenum, up to 0.4% titanium, about 0.001% to 0.1% zirconium, about 0.2% to 0.9% vanadium, up to 0.025% magnesium, up to 3% chromium, up to 0.4% aluminum, up to 0.15 carbon, up to about 1% silicon, up to about 1% manganese, up to 2% tungsten, up to 0.01% boron, up to 1% beryllium, up to 6% copper, up to 3% columbium, up to 4% tantalum, up to 0.4% nitrogen, and the balance essentially iron, said steel being characterized by enhanced toughness and strength by virtue of the copresence of the Zr and V additions.

UNITED STATES PATENTS 3,093,518 6/1963 Bieber 14831 3,093,519 6/1963 Decker et al. 14831 3,262,777 7/1966 Sadowski 75124 CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 75128; l483l

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