CA2073460C - High strength, high fracture toughness alloy - Google Patents

Abstract

A high strength, high fracture toughness steel alloy consisting essentially of, in weight percent, about C 0.2-0.33, Mn 0.20 max., Si 0.1 max., P 0.008 Max,, S 0.004 max., Cr 2-4, Ni 10.5-15, Mo 0.75-1.75, Co 8-17, Ce effective amount-0.030, La effective amount-0.01, Fe balance, and an article made 'therefrom are disclosed. A small but effective amount of calcium can be present in this alloy in substitution for some or all of the cerium and lanthanum. The alloy is an age-hardenable martensitic steel alloy which provides a unique combination of tensile strength and fracture toughness. The alloy provides excellent mechanical properties when hardened by vacuum heat treatment with inert gas cooling and has a low ductile-to-brittle transition temperature.

Description

HIGH STRENGTH, EfIGH FRACTURE TOUGHNESS ALLOY
This invention relates to an age-hardenable, martensitic steel alloy, and in particular to such an alloy and an article made therefrom in which the elements are closely controlled to provide a unique combination of high tensile strength., high fracture toughness and good resistance to stress corrosion cracking in a marine environment.
Heretofore, an alloy designated as 300M has been used in structural components requiring high strength and light weight. The 30C~M alloy has the following composition in weight percent:
wt.

C: 0.40-0.46 M:n 0.65-0.90 Si 1.45-1.80 C'.r 0.70-0.95 Ni 1.65-2.00 M:o 0.30-0.45 V' 0.05 min.

and the balance is essentially iron. The 300M alloy is capable of providing tensile strength in the range of 280-300ksi.

A

2~'~34~0 ' WO 91/12352 PCT/US91/00779 A need has arisen for a high strength alloy such as 300M but having high fracture toughness as represented by a stress intensity factor, KIC, _> , 100ksi in.. The fracture toughness pr-ovided by .
the 300M al~Loy, represented by a KID of about 55-60ksi in, is not sufficient to meet that requirement. Higher fracture toughness is desirable for better reliability in components and because it permits non-destructive inspection of a structural component for flaws that can result in catastrophic; failure.
An allo~~ designated as AF1410 is known to provide good fracture toughness as represented by KIC > 100ksi in. The AF1410 alloy is described in U.S. Patent No. 4,076,525 ('525) issued to Little et al. on February 28, 1978. The AF1410 alloy has the following composition in weight percent, as set forth in they ' 525 patent:
wt.

C 0.12-0.17 Mn .05-.20 S 0.005 max.

Cr 1.8-3.2 Ni 9.5-10.5 Mo 0.9-1.35 Co 11.5-14.5 REM 0.01 max.

REM=rare earth metals and the balance is essentially iron. The AF1410 alloy, however, leaves much to be desired with a regard to tensile strength. It is capable of providing ultimate tensile strength up to 270ksi, a level of strength not suitable for highly stressed ~~'~346~

structural components in which the very high strength to weight ratio provided by 300M is required. 7Ct would be very desirable to have an alloy which provides the good fracture~toughness of the AF1410 alloy in addition-to the high tensile strength provided by the 300M alloy.
Summary of the Invention Accordingly, it is a principal object of this invention tc> provide an age-hardenable, martensitic steel alloy and an article made therefrom which are characterized by a unique combination of high tensile strength and high fracture toughness.
More specifically, it is an object of this invention tc~ provide such an alloy which is characterized by significantly higher tensile strength tha~.n provided by the AF1410 alloy while still maintaining high fracture toughness.
A further object of this invention is to provide an alloy which, in addition to high strength and high fracture toughness, is designed to provide high resistance to stress corrosion cracking in marine environments.
Another object of this invention is to provide a high strength alloy having a low ductile-to-brittle transition temperature. -The foregoing, as well as additional objects and advantages of the present invention, are achieved in an age-hardenable, martensitic steel alloy as summarized in Table I below, containing in weight percent, about:

Table I
Broad Intermediate Preferred C 0.2-0.33 0.20-0.31 x.21-0.27 Mn 0.20 max. 0.15 max. 0.05 max.

0.0040 max. 0.0025 max. 0.0020 max.
S

Cr 2-4 2.25-3.5 2.5-3.3 Ni 10.5-15 10.75-13.5 11.0-12.0 Mo 0.75-1.75 0.75-1.5 1.0-1.3 Co 8-17 10-15 11-14 Ce small but small but effective effective amount up amount up to 0.030 to 0.030 0.01 max.

La small but small but effective effective amount up amount up to 0.01 0.01 0.005 max.

Fe Bal. Bal. Bal.

The balance may include additional elements in amounts which do not detract from the desired combination of properties. For example, about 0.1%
max. silicon, about 0.02% max. titanium, about 0.01% max. aluminum, and not more than about 0.008%
phosphorus m,ay be present in this alloy.
The foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use solely in combination with each other, or to restrict the broad, intermediate or preferred ranges of the elements for use solely in combination with each other. Thus, one or more of the broad, intermediate, and preferred ranges can be used with one or more of the other ranges for the remaining elements. In addition, a broad, intermediate, or preferred minimum or maximum for 2d'~~~-6~

an element c:an be used with the maximum or minimum for that element from one of the remaining ranges.
Here and throughout this application percent means percent by weight, unless otherwise 5 indicated.
The alloy according to the present invention is critically balanced to provide a unique combination of high ten~:ile strength, high fracture toughness, and stress corrosion cracking resistance. For example, they ratio Ce/S is at least about 2 to not more than a~~out 15, preferably not more than about 10. When mare than about 1.3~ molybdenum is present in this alloy, the amount of carbon and/or cobalt are preferably adjusted downwardly so as to be within the lower half of their respective elemental ranges. Carbon and cobalt are preferably balanced in accordance with the following relationships:
a) ~Co < 35-81.8(~C)~
b) ~Co > 25.5-70(~C); and, for best results c) ~Co > 26.9-70(~C).
Detailed Description The alloy according to the present invention contains at least about 0.2~, better yet, at least about 0.20, and preferably at least about 0.21 carbon because it contributes to the good hardness capability and high tensile strength of the alloy primarily by combining with other elements such as chromium and molybdenum to form carbides during heat treatment. Too much carbon adversely affects the fracture toughness of this alloy. Accordingly, ~o~~~o ' WO 91/12352 PCT/US91/00779 carbon is limited to not more than about 0.33%, better yet, to not more than about 0.31%, and preferably t.o not more than about 0.27%.
Cobalt contributes to the hardness and strength of this alloy and benefits the ratio of yield strength to tensile strength (Y.S./U.T.S.).
Therefore, apt least about 8%, better yet at least about 10%, and preferably at least about 11% cobalt is present i.n this alloy. For best results at least about 12% cobalt is present. Above about 17%
cobalt the fracture toughness and the ductile-to-brittle transition temperature of the alloy are adversely affected. Preferably, not more than about 15%, and better yet not more than about 14%
cobalt is present in this alloy.
Cobalt and carbon are critically balanced in this alloy t:o provide the unique combination of high strength and high fracture toughness that is characteristic of the alloy. Thus, to ensure good fracture toughness, carbon and cobalt are preferably k>alanced in accordance with the following relationship:
a) %Co < 35-81.8(%C).
To ensure that the alloy provides the desired high strength and hardness, carbon and cobalt are preferably t>alanced such that:
b) %Co > 25.5-70(%C); and, for best results c) %Co > 26.9-70(%C).
Chromium contributes to the good hardenability , and hardnesss capability of this alloy and benefits the desired low ductile-brittle transition temperature of the alloy. Therefore, at least 2~'~~~-6~

about 2~, better yet at least about 2.25, and preferably ~3t least about 2.5~ chromium is present.
Above about 4~ chromium the alloy is susceptible to rapid overaging such that the unique combination of high tensile strength and high fracture toughness is not attainable with the preferred age-hardening heat treatment. Preferably, chromium is limited to not more than about 3.5$, and better yet to not more than about 3.3$. When the alloy contains more than about 3~ chromium, the amount of carbon present in ~~he alloy is adjusted upwardly in order to ensure that the alloy provides the desired high tensile strE~ngth.
At lease= about 0.75 and preferably at least about 1.0~ rnolybdenum is present in this alloy because it benefits the desired low ductile-brittle transition t=emperature of the alloy. Above about 1.75 molybdenum the fracture toughness of the alloy is adversely affected. Preferably, molybdenum is limited to not more than about 1.5~, and better ~~et to not more than about 1.3~. When more than about 1.3$ molybdenum is present in this alloy the ~ carbon and/or ~ cobalt must be adjusted downwardly in order to ensure that the alloy provides the' desired high fracture toughness.
Accordingly, when the alloy contains more than -about 1.3~ molybdenum, the ~ carbon is not more than the median ~ carbon for a given ~ cobalt~as defined by equations a) and b) or a) and c).
~n Nickel contributes to the hardenability of this alloy such that the alloy can be hardened with or without rapid quenching techniques. Nickel benefits they fracture toughness and stress 20'~34.~~

corrosion cracking resistance provided by this alloy and contributes to the desired low ductile- .
to-brittle transition temperature. Accordingly, at least about 10.5%, better yet, at least about 10.75%, and preferably at least about 11.0% nickel is present. Above about 15% nickel the fracture toughness arid impact toughness of the.alloy can be adversely affected because the solubility of carbon in the allo~~ is reduced which may result in carbide precipitation in the grain boundaries when the alloy is cooled at a slow rate, such as when air cooled follc>wing forging. Preferably, nickel is limited to not more than about 13.5%, and better yet to not more than about 12.0%.
Other elements can be present in this alloy in amounts which do not detract from the desired properties. Not more than about 0.20% manganese can be present because manganese adversely affects the fractures toughness of the alloy. Preferably, manganese is. restricted to about 0.15% max. and better yet t.o about 0.10% max. For best results the alloy contains not more than about 0.05%
manganese. Up to about 0.1% silicon, up to about 0.01% aluminum, and up to about 0.02% titanium can be present a,s residuals from small additions for deoxidizing the alloy.
Small but effective amounts of elements that provide sulfide shape control are present in this alloy to benefit the fracture toughness by combining with sulfur to form sulfide inclusions that do not adversely affect fracture toughness.
For example, the alloy can contain up to about 0.030% cerium and up to about 0.01% lanthanum. The preferred method of providing cerium and lanthanum in this alloy is through the addition of mischmetal during the melting process in an amount sufficient to recover effective amounts of cerium-and lanthanum in the alloy. Effective amounts of cerium and lanthanum are present when the ratio Ce/S is at least about 2. When the Ce/S ratio is more than about 15, the hot workability and tensile ductility of the alloy are adversely affected.
Preferably, the ratio Ce/S is not more than about 10. To ensure good hot workability, for example, when the alloy is to be press forged as opposed to being rotary forged, the alloy contains not more than about 0.01 cerium and not more than about 0.005 lanthanum. A small but effective amount of calcium can be present in this alloy in substitution for some or all of the cerium and lanthanum to benefit the fracture toughness provided by the alloy. Excellent results have been obtained when the alloy contains about 0.002$
calcium. Other rare earth metals, magnesium, or yttrium can also be present in this alloy in place of some or all of the cerium, lanthanum, or calcium to provide the beneficial sulfide shape control.
The balance of the alloy according to the present invention is essentially iron except for the usual impurities found in commercial grades of alloys intended for similar service or use. The levels of such elements must be controlled so as not to adversely affect the desired properties of this alloy. For example, phosphorus is limited to not more than about 0.008. Sulfur adversely ~a'~3~-~~

affects the fracture toughness provided by this alloy. Accordingly, sulfur is restricted to about --0.0040~ max., better yet to about 0.0025 max., and preferably to 0.0020 max. Best results are 5 obtained when the alloy contains not more than about 0.001 sulfur. Tramp elements such as lead, tin, arsenic and antimony are limited to about 0.003 max. each, better yet to about 0.002 max.
each, and preferably to about 0.001 max each.
10 Oxygen is limited to not more than about 20 parts per million (ppm) and nitrogen to not more than about 40 ppm.
The alloy of the present invention is readily melted using conventional vacuum melting techniques. For best results, as when additional refining is desired, a multiple melting practice is preferred. 'The preferred practice is to melt a heat in a vacuum induction furnace (VIM) and cast the heat in the form of an electrode. The alloying addition for sulfide shape control referred to above is preferably made before the molten VIM heat is cast. The electrode is then remelted in a vacuum arc furnace (VAR) and recast into one or more ingots. Prior to VAR the electrode ingots are preferably stress relieved at about 1250F for 4-16 hours and ai.r cooled. After VAR the ingot is _ preferably h~cmogenized at about 2150-2250F for 6-24 hours.
The alloy can be hot worked from about 2250F to about 1500F. The preferred hot working practice is to forge an ingot from about 2150-2250F to obtain at least a 3~D~ reduction in cross sectional area.
The ingot is then reheated to about 1800F and 2~l'~3~-6~

further forged to obtain at least another 30%
reduction in cross sectional area.
The alloy according to the present invention is austenitized and age hardened as follows.
Austenitizing of the alloy is carried out by heating the alloy at about 1550-1650F for about 1 hour plus about 5 minutes per inch of thickness and then quenching in oil. The hardenability of this alloy is good enough to permit air cooling or vacuum heat treatment with inert gas quenching, both of which have a slower cooling rate than oil quenching. Whatever quenching technique is used, the quench rate is preferably rapid enough to cool the alloy from the austenitizing temperature to about 150F in about 2h. When this alloy is to be oil quenched, however, it is preferably austenitized at about 1550-1600F, whereas when the alloy is to be vacuum treated or air hardened it is preferably austenitized at about 1575-1650F. After austenitizing, the alloy is preferably cold treated as by deep chilling at about -100F for 1/2 to 1 hour and then warmed in air.
Age hardening of this alloy is preferably conducted by heating the alloy at about 850-925F
for about 5 hours followed by cooling in air. When austenitized and age hardened the alloy according to the present invention provides an ultimate tensile strength of at least about 280ksi and longitudinal fracture toughness of at least about 100ksi in. Furthermore, the alloy can be aged within the foregoing process parameters to provide a Rockwell hardness of. at least 54 HRC when it is desired for use in ballistically tolerant articles.

~o~~~-oo w munr z~e Five 4001b VIM heats were prepared and each was split cast into two 2001b VAR electrode-ingots.
Prior to casting each of the electrode ingots a predetermined addition of mischmetal or calcium was added to the respective VIM.heats. The amount of each addition was selected to result in a desired retained-amount after refining. The electrode-ingots were cooled in air, stress relieved at 1250F
for 16h and then air cooled. The electrode-ingots were then refined by VAR and vermiculite cooled.
The VAR ingots were stress relieved at 1250F for 16h and cooled in air. The compositions of the VAR
ingots are set forth in weight percent in Table II
below. Heats 1-7 are examples of the present invention and Heats A-C are comparative alloys.
TABLE II
Heat No.

C .243 .210 .210 .226 .228 .228 .221 .229 .215 .221 Mn <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 Si .O1 .O1 <.O1 .O1 .O1 .O1 .O1 .02 <.O1 .O1 P <.005 <.005 <.005 <.005 <.005 <.005 <.005 <.005 <.005 <.005 S .0008 .0006 ..0006 ..0007 .0008 .0007 .0008 .0009 .0005 <.0005 Cr 3.12 3.10 3.11 3.11 3.11 3.10 3.11 3.12 3.09 3.11 Ni 11.06 11.18 11.11 11.16 11.26 11.08 11.22 11.03 11.12 11.16 Mo 1.19 1.19 1.19 1.18 1.19 1.19 1.19 1.20 1.17 1.18 Co 13.46 13.52 13.48 13.46 13.48 13.49 13.51 13.45 13.47 13.50 Ti .O1 .O1 .O1 .O1 .O1 .O1 .O1 .O1 .O1 .Ol A1 <.O1 <.Ol <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 <.O1 Ce .004 .006 .009 .001 <.001 <.001 .001 .001 .024 .029 La .002 .002 .003 <.001 <.001 <.001 <.001 <.001 .005 .006 Ca<.0010<.0010<.0010 .002 .002 .002 .002 <.0010<.0010<.0010 3 0 _Ce 5.0 10.0 15.0 1.4 <1.2 <1.4 <1.2 1.1 48.0 >58.0 S
Fe Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.
Note: The iron charge material was a high purity grade of electrolytic iron.

WO 91/12352 : PCT/US91/00779 Prior to forging, the VAR ingots were homogenized at 2250F for 6h. The ingots were then press forgecl from the temperature of 2250F to Sin high by Sin wide bars. The bars were reheated to 1800F, pres~~ forged to 1-1/2in x 4in bars, and then cooled in ai.r. The forged bars were annealed at 1250F for lE~h and then air cooled.
Standard longitudinal tensile specimens (0.252 inch gage diameter by 1 in gage length) were machined from the annealed bars. The tensile specimens were austenitized in salt for lh at 1625F, vermiculite cooled, deep chilled at -100F
for lh, and then warmed in air. The specimens were then age hardened for 5h at 900F and air cooled.
Standard compact tension fracture toughness specimens were machined with a longitudinal orientation from the remains of the annealed bars.
The fracture toughness specimens were austenitized, deep chilled, and age hardened in the same manner as the tensile specimens except for being air cooled from the austenitizing temperature.
The results of room temperature tensile tests on the duplicate specimens are shown in Table III
including the 0.2$ offset yield strength (0.2~
Y.S.) and the ultimate tensile strength (U.T.S.) in -ksi, as well. as the percent elongation (~ E1.) and percent reduction in area (~ R.A.) The results of room temperature fracture toughness testing in accordance with ASTM Standard Test E399 are also shown in Table III as KID in ksi in. Heats B and C were not tested because they could not be press f orged .

WO 91/12352 ~ ~ ~ ~ ~ PGT/US91/00779 TABLE III
Longitudinal Mechanical erties Prop Ht. Ce xCe %Ca _ K_I Y.S. U.T.S.Z E1. %R. A.
No. _S _ ZS

_ _ _ <.00105.0 1~~.4 262.1291.3 16.4 66.2 1 .0008 .004 115.4 261.6292.4 15.4 65.4 2 .0006 .006<.001010.0 117.2 260.1289.3 15.3 67.1 106.5 260.1288.7 14.8 68.2 3 .0006 .009<.001015.0 109.8 260.5289.0 13.4 63.6 99.0 260.7289.2 13.3 64.0 .0007 .001.002 1.4 130.3 255.5283.0 13.3 69.2 143.4 251.5281.8 16.3 69.2 5 .0008<.001 .002 <1.2 121.2 258.5284.2 15.9 69.2 116.0 257.5283.2 15.3 68.4 6 .0007<.001 .002 <1.4 119.8 255.6283.0 15.5 69.0 124.9 250.0278.2 15.7 69.8 7 .0008<.001 .002 <1.2 129.9 255.1283.0 17.1 67.5 122.2 251.0275.9 16.4 69.3 .0009 .001<.00101.1 93.6 262.1292.5 13.7 66.2 A

86.5 265.6294.5 15.1 65.4 B .0005 .024<.001048.0 --- --- --- --- ---C <.0005.029<.0010>58.0 --- --- --- --- ---The data of Table III show that the alloy according to the present invention provides an ultimate tensile strength of at least 280ksi in combination with high fracture toughness as represented by a KIc of at least about 100ksi in.
The alloy according to the present invention is useful in a variety of applications requiring high strength and low weight, for example, aircraft landing gear components; aircraft structural members, such as braces, beams, struts, etc.;
helicopter rotor shafts and masts; and other aircraft structural components which are subject to high stress in service. The alloy of the present invention could be suitable for use in jet engine shafts. This alloy can also be aged to very high hardness which makes it suitable for use as lightweight armor and in structural components which must be ballistically tolerant. The present alloy is, 01: course, suitable for use in a variety 5 of product i:orms including billets, bars, tubes, plate and sheet.
It is apparent from the foregoing description and accompanying examples, that the alloy according to the present invention provides a unique 10 combination of tensile strength and fracture toughness not provided by known alloys. This alloy is well suited to applications where high strength and low weight are required. The present alloy has a low ductile-to-brittle transition temperature 15 which renders it highly useful in applications where the in.-service temperatures are well below zero degrees Fahrenheit. Because this alloy can be vacuum heat treated, it is particularly advantageous for use in the manufacture of complex, close tolerance components. Vacuum heat treatment of such articles is~desirable because the articles do not undergo any distortion as usually results from oil quenching of such articles made from known alloys.
The terms and expressions which have been employed herein are use as terms of decription and -not of limitation. There is no intention in the use of such terms and expressions to exclude any equivalents of the features described or any portions thereof. It is recognized, however, that various modifications are possible within the scope of the invention claimed.

Claims (25)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An age hardenable, martensitic steel alloy which provides high strength and high fracture toughness, said alloy consisting essentially of in weight percent:
wt. %
Carbon 0.2-0.33 Manganese 0.20 max.
Sulfur 0.004 max.
Chromium 2 - 4 Nickel 10.5-15 Molybdenum 0.75-1.75 Cobalt 8-17 Cerium small but effective amount up to 0.030 max.
Lanthanum small but effective amount up to 0.01 max.
and the balance is essentially iron, wherein the ratio Ce/S
is at least 2 and is not more than 15.

2. An alloy as set forth in claim 1, containing at least 0.20% carbon.

3. An alloy as set forth in claim 1 or 2, containing at least 10.75% nickel.

4. An alloy as set forth in claim 1, 2 or 3, wherein a) % Co < 35-81.8 (% C).

5. An alloy as set forth in claim 4, wherein b) % Co >
25.5-70(% C).

6. An alloy as set forth in claim 5, wherein when %
Mo>1.3, % C is not more than the median % C for a given % Co as defined by relationships a) and b).

7. An alloy as set forth in claim 4, wherein c) % Co <
26 . 9-70 (% C).

8. An alloy as set forth in claim 7, wherein when %
Mo>1.3, % C is not more than the median % C for a given % Co as defined by relationships a) and c).

9. An alloy as set forth in any one of claims 1 to 8, containing 0.15% max. manganese.

10. An allot as set forth in any one of claims 1 to 9, containing calcium in substitution for at least a portion of the cerium and lanthanum.

11. An age-hardenable, martensitic steel alloy which provides high strength and high fracture toughness, said alloy consisting essentially of, in weight percent:

Wt. %

Carbon 0.20-0.31 Manganese 0.15 max.

Sulfur 0.0025 max .

Chromium 2.25-3.5 Nickel 10.75-13.5 molybdenum 0.75-1 Cobalt 10-15 Cerium small but effective amount up to 0.030 max.

Lanthanum small but effective amount up to 0.01 max.

and the balance is essentially iron, wherein the ratio Ce/S
is at least 2 and is not more than 15.

12. An alloy as set forth in claim 11, containing at least 0.21% carbon.

13. An alloy as set forth in claim 11 or 12, containing at least 11.0% nickel.

14. An alloy as set forth in claim 11, 12 or 13, containing 0.10% max, manganese.

15. An age-hardenable, martensitic steel alloy which provides high strength and high fracture toughness, said alloy consisting essentially of, in weight percent:

Wt. %

Carbon 0.21-0.27 Manganese 0.05 max.

Silicon 0.1 max.

Phosphorus 0.008 max.

Sulfur 0.0020 max.

Chromium 2.5-3.3 Nickel 11.0-12.0 Molybdenum 1.0-1.3 Cobalt 11-14 Cerium 0.01 max.

Lanthanum 0.01 max.

and the balance is essentially iron, wherein the ratio Ce/S
is between 2 and 10.

16. An alloy as set forth in claim 15, wherein a) % Co ~ 35-81.8(% C).

17. An alloy as set forth in claim 16, wherein b) % CO
~ 25.5-70(% C).

18. An allot as set forth in claim 15, 16 or 17, containing calcium in substitution for at least a portion of the cerium and lanthanum.

19. An age-hardened article having high strength and high fracture toughness, said article being formed of a martensitic steel alloy consisting essentially of, in weight percent:

wt. %
Carbon 0.2-0.33 Manganese 0.15 max.
Silicon 0.1 max.
Phosphorus 0.008 max.
Sulfur 0.004 max.
Chromium 2-4 Nickel 10.5-15 Molybdenum 0.75-1.75 Cobalt 8-17 Cerium 0.030 max.
Lanthanum 0.01 max.
and the balance is essentially iron, wherein the ratio Ce/S
is between 2 and 15, said article being characterized by a longitudinal, room temperature, tensile strength of at least 280 ksi and a room temperature, longitudinal, KIC fracture toughness of at least 100 ksi ~in.

20. An article as set forth in claim 19, wherein the alloy contains at least 0.21% carbon.

21. An article as set forth in claim 19 or 20, wherein the alloy contain, at least 11.0% nickel.

22. An article as set forth in claim 19, 20 or 21, wherein a) % Co s 35-81.8(% C).

23. An article as set forth in claim 22, wherein b) Co > 25.5-70 (% C).

24. An article as set forth in claim 23, wherein when %
Mo>1.3, % C is not: more than the median % C for a given % Co as defined by relationships a) and b).

25. An article as set forth in any one of claims 19 to 24, wherein the alloy contains not more than 0.05% manganese.