EP0514480B1

EP0514480B1 - High strength, high fracture toughness alloy

Info

Publication number: EP0514480B1
Application number: EP91904760A
Authority: EP
Inventors: Raymond M. Hemphill; David E. Wert; Paul M. Novotny; Michael L. Schmidt
Original assignee: CRS Holdings LLC
Current assignee: CRS Holdings LLC
Priority date: 1990-02-06
Filing date: 1991-02-05
Publication date: 2001-04-04
Anticipated expiration: 2011-02-05
Also published as: EP0514480A1; WO1991012352A1; JPH03243747A; ATE200309T1; JPH0689436B2; CA2073460C; DE69132572D1; JPH05502477A; JP2683599B2; IL97154A; IL97154A0; DE69132572T2; EP0514480A4; CA2073460A1; ES2156854T3

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 vaccum heat treatment with inert gas cooling and has a low ductile-to-brittle transition temperature.

Description

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 300M alloy has the following composition in weight percent:

wt. %

C 0.40-0.46

Mn 0.65-0.90

Si 1.45-1.80

Cr 0.70-0.95

Ni 1.65-2.00

Mo 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 1930-2068mMPa (280-300ksi).
A need has arisen for a high strength alloy such as 300M but having high fracture toughness as represented by a stress intensity factor, K_IC, ≥ 110MPa m (100ksi in ). The fracture toughness provided by the 300M alloy, represented by a K_IC of about 60-66MPa m (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 alloy designated as AF1410 is known to provide good fracture toughness as represented by K_IC ≥ 1100MPa m (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 the '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 regard to tensile strength. It is capable of providing ultimate tensile strength up to 1862 MPa (270ksi), a level of strength not suitable for highly stressed structural components in which the very high strength to weight ratio provided by 300M is required. It 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.

Our copending European Patent Application No. 90303201.9, published on 3rd October 1990 (Publication No. 0,390,468), discloses age-hardenable, martensitic steel alloys as summarised in Table A below, containing in weight percent

carbon	0.2 - 0.33
chromium	2 - 4
nickel	10.5 - 15
molybdenum	0.75 - 1.75
cobalt	8 - 17
manganese	0.2 max
silicon	0.1 max
titanium	0.01 max
aluminum	0.01 max
phosphorus	0.008 max
sulfur	0.004 max
lead	0.003 max
tin	0.003 max
arsenic	0.003 max
antimony	0.003 max
rare earth metals such as cerium and lanthanum
	0.001 max each
nitrogen	40 ppm max
oxygen	20 ppm max
iron	balance (exc. usual impurities)

That application does not refer to the presence of calcium, magnesium or yttrium in the alloys.

Summary of Invention

It is a principal object of this invention to provide an age-hardenable, martensitic steel alloy and an article made thereform which have a unique combination of high tensile strength and high fracture toughness.
More specifically, it is an object of this invention to provide such an alloy which has a significantly higher tensile strength than 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 a 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 objects of the present invention are achieved by the steel alloys in the claims. Table 1 below summarizes compositions of age-hardenable martensitic steel alloys as claimed in claim 1, in weight percent:

	Broad	Intermediate	Preferred
C	0.2-0.33	0.20-0.31	0.21-0.27
Mn	0.05 max.	0.05 max.	0.05 max.
Si	0.1 max.	0.1 max.	0.1 max.
P	0.008 max.	0.008 max.	0.008 max.
S	0.002 max.	0.002 max.	0.0020 max.
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
Ti	0.02 max.	0.02 max.	0.02 max.
Al	0.01 max.	0.01 max.	0.01 max.
Ce	up to 0.030	up to 0.030	0.01 max.
La	up to 0.01	up to 0.01	0.005 max.
Ce/S ratio	2-10	2-10	2-10
Pb	0.003 max.	0.003 max.	0.003 max.
Sn	0.003 max.	0.003 max.	0.003 max.
As	0.003 max.	0.003 max.	0.003 max.
Sb	0.003 max.	0.003 max.	0.003 max.
O	20 ppm max.	20 ppm max.	20 ppm max.
N	40 ppm max.	40 ppm max.	40 ppm max.
Fe	Bal. (exc. usual impurities)	Bal. (exc. usual impurities)	Bal. (exc. usual impurities)

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 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 an element can 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 indicated.
The alloy according to the present invention as given in the claims is critically balanced to provide a unique combination of high tensile strength, high fracture toughness, and stress corrosion cracking resistance. For example, the ratio Ce/S is at least 2 to not more than 10 in the first aspect of the invention (claim 1).
When more than 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 as given in the claims contains at least 0.2%, better yet, at least 0.20%, and preferably at least 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, carbon is limited to not more than 0.33%, better yet, to not more than 0.31%, and preferably to not more than 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, at least 8%, better yet at least 10%, and preferably at least 11% cobalt is present in this alloy. for best results at least 12% cobalt is present. Above 17% cobalt the fracture toughness and the ductile-to-brittle transition temperature of the alloy are adversely affected. Preferably, not more than 15%, and better yet not more than 14% cobalt is present in this alloy.
Cobalt and carbon are critically balanced in this alloy to 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 balanced 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 balanced 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 hardness capability of this alloy and benefits the desired low ductile-brittle transition temperature of the alloy. Therefore, at least 2%, better yet at least 2.25%, and preferably at least 2.5% chromium is present. Above 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 3.5%, and better yet to not more than 3.3%. When the alloy contains more than 3% chromium, the amount of carbon present in the alloy is preferably adjusted upwardly in order to ensure that the alloy provides the desired high tensile strength.
At least 0.75% and preferably at least 1.0% molybdenum is present in this alloy because it benefits the desired low ductile-brittle transition temperature of the alloy. Above 1.75% molybdenum the fracture toughness of the alloy is adversely affected. Preferably, molybdenum is limited to not more than 1.5%, and better yet to not more than 1.3%. When more than 1.3% molybdenum is present in this alloy the % carbon and/or % cobalt is preferably adjusted downwardly in order to ensure that the alloy provides the desired high fracture toughness. Accordingly, when the alloy contains more than 1.3% molybdenum, the % carbon is preferably not more than the median % carbon for a given % cobalt as defined by equations a) and b) or a) and c).
Nickel contributes to the hardenability of this alloy such that the alloy can be hardened with or without rapid quenching techniques. Nickel benefits the fracture toughness and stress corrosion cracking resistance provided by this alloy and contributes to the desired low ductile-to-brittle transition temperature. Accordingly, at least 10.5%, better yet, at least 10.75%, and preferably at least 11.0% nickel is present. Above 15% nickel the fracture toughness and impact toughness of the alloy can be adversely affected because the solubility of carbon in the alloy 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 following forging. Preferably, nickel is limited to not more than 13.5%, and better yet to not more than 12.0%.
Certain other elements can be present in this alloy in amounts which do not detract from the desired properties. Not more than 0.05% manganese is present because manganese adversely affects the fracture toughness of the alloy. Up to 0.1% silicon, up to 0.01% aluminum, and up to 0.02% titanium can be present as 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, in the first aspect of the invention (claim 1) the alloy can contain up to 0.030% cerium and up to 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 in this aspect when the ratio Ce/S is at least 2. When the Ce/S ratio is more than 15, the hot workability and tensile ductility of the alloy are adversely affected. According to the invention the ratio Ce/S is not more than 10 in this aspect. 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 0.01% cerium and not more than 0.005% lanthanum. According to a further aspect of the invention (claim 9) 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. Thus, the amount of calcium in the alloy is in the range from 0.002% up to the combined maximum amounts of cerium and lanthanum in the Table I above, such that the total amount of cerium, lanthanum and calcium is not greater than the combined maximum amounts of cerium and lanthanum in the Table I above. Excellent results have been obtained when the alloy contains 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 in yet another aspect of the invention (claim 11) to provide the beneficial sulfide shape control.
The balance of the alloy according to the present invention is 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 0.008%. Sulfur adversely affects the fracture toughness provided by this alloy. Accordingly, sulfur is restricted to 0.002% max. Best results are obtained when the alloy contains not more than 0.001% sulfur. Tramp elements such as lead, tin, arsenic and antimony are limited to 0.003% max. each, better yet to 0.002% max. each, and preferably to 0.001% max each. Oxygen is limited to not more than 20 parts per million (ppm) and nitrogen to not more than 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 677°C (1250F) for 4-16 hours and air cooled. After VAR the ingot is preferably homogenized at about 1177-1232°C (2150-2250F) for 6- 24 hours.
The alloy can be hot worked from about 1232°C (2250F) to about 816°C (1500F). The preferred hot working practice is to forge an ingot from about 1177-1232°C (2150-2250F) to obtain at least a 30% reduction in cross sectional area. The ingot is then reheated to about 982°C (1800F) and 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 843-599°C (1550-1650F) for about 1 hour plus about 2 minutes per cm (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 65°C (150F) in about 2h. When this alloy is to be oil quenched, however, it is preferably austenitized at about 843-871°C (1550-1600F), whereas when the alloy is to be vacuum treated or air hardened it is preferably austenitized at about 857-899°C (1575-1650F). After austenitizing, the alloy is preferably cold treated as by deep chilling at about -73°C (-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 454-496°C (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 1930MPa (280ksi) and longitudinal fracture toughness of at least about 110MPa m (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.

EXAMPLES

Five 180kg (400lb) VIM heats were prepared and each was split cast into 90kg (200lb) 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 677°C (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 677°C (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 and 2 are examples of the present invention (claim 1) , Heats 4 - 7 are examples of the present invention (claim 9) and Heats 3 and A-C are comparative alloys.

Heat No.
	1	2	3	4	5	6	7	A	B	C
C	.243	.210	.210	.226	.228	.228	.221	.229	.215	.221
Mn	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01
Si	.01	.01	<.01	.01	.01	.01	.01	.02	<.01	.01
P	<.005	<.005	<.005	<.005	<.005	<.005	<.005	<.005	<.005	<.005
S	.0008	.0006	.0006	.0007	.0008	.0007	.0008	.0009	.0005	<.005
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	.01	.01	.01	.01	.01	.01	.01	.01	.01	.01
Al	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.01
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
Ce / S	5.0	10.0	15.0	1.4	<1.2	<1.4	<1.2	1.1	48.0	>58.0
Fe	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.
Note : The iron charge material was a high purity grade of electrolytic iron.

Prior to forging, the VAR ingots were homogenized at 1232°C (2250F) for 6h. The ingots were then press forged from the temperature of 1232°C (2250F) to 7.6 cm high by 12.7 cm. (3 in high by 5in wide) bars. The bars were reheated to 982°C (1800F), press forged to 3.8 x 10.2 cm (1-1/2in x 4in) bars, and then cooled in air. The forged bars were annealed at 677°C (1250F) for 16h and then air cooled.
Standard longitudinal tensile specimens (0.64cm-0.252 inch-gage diameter by 2.5cm-1 in-gage length) were machined from the annealed bars. The tensile specimens were austenitized in salt for 1h at 885°C (1625F), vermiculite cooled, deep chilled at -73°C (-100F) for lh, and then warmed in air. The specimens were then age hardened for 5h at 482°C (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, 20 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 25 including the 0.2% offset yield strength (0.2% Y.S.) and the ultimate tensile strength (U.T.S.) in MPa and ksi, as well as the percent elongation (% El.) and percent reduction in area (% R.A.) The results of room temperature fracture toughness testing in 30 accordance with ASTM Standard Test E399 are also shown in Table III as K_IC in MPa m and ksi in . Heats B and C were not tested because they could not be press forged.
The data of Table III show that the alloy according to the present invention provides an ultimate tensile strength of at least 1930MPa (280ksi) in combination with high fracture toughness as represented by a K_IC of at least 110MPa m (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, of course, suitable for use in a variety of product forms 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 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 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.

Claims

An age-hardenable, martensitic steel alloy which provides high strength and high fracture toughness, said alloy comprising, in weight percent, Carbon 0.2-0.33 Manganese 0.05 max. Silicon 0.1 max. Phosphorus 0.008 max. Sulfur 0.002 max. Chromium 2-4 Nickel 10.5-15 Molybdenum 0.75-1.75 Cobalt 8-17 Titanium 0.02 max. Aluminum 0.01 max. Lead 0.003 max. Tin 0.003 max. Arsenic 0.003 max. Antimony 0.003 max. Oxygen 20 ppm max. Nitrogen 40 ppm max.

wherein, in order to combine with available sulfur for sulfide shape control so that said alloy provides a room temperature, longitudinal, K_Ic fracture toughness of at least 110MPa√m (100 ksi √in) at a room temperature tensile strength of at least 1930 MPa (280ksi) when said alloy is in the age-hardened condition, the alloy additionally contains cerium in an amount up to 0.030wt% and lanthanum in an amount up to 0.01wt%, the ratio Ce/S being from 2 to 10, and the balance of said alloy is iron apart from usual impurities.
An alloy as set forth in claim 1 containing at least 0.20% carbon.
An alloy as set forth in claim 1 containing at least 10.75% nickel.
An age-hardenable martensitic steel alloy as set forth in claim 1 containing wt% Carbon 0.20-0.31 Manganese 0.05 max Sulfur 0.002 max Chromium 2.25 - 3.5 Nickel 10.75 - 13.5 Molybdenum 0.75 - 1.5 Cobalt 10-15
An alloy set forth in any of claims 1 to 45 wherein
a) %Co≤ 35 - 81.8 (%C).
An alloy set forth in any of claims 1 to 5 wherein
b) %Co ≥ 25.5 - 70 (%C).
An alloy set forth in any of claims 1 to 6 wherein
c) %Co≥ 26.9 - 70 (%C) .
An alloy as set forth in claim 6 or 7 wherein, when %Mo > 1.3, %C is not more than the median %C for a given %Co 25 as defined by relationships a) and c) .
An age-hardenable, martensitic steel alloy which provides high strength and high fracture toughness, said alloy comprising, in weight percent, Carbon 0.2-0.33 Manganese 0.05 max. Silicon 0.1 max. phosphorus 0.008 max. Sulfur 0.002 max. Chromium 2-4 Nickel 10.5-15 Molybdenum 0.75-1.75 Cobalt 8-17 Titanium 0.02 max. Aluminum 0.01 max. Lead 0.003 max. Tin 0.003 max. Arsenic 0.003 max. Antimony 0.003 max. Oxygen 20 ppm max. Nitrogen 40 ppm max.

wherein,in order to combine with available sulfur for sulfide shape control so that said alloy provides a room temperature, longitudinal K_Ic fracture toughness of at least 110MPa m (100 ksi in ) at a room temperature tensile strength of at least 1930 MPa (280ksi) when said alloy is in the age-hardened condition, the alloy additionally contains up to a maximum amount of 0.030wt% cerium, up to a maximum amout of 0.01wt% lanthanum, and an effective amount of calcium in the range from 0.002wt% up to the combined maximum amounts of carium and lanthanum, such that the total amount of carium, lanthanum and calcium is not greater than the combined maximum amounts of carium and lanthanum, and the balance of said alloy is iron apart from usual impurities.
An alloy as set forth in Claim 9 containing not more than 0.001wt% sulfur.
An alloy as set forth in any of Claims 1 to 10 which is modified by the substitution of magnesium, yttrium or a rare earth metal other than cerium or lanthanum for at least a portion of the cerium and lanthanum or calcium, the additional element being used in an amount sufficient to combine with available sulfur for sulfide shape control so that the alloy provides a room temperature longitudinal K_Ic fracture toughness of at least 110MPa√m (100 ksi √in) at a room temperature tensile strength of least 1930 MPa (280ksi) when said alloy is in the age-hardened condition.
An age-hardenable, martensitic steel alloy as set forth in claim 1 containing wt% Carbon 0.21 - 0.27 Manganese 0.05 max Sulfur 0.0020 max Chromium 2.5 - 3.3 Nickel 11.0 - 12.00 Molybdenum 1.0 - 1.3 Cobalt 11 - 14 Cerium 0.01 max.

and wherein the ratio Ce/S is 2 - 10.
An age-hardened article formed of a martensitic steel alloy as claimed in any of claims 1 to 12, said article having a longitudinal, room-temperature, tensile strength of at least 1930 MPa (280 ksi) and a longitudinal room temperature, K_Ic fracture toughness of at least 110 MPa m (100 ksi in ).