High strength stainless chromium-nickel steel without aluminium and titanium, and method for making same

Description

FIELD OF THE INVENTION

[0001] The present invention relates to ultra-high strength steels and, more particularly, to ultra-high strength steels that are air castable and that provide improved properties of strength, ductility, toughness, and corrosion resistance when compared to conventional ultra-high strength steels.

BACKGROUND OF THE INVENTION

[0002] Conventional ultra-high strength steels suffer from a number of drawbacks. For example, most martensitic and maraging steels are not resistant to atmospheric corrosion, particularly during the casting process. Conventional maraging steels are melted under vacuum conditions using high-purity raw materials in order to strictly control the presence of unwanted interstitial and tramp elements.

[0003] Maraging steels and other types of ultra-high strength precipitation hardening steels are melted and poured under vacuum conditions in an inert environment to protect essential and very reactive elements used in making such steels, such as titanium and aluminum, from exposure with atmospheric air. Air exposure during the melting and pouring process is intentionally avoided for the purpose of preventing the reactive elements from undergoing oxidation reactions. Presence of the reactive elements in the oxidized state is known to severely degrade the desired mechanical properties of the resulting steels.

[0004] However, this vacuum melting and pouring process is expensive. Additionally, many conventional ultra-high strength steels must be vacuum melted and poured multiple times in order to achieve a desired level of purity. Once the melting and pouring process is complete, the steels must go through significant wrought processing to achieve the desired mechanical properties. Because wrought processing is required, the steel manufacturer must have and use progressive dies to achieve the desired shape and mechanical properties, further adding to the cost and time involved in making the desired steel product. Still further, many conventional ultra-high strength steels require chrome or cadmium plating for most applications to provide a level of corrosion protection to protect the alloy from rusting.

[0005] Conventional ultra-high strength maraging steels typically include the following alloying elements provided below in Table 1 (in weight percent):

Table 1

13-8 Mo	8 Ni	2.25 Mo	13 Cr	.05 C	1.1 Al			Bal. Fe
Maraging 250	18 Ni	5 Mo	8.5 Co	0.4 Ti	0.1 Al			Bal. Fe
Maraging 300	18 Ni	5 Mo	9 Co	0.7 Ti	0.1 Al			Bal. Fe
Cobalt Free 250	18.5 Ni	3 Mo		0.7 Ti	0.1 Al			Bal. Fe
Custom 455	8.5 Ni	0.5 Mo	12 Cr	2 Cu	1.1 Ti	0.3 Cb	0.05 C	Bal. Fe
Custom 465	11 Ni	1 Mo	12 Cr	1.65 Ti	0.02 C			Bal. Fe

[0006] Titanium and aluminum are mandatory ingredients that are used in making maraging steels for the purpose of providing properties of increased strength and hardness to the resulting steel. However, any steels that are made using titanium and aluminum cannot be melted in air (as explained above) because of their susceptibility to form embrittling compounds. Thus, ultra-high strength steels formed using titanium and aluminum must be subjected to vacuum melting and pouring.

[0007] Also, conventional martensitic ultra-high strength steels such as AISI 4340, H 11, and 300 M possess an insufficient level of toughness and poor corrosion resistance against atmospheric elements for acceptable use in many high strength steel applications calling for both a desired degree of toughness and some degree of corrosion resistance, e.g., use in an outdoor exposure application. In addition, the alloys used in making these steels make it generally difficult to weld the resulting steel (because of the high carbon content - generally over 0.25%), and produce steel having inferior mechanical properties as castings in comparison to their wrought counterparts.

[0008] Ultra-high strength maraging steels on the other hand are known to possess good properties of toughness and weldability. However, as discussed above, maraging steels are sensitive to atmospheric corrosion and related stress corrosion cracking. These steels are also are very expensive to acquire.

[0009] As briefly discussed above, the most common process for manufacturing ultra-high strength maraging steels is by double or triple melting process that includes a minimum of one vacuum melting/refining cycle. The melting/refining cycle is generally followed by forging and rolling into sheet or bar stock. Maraging steels achieve their best properties via wrought processing. Although casting maraging steels is know to provide a steel product having decent strength, casting into net shape results in inferior ductility (compared to its wrought and machined counterparts), as shown in Table 2 below.

Table 2

Alloy	Yield Strength PSI	Tensile Strength PSI	Elongation %	Reduction % (Ductility)
Maraging 250 (wrought)	247,000	260,000	8	55
Maraging 250 (cast)	242,102	261,233	5.8	10

[0010] It is, therefore, desired that new ultra-high strength steels be developed that provide improved combined properties of strength, toughness, ductility and resistance to atmospheric corrosion when compared to those conventional ultra-high strength steels noted above. It is also desired that such new ultra-high strength steels be capable of being made without the need for expensive and time consuming vacuum melt and pour processing, in a manner that permits air casting.

SUMMARY OF THE PREFERRED EMBODIMENTS

[0011] In accordance with one aspect of the present invention, there is provided an air-cast ultra-high strength stainless steel prepared by combining Si, Cr, Mo, Ni, Co, Cu, W, and Fe, wherein the steel has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent. The air-cast ultra-high strength stainless steel may additionally include one or more ingredient selected from the group consisting of C, Mn, V, and mixtures thereof.

[0012] In accordance with one aspect of the present invention, ultra-high strength stainless steels of this invention may comprise in the range of from about 0.3 to 1.5 percent by weight Si, in the range of from about 9 to 13 percent by weight Cr, in the range of from about 2.5 to 4.5 percent by weight Mo, in the range of from about 4 to 7 percent by weight Ni, in the range of from about 7 to 13 percent by weight Co, in the range of from about 0.3 to 3 percent by weight Cu, and in the range of from about 0.2 to 2 percent by weight W. Additionally, C may be present up to about 0.08 percent by weight, Mn may be present up to about 1.5 percent by weight, and V may be present up to about 0.5 percent by weight. Fe is present in a remaining amount.

[0013] In accordance with another aspect of the present invention, ultra-high strength stainless steels of this invention may comprise in the range of from about 0.4 to 1.2 percent by weight Si, in the range of from about 10.5 to 13 percent by weight Cr, in the range of from about 2.75 to 4.25 percent by weight Mo, in the range of from about 4.5 to 6 percent by weight Ni, in the range of from about 8 to 12.5 percent by weight Co, in the range of from about 0.4 to 1.5 percent by weight Cu, and in the range of from about 0.2 to 1 percent by weight W. Additionally, C may be present up to about 0.05 percent by weight, Mn may be present in the range of from about 0.2 to 1 percent by weight, and V may be present up to about 0.25 percent by weight. Fe is present in a remaining amount.

[0014] In accordance with yet another aspect of the present invention there is provided an ultra-high strength stainless steel including in the range of from about 0.4 to 0.9 percent by weight Si, in the range of from about 10.5 to 12 percent by weight Cr, in the range of from about 3.5 to 4.1 percent by weight Mo, in the range of from about 4.5 to 5.5 percent by weight Ni, in the range of from about 8.3 to 10.6 percent by weight Co, in the range of from about 0.4 to 1 percent by weight Cu, in the range of from about 0.3 to 0.6 percent by weight W, and a remaining amount Fe. Additionally, C may be present up to about 0.03 percent by weight, Mn may be present in the range of from about 0.2 to 0.6 percent by weight, and V may be present up to about 0.15 percent by weight. Fe is present in a remaining amount.

[0015] In accordance with another aspect of the present invention there is provided a casting made from a stainless steel that is prepared according to the steps of melting the combined ingredients to form a stainless steel mixture and pouring the stainless steel mixture to form the cast part, which can both be conducted in an open air environment; homogenating the cast part at a temperature of above about 2,100°F for a period of from about 1 to 6 hours; solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100°F for about 1 to 4 hours, followed by cooling to room temperature; cooling the cast part by refrigeration for about 1 to 8 hours at a temperature below about -50°F; and aging the cast part at a temperature of between about 800°F and about 1,000°F for about 4 to about 5 hours; if required to further enhance the mechanical properties of this alloy a hot isostatic pressing of a cast part made from this stainless steel at a temperature of above about 1,900°F for about 3 to 5 hours at a pressure of greater than about 14,000 psi, is acceptable. This should be performed prior to the homogenation step.

[0016] Ultra-high strength steels prepared in accordance with the principles of this invention provide an air-castable product that demonstrates improved combined properties of strength, toughness, ductility and resistance to atmospheric corrosion when compared to those conventional ultra-high strength steels noted above. Additionally, such new ultra-high strength stainless steels of this invention can be made in an open air environment, thereby obviating the need for employing expensive and time consuming vacuum melt, and multiple melt and pour processing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The present invention is directed to ultra-high strength stainless steels (hereafter "stainless steel") and methods of making the same. Stainless steels of this invention generally comprise silicon, chromium, molybdenum, nickel, cobalt, copper, and tungsten as principle alloying elements along with small additions of carbon, manganese and vanadium if so desired, with the remaining ingredient being iron. The specific constitution of the alloying ingredients used to form stainless steels of this invention, and the thermal treatments used for processing the same, provide an air castable steel having improved combined properties of strength, ductility, toughness, and resistance to atmospheric corrosion when compared to conventional ultra-high strength steels, and results in a predominantly martensitic microstructure that is subsequently hardened by precipitation of intermetallic compounds. If desired, stainless steels of this invention can be vacuum melted and poured directly into a mold (investment cast) or made into ingots that could be subsequently forged to further improve its toughness and fatigue properties.

[0018] A key feature of stainless steels of this invention is the ability to achieve such performance improvements while intentionally avoiding use of reactive elements such as titanium and/or aluminum. The use of these elements is common in making typical maraging steels for the purpose of providing contributions in hardness. Therefore, the ability to provide a stainless steel having a high level of hardness while intentionally avoiding the use of these elements was something unexpected, surprising, and not before thought possible.

[0019] These reactive elements are intentionally avoided for the purpose of permitting the stainless steels to be air melted. Other alloying elements are judicially selected and/or adjustments are made to the overall balance of alloying ingredients used to make stainless steels to achieve the desired combination of mechanical properties. The strategic substitution of these elements avoids having to conduct multiple melt and pour cycles in a nonreactive environment, thereby enabling formation of stainless steels in an open air environment that is both less expensive and more time efficient.

[0020] Stainless steels can be formed from the following atomic elements: Fe, Si, Cr, Mo, Ni, Co, Cu, and W. As discussed above, a key feature distinguishing steels of this invention from conventional ultra-high strength steels, such as maraging steels or the like, is the intentional avoidance of the reactive elements titanium and aluminum.

[0021] Stainless steels of this invention comprise alloying elements provided in respective weight percent ranges as recited below in Table 3. It is to be understood that all provided values are approximate.

Table 3

Element	Example Range	Preferred Range	Most Preferred Range
C	Up to 0.08	Up to 0.05	Up to 0.03
Si	0.3 to 1.5	0.4 to 1.2	0.4 to 0.9
Cr	9 to 13	10.5 to 13	10.5 to 12
Cu	0.3 to 3	0.4 to 1.5	0.4 to 1
Mo	2.5 to 4.5	2.75 to 4.25	3.5 to 4.15
Ni	4 to 7	4.5 to 6	4.5 to 5.5
Mn	Up to 1.5	0.2 to 1	0.2 to 0.6
Co	7 to 13	8 to 12.5	8.3 to 10.6
V	Up to 0.5	Up to 0.25	Up to 0.15
W	0.2 to 2	0.2 to 1	0.2 to 0.6
S	0.01 Max	0.01 Max	0.01 Max
P	0.03 Max	0.03 Max	0.03 Max
N	0.05 Max	0.05 Max	0.05 Max
Fe	Balance	Balance	Balance

[0022] Stainless steels of this invention may comprise the usual impurities found in commercial grades of maraging steels. For example, as shown in Table 3, stainless steels of this invention may include the presence of trace elements such as up to about 0.01 percent by weight S, up to about 0.03 percent by weigh P, and up to about 0.05 percent by weight.

[0023] Silicon is used for the purpose of providing improved properties of castability and fluidity during processing of the steel. Stainless steels of this invention may comprise in the range of from about 0.3 to 1.5 percent by weight Si. Chromium is used for the purpose of providing improved properties of corrosion resistance, strength and hardenability. Stainless steels of this invention comprise in the range of from about 9 to 13 percent by weight Cr. Copper is used for the purpose of providing improved corrosion resistance and weatherability to the finished steel product. It, works along with Cr, Ni and Mo, to provide the desired degree of corrosion resistance. Stainless steels of this invention comprise in the range of from about 0.3 to 3 percent by weight Cu.

[0024] Molybdenum is used for the purpose of improving the hardenability of stainless steels of this invention. Thus, stainless steels of this invention comprise in the range of from about 2.5 to 4.5 percent by weight Mo. Nickel is used for the purpose of adding strength and toughness to the alloy. Stainless steels of this invention comprise in the range of from about 4 to 7 percent by weight Ni. Cobalt is used for the purpose of increasing strength. Thus, stainless steels of this invention comprise in the range of from about 7 to 13 percent by weight Co. Tungsten is used for the purpose of providing improved properties of strength and hardness. Stainless steels of this invention comprise in the range of from about 0.2 to 2 percent by weight W.

[0025] Use of the alloying elements C, Mn and V is understood to be optional. Carbon is used for the purpose of increasing strength & hardenability of the alloy by heat treatment. Thus, stainless steels of this invention may comprise up to about 0.08 percent by weight C. Manganese is used for the purpose of castability. Stainless steels of this invention may comprise up to about 1.5 percent by weight Mn. Vanadium is used for the purpose of increasing hardness, or to enhance toughness at a given hardness level. Stainless steels of this invention may comprise up to about 0. 5 percent by weight V.

[0026] Stainless steels of this invention are rendered more robust against atmospheric corrosion by substitution of the highly reactive elements, e.g., aluminum and/or titanium, contained in conventional maraging or other ultra-high strength precipitation hardening alloys with more benign elements. Purposeful substitution of the highly reactive elements with less reactive elements results in the creation of an alloy that can be single melted and poured in air while exhibiting superior stability and castability (fluidity).

[0027] In a preferred embodiment, stainless steels of this invention are investment cast into a desired form. Preferably, the stainless steel is melted and poured in a single operation that, unlike conventional maraging steel, does not have to take place in a non-reactive environment. It will be appreciated that stainless steels, prepared according to the principles of the invention, can be readily air cast into complex configurations.

[0028] It should be understood that although air melting is preferably used, a protective atmosphere can also be used. For example, Argon, used as either a liquid or in a gas state, can be applied over the surface of the charge material during melting in order to enhance its castability. Additionally, during the first ten minutes or so of the solidification of the casting, if air is eliminated from around the cooling shell by placing a can over the cooling shell a better casting surface may be obtained. However, this is not a limitation on the invention.

[0029] Generally speaking, stainless steels of this invention display the following desired combined mechanical properties: (i) yield strength of greater than about 215,000 psi, preferably in the range of from about 230,000 to 260,000 psi; (ii) tensile strength of greater than about 240,000 psi, preferably in the range of from about 250,000 to 280,000 psi; and (iii) elongation of over about 6 percent, preferably from 8 to 14 percent. In addition to these mechanical properties, stainless steels of this invention also display good properties of castability, fluidity, weldability, and resistance to atmospheric corrosion.

[0030] Stainless steels of this invention can be formulated to provide desired levels of strength, toughness, elongation, and corrosion resistance within the above-noted ranges as needed to accommodate a particular steel application. The different levels of performance in these areas can be obtained by selective use of alloying ingredients and/or by varying the amounts of such alloying ingredients.

[0031] As noted above, stainless steels of this invention can be melted and poured in an air environment, i.e., air melted, poured, and air cast into a desired configuration. After the part is cast, the typical process for processing the stainless steel includes the following steps: (a) homogenation; (b) solution heat treating; (c) cooling; and (d) aging; (e) a high-temperature/high-pressure consolidation can be performed if desired, but should be done prior to the homogenation step.

[0032] The step of high-temperature/high-pressure consolidation is preferably carried out by hot isostatic pressing (hipping). During the hipping step, the investment cast product is placed in a special pressure vessel that is capable of heating the product to specific temperatures under inert gas pressures of over about 14,000 psi. The high temperatures and pressures used in this process cause the collapse of internal casting discontinuities within the product, such as voids, gas, or shrinkage porosity. Thereby, resulting in a casting that has substantially lower internal defects and better mechanical properties.

[0033] The cast product is preferably Hipped at a pressure of about 15,000 psi, and at a temperature of greater than about 1,900°F and, more preferably within the range of from about 1,900°F to 2,250°F in an inert gas-filled chamber, e.g., using argon gas. A pressure chamber is filled or charged with argon gas to a specific pressure range before the chamber is heated. The temperature within the chamber is raised to a desired temperature, causing the pressure within the chamber to increase to a desired level. The desired temperature is held for a period of from about 3 to 5 hours followed by cooling of the castings to room temperature. Products are normally inspected after hipping to evaluate pore closure.

[0034] The homogenation step is conducted in a neutral atmosphere at a temperature range of from about 2,000°F and about 2,225°F for a period of about 1 to 6 hours. The homogenation step serves to reverse and or neutralize the micro-structural changes produced during the hip cycle and minimizes any chemical segregation caused during casting.

[0035] The solution heat treating step is conducted in a neutral atmosphere at a temperature in the range of from about 1,600°F to 2,100°F for a period of from about 1 to 4 hours, followed by rapid gas fan cooling to room temperature. The solution heat treating step serves to put the micro-structure in an Austenitic condition and by rapid cooling transforming the structure into Martensite.

[0036] The cooling step is conducted by refrigerating the room temperature cast product within 24 hours of completing the solution heat treat for a period of from about 1 to 4 hours at a temperature below about -50°F preferably (-90°F to -100°F) to complete the transformation of the Austenite into Martensite. The aging step is conducted at a temperature range of from about 800°F to 1,000°F for a period of from about 1 to 6 hours to achieve the desired combination of above-described mechanical properties.

[0037] Table 4, provided below, sets forth preferred, more preferred and most preferred ranges of the processing parameters used for making stainless steel according to the principles of this invention. It will be understood that all values are approximate.

Table 4

Process Step	Preferred	More Preferred	Most Preferred
Hipping (optional)	1,900°F to 2,250°F @ 14,000 to 16,000 psi for 3 to 5 Hours	2,000°F to 2,240°F @ 14,000 to 16,000 psi for 3 to 5 Hours	2,125°F to 2,220°F @ 14,000 to 16,000 psi for 3 to 5 Hours
Homogenation	2,100°F to 2,225°F for 1 to 6 Hours	2,150°F to 2,220°F for 2 to 5 Hours	2,205°F to 2220°F for 4 Hours ± 15 Minutes
Solution Heat Treating	1,600°F to 2,100°F for 1 to 4 Hours	1,800°F to 1,950°F for 1 to 3 Hours	1,875°F to 1,925°F for 1 Hour ± 15 Minutes
Cooling	-350°F to -50°F for 1 to 8 Hours	-110° to -75°F for 1 to 3.5 Hours	-110°F to -90°F for 3 Hours ±15 Minutes
Aging	800°F - 1000°F for 1 to 6 Hours	900°F to 1,000°F for 3 to 5 Hours	925°F to 950°F for 4 Hours ± 15 Minutes

[0038] The following example is provided for the purpose of reference for better understanding stainless steels and methods for making the same according to principles of this invention.

EXAMPLE

[0039] A number of example embodiments of stainless steels of this invention were prepared according to the practice of this invention. These examples are designated MR04-1, MR04-2, MR04B, MR04C, MR04D, MR04E, MR04L, MR04C, MR08A, MR08B, in Table 5 provided below. These examples were prepared by combining alloying ingredients within the amounts presented in Table 5, and air melting, pouring and casting the combined alloy mixture into a desired cast part.

Table 5

Alloy	MR04 -1	MR04 -2	MR04 B	MR04 C	MR04 D	MR04 E	MR04 L	MR06 C	MR08 A	MR08 B
Element
C	0.02	0.02	0.02	0.02	0.02	0.01	0.02	0.02	0.02	0.02
Mn	0.35	0.35	0.25	0.25	0.26	0.26	0.24	0.24	0.21	0.20
Si	0.49	0.49	0.48	0.47	0.48	0.51	0.47	0.45	0.55	0.56
Cr	9.51	9.51	10.14	10.67	11.14	11.66	10.09	10.76	11.00	11.05
Ni	6.94	6.94	5.89	5.40	5.37	5.26	6.30	5.14	4.85	4.84
Mo	4.00	4.00	4.15	4.13	4.02	4.20	4.12	4.13	3.98	4.02
Cu	0.48	0.48	0.51	0.52	0.50	0.44	0.51	0.49	1.41	0.94
Co	8.49	8.49	8.51	8.48	8.59	8.22	8.48	10.60	10.39	10.40
W	-	-	-	-	-	-	-	-	0.51	0.98
N	0.020	0.020	-	-	-	0.011	-	-	0.01	0.01
V	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.03	-	-
P	0.018	0.018	0.021	0.021	0.021	0.021	0.021	0.021	0.021	0.021
S	0.011	0.011	0.009	0.009	0.009	0.010	0.009	0.009	0.005	0.006
Fe	Bal	Bal	Bal	Bal	Bal	Bal	Bal	Bal	Bal	Bal

[0040] The example stainless steels of this invention were tested and were shown to provide properties of yield strength and tensile strength that are comparable to conventional vacuum melted wrought ultra-high strength stainless steel alloys. However, the example stainless steels of this invention were provided in the form of investment castings that were air melted and air cast.

[0041] Table 6 below provides physical properties of the example stainless steels prepared as described above along with the heat treatment process data for each of the samples. The cast part was homogenized for a period of about 4 hours at a temperature in the range of from 2,100°F to about 2,220°F (see the table for each sample), followed by solution heat treating at a temperature of about 1,900°F for a period of about 1.5 hours. The solution heat treated part was then rapid gas fan cooled to room temperature, and subsequently refrigerated at a temperature of about 100°F for a period of about 4 to about 6 hours (see the table for each sample) to transform retained austenite to martensite. The part was then aged at a temperature of about 950°F for a period of from 4 to 6 hours to achieve the desired mechanical properties.

[0042] In the Heat Treatment column of Table 6, the first time and temperature numbers represent the homogenization data, the second time and temperature numbers represent the solution heat treating data, the third time and temperature numbers represent the refrigeration data, and the fourth time and temperature numbers represent the aging data.

Table 6

Specimen	Yield KSI	UTS KSI	%EL	%RA	Heat Treatment
MR04-1	231. 5	253.5	13	40	2220°F 4 hrs,1900°F 1. 5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04-2	236.0	257.0	10.3	35.25	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 6 hrs
MR04-1	226.0	249.0	11.5	40.0	2120°F 4 hrs, 1900°F 1. 5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04-2	233.0	256.8	11.5	38.5	2120°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 6 hrs
MR04B	235.0	254.6	10.2	39.4	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04C	231.6	252.0	10.8	41.0	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04D	232.4	251.2	10.2	41.2	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04E	232.8	252.2	11.2	41.6	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR04L	237.0	256.6	10.4	37.4	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR06C	252.5	271.0	9.5	33.0	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR08A	257.7	278.0	7.1	21.5	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs
MR08B	259.9	280.1	6.9	20.9	2220°F 4 hrs, 1900°F 1.5 hrs, -100°F 2 hrs, 950°F 4 hrs

[0043] Table 7 below provides data comparing the physical/mechanical properties of the example stainless steels prepared as described above with other conventional steels:

Table 7

Alloy	YS, PSI	UTS, PSI	% El	Density	Strength to Weight	Corrosion Rating	Castability Rating
MR04-1 Avg.	231,500	253,500	13	0.285	889474	8	8
MR04-2 Avg.	236,000	257,000	10.3	0.285	901754	8	8
MR04B Avg.	235,000	254,600	10.2	0.285	893333	8	8
MR04C Avg.	231,600	252,000	10.8	0.285	884211	8	8
MR04D Avg.	232,400	251,200	10.2	0.285	881403	8	8
MR04E Avg.	232,800	252,200	11.2	0.285	884912	8	8
MR04L Avg.	237,00	256,600	10.4	0.285	900351	8	8
MR06C Avg.	252,500	271,000	9.5	0.285	950877	8	8
MR08A Avg.	257,700	278,000	7.1	0.285	975438	8	8
MR08B Avg.	259,900	280,100	6.9	0.285	982807	8	8
17-4 Cast	159300	167000	8	0.282	592199	8	8
17-4 Wrought	178000	192000	12	0.282	680851	8
Super steel C	170000	185000	7	0.282	656028	9	5
Ti-6-4 sheet	140000	150000	10	0.16	937500	10
Ti-6-4 Avg.	122000	134000	7	0.16	837500	10	3
Custom 465	234000	257000	12	0.282	911348	7	2
13-8 PH	210000	225000	12	0.282	797872	8	2
304 stainless	30000	70000	35	0.282	248227	9	7
IC 316	35000	75000	35	0.282	265957	10	6
IC 8620	120000	140000	10	0.280	500000	1	5

[0044] According to the performance data set forth above, stainless steels of this invention display combined properties of yield strength and tensile strength that are both superior to other compared conventional steels, while at the same time providing properties of percent elongation and corrosion resistance that are comparable to the other compared conventional steels.

[0045] Accordingly, the present invention provides stainless steels that are air-castable and that are specially designed to overcome the noted drawbacks of conventional ultra-high strength steels discussed above. Stainless steels of this invention are more resistant to atmospheric corrosion than their martensitic and maraging counterparts, and offer a better combination of mechanical properties than most wrought hardened and precipitation hardened stainless steels, providing a yield strength of over about 215,000 psi and tensile strength of more than about 240,000 psi with over 6% elongation and 20 % reduction in area.

[0046] Stainless steels of this invention exhibit excellent fluidity and castability, which permits their use in being cast into complex thin-walled configurations. Stainless steels are less reactive than other maraging steels, enabling air melting and pouring, and achieving the desired combination of mechanical properties without the need for multiple vacuum melting and wrought processing. The less reactive nature of such steels results in less metal-mold reaction and porosity. Additionally, stainless steels of this invention can be produced to near net shape directly via investment casting. These alloys are significantly cheaper and easier to obtain than conventional maraging alloys. They can achieve their desired mechanical properties without the need for high purity raw material. They can easily be recycled to reduce costs and help the environment. They exhibit better weldability than their low-alloy martensitic counterparts (4140 or 300M alloys). They do not require chrome or cadmium plating for most applications, in comparison to conventional ultra high strength steels, thereby making them more environmentally friendly.

[0047] Stainless steels of this invention can be single melted, thereby significantly reducing processing costs. They can be air melted and air cast into complex configurations, thereby reducing the capital expenditure needed for vacuum casting furnaces. As an air cast alloy, productivity (output per hour) is significantly higher than that associated with using vacuum melted alloys. Stainless steels require lower acquisition lead-times, faster production cycle times, and lower manufacturing costs. Due to their simpler processing and faster production throughput and cycle times (set up and run times), less inventory is required. Since they achieve their properties without the need for wrought processing, they do not require progressive dies to achieve the desired shape and mechanical properties. Wrought processing and vacuum melting can improve their properties for certain applications, at the expense of acquisition and processing costs.

[0048] While specific examples have been disclosed and illustrated, it is to be understood that stainless steels ofthis invention can have one of a number of different chemical make-ups, depending on the particular application for the final product. Furthermore, it should be understood that the temperatures, times and pressures, and the like described for making these steels are only exemplary, and that those skilled in the art will be able to configure the process and chemistry differently than disclosed and illustrated without departing from the spirit of this invention.

Claims

1. An air-cast ultra-high strength stainless steel prepared by combining Si, Cr, Mo, Ni, Co, Cu, W and Fe, wherein the steel has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent.

2. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein the steel has a yield strength in the range of from about 230,000 to 260,000 psi, a tensile strength in the range of from between 250,000 and 280,000 psi, and an elongation of between about 8 to 14 percent.

3. The air-cast ultra-high strength stainless steel as recited in claim 1 further comprising at least one ingredient selected from the group consisting of C, Mn, V and mixtures thereof.

4. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein Si is present in the range of from about 0.3 to 1.5 percent by weight, Cr is present in the range of from about 9 to 13 percent by weight, Mo is present in the range of from about 2.5 to 4.5 percent by weight, Ni is present in the range of from about 4 to 7 percent by weight, Co is present in the range of from about 7 to 13 percent by weight, W is present in the range of from about 0.2 to 2 percent by weight, and Fe is present in a remaining amount.

5. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein C is present up to about 0.08 percent by weight, Mn is present up to about 1.5 percent by weight, and V is present in up to about 0.5 percent by weight.

6. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein the steel is essentially free of Al and Ti.

7. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein the steel comprises:

in the range of from about 0.4 to 1.2 percent by weight Si;

in the range of from about 10.5 to 13 percent by weight Cr;

in the range of from about 2.75 to 4.25 percent by weight Mo;

in the range of from about 4.5 to 6 percent by weight Ni;

in the range of from about 8 to 12.5 percent by weight Co

in the range of from about 0.4 to 1.5 percent by weight Cu;

in the range of from about 0.2 to 1 percent by weight Mn; and

in the range of from about 0.2 to 1 percent by weight W.

8. The air-cast ultra-high strength stainless steel as recited in claim 7 further comprising up to about 0.05 percent by weight C, and up to about 0.25 percent by weight V.

9. The air-cast ultra-high strength stainless steel as recited in claim 1 wherein the steel comprises:

in the range of from about 0.4 to 0.9 percent by weight Si;

in the range of from about 10.5 to 12 percent by weight Cr;

in the range of from about 3.5 to 4.15 percent by weight Mo;

in the range of from about 4.5 to 5.5 percent by weight Ni;

in the range of from about 8.3 to 10.6 percent by weight Co;

in the range of from about 0.2 to 0.6 percent by weight Mn;

in the range of from about 0.4 to 1 percent by weight Cu;

in the range of from about .2 to .6 percent by weight W.

10. The air-cast ultra-high strength stainless steel as recited in claim 9 further comprising up to about 0.03 percent by weight C, and up to about 0.15 percent by weight V.

11. An ultra-high strength stainless steel consisting essentially of

in the range of from about 0.3 to 1.5 percent by weight Si;

in the range of from about 9 to 13 percent by weight Cr;

in the range of from about 2.5 to 4.5 percent by weight Mo;

in the range of from about 4 to 7 percent by weight Ni;

in the range of from about 7 to 13 percent by weight Co

in the range of from about 0.3 to 3 percent by weight Cu;

in the range of from about .2 to 2 percent by weight W; and

a remaining amount Fe.

12. The ultra-high strength stainless steel as recited in claim 11 further including at least one ingredient selected from the group consisting of C, Mn, V, and mixtures thereof.

13. The ultra-high strength stainless steel a recited in claim 11 wherein the steel has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent.

14. An ultra-high strength stainless steel comprising:

up to about 0.08 percent by weight C;

in the range of from about 0.3 to 1.5 percent by weight Si;

in the range of from about 9 to 13 percent by weight Cr;

in the range of from about 0.3 to 3 percent by weight Cu;

in the range of from about 2.5 to 4.5 percent by weight Mo;

in the range of from about 4 to 7 percent by weight Ni;

in the range of from about 7 to 13 percent by weight Co;

in the range of from about 0.2 to 2 percent by weight W;

up to about 0.5 percent by weight V;

up to about 1.5 percent by weight Mn; and

a remaining amount Fe.

15. The ultra-high strength stainless steel a recited in claim 14 wherein the steel has a yield strength in the range of from about 255,000 to 300,000 psi, a tensile strength in the range of from between 255,000 and 320,000 psi, and an elongation of between about 10 to 14 percent.

16. A casting made from a stainless steel that is prepared according to the steps of:

homogenating a cast part made from stainless steel at a temperature of above about 2,000°F for a period of from about 1 to 6 hours;

solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100°F for about 1 to 4 hours, followed by cooling to room temperature;

cooling the cast part by refrigeration for about 1 to 8 hours at a temperature below about -5 0°F; and

aging the cast part at a temperature of between about 800°F and about 1,000°F for about 4 to about 5 hours.

17. The casting as recited in claim 16 wherein, prior to the step of homogenating, the following step is performed:

hot isostatic pressing the cast part at a temperature of above about 1,900°F for about 3 to 5 hours at a pressure of greater than about 14,000 psi.

18. The casting as recited in claim 16 wherein, prior to the step of pressing, the casting is formed by:

combining within the range of from about 0.3 to 1.5 percent by weight Si, in the range of from about 9 to 13 percent by weight Cr, in the range of from about 0.3 to 3 percent by weight Cu, in the range of from about 2.5 to 4.5 percent by weight Mo, in the range of from about 4 to 7 percent by weight Ni, in the range of from about 7 to 13 percent by weight Co, in the range of from about .2 to 2 percent by weight W, and a remaining amount Fe;

melting the combined ingredients to form a stainless steel mixture; and

pouring the stainless steel mixture to form the cast part.

19. The casting as recited in claim 17 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting of C, V, Mn, and mixtures thereof

20. The casting as recited in claim 17 wherein the steps of melting and pouring are conducted in an air environment.

21. The casting as recited in claim 16 wherein the cast part after the step of aging has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent.

22. The casting as recited in claim 16 wherein the homogenating step is conducted in the range of from about 2,150°F to 2,220°F, the solution heat treating step is conducted in the range of from about 1,800°F to 1,950°F, the cooling step is conducted in the range of from about -110°F to -75°F, and the aging steps is conducted in the range of from about 900°F to 1,000°F.

23. The casting as recited in claim 17 wherein the hipping step is conducted in the range of from about 2,000°F to 2,240°F.

24. The casting as recited in claim 16 wherein, prior to the step of pressing, the casting is formed by:

combining up to 0.08 percent by weight C, with in the range of from about 0.4 to 1.2 percent by weight Si, in the range of from about 10.5 to 13 percent by weight Cr, in the range offrom about 0. 4 to 1.5 percent by weight Cu, in the range of from about 2.75 to 4.25 percent by weight Mo, in the range of from about 4.5 to 6 percent by weight Ni, in the range of from about 8 to 12.5 percent by weight Co, in the range of from about 0.2 to 1 percent by weight W, and a remaining amount Fe;

melting the combined ingredients to form a stainless steel mixture; and

pouring the stainless steel mixture to form the cast part.

25. The casting as recited in claim 22 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting of V, Mn and mixtures thereof.

26. The casting as recited in claim 22 wherein the steps of melting and pouring are conducted in an air environment.

27. The casting as recited in claim 22 wherein the cast part after the step of aging has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent.

28. A method for making a cast part from a stainless steel comprising the steps of:

combining up to 0.08 percent by weight C, with in the range of from about 0.3 to 1.5 percent by weight Si, in the range of from about 9 to 13 percent by weight Cr, in the range of from about 0.3 to 3 percent by weight Cu, in the range of from about 2.5 to 4.5 percent by weight Mo, in the range of from about 4 to 7 percent by weight Ni, in the range of from about 7 to 13 percent by weight Co, in the range of from about 0.2 to 2 percent by weight W, and a remaining amount Fe;

melting the combined ingredients to form a stainless steel mixture; and

pouring the stainless steel mixture to form the cast part.

29. The method as recited in claim 26 further comprising, during the step of combining, adding one or more ingredient selected from the group consisting ofV, Mn and mixtures thereof.

30. The method as recited in claim 26 wherein the steps of melting and pouring are conducted in an air environment

31. The method as recited in claim 26 further comprising the steps of:

homogenating the cast part at a temperature of above about 2,100°F for a period of from about 1 to 6 hours;

solution heat treating the cast part at a temperature in the range of from about 1,600° to about 2,100°F for about 1 to 4 hours, followed by cooling to room temperature;

cooling the cast part by refrigeration for about 1 to 4 hours at a temperature below about -50°F; and

aging the cast part at a temperature of between about 800°F and about 1,000°F for about 4 to about 5 hours.

32. The method as recited in claim 31 further comprising the step of:

hot isostatic pressing the cast part at a temperature of above about 1,900°F for about 3 to 5 hours at a pressure of greater than about 14,000 psi prior to the homogenating step.

33. The method as recited in claim 29, wherein after the step of aging, the cast part has a yield strength of greater than about 215,000 psi, a tensile strength of greater than about 240,000 psi, and an elongation of greater than about 6 percent.