MX2008006428A - Ultra-high strength martensitic alloy - Google Patents

Abstract

An age hardenable martensitic steel alloy is disclosed. The alloy has the following composition in weight percent. C 0.30-0.36;Mn 0.05 max.;Si 0.05 max.;P 0.01 max.;S 0.0010 max.;Cr 1.30-3.2;Ni 10.0-13.0;Mo 1.0-2.70;Co 13.8-17.4;Ti 0.02 max.;Al 0.005 max.;Ce 0.030 max.;La 0.010 max. The balance is iron and the usual impurities. The composition of this alloy is balanced to provide a unique combination of very high strength, together with good toughness, ductility, and resistance to fatigue.

Description

MARTENSITIC ALLOY OF ULTRA-HIGH RESISTANCE " FIELD OF THE INVENTION The present invention relates to an alloy of martensitic steel hardenable by aging, and in particular, to such an alloy that it provides a unique combination of very high strength together with good strength, toughness, ductility and strength. to fatigue.

BACKGROUND OF THE INVENTION It is known that martensitic steel alloys that are curing by aging provide a combination of high strength and good fracture toughness. The alloy is sold under the trademark AERMET® 310 and has been found to be quite useful in structural components for the aerospace industry, in the armoring of both aerial and ground vehicles and in machine tool components. The alloy AERMET 310 escapes provide a tensile strength of approximately 2137 MPa (310 ksi) in combination with a fracture toughness of approximately 65.9 MPaVm (60 ksiVin). The weight and size of the structural components are varied critical of the design for the aerospace industry. This also applies to the industry automotive, particularly in the field of high-performance racing cars. Therefore, aerospace design engineers and automotive engineers have continuously sought ways to reduce the size of the components, and therefore the weight, without forgetting the important mechanical properties, particularly the mechanical strength, toughness and ductility. Due to the ongoing demand for materials that allow the use of lighter weight structural components, it is desirable to have a steel alloy that provides even greater strength than the AER ET 310 alloy. However, it is well known that typically the toughness and Steel ductility are inversely related to the resistance property. It is therefore important that any alloy provides a higher strength property without a significant loss in toughness and ductility properties. The function of some structural parts for high-performance automotive applications, such as springs, subjects such parts to high frequencies, repetitive actions, for long periods of time. Consequently, a property of critical design for the material used in such components is the resistance to fa 1 derived from fatigue. Conveniently, for the sake of. utility in high-speed racing performance, good fatigue strength is required in addition to the aforementioned combination of high strength, toughness and ductility.

BRIEF DESCRIPTION OF THE INVENTION The. The combination of properties desired for the fields of use described above is implemented to a significant degree by the steel alloy according to the. present invention. The. alloy according to this invention is an age-hardenable martensitic steel alloy that provides significantly greater strength than the known alloy and maintains simultaneously acceptable levels of toughness and ductility relative to the known alloy. In particular, the alloy of the present invention will escape from providing a tensile strength (UTS) of at least about 2344 MPa (340 ksi) with ductility and overall toughness at least similar to the AERMET 310 alloy. In addition, the alloy of this invention provides excellent resistance to fatigue. The alloy according to this invention is an age-hardenable martensitic steel alloy having the following broad, intermediate and preferred weight percentage compositions. b OR The balance of the alloy is essentially iron except for the normal impurities found in commercial grades of such steels and minimum amounts of additional elements which can vary from a few thousandths of a percentage to larger quantities that do not unacceptably deplete from the desired combination of the properties provided by this, alloy. The previous tabulation is provided as a convenient summary and is not intended to be restricted to the lower and upper values of the ranges of the individual elements of the alloy of this invention for use in combination with one another, or to restrict the ranges of the elements for use in combination with one another. Consequently, one or more of the element ranges of the. Broad composition can be used with one or more of the higher ranges for, the remaining elements in. the. preferred composition. In addition, a minimum or maximum may be used for an element of a preferred embodiment with the maximum or minimum for that element from another preferred embodiment. Here and throughout this specification, the term "percentage" or the symbol "%" refers to a percentage by weight, unless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION The alloy according to the present invention contains at least about 0.30% and preferably at least about 0.32% carbon. The carbon contributes to the good strength and toughness of the alloy by basically combining with other elements, such as bromine and molybdenum, to form 2C carbides during an aging heat hardening process. Too much carbon adversely affects the fracture toughness, Charpy notch impact resistance (CVN - Charpy V-notch) at ambient temperature, and crack corrosion crack resistance. Conveniently, the carbon is limited to no more than about 0.36%, even no more than about 0.35%, and preferably no more than about 0.34% in this alloy. The coba.lto contributes to the very high strength provided by this alloy and benefits the aging hardening of the alloy by improving the heterogeneous nuclcation sites for the 2C carbides. In addition, the contribution of cobalt or the very high strength given by this lesson is less detrimental to the tenacity of the alloy than the addition of carbon. Suitably, the alloy contains at least about 13.8%, even at least about 15.0% cobalt. Preferably, at least about 15.4% cobalt is present in the alloy. Because cobalt is an expensive element, the benefit obtained from cobalt may not justify the use of very large amounts of cobalt in this alloy. Therefore, cobalt is restricted to no more than about 17.4%, even neither a. no more than about 16.0% and preferably a. no more than approximately 15.6%.

Carbon and cobalt are controlled in the alloy of the present invention to benefit the superior combination of very high strength and high tenacity that is characteristic of the alloy. I have observed that increasing the ratio of cobalt to carbon (Co / C) stimulates a greater tenacity and a better combination of strength, and tenacity in this alloy. Also, increase the. Co / C ratio benefits the tenacity to the en.1.1.a of La. alloy. Conveniently, cobalt and carbon are controlled in this alloy such that the Co / C ratio is at least about 43 and about at least about 52. However, the benefits derived from a high Co / C ratio are counteracted by the high cost of producing an alloy that has a too high Co / C ratio. Therefore, the Co / C ratio is restricted to no more than about 100 and preferably no more than 75. Chromium contributes to the good strength and hardness of this alloy when combined with carbon to form M2C carbides during the hardening process by aging. Therefore, at least about 1.30%, even at least about .1.0%, and preferably at least about 2.20% chromium is present in the alloy. Too Chromium increase the sensitivity of the alloy to over-aging which may result in lower strength. further, too much chromium results in greater precipitation of the carbides in the grain boundaries, which adversely affects the toughness and ductility of the alloy. Conveniently, the. chrome is limited to no more than about 3.20%, i. not more than approximately 2.50%, and preferably a. no more than approximately 2.30% in this 10 aluation. Molybdenum, like chromium, is present in this alloy because it contributes to the very high strength and toughness of this alloy when combined with carbon to form MC ib carbides during the aging process. In addition, molybdenum reduces the sensitivity of the alloy to over-aging and benefits the crack resistance of stress corrosion. Therefore, at least, at least about 1.00%, even at least about 1.50%, and preferably at least about 1.80% molybdenum is present in the alloy. Too much molybdenum increases the risk of undesirable border carbide precipitation from gra.no, la. which, as noted previously, may result in reduced tenacity and ductility.

Therefore, molybdenum is restricted to not more than about 2.70%, even not more than about 2.2% and preferably not more than about 1.90%. At least about 10.0%, even at least about 11.0%, and preferably at least about 11.5% nickel is present in a. The purpose is to benefit the hardenability and reduce the sensitivity of the. alloy at a cooling rate in such a way that the CV tenacity. Nickel also benefits stress corrosion crack resistance and fracture toughness KIC. Too much nickel improves and increases sensitivity to aging. Therefore, nickel is restricted in the alloy to not more than about 13.0% and not more than about 12.5%. Other elements may be present in the alloy in insignificant amounts with respect to the desired properties. Not more than about 0.05% and even not more than about 0.02% manganese is present because the manganese adversely affects the fracture toughness of the alloy. Preferably, the manganese is restricted to no more than about 0.01%. Also, up to about 0.05% silicon, up to about 0. 005% aluminum and up to approximately 0.002% titanium may be present as residues from small additions for the deoxidation of the alloy during casting. Preferably, the silicon is restricted to no more than about 0.03%, 1. aluminum is restricted to no more than about 0.003%, and titanium is restricted to. no more than about 0.015%. In the. alloy there are present small but effective amounts of elements that provide a control of sulfide form to benefit the property of fracture toughness when combined with some sulfur present in the alloy. Such elements are effective to form sulfide inclusions that do not adversely affect the fracture toughness property. A similar effect is described in the U.S. Patent. 5,268,044, which is incorporated herein by reference. In one embodiment of the present invention, the alloy contains up to about 0.030% cerium and up to about 0.010% lanthanum. Preferably, the alloy contains no more than about 0.020%, even no more than about 0.010% cerium, and no more than about 0.005% lanthanum. The balance of the alloy is essentially iron except for the common impurities found in Commercial grades of alloys intended for service or similar use. The levels of such elements are controlled to avoid adversely affecting the desired properties. For example, phosphorus is restricted to no more than about 0.01% and preferably a. no more than about 0.05% due to its brittle effect on the alloy. Sulfur, although inevitably present, is restricted to. not more than about 0.0010% and preferably a. no more than about 0.0005%, because the sulfur adversely affects the fracture toughness of the alloy. The alloy of the present invention is easily melted using vacuum casting techniques. For best results, a multiple casting practice is preferred and ultra clean raw materials, such as electrolytic iron, are preferably used to charge the casting furnace. The preferred practice is to melt the cast by vacuum induction melting (VIM - vacuum induction melting) and melt the cast in the form of an electrode. Then, the electrode is refined by vacuum arc remelting (VAR) in one or more ingots for further processing. The preferred method to introduce cerium and 1.antaño to this, alloy is through the addition of grades of high purity of cerium and lanthanum during the VIM, before melting the VAR ingot electrode. Rare earth alloys such as NiLa can also be used. Effective amounts of cerium and lanthanum are present when the ratio of cerium to sulfur (Ce / S) in the VIM smelter is at least about 4. When the Ce / S ratio is greater than about 20, the retained amounts of earth Rarely present in the melted VAR electrode can adversely affect the hot workability and ductility of the alloy. Preferably, the Cc / S ratio in VIM is at least about 8, and not greater than about 10. In another embodiment of this alloy, a small but effective amount of one or more of calcium, magnesium, yttrium, or other element of Sulfur removal is present in the alloy instead of a portion or all of the cerium and lanthanum in order to provide control of beneficial sulfide form. Prior to VAR, the electrode ingots are preferably tension-free at approximately 677 ° C (1250 ° F) for 4-16 hours and cooled with air. After the VAR, the ingot is preferably homogenized at about 1177-1232 ° C (2150-2250 ° F) for about 6-24 hours. The alloy can be hot worked from approximately 1232 ° C (2250 ° F) to approximately 816 ° C (1500 ° F). The preferred practice for hot work is to set an ingot from approximately 1177-1232 ° C (2150-2250 ° F) in order to obtain at least approximately a 30% reduction in cross-sectional area. The ingot is then reheated to approximately 982 ° C (1800 ° F) and further forged to obtain at least another 30% reduction in the cross-sectional area. The heat treatment to obtain the desired combination of properties includes a solution treatment (austenization) and cooling, a deep cooling treatment, and then a term hardening treatment by aging. The alloy is austenized by heating it to approximately 843-982 ° C (1550-1800 ° F). for about 1 hour plus about 5 minutes per inch of thickness, followed by cooling. Preferably, the cooling rate is fast enough to cool the alloy from the austenitization temperature to about 66 ° C (1.40 ° F) in no more than about 2 hours. The preferred cooling technique will depend on the size of the cross-sectional area of the manufactured part. However, the temp! a.bi 1. this alloy is good enough to allow air cooling, cooling vermiculite, or cooling by inert gas in a vacuum oven, as well as oil cooling. After the austenitization and cooling treatment, the alloy is preferably cold treated with deep cooling to about -196 ° C (-320 ° F) for about 0.5-1 hour and then heated with. he. air. The hardening by age of this a. The process is preferably carried out when heating. alloy at approximately 454-510 ° C (850-950 ° F) for about 5 hours followed by air cooling. Preferably, the hardened alloy by aging is given an additional cold treatment under the same conditions specified above. The alloy of the present invention is useful in a wide variety of applications. The very high strength and good fracture toughness of the alloy makes it useful for structural components of aerial vehicles and machine tool components. The alloy of this invention is also useful for automotive components including, but not limited to, structural members, drive shafts, springs and crankshafts.

Operational examples In order to demonstrate the. novel combination of properties provided by the alloy according to the present invention, two experimental 400 Ib washes were melted and processed. (181.4 kg), Castings 1 and 2, in specimens for mechanical tests. The compositions in percent by weight of the experimental casts are set forth in Table 1 shown below. Table 1 also shows the chemistries in percentage by weight of four castings of production of the alloy AER ET 310 (Castings A, B, C and D) which were tested for comparison.

Table 1 The balance of each pour is iron and the common impurities Castings 1 and 2 were processed into forged bars that measured approximately 1.5 inches by 4.5 inches (3.81 cm by 11.4 cm). Duplicate specimens were prepared for the tensile tests, the Charpy notch shock resistance test and the fracture tenacity test from the forged bars of each. a. of Castings 1, 2 and A-D. All specimens of test were thermally treated to provide maximum tensile strength. For Castings 1 and 2, the specimens were austenitized at 1775 ° F (968.3 ° C) for one hour and then cooled with air. Afterwards, the specimens received a deep cooling treatment at -320 ° F (-196 ° C) followed by heating with air. The specimens were hardened by aging at 900 ° F (482.2 ° C) for 5 hours, and then cooled with air. For, Comparative Castings A-D, specimens were austenitized at 1675 ° F (912.8 ° C) for one hour, and then cooled with air. Afterwards, the specimens are subjected to a deep cooling treatment at -100 ° F (-73.3 ° C) and then heated with air. The specimens were hardened by aging at 875 ° F (468.3 ° C) for 6 hours, and then cooled with air. Table 2 below shows the results of the mechanical tests of specimens including traction resistance (UTS) and yield strength (YS - yield strength), both in ksi (MPa), percentage elongation (% El .), area reduction (% RA), Charpy notch impact resistance (CVN), as fracture toughness (KIC) in ksi Vm (MPa Vm). The Charpy notch shock resistance test was performed in accordance with the Standard Test. ASTM E23. The fracture tenacity test is performed in accordance with the Test of ASTM E23 Standard. An objective of the alloy according to the present invention is to maximize both strength and toughness. The relevant resistance parameter is the resistance to tractional breakage. However, tenacity can be measured in various ways. Mechanical engineers often use a tenacity measurement la. which is an approximation of the area under the curve, of stresses and deformations. This measurement allows you to design one. part to "bend without breaking". Parts that use ultra-high strength alloys are often designed with tenacity measurements that take stress concentrations into account. The two most common tests to measure the effects of tenacity stress concentrations are the Charpy notch shock resistance test and the fracture toughness test. In order to provide an indication of the general tenacity of the alloy according to this invention, three parameters of tenacity were considered: the area under the stress and strain curve, the impact energy of CV, and the fracture toughness ( KIC). Three measurements are combined in one parameter: the Tenacity index. The Tenacity Index is the geometric mean, of the three standardized tenacity measurements and it is calculated as follows: Tenacity index = yf ([((Elong.) x (YS + UTS) ÷ 2) ÷ 50] x [CVN x 3] x [K / c]) Two of the tenacity measurements were "normalized" in such a way that each of their values is within a scale of 0 - 100. Normalization was used in such a way that the Tenacity index did not surpass too much a measure of tenacity with rel .a.tion to the other. In the previous equation, the area under 1.a. curve, stress and deformation ((Elong.) Z (YS + UTS) / 2) is normalized by dividing by 50 and the value of impact energy of CVN is multiplied by 3. The value of fracture toughness is used without normalize. The Tenacity index values calculated for each of the experimental casts and two of the comparative casts are also present in Table 2.

COLADA YS UTS% EL. % RA CV Klc IND. OF TENAC. 1 314.3 349.7 10.3 53.1 8.8 31.1 38.3 (2167) (2411) (11.9) 354.2 10.4 49.6 11.0 31.6 41.7 (2442) (14.9) 2 307.9 349.0 10.0 45.8 13.8 31.5 44.1 (2123) (2406) (18.7) 312.5 348.7 9.6 50.9 11.0 33.5 41.2 (2155) (2404) (14.9) A 290.3 335.0 9.4 40.6 12.0 (1) (1) (2002) (2310) (16.3) 296.0 335.9 8.4 39.3 12.5 (2041) (2316) (16.9) B 285.8 332.3 6.0 20.9 20.0 40.7 44.9 (1971) (2291) (27.1) 289.5 331.5 9.2 38.4 25.5 42.5 48.3 (1996) (2286) (34.6) C 296.1 333.5 9.3 51.8 (1) 41.3 (1) (2042) (2299) 287.2 330.4 11.3 54.7 40.7 (1980) (2278) D 287.0 335.4 9.3 33.8 7.3 40.6 37.2 (1979) (2313) (9.9) 284.6 335.4 9.3 22.2 6.2 34.0 33.2 (1962) (2313) (8.4) (1) Insufficient material to test. The data presented in Table 2 show that Examples 1 and 2 of the alloy according to the present invention provide a tensile strength significantly higher than any of the comparative washes A, B, C and D. The data, particularly the Tenacity index, also clarify that 1. to. The toughness and ductility of Examples 1 and 2 are at least as good as the known alloy represented by the comparative wastes. Accordingly, the combination of strength, ductility and toughness provided by the alloy according to this invention is superior to the combination of those properties provided by the castings of the known alloy. In order to demonstrate the good fatigue resistance of the alloy according to this invention, a fatigue test was carried out on the specimens of Example 1 and an additional comparative casting, Casting E, having the following weight percentage composition . Casting E is a production cast of the AERMET 310 alloy described above and was prepared in a similar manner with the other comparative washes.

Casting E C 0.258 Mn 0.01 Si 0.02 P 0.002 S < 0.0005 Cr 2.46 Ni 11.1 Mo 1.45 Co 15.23 Al 0.004 Ti 0.010 N < 0.0010 O < 0.0010 Ce * 0.006 The * 0.002 * In VIM The balance is iron and common impurities. In Table 3 shown below the results of the fatigue test are exposed to a rotating beam R.R. Moore for each of the alloys, including the number of cycles to fail (Cycles) and the applied voltage (Voltage) in ksi (MPa). ? 3 TABLE 3 Colada. E Casting 1 Cycles Tension Cycles Voltage 8. 00 * 104 200 (1379) 1 .00 * 105 200 (1379) 8. 20 * 104 200 (1379) 1 .83 * 105 200 (1379) 1. 12 * 106 180 (1241) 6 .39 * 105 180 (1241) 1. 04 * 106 180 (1241) 6 .99 * 105 180 (1241) 3. 01 * 10s 160 (1103) 7 .06 * 105 160 (1103) .86 * 106 160 (1103) 3 .01 * 106 150 (1034) 4. 94 * 106 150 (1034) 6 .70 * 106 160 (1103) 9. 75 * 10¾ 150 (1034) 6 .84 * 106 150 (1034) 2. 29 * 107 140 (965) 1 .46 * 107 140 (1379) 3. 36 * 107 140 (965) 1 .83 * 107 140 (1379) The data set forth in Table 3 show that the fatigue strength of Example 1 of the alloy according to the present invention is at least as good as the fatigue resistance of the known alloy. The terms and expressions that have been used in the above description are used as terms of description and not limitation. There is no intent in the use of such terms or expressions to exclude any equivalents of the elements or characteristics of these or any portions thereof. However, it must be recognized that diverse modifications within the scope of the invention as describes and claims. b or I 20

Claims (20)

NOVELTY OF THE INVENTION The invention having been described as antecedent, property is claimed as contained in the following claims CLAIMS . A marthasitic steel process Hardened by aging, characterized because consists essentially of, percentage by weight, of approximately c 0.30- 0.36 Mn 0.05 má Yes 0.05 m P 0.005 max. S 0.0010 max Cr 1.30-

1.32 Ni 10.0- 13.0 MO í. o-: 2.70 CO 13.8- 17.4 Ti 0.02 max. At 0.005 max. Ce 0.030 max. The 0.010 max. being the iron balance and common impurities.

2. An alloy of martensitic steel hardenable by aging according to claim 1, characterized, because it contains at least approximately 11.0 percent by weight of nickel.

3. An age-hardenable martensite steel alloy according to claim 1, characterized in that it contains at least about 2.0 weight percent chromium.

4. An age-hardenable martensite steel alloy according to claim 1, hoisted character, because it contains no more than about 2.5 weight percent chromium.

5. An age-hardenable martensitic steel alloy according to claim 1, characterized in that it contains no more than about 16.0 weight percent cobalt.

6. An age-hardenable martensitic steel alloy according to claim 1, characterized in that it contains no more than about 15.0 weight percent cobalt.

7. An age-hardenable martensitic steel alloy according to claim 1, characterized in that it contains no more than about : ic 1. 0 weight percent molybdenum.

8. An age-hardenable martensitic steel alloy according to claim 1, characterized in that it contains no more than about 2.2 weight percent molybdenum. 25

9. An alloy of martensitic steel hardenable by aging according to claim 1, characterized in that it contains at least about 0.32 percent by weight of carbon.

10. An alloy of martensitic steel Hardened by aging, characterized because consists essentially, in percentage by weight, of approximately C 0.32-0.35 Mn 0.05 max. Yes 0.05 max. P 0.005 max. S 0.0010 max. Cr 2.0-2.5 Ni 11.0-13.0 Mo 1.5-2.2 Co 15.0-16.0 Ti 0.015 max. At 0.003 max. Ce 0.020 max. The 0.010 max. being the iron balance and common impurities.

11. An alloy of martensitic steel curable by aging according to claim 10, characterized in that it contains at least about 11.5 percent by weight of nickel.

12. An alloy of martensitic steel

Hardened by aging according to the. claim 10, characterized, because it contains at least about 2.20 percent by weight of chromium. 13. An age-hardening martensitic steel alloy according to claim 10, characterized in that it contains no more than about 2.30 weight percent chromium.

14. One. martensitic steel alloy endurable by aging according to claim 10, characterized in that it contains no more than about 15.6 weight percent cobalt. 15. One. martensitic steel alloy curable according to claim 10, characterized in that it contains at least about

15.4 weight percent cobalt.

16. An age-hardenable martensitic steel alloy according to claim 10, characterized in that it contains at least about 18.0 weight percent molybdenum.

17. An age-hardenable martensitic steel alloy according to claim 10, characterized in that it contains not more than about 2.0 weight percent molybdenum.

18. An age-hardenable martensitic steel alloy according to claim 10, characterized in that it contains not more than about 0.34 percent in. carbon weight.

19. An alloy of martensitic steel Hardened by aging, characterized because consists essentially of, percentage by weight, of approve c 0.32- 0.34 Mn 0.04 max. Yes 0.03 max. P 0.003 max. S 0.0005 max Cr 2.20- 2.30 Ni 11.5- 12.5 Mo 1.80- 1.90 Co 15.4- 15.6 Ti 0.01 max. At 0.001 m Ce 0.010 max. The 0.005 max. being the iron balance and common impurities.

20. An alloy of martensitic steel hardenable by aging according to claim 1, characterized in that it comprises an element selected at from the group consisting of calcium, magnesium, yttrium, and combinations thereof, instead of at least one portion of cerío and lanthanum.