Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64C—AEROPLANES; HELICOPTERS
  • B64C25/00—Alighting gear
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64C—AEROPLANES; HELICOPTERS
  • B64C9/00—Adjustable control surfaces or members, e.g. rudders
  • B64C9/02—Mounting or supporting thereof
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/04—Hardening by cooling below 0 degrees Celsius
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working

Definitions

• This invention relates generally to steel alloys that provide very high strength in combination with good toughness in the quenched and tempered condition and in particular to such a steel alloy that also provides good corrosion resistance.
• Aircraft landing gear are critical components that are highly stressed and subject to adverse environmental conditions in use.
• Steel alloys such as AISI 4340 and the 300M alloy have long been used to make landing gear for aircraft because those alloys can be quenched and tempered to provide very high strength (ultimate tensile strength of at least 280 ksi) in combination with fracture toughness (K Ic ) of at least 50 ksi ⁇ in.
• K Ic fracture toughness
• neither of those alloys provides effective corrosion resistance. Therefore, it has been necessary to plate the landing gear components with a corrosion resistant metal such as cadmium.
• Cadmium is a highly toxic, carcinogenic material and its use has presented significant environmental risks in the manufacture and maintenance of aircraft landing gear and other components made from these alloys.
• a known alloy that is sold under the registered trademark FERRIUM S53 was developed to provide a combination of strength and toughness similar to that provided by the 4340 and 300M alloys and to also provide corrosion resistance.
• the FERRIUM S53 alloy was designed to overcome the problems associated with using cadmium plating to provide adequate corrosion resistance in aircraft landing gear made from either the 4340 alloy or the 300M alloy.
• the FERRIUM S53 alloy includes a significant addition of cobalt which is a rare and thus, expensive element.
• Cobalt-free martensitic steel alloys that can be quenched and tempered to provide strength and toughness comparable to the FERRIUM S53 alloy and which also provide corrosion resistance are described in U.S. Pat. No. 8,071,017 and in U.S. Pat. No. 8,361,247. However, it has been found that the corrosion resistance provided by those steels leaves something to be desired. Enhanced corrosion resistance is especially important for aircraft landing gear because they are exposed to many different types of corrosive environments, some of which are more aggressive than others at causing corrosion in steel.
• a quench-and-temper alloy in accordance with the present invention, there is provided a high strength, high toughness, corrosion resistant steel alloy that has the following broad and preferred weight percent compositions.
• the balance of the alloy is iron and the usual impurities found in similar grades of quench and temper steels intended for similar use or service, including not more than about 0.01% phosphorus, not more than about 0.010% sulfur, and not more than about 0.10% nitrogen.
• 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 for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other.
• one or more of the ranges can be used with one or more of the other ranges for the remaining elements.
• a minimum or maximum for an element of the broad ranges can be used with the minimum or maximum for the same element in the preferred ranges, and vice versa.
• the alloy according to the present invention may comprise, consist essentially of, or consist of the constituent elements described above and throughout this application.
• percent or the symbol “%” means percent by weight or mass percent, unless otherwise specified.
• a quenched and tempered steel article that is made from either of the steel alloy compositions set forth above.
• the steel article is characterized by having a tensile strength of at least about 280 ksi and a fracture toughness (K Ic ) of at least about 65 ksi ⁇ in.
• the steel article is further characterized by having good resistance to general corrosion as determined by the salt spray test (ASTM B117) and good resistance to pitting corrosion as determined by the cyclic potentiodynamic polarization method (ASTM G61 Modified).
• At least about 0.2% and preferably at least about 0.35% carbon is present in this alloy.
• Carbon combines with iron to form an Fe—C martensitic structure that benefits the high hardness and strength provided by the alloy.
• Carbon also forms carbides with molybdenum, vanadium, titanium, niobium, and/or tantalum that further strengthen the alloy during tempering.
• the carbides that form in the present alloy are predominantly MC-type carbides, but some M 2 C, M 6 C, M 7 C 3 , and M 23 C 6 carbides may also be present. Too much carbon adversely affects the toughness and ductility provided by this alloy. Therefore, carbon is restricted to not more than about 0.5% and preferably to not more than about 0.45%.
• the alloy according to this invention contains at least about 9% chromium to benefit the corrosion resistance and hardenability of the alloy.
• the alloy contains at least about 9.5% chromium. More than about 14.5% chromium in the alloy adversely affects the toughness and ductility provided by the alloy.
• the alloy contains not more than about 12.5% chromium.
• Nickel is beneficial to the toughness and ductility provided by the alloy according to this invention. Therefore, the alloy contains at least about 3.0% nickel and preferably at least about 3.2% nickel. In order to limit the upside cost of the alloy, the amount of nickel is restricted to not more than about 5.5%. Preferably the alloy contains not more than about 4.3% nickel.
• Molybdenum is a carbide forming element that forms M 6 C and M 23 C 6 carbides that are beneficial to the temper resistance provided by this alloy. Molybdenum also contributes to the strength and fracture toughness provided by the alloy. Furthermore, molybdenum contributes to the pitting corrosion resistance provided by the alloy. The benefits provided by molybdenum are realized when the alloy contains at least about 1% molybdenum and preferably at least about 1.25% molybdenum. Like nickel, molybdenum does not provide an increasing advantage in properties relative to the increased cost of adding larger amounts of molybdenum. For that reason, the alloy contains not more than about 2% molybdenum and preferably not more than about 1.75% molybdenum.
• the alloy of this invention contains a positive addition of cobalt to benefit the strength and toughness provided by the alloy. Cobalt also benefits the temper resistance of the alloy in a manner similar to molybdenum. Unexpectedly, cobalt appears to be beneficial for the corrosion resistance provided by the alloy. For these reasons, the alloy contains at least about 1% cobalt and preferably at least about 2% cobalt. Cobalt is a rare and thus, very expensive element. Therefore, in order to obtain the benefits of cobalt in this alloy and yet maintain a cost advantage relative to other high strength steel alloys that contain 6% or more cobalt, this alloy contains not more than about 4% cobalt. Preferably, the alloy contains not more than about 3% cobalt.
• Vanadium and titanium combine with some of the carbon to form MC-type carbides that limit the grain size which in turn benefits the strength and toughness provided by the alloy according to this invention.
• the MC-type carbides formed by vanadium and titanium in this alloy also benefit the temper resistance and secondary hardening of the alloy. Therefore, the alloy contains at least about 0.1% vanadium and at least about 0.01% titanium. Preferably, the alloy contains at least about 0.3% vanadium. Too much vanadium and/or titanium adversely affects the strength of the alloy because of the formation of larger amounts of carbides in the alloy that depletes carbon from the martensitic matrix material. Accordingly, vanadium is preferably restricted to not more than about 0.6% and titanium is preferably restricted to not more than about 0.2% in this alloy. When the alloy is produced by powder metallurgy, titanium may not be needed. Therefore, it is expected that titanium would not be intentionally included when the alloy is produced in powder form.
• manganese may be present in this alloy primarily to deoxidize the alloy. It is believed that manganese may also benefit the high strength provided by the alloy. If too much manganese is present, then an undesirable amount of retained austenite may remain after quenching such that the high strength provided by the alloy is adversely affected. Therefore, the alloy contains not more than about 1.0% and preferably not more than about 0.7% manganese.
• the alloy preferably contains at least about 0.1% silicon. Too much silicon adversely affects the hardness, strength, and ductility of the alloy. In order to avoid such adverse effects silicon is restricted to not more than about 1.2% and preferably to not more than about 1.0% in this alloy.
• Copper may be present in this alloy because it contributes to the hardenability, toughness, and ductility of the alloy. Copper may also benefit the alloy's machinability and corrosion resistance.
• the alloy preferably contains at least about 0.1% and better yet at least about 0.3% copper.
• the inventors have discovered that copper and nickel should be balanced in this alloy, particularly when the alloy contains very low or no positive addition of copper. Thus, when the alloy contains less than 0.1% copper, for example, not more than about 0.01% copper, at least about 3.75% and preferably not more than about 4.0% nickel should be present to ensure that the desired combination of strength, toughness, and ductility are provided. Too much copper can result in precipitation of an undesirable amount of free copper in the alloy matrix and adversely affect the fracture toughness of the alloy. Therefore, when copper is present in the alloy, copper is restricted to not more than about 1.0% and preferably to not more than about 0.7%.
• Tungsten is a carbide forming element which, like molybdenum, contributes to the hardness and strength of this alloy when present.
• a small amount of tungsten, up to about 0.2% may be present in this alloy in substitution for some of the molybdenum.
• tungsten does not appear to benefit the corrosion resistance of the alloy. Therefore, the alloy preferably contains not more than about 0.1% tungsten.
• Niobium and tantalum are carbide forming elements that combine with carbon to form M 4 C 3 carbides that benefit the temper resistance and hardenability of the alloy. Therefore, the alloy may contain niobium and/or tantalum provided that the combined amount of niobium and tantalum (Nb+Ta) is not more than about 0.5%. However, in order to avoid the formation of excessive amounts of carbides, the alloy preferably contains not more than about 0.01% of niobium and/or tantalum.
• the alloy may be present in the alloy from deoxidation additions during melting.
• the alloy contains not more than about 0.01% aluminum.
• the alloy contains not more than about 0.006% cerium and not more than about 0.005% lanthanum from such additions.
• the balance of the alloy is iron and the usual impurities found in known grades of steels intended for similar purpose or service.
• phosphorus is restricted to not more than about 0.01% and preferably to not more than about 0.005% in this alloy.
• Sulfur is restricted to not more than about 0.001% in this alloy and preferably to not more than about 0.0005%.
• the alloy may contain up to about 0.010% max. sulfur.
• Nitrogen is preferably maintained as low as practicable in this alloy.
• nitrogen is restricted to not more than about 0.05% and better yet to not more than about 0.03%.
• the alloy is expected to contain up to about 0.10% nitrogen in the nitrogen-atomized powder form of the alloy.
• the alloy according to this invention is preferably prepared by vacuum induction melting (VIM) and refined by vacuum arc remelting (VAR).
• VIM vacuum induction melting
• VAR vacuum arc remelting
• the alloy can be refined by electroslag remelting (ESR) after VIM.
• ESR electroslag remelting
• the alloy can be arc melted and refined by VAR.
• this alloy can be produced by powder metallurgy techniques.
• the VAR or ESR ingot is preferably given a homogenization heat treatment after removal from the mold.
• the homogenization is preferably carried out by heating the ingot at about 2200° F. to about 2375° F. for about 9 to 18 hours depending on the size of the ingot.
• the ingot is then hot worked to a billet having a smaller cross-sectional area.
• the billet is further hot worked such as by forging or rolling to provide an intermediate product form having a desired cross-section dimension and shape, for example, round or square bar.
• the intermediate product form is preferably normalized by heating the alloy under temperature and time conditions sufficient to dissolve Cr-rich carbides that may have precipitated during solidification.
• the intermediate product is normalized by heating at about 1925-2050° F.
• the alloy is then annealed by further heating the alloy at about 1100-1250° F. for about 2 to 12 hours. This low annealing temperature helps to keep the dissolved chromium carbides in solution.
• the alloy is preferably formed into final or near-final product forms in the annealed condition. Final product forms made from the alloy are hardened by heating the alloy at a temperature of about 1950-2050° F., preferably at about 2000° F., for a time sufficient to fully austenitize the alloy and to dissolve most, preferably all, of the remaining chromium carbides so that the amount of chromium present in the alloy matrix can be maximized.
• the alloy is then preferably oil quenched from the austenitizing temperature.
• the alloy is preferably deep chilled at about ⁇ 100° F. for at least about 1 hour and then warmed in air.
• the alloy is then tempered to final hardness by heating at about 350-550° F., preferably at about 400° F., for 1-6 hours, and then cooled to room temperature.
• the tempering temperature is selected to maximize toughness while minimizing the re-precipitation of chromium carbides in the alloy.
• the alloy comprises a dispersion of carbides as discussed above in the Fe—C martensitic matrix.
• the carbides present in the alloy and articles made therefrom are predominantly, if not entirely, greater than 10 nm in major cross-sectional dimension.
• the heat treating parameters are controlled so that the carbide size is not greater than about 15 ⁇ m in major cross-sectional dimension.
• a steel article made from the alloy described above and processed in accordance with the foregoing processing steps provides a combination of properties that make it particularly useful for aircraft landing gear and other aeronautical or aerospace structural components, including but not limited to flap tracks, slat tracks, rotating shafts, and actuators, and for other applications where the non-corrosion resistant steels 300M and 4340 are currently used.
• a steel article fabricated from the alloy that is hardened and tempered as set forth above provides a tensile strength of at least 280 ksi, preferably at least 285 ksi, and a fracture toughness (K Ic ) of at least 65 ksi ⁇ in when tested with a test machine that meets the requirements of ASTM Standard Test Procedure E1290.
• a steel article in accordance with this invention is also characterized by a Charpy V-notch impact energy of at least 20 ft-lbs when tested in accordance with ASTM Standard Test Procedure E23. Further, a steel article in accordance with this invention is characterized by general corrosion resistance such that the article does not rust when tested in accordance with ASTM Standard Test procedure B 117 and by sufficient pitting corrosion resistance such that the article has a pitting potential of at least 90 mV when tested in accordance with a modified ASTM Standard Test procedure G61.
• the ASTM G61 test procedure was modified by using round bar rather than flat samples. The use of round bar samples exposes the end grains and can be considered to be a more severe test than the standard G61 procedure.
• Heat F C 0.40 0.40 0.40 .040 .041 Mn 0.62 0.61 0.60 0.60 0.60 Si 0.90 0.90 0.89 0.88 0.88 P ⁇ 0.005 ⁇ 0.005 ⁇ 0.005 ⁇ 0.005 ⁇ 0.005 S 0.003 0.0014 0.001 0.0012 0.0013 Cr 9.96 9.95 9.92 10.10 9.98 Ni 3.76 4.01 3.50 4.23 4.49 Mo 1.50 1.50 1.49 1.50 1.50 Cu ⁇ 0.01 ⁇ 0.01 ⁇ 0.01 ⁇ 0.01 ⁇ 0.01 Co 2.49 2.50 2.45 2.50 2.50 W — — — — V 0.50 0.50 0.49 0.50 0.50 Ti 0.08 0.09 0.01 0.09 0.09 Ce — — — — — La — — — — — — — — The balance of each heat is iron and usual impurities. Heats 1 to 4 are representative of the alloy according to the present invention. Heats A to F are comparative heats. In particular, Heats A to F are comparative heats
• the VIM heats were melted and cast as 6-inch round electrodes for remelting.
• the 6-inch round electrodes were remelted by VAR into 8-inch round ingots.
• the VAR ingots were cooled in air after being stripped from the molds, stress relieved at 1150° F. for 3 hours, and then air cooled from the stress relieving temperature.
• the ingots were then charged into a furnace running at 1200° F.
• the furnace temperature was ramped up to 1600° F. and held for a time sufficient to equalize the temperature of the ingots.
• the furnace temperature was then ramped up to 2300° F. and the ingots were heated at 2300° F. for 16 hours.
• the furnace temperature was decreased to 2200° F.
• Test specimens for the nominal copper heats were prepared as follows. A 3-inch thick cut was made from one end of each of the billets, and then a 24-inch long piece was cut from each billet. The 24-inch long pieces were charged into a furnace running at 1200° F. The furnace temperature was ramped up to 1600° F. and held at that temperature to equalize the temperature of the pieces. The furnace temperature was then ramped up to 2200° F. and held at that temperature for 1 hour. The billet pieces were double-end forged to 3-inch square bars with one reheat at 2200° F. The 3-inch square bars were hot cut into four pieces each approximately 6 in. long with the remainder cooled in a hot box.
• the 3-inch square bar pieces were reheated at 2200° F., double end forged to 13 ⁇ 8” square with one reheat, and then hot cut into two pieces.
• the 13 ⁇ 8′′ square bars were reheated at 2200° F., and then single end forged to 3 ⁇ 4-inch square bars without a reheat.
• the bars were cooled in a hot box overnight and then air cooled to room temperature.
• the 3 ⁇ 4-inch bars were normalized by heating them at 1950° F. for 4 hours and then cooling in air.
• the bars were then overage annealed at 1150° F. for 6 hours and cooled in air.
• Samples from the nominal copper heats were rough machined for corrosion testing. Pitting potential samples, salt spray cone samples, and RSL stress corrosion cracking (SCC) samples were preheated at 800° F. for 15 minutes in air, austenitized at 1975° F. (2000° F. for Heat A) for 1 hour, oil quenched, refrigerated at ⁇ 100° F. for 1 hour, air warmed, and tempered at 350° F. for 3 hours, air cooled. All samples were finish machined to final dimension after heat treatment.
• SCC RSL stress corrosion cracking
• Test specimens for the low-copper heats were prepared as follows. A 3-inch trim cut was made from one end of the billets, and then two pieces at 8-inch long were cut from each billet. The 8-inch long pieces were charged into the furnace at 1200° F., ramped up to 1600° F., equalized, ramped up to 2200° F., and held at temperature for 1 hour. The billets were double end forged to 3-inch square bars with one reheat at 2200° F. The 3-inch square bars were each hot cut into 2 pieces. The 3-inch square pieces were reheated at 2200° F., double end forged to 13 ⁇ 8 inch square bars with one reheat, and then hot cut into 2 pieces.
• the 13 ⁇ 8 inch square bars were reheated at 2100° F., and then single end forged to 0.725 inch square, with no reheats.
• the bars were cooled in a hot box overnight, and then air cooled the next day.
• the bars were then normalized at 1950° F. for 4 hours, air cooled, overage annealed at 1150° F. for 6 hours, and air cooled.
• RSL stress corrosion cracking tests were performed in accordance with ASTM Standard Test Procedure F1624. Samples from all heats were tested in 3.5% NaCl solution, natural pH, at room temperature. The first test of each heat was run using 1 hour steps and the second run used 2 hour steps. A further sample from each of Heats 3, 4, E, and F was run using 4 hour steps. The results of the stress corrosion cracking tests are shown in Tables VIIA and VIIB below including the stress corrosion cracking resistance (K ISCC ) in ksi ⁇ in.
• K ISCC stress corrosion cracking resistance
• Heats 1, 2, 3, and 4 provide a good combination of strength, ductility, toughness, and corrosion resistance.
• the data also show that although the comparative Heats A-D provide acceptable strength in general, they leave something to be desired with respect to other important properties. More specifically, Heat A has tensile ductility, fracture toughness, and pitting and general corrosion resistance that are inferior to Heats 1 and 2. Heat B has less than desirable pitting corrosion resistance and stress corrosion cracking resistance compared to Heats 1 and 2. Heat C has tensile strength, notch tensile strength, and general and pitting corrosion resistance that are comparable to Heat 1 and 2.
• Heat C is inferior to Heats 1 and 2.
• Heat D has several properties that are inferior relative to Heats 3 and 4, including tensile ductility, fracture toughness, and pitting resistance.
• Heats E and F have tensile strength that is less than acceptable relative to Heats 2 and 3. The yield strength provided by those heats would likely render those alloys unsuitable for the primary application for this alloy, namely, structural components for aircraft.

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