Patent Application
20140161658
1. Field of the Invention
This invention relates to precipitation hardenable, martensitic stainless steel alloys and in particular to a martensitic stainless steel alloy and an article made therefrom, having a novel combination of strength and corrosion resistance.
2. Description of the Related Art
The aerospace industry has been looking for a stainless steel alloy for landing gear for many years. The main alloy currently used for the commercial landing gear application is 300M alloy. 300M alloy can be quenched and tempered to provide an ultimate tensile strength of at least 280 ksi and fracture toughness (KIc) of at least 50 ksi√in. However, 300M alloy does not provide 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 300M alloy.
Precipitation hardenable stainless steel alloys having commercially acceptable combinations of strength and toughness are known and used for various aerospace applications. However, some of those alloys do not provide strength equivalent to 300M, so they cannot be considered as “drop-in” replacements for that alloy. The other known precipitation hardenable stainless steels may provide adequate strength for the landing gear application, but leave something to be desired in the resistance to corrosion they provide. The corrosion resistance desired for the aircraft landing gear application includes general corrosion resistance, pitting corrosion resistance, and resistance to stress corrosion cracking.
In view of the foregoing discussion, there is a need for a steel alloy with mechanical properties comparable to those of 300M, so the new alloy can be used as a drop-in replacement, combined with effective corrosion resistance in the variety of environments in which commercial aircraft are used.
The disadvantages associated with the known precipitation hardenable, martensitic stainless steel alloys are solved to a large degree by the alloy in accordance with the present invention. The alloy according to the present invention is a precipitation hardening Cr—Ni—Ti—Mo martensitic stainless steel alloy that provides a unique combination of strength, toughness, and corrosion resistance.
The broad, intermediate, and preferred compositional ranges of the alloy according to the present invention are set forth below in weight percent.
The balance of the alloy is essentially iron except for the usual impurities found in commercial grades of such steels and minor amounts of additional elements which may vary from a few thousandths of a percent up to larger amounts that do not adversely affect the desired combination of properties provided by this alloy.
The foregoing tabulation is provided as a convenient summary and is not intended thereby to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the element ranges of the broad composition can be used with one or more of the other ranges for the remaining elements in the preferred composition. In addition, a minimum or maximum for an element of one preferred embodiment can be used with the maximum or minimum for that element from another preferred embodiment. Moreover, the alloy according to this invention may comprise, consist essentially of, or consist of the constituent elements described above and throughout this specification. Here and throughout this application, unless otherwise indicated, the term percent or the symbol “%” means percent by weight or mass percent.
The alloy according to the present invention provides a unique combination of strength, toughness, and corrosion resistance which results from a novel balancing of the elements chromium, nickel, cobalt, molybdenum and also the elements titanium, aluminum, and columbium. At least about 10%, better yet at least about 10.5%, and preferably at least about 11.0% chromium is present in the alloy to provide corrosion resistance similar to that of a conventional stainless steel. At least about 10.5%, better yet at least about 10.75%, and preferably at least about 10.85% nickel is present in the alloy because nickel benefits the toughness and notch toughness of the alloy. Nickel also contributes to the corrosion resistance by enhancing the ability of the alloy to repassivate. This alloy contains at least about 0.5%, better yet at least about 0.75%, and preferably at least about 0.9% cobalt because cobalt contributes to the high strength and corrosion resistance provided by the alloy. At least about 0.25%, better yet at least about 0.75%, and preferably at least about 0.9% molybdenum is also present in the alloy because molybdenum contributes to the alloy's notch toughness. Molybdenum also benefits the alloy's corrosion resistance in reducing media and in environments which promote pitting attack and stress-corrosion cracking.
The alloy of this invention also contains at least about 1.5% titanium to benefit the strength of the alloy through the precipitation of a nickel-titanium-rich phase during aging. Columbium and aluminum also contribute to the strength provided by this alloy. Therefore, the alloy contains at least about 0.3% and better yet at least about 0.4% of each of columbium and aluminum. Preferably the alloy contains at least about 0.45% aluminum.
When chromium, nickel, cobalt, molybdenum, titanium, columbium, and aluminum are not properly balanced, the alloy's ability to transform fully to a martensitic structure using conventional processing techniques is inhibited. Furthermore, the alloy's ability to remain substantially fully martensitic when solution treated and age-hardened is impaired. Under such conditions the strength provided by the alloy is significantly reduced. Therefore, the amounts of chromium, nickel, cobalt, molybdenum, titanium, columbium, and aluminum present in this alloy are restricted. More particularly, chromium is limited to not more than about 13%, better yet to not more than about 12.5%, and preferably to not more than about 12.0%. Nickel is limited to not more than about 11.6% and preferably to not more than about 11.25%. Too much cobalt adversely affects the strength and toughness provided by this alloy. Therefore, cobalt is restricted to not more than about 1.5%, better yet to not more than about 1.25%, and preferably to not more than about 1.1%. Molybdenum is restricted to not more than about 1.5%, better yet to not more than about 1.25%, and preferably to not more than about 1.1%.
Too much titanium adversely affects the toughness and notch toughness of the alloy. Therefore, titanium is restricted to not more than about 1.8% and preferably to not more than about 1.7% in this alloy. Too much aluminum can adversely affect the toughness and corrosion resistance provided by the alloy. Therefore, aluminum is restricted to not more than about 0.8%, better yet to not more than about 0.7%, and preferably to not more than about 0.65%. Too much columbium is likely to result in undesirable alloy segregation and the precipitation of unwanted secondary phases such as Laves phase. Therefore, columbium is restricted to not more than about 0.8%, better yet to not more than about 0.7%, and preferably to not more than about 0.6% in this alloy.
Additional elements such as manganese, silicon, and boron may be present in controlled amounts to benefit other desirable properties provided by this alloy. More specifically, up to about 1.0%, better yet up to about 0.5%, still better up to about 0.25%, and preferably up to about 0.10% manganese and/or up to about 0.75%, better yet up to about 0.5%, still better up to about 0.25%, and preferably up to about 0.10% silicon can be present in the alloy as residuals from scrap sources or deoxidizing additions. Such additions are beneficial when the alloy is not vacuum melted. Manganese and/or silicon are preferably kept at low levels because of their adverse effect on toughness, corrosion resistance, and the austenite-martensite phase balance in the matrix material.
Up to about 0.010% boron, better yet up to about 0.005% boron, and preferably up to about 0.0035% boron can be present in the alloy to benefit the hot workability of the alloy. In order to provide the desired effect, at least about 0.001% and preferably at least about 0.0015% boron is present in the alloy.
The balance of the alloy is essentially iron apart from the usual impurities inevitably found in commercial grades of alloys intended for similar service or use. The levels of such elements are controlled so as not to adversely affect the desired properties.
In particular, too much carbon and/or nitrogen impair the corrosion resistance and adversely affect the toughness provided by this alloy. Accordingly, not more than about 0.03%, better yet not more than about 0.02%, and preferably not more than about 0.015% carbon is present in the alloy. Also, not more than about 0.030%, better yet not more than about 0.015%, not more than about 0.010% nitrogen is present in the alloy. When carbon and/or nitrogen are present in larger amounts, the carbon and/or nitrogen bond with titanium, aluminum, and/or columbium to form undesirable non-metallic inclusions such as carbides or nitrides and/or carbonitrides. Those reactions inhibit the formation of the nickel-titanium/aluminum/columbium intermetallic phases which are a primary factor in the development of the high strength provided by this alloy.
Phosphorus is maintained at a low level because of its adverse effect on toughness and corrosion resistance. Accordingly, not more than about 0.040%, better yet not more than about 0.015%, and preferably not more than about 0.010% phosphorus is present in the alloy.
Not more than about 0.020%, better yet not more than about 0.010%, and preferably not more than about 0.005% sulfur is present in the alloy. Larger amounts of sulfur promote the formation of non-metallic sulfide inclusions which, like carbon and nitrogen, inhibit the desired strengthening effect provided by titanium, aluminum, and columbium. These sulfide inclusions impair the toughness of the alloy, especially in the transverse direction. Also, a greater amount of sulfur adversely affects the hot workability and corrosion resistance of this alloy.
Although sulfur and phosphorus can be reduced to very low levels through the selection of high purity charge materials or by employing alloy refining techniques, their presence in the alloy cannot be entirely avoided under large scale production conditions. Therefore, a small amount of calcium may be added in controlled amounts to combine with phosphorus and/or sulfur to facilitate the removal and stabilization of those two elements in the alloy. Calcium is also used to deoxidize the alloy. When used, the retained amount of calcium is not more than about 0.010% and preferably to not more than about 0.005% in this alloy. As an alternative to the calcium treatment, one or more rare earth metals (REM), particularly cerium and lanthanum, can be added to the alloy. In this regard, the alloy may contain at least about 0.001% REM and better yet, at least about 0.002% REM. Too much REM recovery adversely affects the hot workability and the toughness of this alloy. Excessive REM content also results in the formation of undesirable oxide inclusions in the alloy. Therefore, the amount of REM present in this alloy is limited to not more than about 0.025%, better yet to not more than about 0.015%, and preferably to not more than about 0.010%, in this alloy. It is further contemplated that magnesium can be added as an alternative to calcium or REM for desulfurization and deoxidation.
Too much copper adversely affects the notch toughness, ductility, and strength of this alloy. Therefore, the alloy contains not more than about 0.75%, better yet not more than about 0.50%, and preferably not more than about 0.25% copper.
No special techniques are required for melting, casting, or working the alloy of the present invention. Vacuum induction melting (VIM) and vacuum induction melting followed by vacuum arc remelting (VAR) are the preferred methods of melting and refining this alloy, but other practices can be used. In addition, this alloy can be made using powder metallurgy techniques, if desired. Further, although the alloy of the present invention can be hot or cold worked, cold working enhances the mechanical strength of the alloy.
The preferred method of providing calcium in this alloy is through the addition of a nickel-calcium compound during VIM. The nickel-calcium compound, such as the Ni-Cal® alloy sold by Chemalloy Co. Inc., is added in an amount effective to combine with available phosphorus, sulfur, and oxygen. Other techniques for adding calcium may also be used. For example, capsules of elemental calcium or calcium master alloys can be added to the melt. It is believed that a slag containing calcium or a calcium compound may also be used. The chemical reactions result in the formation of secondary phase inclusions such as calcium sulfides, calcium oxides, and calcium oxysulfides that are readily removed during primary or secondary melting. When used, REM are added to the molten alloy in the form of mischmetal which is a mixture of rare earth elements, an example of which contains about 50% cerium, about 30% lanthanum, about 15% neodymium, and about 5% praseodymium.
The precipitation hardenable alloy of the present invention is processed in multiple steps to develop the desired combination of properties. In a first step, the alloy is solution annealed. The solution annealing temperature is selected to be high enough to dissolve essentially all of the undesired precipitates into the alloy matrix material and to ensure that the grain structure is fully recrystallized. Unrecrystallized grains can lead to increased anisotropy of the mechanical properties, particularly the ductility and toughness, of the alloy. However, if the solution annealing temperature is too high, it will impair the fracture toughness of the alloy by promoting excessive grain growth. Preferably, the alloy of the present invention is solution annealed at 1850° F.-1950° F. (1010° C.-1066° C.) for a time sufficient to substantially completely dissolve any precipitates in the alloy matrix and to fully recrystallize the grain structure. The time at the solution temperature depends on the thickness of the part. The alloy is then quenched, preferably in oil.
To further develop the high strength of the alloy, it is subjected to a refrigeration treatment after it is quenched. The refrigeration treatment cools the alloy to a temperature sufficiently below the martensite finish temperature to ensure the completion of the martensite transformation. Preferably, the refrigeration treatment comprises cooling the alloy to about −100° F. (−73° C.) or lower for a time sufficient to ensure that the alloy has substantially completely transformed to martensite. The need for a refrigeration treatment will be affected, at least in part, by the martensite finish temperature of the alloy. If the martensite finish temperature is sufficiently high, the transformation to a martensitic structure can proceed without the need for a refrigeration treatment. In addition, the need for a refrigeration treatment may also depend on the section size of the piece being manufactured. As the section size of the piece increases, segregation in the alloy becomes more significant and the use of a refrigeration treatment becomes more beneficial. Further, the length of time that the piece is chilled may need to be increased for large pieces in order to complete the transformation to martensite. For example, it has been found that a refrigeration treatment lasting a minimum of about 8 hours is preferred for developing the high strength that is characteristic of this alloy.
The alloy of the present invention is age hardened in accordance with techniques used for the known precipitation hardening, stainless steel alloys, which treatments are known to those skilled in the art. For example, the alloys are preferably aged at about 950-975° F. (510-524° C.) for a time sufficient to ensure that the alloy is substantially uniformly heated to the aging temperature depending on the thickness of the part and typically for an additional 4 to 8 hours to complete the aging reaction and to reach the desired combination of strength and toughness. The specific aging temperature used is selected by considering that: (1) the ultimate tensile strength of the alloy decreases as the aging temperature increases; and (2) the time required to age harden the alloy to a desired strength level increases as the aging temperature decreases.
The alloy of the present invention can be formed into a variety of product shapes for a wide variety of uses and lends itself to the formation of billets, bars, rod, wire, strip, plate, or sheet using conventional practices. The alloy of the present invention is useful in a wide range of practical applications which require an alloy having a good combination of corrosion resistance, strength, and toughness. In particular, the alloy of the present invention can be used to produce structural members for aircraft, including but not limited to landing gear components and fasteners. The alloy is also well suited for use in medical and dental applications such as dental tools and medical scrapers, cutters, and suture needles.
In order to demonstrate the novel combination of strength, toughness, and corrosion resistance provided by the alloy according to this invention, a comparative testing program was carried out. Seven 35 lb. heats having the weight percent compositions set forth in Table I below were produced by VIM.
The balance of each heat is iron and usual impurities. Examples 1 and 2 are representative of the alloy according to the present invention. Examples A to E are comparative alloys. In particular, Example A is within the scope of the alloy described in U.S. Pat. No. 5,681,526.
The VIM heats were melted and cast into 4″ square ingots. The ingots were charged into a furnace operating at 1500° F. and the furnace temperature was ramped up to 2300° F. Ingots were held at 2300° F. for 16 hours after which the furnace temperature was lowered to 2000° F. The ingots were held at 2000° F. until they were substantially fully equalized in temperature. The ingots were then double-end forged to 2¾″ square billet from starting temperature of 2000° F. and then hot cut into 3 pieces each. Pieces were re-heated at 2000° F., and double-end forged to 1¼″ square. The bars were again hot cut into 3 pieces and reheated at 2000° F. The bars were then single-end forged to 11/16″ square with no reheats. The bars were cooled in air, overage annealed at 1250° F. for 8 hours, and then air cooled.
Longitudinal smooth and notched (Kt=3) tensile samples, longitudinal Charpy V-notch (CVN) samples, and longitudinal rising step load (RSL) fracture toughness samples were machined from the bars of each heat. The samples from Examples 1, 2, B, C, D, and E were solution treated at 1900° F. for 1 hour and oil quenched. The samples from Example A were solution treated at 1800° F. in accordance with the usual practice for that alloy. After solution treatment, all samples were refrigerated at −100° F. for 24 hours then warmed in air to room temperature. The samples were then age-hardened at various temperatures ranging from 900° F. to 1000° F. Aging was conducted by holding the samples at temperature for 4 hours in air and then quenching the samples in water.
The results of room temperature tensile testing on the samples of each heat are shown in Tables IIA and IIB below including the 0.2% offset yield strength (Y.S.) and the ultimate tensile strength (U.T.S) in ksi, the percent elongation (% El.), the percent reduction in area (% R.A.), and the notch tensile strength (N.T.S.) in ksi.
The results of Charpy V-notch (CVN) impact testing of Examples 1, 2, and D are shown in Table III below including the aging temperature, the Rockwell C-scale hardness (HRC), and the impact toughness (CVN) in foot-pounds. CVN testing was performed in accordance with ASTM Standard Test Procedure E23.
Rising Step Load (RSL) samples for plane-strain fracture toughness testing and stress corrosion cracking resistance (SCC) were finish machined from the age-hardened bars of Examples 1, 2, A, and D. Two samples from each heat were tested in air to provide a fracture toughness value (KO. Additional samples were tested in 3.5% NaCl solution, natural pH, at room temperature, to provide a threshold stress intensity value (KISCC). Testing was performed on a test machine that meets the requirements of ASTM Standard Test Procedure E1290. The results of room temperature fracture toughness testing (KIc) and stress corrosion cracking testing for Examples 1, 2, A, and D are presented in Table IV below including the plane-strain fracture toughness (KIc) in ksi√in and the threshold stress intensity to produce stress corrosion cracking (KISCC) in ksi√in. KISCC is reported for each step interval and as a final value. The lowest of the measured values for each example is designated as the final value of KISCC in accordance with the standard test procedure. The tensile strength values for each example are also reported in Table IV to show that the fracture toughness and stress corrosion cracking resistance were measured on alloys having similar levels of strength.
Duplicate salt spray corrosion test cones were finish machined from the bars of Examples 1, 2, A, D after age-hardening. The cone samples were prepared by turning and hand polishing to a 600 grit finish. Prior to testing, all salt spray cones were passivated using 20% Nitric acid+3 oz./gallon Sodium Dichromate at 120/140° F. for 30 minutes. Samples were tested in accordance with ASTM B117, using a 5% NaCl concentration, natural pH, at 95° F. for 200 hour test duration. Time to first rust was noted for all samples, as well as a final rating after the completion of 200 hours test duration. The results of the salt-spray testing are shown in Table V below including the time to first appearance of rust and a final rating after the completion of the test duration. The ratings are defined as follows: 1=no rust, 2=1-3 rust spots, 3=<5% rust, 4=5-10%, 5=10-20%, 6=20-40%, 7=40-60%, 8=60-80%, 9=>80%.
Cyclic polarization (pitting) test samples were finish machined from the aged bars of Examples 1, 2, A, and D. Scans to measure pitting resistance were run on duplicate samples from each of those examples. The samples were tested in 3.5% NaCl solution, natural pH, at room temperature and were cleaned but not passivated prior to testing. Testing was performed with a modified ASTM Standard Test procedure G61 as described below. Voltage values at the knee of the curve and protection potentials were measured for all samples. The results of the potentiodynamic pitting tests are shown in Table VI below including the pitting potential and the protection potential in millivolts (mV).
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 aircraft structural components, including but not limited to flap tracks and slat tracks, and for other applications where both high strength and corrosion resistance are required. In particular, a steel article fabricated from the alloy that is solution heat treated and age hardened as described above provides a tensile strength of at least 280 ksi and a fracture toughness (KIc) of at least 45 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 about 4 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 62 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.
The terms and expressions which are employed in this specification are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications are possible within the invention described and claimed herein.
