Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/053—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 30% but less than 40%

Definitions

• This invention relates to a corrosion resistant nickel base alloy, and more particularly to an improved hot and cold workable nickel base alloy which has excellent corrosion resistance under a broad range of corrosive conditions, and which is particularly suited for use in highly corrosive deep sour gas well applications.
• alloys used commercially in applications requiring good corrosion resistance are nickel base alloys. Such alloys generally contain relatively large amounts of chromium and molybdenum, and usually also contain substantial proportions of iron, copper or cobalt. Alloy C-276 for example, a well known corrosion resistant nickel base alloy used in a variety of corrosive applications, has a nominal composition of about 15.5% chromium, 15.5% molybdenum, 3.5% tungsten, 6% iron, 2% cobalt and the balance nickel.
• alloy B-2 which has a nominal composition of about 28% molybdenum, 1% chromium, 2% iron, 1% cobalt, and the balance nickel
• alloy 625 which contains about 21.5% chromium, 9% molybdenum, 4% iron, 3.6% columbium, and the balance nickel
• alloy 718 which contains about 19% chromium, 3% molybdenum, 19% iron, 5.1% columbium, and the balance nickel.
• nickel base alloy having outstanding corrosion resistance over a broad range of corrosive conditions ranging from oxidizing conditions to reducing conditions, and which performs particularly well in tests designed to simulate the extremely severe corrosive environment found in deep sour gas well operations. Additionally, this alloy exhibits excellent hot and cold workability, and has a relatively low content of expensive alloying elements.
• Nickel base alloys having this critical balance of chromium, molybdenum and tungsten exhibit superior corrosion resistance in a variety of solutions when compared to other commercially available corrosion resistant alloys, including alloy C-276, alloy B-2, alloy 718 and alloy 625. Further, based upon the cost of the metals contained therein, alloys in accordance with this invention are less expensive than certain other commercial nickel base alloys which have poorer corrosion resistance. Alloys of the invention are easily hot workable so that they can be formed into various desired shapes, and also exhibit excellent cold workability so that high strength can be imparted to the final product by cold working.
• the alloy consists essentially of about 27 - 33% chromium, about 8 - 12% molybdenum, about 0 - 4% tungsten, up to about 1.5% iron, up to about 12% cobalt, up to about .15% carbon, up to about 1.5% aluminum, up to about 1.5% titanium, up to about 2% columbium, and the balance nickel.
• the term "consisting essentially of” we mean that in addition to the elements recited, the alloy may also contain incidental impurities and additions of other unspecified elements which do not materially affect the basic and novel characteristics of the alloy, particularly the corrosion resistance of the alloy.
• Chromium is an essential element in the alloy of the present invention because of the added corrosion resistance that it contributes. It appears from testing that the corrosion resistance is at an optimum when the chromium is at about 31% of the composition. When the chromium is raised above about 33%, both the hot workability and the corrosion resistance worsen. Corrosion resistance also worsens below about 27% chromium.
• molybdenum provides improved pitting corrosion resistance.
• An optimum content of about 10% molybdenum appears to yield the lowest corrosion rate in the.solutions tested.
• the molybdenum content is decreased below about 8%, the pitting and crevice corrosion increases significantly.
• the molybdenum is increased above about 12%, and in addition, the hot and cold workability decrease noticeably.
• Tungsten is not generally included in commercial alloys developed for corrosion resistant applications. This element is usually provided in applications where enhanced strength, particularly at high temperature, is of primary concern, and is not generally thought to have any beneficial effect on corrosion resistance.
• the presence of tungsten has been found to significantly enhance the corrosion resistance. Corrosion testing shows that the absence of tungsten results in a significantly higher corrosion rate, while a tungsten content in excess of about 4X causes the material to corrode at a higher rate in certain solutions, as well as making the alloy more difficult to hot work.
• the optimum tungsten content at the 10% molybdenum level appears to be about 2%, although replacement of some or all of the tungsten with additional molybdenum, for example, provides good corrosion resistance in some test media (see Table I, alloy M).
• the alloy will normally also contain carbon at a level of up to 0.15% by weight, either as an incidental impurity or as ' a purposeful addition for forming stable carbides.
• the carbon level should be maintained at a level up to a maximum of about.0.08% by weight, and most desirably to about 0.04%.
• Cobalt and nickel are generally regarded as being interchangeable and provide similar properties to the alloy. Tests have shown that the substitution of cobalt for a portion of the nickel content does not adversely affect the corrosion resistance and workability characteristics of the alloy. Therefore cobalt may be included in the alloy if desired, even at levels up to about 12% by weight. However, because of the present high cost of cobalt, substitution of cobalt for nickel would not be economically attractive.
• Aluminum may be present in small amounts to serve as a deoxidant. However, higher additions of aluminum adversely affect the workability of the alloy. Preferably, aluminum is present in amounts up to about 1.5% by weight, and most desirably up to about 0.25%.
• Titanium and columbium may also be present in small amounts to serve as carbide formers. These elements are included at levels preferably up to about 1.5% by weight of titanium and about 2% by weight of columbium, and most desirably up to about 0.40% by weight. However, addition of significantly larger amounts of these elements has been found to have deleterious effects on hot workability.
• Alloys in accordance with this invention may also contain minor amounts of other elements as impurities in the raw materials used or as deliberate additions to improve certain characteristics as is well known in the art.
• minor proportions of magnesium, cerium, lanthanum, yttrium or misch metal may be optionally included to contribute to workability.
• Tests have shown that magnesium can be tolerated up to about 0.107; by weight, preferably 0.07%, without significant loss of corrosion resistance.
• Boron may be added, preferably up to about .005%, to contribute to high temperature strength and ductility.
• Tantalum may be present at levels up to about 2% by weight without adversely affecting the corrosion resistance or workability, but the presence of tantalum at these levels has not been observed to benefit these properties of the alloy.
• vanadium can be present up to about 1% and zirconium up to 0.1% by weight.
• Iron in significant amounts lowers the corrosion. resistance of the alloy. Iron can be tolerated at levels up to about 1.5% by weight, but the corrosion resistance drops quite significantly at higher levels. Copper, manganese, and silicon, when present in small amounts or as impurities, can be tolerated. However, when added in significant amounts as alloying elements to the basic composition of this alloy, the elements have been found either to lower the corrosion resistance or to decrease the workability of the alloy or a combination of both. For example, the corrosion resistance of the alloy worsens significantly when copper is present at levels of about 1.5% by weight or greater, or manganese is present at levels of about 2% by weight or greater. Silicon is preferably maintained at levels less than 1%.
• Alloys in accordance with the invention are produced by introducing into a furnace metallic raw materials containing nickel and the other specified metallic elements within the percentage ranges stated. Heating the raw materials to form a melt, and pouring the melt from the furnace into a mould for solidification. Preferably, the melting is carried out under vacuum conditions. If desired, the thus formed alloy ingot may be further refined by remelting under vacuum conditions.
• Test 1 is a standard test method for determining pitting and crevice corrosion resistance by the use of a ferric chloride solution.
• the test specimens were immersed in a 10% by weight solution of ferric chloride for 72 hours at 50°C. This test method is similar to ASTM Standard Test Method G 48-76, except that the ASTM test uses 6% by weight ferric chloride.
• test 2 the samples are immersed in a boiling aqueous solution of 10% sodium chloride and 5% ferric chloride for 24 hours.
• Test 3 is a standard test method for detecting susceptibility to intergranular attack in wrought nickel-rich chromium bearing alloys (ASTM Test Method G 28-72). In this test, the samples are immersed in a boiling ferric sulfate - 50% sulfuric acid solution for 24 hours. In test 4 the samples are immersed in boiling 65% nitric acid for 24 hours.
• Alloys N and 0 had a nominal chemical composition as follows: 31X Cr, 10% Mo, 2% W, .40% Cb, .25% Ti, .25% Al, .001% max B, .10% max Fe, .10% max Cu, .04% C, .015% max S, .25% max Co, .015% max P, .10% max Ta, .10% max Zr, .10% max Mn, .01% max V, .25 max Si, balance nickel.
• specimens of alloy C-276 were evaluated under similar conditions. All three materials were studied in the 500°F (260°C) aged and unaged conditions following unidirectional cold working.
• Test B Hydrogen Embrittlement in NACE Solution at 24°C.
• Nickel-chrome wire was spot welded to the ends of beams stressed to 80 or 100 percent of yield strength. The beam specimens were then placed in the test solution and cathodically charged with hydrogen at a current of 25 mA/cm 2 . At the end of 13 days, alloy C-276 in the aged condition stressed at 100 percent of yield was found to have failed. Alloy C-276 in the unaged condition stressed to 100 percent yield strength failed after 21 days. Specimens of alloys N and 0 were retrieved unbroken at the end of the 28 day test.
• Test D Weight-Loss Corrosion in "Green Death” Solution (Boiling 1% H 2 SO 4 + 3% HC1 + 1% FeCl + 1% CuCl 3 ) Weight-loss corrosion coupons of each material were weighed, creviced, and placed in the "Green Death” solution. The coupons were cleaned and reweighed at 24 hours, 72 hours, and 168 hours. The coupons of alloys N and 0 had significantly less corrosion weight loss than the coupons of alloy C-276, as shown in Table IV.
• the basic alloy composition (heat 367) was as follows:

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• 1983
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