Corrosion resistant high strength nickel-base alloy

Abstract: Nickel-base alloys containing special and correlated percentages of chromium, iron, molybdenum, titanium, niobium, aluminium that can be processed by cold working and age hardening to achieve high yield strengths and tensile elongations, are resistant to such corrosive media as hydrogen sulphide and acid chloride solutions, and to hydrogen embittlement, and are useful for, inter alia, oil and gas well production tubing and hardware.

Description: This invention relates to nickel-base alloys and articles made therefrom, and particularly to such alloys which offer a combination of properties, including high resistance to various corrosive agents and high levels of strength, ductility and workability, that are useful in the production of tubing and associated hardware, including packers, hangers and valves, for deep sour gas and/or oil well applications and for other articles and parts exposed in use to similar corrosive environments. Alloys having high strength, for example 689.5 MPa or advantageously even 1034 MPa, are required in some applications for sustaining stress in load-bearing service in chemically adverse environments. Some plastic ductility is also needed for enduring or permitting modest amounts of deformation without sudden fracture, for example to safeguard against accidental bending, or to enable cold forming to be carried out. In the specific and principal area of application to which the subject invention is directed, i.e. gas and/or oil well tubing and associated hardware, e.g. packers, hangers and valves, complex corrosive environments are encountered. For example, hydrogen sulphide attack can occur whereby hydrogen is evolved, and should the hydrogen permeate tubing "hydrogen embrittlement" can ensue. Chloride ions can be present in wells and, as a consequence, stress-corrosion cracking is often experienced. And there is virtually always the troublesome corrosion problem involving pitting brought on by, for example, chloride attack. Thin tubing is often desirable, but greater attention then has to be focused on the pitting problem. Thus, resistance to hydrogen embrittlement, pitting corrosion and stress-corrosion cracking are among the characteristics that are important for high-strength metal articles such as petroleum production tubing and hardware for oil and/or gas wells. In our application EP-A 0 066 361 we have disclosed the use of an alloy consisting, by weight, of from 15 to 22% chromium, 10 to 28% iron, 6 to 9% molybdenum, 2.5 to 5% niobium, 1 to 2% titanium, up to 1% aluminium, up to 0.1% carbon, up to 0.35% silicon, up to 0.35% manganese, up to 0.01% boron, with or without residual amounts not exceeding 0.2% in total of cerium, calcium, lanthanum, mischmetal, magnesium, neodymium and zirconium, the balance, apart from impurities, being nickel in a proportion of from 45 to 55% of the alloy, for wrought and age-hardened articles and parts requiring high resistance in use to corrosive conditions such as obtain in deep oil or gas wells or in environments containing sulphur dioxide. A problem with such alloys is that increasing the contents of chromium and molybdenum with the object of improving corrosion resistance adversely affects the workability, particularly at higher niobium contents. In particular there is a risk that objectionable precipitates may form, e.g. Laves phase, in detrimental quantities which, in turn, can lead to cracking during, for example, hot and/or cold rolling to produce sheet and strip. EP-A-0 056 480 discloses articles such as springs having a high resistance to stress corrosion cracking for use under stress in an atmosphere such as high purity water in a nuclear reactor at a temperature below the creep temperature, made from a Ni base alloy that consists essentially of, by weight, 15-25% Cr, 1-8% Mo, 0.4-2% Al, 0.7-3% Ti, 0.7-4.5% Nb, less than 1% Si, less than 1% Mn and less than 40% Fe, the balance being more than 40% Ni, and has an austenitic matrix containing at least one of γ' phase and γ'' phase. The ranges of Cr and Fe set forth are selected to minimise the formation of detrimental phases such as Laves phase, and the Mo content is preferably from 1.5 to 5%. EP-A-0 247 577, which was published on 02. 12. 87 but was filed on 26. 05. 87 and therefore belongs to the state of the art according to Article 54(3) EPC in respect of the designated States AT, DE, FR, GB, SE, discloses an alloy containing 16 to 24% chromium, 7 to 12% molybdenum, 2 to 6% niobium, 0.5 to 2.5% titanium, a trace to 1% aluminium, up to 20% iron and at least 55% nickel, preferably at least 57% nickel, better yet at least 59% nickel, and the use of this alloy for making articles for sour well application. The content of silicon is limited to no more than 1%, as silicon is said to promote the formation of Laves phase. It has now been found that the occurrence of Laves phase may be held to very low levels while obtaining the desired combination of corrosion resistant properties by controlling the balance of the contents of nickel, molybdenum, chromium, niobium and iron in alloys of relatively high nickel content. The amounts of titanium and niobium present are further controlled in the interest of ductility and workability. According to the invention an alloy exhibiting good workability and fabricability, having, in both the cold-rolled and aged conditions, high strength, good ductility and resistance to hydrogen embrittlement, pitting corrosion and stress-corrosion cracking consists, by weight, of from 15 to 25% chromium, from 5 to 15% iron, from 6 to 8% molybdenum, from 2.5 to 5% niobium, from 0.5 to 2.5% titanium, up to 0.3% aluminium, from 0 to 0.1% carbon, from 0 to 0.35% silicon, from 0 to 0.5% manganese, from 0 to 3% vanadium, from 0 to 0.01% boron, from 0 to 0.2% in total of cerium, calcium, lanthanum, mischmetal, magnesium and zirconium, from 0 to 1% copper, from 0 to 0.1% tungsten, from 0 to 0.1% tantalum, from 0 to 0.015% sulphur, from 0 to 0.015% phosphorus and from 0 to 0.2% nitrogen, the balance being nickel in an amount of from 54 to 58%, the contents of nickel, chromium, molybdenum, niobium and titanium being correlated so that %Mo + %Cr + 2 (%Nb) ≦ (%Ni +71)/3.3 and 3 ≦ %Ti + 0.5 (%Nb) ≦ 4. and the value of the expression 0.00929(%Fe x %Mo) + 0.2075(%Mo x %Nb) - 0.01881(%Ni x %Nb) being restricted so that the alloy contains not more than 5% of Laves phase. This alloy is used according to the invention as material containing not more than 5% Laves phase for oil or gas well tubing, packers, hangers and valves and other articles and parts exposed to similar corrosive environments, and the invention also includes such articles and parts made from the alloy. The amount of manganese present may for example be not more than 0.35%, that of copper not more than 0.5% and that of nitrogen not more than 0.15%. The amounts of up to 0.1% tungsten and 0.1% tantalum that may be present are the incidental percentages such as are often associated with commercial sources of molybdenum and niobium respectively. Tungsten in larger amounts may lead to the occurrence of deleterious amounts of undesired phases, e.g. Laves phase, particularly at the higher percentages of chromium, molybdenum and iron. Tantalum is not desired in view of its high atomic weight. For optimum corrosion resistance the molybdenum content advantageously should be at least 6.5% and preferably at least 7%, together with a chromium content of at least 20%, the sum of the chromium plus molybdenum being 27% or more. However, as mentioned above, increasing the molybdenum and chromium tends to impair workability, particularly when high percentages of niobium, e.g. 4 to 5%, are present together with molybdenum percentages of 7 to 7.5% or more. Niobium has a greater adverse effect on workability than molybdenum. This undesirable effect is countered by the use of nickel contents of at least 54%, and preferably more than 55%, but not exceeding 58%. At nickel levels exceeding 60% strength tends to drop off. Alloys satisfying the relationship %Mo + %Cr + 2 (%Nb) ≦ (%Ni + 71)/3.3 can be fabricated by hot working operations, but may still exhibit low ductility during subsequent processing to desired end product forms or during tensile testing of the final product due to the occurrence of Laves phase. To minimise this the value of the expression 0.00929(%Fe x %Mo) + 0.2075(%Mo x %Nb) - 0.01881(%Ni x %Nb) is restricted so that the content of Laves phase is less than 5%. Compositions having amounts of Laves phase greater than this are likely to exhibit marginal cold workability, so as to be commercially unattractive. To ensure adequate tensile ductility the value of the expression should not exceed 2.6, and most preferably it does not exceed zero. For example, an alloy having the composition, in % by weight: Cr 20, Fe 16, Mo 7, Nb 5, Ti 1.5, C 0.02, Al 0.10, Ni balance (about 50%), for which the value of the expression is 3.6, cracked on hot working 12.7 mm (0.5 inch) plate to 4 mm (0.16 inch) strip at 1121°C (2050°F) owing to the presence of excessive amounts of Laves phase. With regard to other constituents of the alloy, for resistance to stress corrosion cracking the iron range of 5 to 15% is advantageous. Aluminium imparts strength and hardness characteristics, but detracts from pitting resistance if present in excess. Accordingly, it should be less than 0.3% and preferably is held below about 0.25%. Whilst it is preferred that 1% or more titanium be present in the alloys of the instant invention, percentages as low as 0.5% can be employed, particularly in conjunction with niobium at the higher end of its range, say 3.5% or 4% and above. Titanium up to 2.5% can be utilized in the interests of strength. Balancing the titanium and niobium present in the alloy such that 3 ≦ %Ti + 0.5 (%Nb) ≦ 4 is advantageous for achieving high strength and maintaining good ductility, workability and other desired results. For instance, about 1.5% titanium and about 4% niobium, such as 1.3 to 1.7% Ti and 3.6 to 4.4% Nb, are advantageous in alloys of the invention. Where particularly close control is desired, possibly for promoting consistency of desired results, the composition can be specially restricted with one or more of the ranges of 18.5% to 20.5% chromium, 6.5 to 8% molybdenum, 3 to 4.5% niobium, 1.3 to 1.7% titanium and at least 0.05% aluminium. Attention to the compositional relationships set forth above enables alloys with good workability, both hot and cold, to be obtained for production of articles such as wrought products, e.g. hot or cold drawn rod or bar, cold rolled strip and sheet and extruded tubing. Where desired, the yield and tensile strengths of articles manufactured from the alloy can be enhanced by cold working or age-hardening or combinations thereof, e.g. cold working followed by age hardening. Heat treatment temperatures for the alloy are, in most instances, about 870°C to 1148°C (1600°F to 2100°F) for annealing and about 593°C to 816°C (1100°F to 1500°F) for ageing. Direct ageing treatments of at 648°C to 760°C (1200°F to 1400°F) for 1/2 hour to about 2 or 5 hours directly after cold working are particularly beneficial for obtaining desirable combinations of good strength and ductiity. As indicated, alloys contemplated herein can be hot worked (or warm worked) and then age hardened. Generally speaking, it is thought hot working or warm working followed by ageing lends to better resistance to stress corrosion, albeit yield strength is lower. Cold working followed by ageing lends to the converse. In this connection, an annealing treatment followed by ageing seems to afford better stress corrosion cracking resistance, the yield strength being somewhat lower. Among the articles of the invention are mechanithermo-processed (wrought and age-hardened) high-strength, corrosion-resistant products characterised by yield strengths (at 0.2% offset) upwards of 827 to 1034 MPa (120 000 to 150 000 pounds per square inch) and elongations of 8%, and higher, e.g. 1103, 1241 or 1310 MPa (160 000, 180 000 or 190 000 psi) and 10, 12 or 15% and even greater strengths and elongations. By way of example, the composition of an alloy No. 1 in accordance with the invention is set forth in the Table below, together with those of two comparative alloys D and E having higher contents of aluminium. Alloy Cr Fe Mo Nb Ti C Al Ni Wt.loss (mg/cm²) 1 18.8 13.06 6.51 3.68 1.61 0.012 0.27 55.4 8.04 D 18.8 12.14 6.63 3.75 1.73 0.009 0.67 55.8 11.9 E 18.1 11.95 6.72 3.83 1.72 0.010 0.98 55.9 82.6 The weight losses reported in the last column of the Table indicate the benefit of low aluminium contents on resistance to pitting corrosion. The testing involved immersing alloy specimens in 10% ferric chloride solution at 67°C (152°F) using an exposure period of 24 hours. The alloy specimens were 3.8 mm thick x 7.5 cm wide x 10 cm long and were tested in the cold-rolled (20%) plus 760°C (1400°F) 12 hours, air-cooled condition. While this test does not duplicate service conditions in a sour gas well, it has been reported that there is a reasonably good correlation between pitting behaviour in ferric chloride solution and other test environments that more closely simulate deep sour gas well environments. The present invention is applicable to providing metal articles, e.g. tubes, vessels, casings and supports, needed for sustaining heavy loads and shocks in rough service while exposed to corrosive media, and is particularly applicable in the providing of production tubing and associated hardware, such as packers and hangers, to tap deep natural reservoirs of hydrocarbon fuels. In deep oil or gas well service, possibly in off-shore installations, the invention is especially beneficial for resistance to media such as hydrogen sulphide, carbon dioxide, organic acids and concentrated brine solutions sometimes present with petroleum. Also the invention is applicable to providing good resistance to corrosion in sulphur dioxide gas scrubbers and is considered useful for seals, ducting fans and stack liners in such environments. Articles of the alloy can provide useful strength at elevated temperatures up to 648°C (1200°F) and possibly higher. Where English and S.I. units are used herein, original observations were obtained in English units, S.I. units being obtained by conversion. If any discrepancies exist between these units, the English units shall control.

Claims: An alloy exhibiting good workability and fabricability having, in both the cold-rolled and aged conditions, high strength, good ductility and resistance to hydrogen embrittlement, pitting corrosion and stress-corrosion cracking and consisting, by weight, of from 15 to 25% chromium, from 5 to 15% iron, from 6.5 to 8% molybdenum, from 2.5 to 5% niobium, from 0.5 to 2.5% titanium, less than 0.3% aluminium, from 0 to 0.1% carbon, from 0 to 0.35% silicon, from 0 to 0.5% manganese, from 0 to 3% vanadium, from 0 to 0.01% boron, from 0 to 0.2% in total of cerium, calcium, lanthanum, mischmetal, magnesium and zirconium, from 0 to 1% copper, from 0 to 0.1% tungsten, from 0 to 0.1% tantalum, from 0 to 0.015% sulphur, from 0 to 0.015% phosphorus and from 0 to 0.2% nitrogen, the balance being nickel in an amount of from more than 55 to 58%, the contents of nickel, chromium, molybdenum, niobium and titanium being correlated so that %Mo + %Cr + 2 (%Nb) ≦ (%Ni +71)/3.3 and 3 ≦ %Ti + 0.5(%Nb) ≦ 4 and the value of the expression 0.00929(%Fe x %Mo) + 0.2075(%Mo x %Nb) - 0.01881(%Ni x %Nb) being restricted so that the alloy contains not more than 5% of Laves phase. An alloy according to claim 1 wherein the molybdenum content is at least 7%. An alloy according to claims 1 or 2 wherein the value of the expression 0.00929(%Fe x %Mo) + 0.2075(%Mo x %Nb) - 0.01881(%Ni x %Nb) does not exceed 2.6. An alloy according to any preceding claim wherein the aluminium content is at least 0.05%. An alloy according to any preceding claim wherein the chromium content is at least 20%. An alloy according to any preceding claim wherein the manganese content does not exceed 0.35%, the nitrogen content does not exceed 0.15% and the copper content does not exceed 0.5%. An alloy according to claim 1 wherein the niobium content is from 3 to 4.5%, the titanium content is from 1.3 to 1.7% and the aluminium content is at least 0.05%. An alloy according to any preceding claim in the condition resulting from cold working and ageing. The use of an alloy according to any preceding claim as material containing not more than 5% Laves phase for oil or gas well tubing, packers, hangers and valves and other articles and parts exposed to similar corrosive environments. Oil or gas well tubing, packers, hangers and valves and other articles and parts exposed to similar corrosive environments and containing not more than 5% Laves phase, made from an alloy according to any one of claims 1 to 8.

Inventors: CLATWORTHY EDWARD FREDERICK, DAVIDSON JEFFREY MAX, EISELSTEIN HERBERT LOUIS, FLOREEN STEPHEN, HARRIS JERRY ARDON, SMITH DARRELL FRANKLIN JR, CLATWORTHY, EDWARD FREDERICK, DAVIDSON, JEFFREY MAX, EISELSTEIN, HERBERT LOUIS, FLOREEN, STEPHEN, HARRIS, JERRY ARDON, SMITH, DARRELL FRANKLIN, JR.
Applicants: INCO ALLOYS INT, INCO ALLOYS INTERNATIONAL, INC.
Application year: 1987
Main classification: C22C19
IPC codes: C22C19