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
• • 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

Definitions

• the present invention relates to nickel-chromium-molybdenum alloys of high yield strength and resistance to corrosion in environments such as are found in sour gas wells.
• sour gas wells typically contain hydrogen sulphide, carbon dioxide, methane and brine, often at operating temperatures of 250 to 300°C.
• HSSC hydrogen sulphide stress cracking
• CSCC chloride stress corrosion cracking
• Well aging may also be a cause of early failure.
• Hydrogen sulphide stress cracking appears to result from the presence of both H 2 S and brine causing hydrogen evolution. This permeates the tubing causing "hydrogen embrittlement" which with tensile stress leads to cracking.
• the susceptibility of low alloy steels prevents their prolonged use in sour gas wells. Chloride stress corrosion cracking, from release of chloride ions is particularly troublesome at higher operating temperatures, and the susceptibility of stainless steels to this form of corrosion prevents their use in sour gas wells. Pitting corrosion is also caused by chloride attack, and is a particular problem for thin wall tubing. General corrosion causes weight loss of metal affecting the ability of the material to sustain load and its pressure bearing capabilities. Well aging is the time-dependent degradation in properties during prolonged exposure at elevated temperatures, of 250° or even 300°C found in some wells. This affects the ability of the alloy to resist growth propagation.
• An alloy for use in sour gas wells particularly those greater than 15,000 feet deep must offer both resistance to the hostile corrosive environment and high yield strength, to allow the use of thin wall tubing allowing high volume gas flow but resisting tensile failure under high axial loading.
• the alloys must be cold workable to generate the strength required necessitating a high level of work hardening. Also required is an acceptable level of "residual ductility", the ductility remaining after cold working, since considerable distortion is likely to be encountered in sour gas well tubing in service.
• the alloy also needs hot workability and the ability to be fabricated.
• the present invention is based on the discovery that certain Ni-Cr-Mo alloys can be produced satisfying these requirements even at applied stresses in excess of 1000 MN/m 2 .
• a wrought alloy having a yield strength in excess of 1000 MN/ M 2 and resistance to corrosive environments such as those found in a sour gas well having the composition by weight 15 to 30% chromium, 5 to 15% molybdenum the total content of chromium and molybdenum being in the range 29 to 40%, 5 to 15% iron the total content of iron, chromium and molybdenum being not in excess of 46%, carbon up to 0.06%, up to 1% aluminium and/or titanium, up to 1% silicon, up to 0.5% niobium, less than 0.3% manganese balance nickel apart from incidental elements and impurities.
• Such alloys exhibit a high degree of resistance to hydrogen sulphide stress cracking, chloride stress corrosion cracking, pitting and general corrosion, and have good ductility and resistance to "well aging".
• phosphorous and sulphur levels be kept as low as possible. Whilst manganese may be present up to 0.3%, it is preferably kept below 0.2%. Incidental elements may include copper which is not required and may be kept to low levels, and cobalt up to about 25%. Boron up to 0.1% and mischmetal up to 0.1% may provide useful refining additions. Carbon, while virtually unavoidably present, affects ductility. Magnesium and zirconium can be used for grain refinement. Tungsten does not offer any particular advantage, given its density and added cost. The carbon content is preferably held to not more than 0.03%, and amounts up to about 0.1% of magnesium and/or zirconium may be present.
• the chromium level does not fall below 20% to provide sufficient pitting resistance and HSSC and CSCC resistance.
• the chromium need not exceed 30%. When chromium levels of below 15% are used it is necessary to provide high levels of molybdenum and this can affect working characteristics.
• Molybdenum markedly contributes to corrosion resistance but imparts a large degree of work hardening. Levels as low as 5% may be used in comparatively less severe conditions of temperature and pressure but levels of 7% or more are preferred.
• the content of chromium plus molybdenum should preferably be above 32% but preferably does not exceed 40%. This is because alloy brittleness, and other hot working problems can be caused at such levels. Also above 15% of molybdenum, the ductility of the alloy may be affected. The preferred content of molybdenum is 7 to 12%.
• the content of chromium and molybdenum also affects residual ductility. It has found to be desirable that the quantity % Cr - 2 (% Mo) is from 2 to 12 provides for optimum residual ductility.
• Iron is present in alloys of the present invention at levels of from 5 to 15%, more preferably 8 to 12%. Excessive iron may produce unwanted morphological phases, such as sigma, and to prevent this the sum of molybdenum, chromium and iron is preferablyibelow 46%.
• Aluminium and titanium may be used as refining additions, and they contribute to workability.
• alloys for use in the present invention contain 0.05 to 0.5% of either or both of these elements.
• the presence of silicon may not be deleterious, but it is preferably kept below 0.5% to avoid affecting the hydrogen stress cracking resistance.
• the preferred wrought alloy of the present invention having a yield strength in excess of 1000 MN/ M 2 and intended for use in corrosive environments such as sour gas wells, consists of, by weight, 20 to 30% chromium,7 to 12% molybdenum, the sum of chromium plus molybdenum being in the range 29 to 40%, the quantity of % chromium less twice the % molybdenum being in the range 2 to 12%, from 5 to 15% iron, the sum of chromium, molybdenum and iron not exceeding 46%, from 0.05 to 0.5% of either or both of aluminium and titanium, up to 0.06% carbon, up to 0.5% niobium, up to 0.5% silicon, up to 0.2% manganese the balance apart from impurities being nickel.
• Alloys for use in the present invention are solution annealed at temperatures in the range 1066 to 1177°C, preferably 1093 to 1177°C for 0.5 to 5 hours, normally 1 to 2 hours.
• the alloys are cooled, for example by air cooling and are cold worked in the range 40 to 50% or more to provide yield strengths of the order of 1200 MN/ M 2 or more. Since only low levels of aluminium and titanium are present the alloys are not age-hardenable, so that aging treatments are not required.
• a range of alloys both inside and outside the invention were produced in approximately 45 kg heats by induction melting high purity charge materials, hot rolling the ingots to plate stock approximately 15 mm thick, and solution annealing followed by cold rolling to develop strength. The amount of cold rolling was varied. Test specimens were machined from the cold rolled material normally in the transverse direction. Tables I sets out the chemical composition of the alloys and includes alloys in which major element concentrations were varied, and alloys based nominally on Ni-25% Cr - 10% Mo in which minor element concentrations were varied.
• alloys 1 to 6 and 8 to 23 are alloys of the present invention and alloys A to X are outside the present invention. Alloys 8 to 23 are the preferred alloys of the invention.
• H 2 S stress corrosion tests NACE Spec. Standard TM-01-77
• H 2 S stress corrosion tests NACE Spec. Standard TM-01-77
• 5g glacial acetic acid and 50g NaCl 945g H 2 0 saturated with H 2 S gas. This allows sensitivity to H 2 S gas at ambient temperatures to be tested.
• the specimens were 3-point bent beam samples loaded in small electrically insulated test fixtures stressed to various percentage of the yield strength, usually 100%.
• the cold rolled materials were given "well.aging" heat treatments at 260°-315°C for various times before testing.
• the samples were oriented in the transverse direction from the cold worked plate. (Note: extra specimens were first deformed to determine the load-deflection characteristics.)
• Specimens for test were then loaded in the fixtures to predetermined deflection corresponding to desired stress levels.
• Some U-bend specimen were also tested. All samples were attached to small pieces of steel to provide galvanic coupling.
• Alloy D is such an alloy.
• Alloy 1 is a marginal composition.
• % Cr - 2 is in the range 2 to 12.
• Alloy 1 would not be recommended for sour gas well applications. Alloys 11, 12 and 13 were excellent.
• the alloys described hereinbefore can be used in other corrosive environments in which high strength is required.
• Such applications include, intermediate gas wells, aqueous and marine environments, scrubbers, chemical plant equipment (such as tubing and piping), aircraft and aerospace and applications.
• Mill product forms include forgings, bar plate, extrusions and sheet. Among other structural shapes might be mentioned fasteners, valves, pins, shafts and rotors.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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• Treatment Of Steel In Its Molten State (AREA)
• Heat Treatment Of Steel (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
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• 1983
  • 1983-04-15 EP EP83302135A patent/EP0092397A1/fr not_active Ceased
  • 1983-04-19 ES ES521616A patent/ES8502167A1/es not_active Expired
  • 1983-04-20 KR KR1019830001666A patent/KR840004458A/ko not_active Application Discontinuation
  • 1983-04-20 JP JP58069860A patent/JPS58221252A/ja active Pending

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