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
  • C22C38/001—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
• • 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

Definitions

• the invention is based on a heat and creep-resistant steel with a martensitic structure produced by a tempering process, which contains at least silicon, manganese, nickel, molybdenum, vanadium, niobium and tungsten in addition to iron and approx.
• a steel can be produced by forging or casting or by powder metallurgy and, because of its properties, can be used with particular advantage for the production of heat and creep-resistant parts of gas and steam power plants, such as in particular thermal turbo machines, for example gas or steam turbines or compressors, or steam generators and other high temperature equipment and machines.
• a martensitic steel known from this prior art contains iron in percent by weight 0.05-0.25 carbon, 0.2-1.0 silicon, up to 1 manganese, 0.3-2.0 nickel, 8.0- 13 chromium, 0.5 - 2.0 molybdenum, 0.1 to 0.3 vanadium, 0.03 - 0.3 niobium, 0.01 - 0.2 nitrogen, 1.1 - 2.0 tungsten.
• This steel has an elongation at break of at least 18% at room temperature and is characterized by a high creep resistance at temperatures of up to 600 ° C. At temperatures of 600 ° C and higher, the steel used, in addition to high creep resistance, also requires high structural stability, low embrittlement tendencies and, in particular, high oxidation resistance.
• the invention has for its object to provide a heat and creep-resistant steel with a martensitic structure produced by a tempering process, which is characterized by properties that its use in thermal turbomachines, such as steam and Make gas turbines appear extremely promising at temperatures of 600 ° C and more.
• the steel according to the invention has a thermally extremely stable and homogeneous structure. It is therefore characterized by a significantly improved creep resistance compared to comparable alloys according to the prior art and a particularly good resistance to oxidation.
• the steel according to the invention has an unusually high strength and toughness at room temperature. In the temperature range between room temperature and A c1 temperature, it also has an unexpectedly high hot stretch.
• carbon is the most important alloying element for hardenability. Carbon forms the carbides normally required for creep resistance during the tempering process, e.g. M23C6. In contrast, in the steel according to the invention, carbon is replaced by nitrogen. Instead of carbides, thermally stable nitrides are formed in the steel according to the invention. In order to avoid the precipitation of carbon-dominated phases, the carbon content should be low, at most 0.05, preferably 0.001 to 0.03 percent by weight.
• Silicon promotes the formation of ⁇ -ferrite and the Laves phase.
• silicon preferentially segregates at the grain boundary and reduces toughness.
• the silicon content should therefore be less than 0.5, preferably less than 0.2, percent by weight.
• Manganese suppresses the formation of ⁇ -ferrite and should therefore be kept at a value greater than 0.05 percent by weight. However, manganese also promotes the formation of the Laves phase and worsens the oxidation behavior. For this reason, the manganese content should not exceed 2 percent by weight. The manganese content should preferably be between 0.05 and 1 percent by weight.
• Nickel suppresses the formation of ⁇ -ferrite and should therefore be kept above 0.05 percent by weight. High nickel contents lead to an inadmissible lowering of the A c1 temperature, so that tempering treatment at high temperatures is no longer possible. For this reason, the nickel content should be between 0.05 and 2, preferably between 0.3 and 1, percent by weight.
• Chromium is the decisive alloying element for increasing the oxidation resistance, i.e. to form a heat-resistant steel.
• the chromium content should be at least 8 percent by weight. Too high a chromium content leads to the formation of ⁇ -ferrite.
• the chromium content should therefore be between 8 and 13, preferably between 8.5 and 11, percent by weight.
• Molybdenum promotes the formation of stable nitrides of the M6X type and thus contributes to increasing the creep resistance. To ensure this, the molybdenum content should be greater than 0.05 percent by weight. However, high molybdenum contents promote the formation of ⁇ -ferrite and Laves phase. Accordingly, the molybdenum content should be between 0.05 and 1, preferably between 0.05 and 0.5, percent by weight.
• Tungsten contributes significantly to the formation of stable nitrides.
• tungsten makes a contribution to the solid solution hardening of the matrix.
• tungsten increases the nitrogen solubility and thus enables the steel according to the invention to be produced economically.
• the tungsten content should be more than 1% by weight.
• excessively high tungsten contents promote the formation of ⁇ -ferrite and Laves phase. Accordingly, the tungsten content should be between 1 and 4, preferably between 1.5 and 3, percent by weight.
• Vanadium is an important element in the formation of stable vanadium nitrides in the steel according to the invention.
• the vanadium content must be greater than 0.05 percent by weight in order to achieve a sufficient hardening effect. With a high vanadium content, the tendency to form ⁇ -ferrite increases.
• the vanadium content should therefore expediently range from 0.05 to 0.5, preferably 0.15 to 0.35, percent by weight.
• Niobium combines with nitrogen to form niobium nitride and thus helps to form a fine structure.
• a small proportion of niobium dissolves in the hardening annealing and is eliminated as niobium nitride in the tempering treatment. This phase significantly improves creep resistance.
• the niobium content should be more than 0.01 percent by weight.
• the niobium content is above 0.2% by weight, niobium binds too much nitrogen, so that the precipitation of other nitrides is prevented too much.
• the niobium content should accordingly be between 0.01 and 0.2, preferably between 0.04 and 0.1, percent by weight.
• Cobalt increases the creep resistance of the steel according to the invention by preventing the formation of dislocation substructures favorably influenced and by preventing or at least significantly delaying the formation of ⁇ -ferrite and Laves phase.
• the cobalt content should be more than 2 percent by weight. Excessive cobalt contents lower the A c1 temperature too much and make the steel considerably more expensive. Accordingly, the cobalt content should be between 2.0 and 6.5, preferably between 3.0 and 5.0, percent by weight.
• Nitrogen forms nitrides with the elements V, Nb, Cr, W and Mo, which are thermally extremely stable as a hardening phase.
• nitrogen stabilizes the austenite present in the steel according to the invention and thus prevents the formation of ⁇ -ferrite.
• the beneficial effect of nitrogen is guaranteed with a nitrogen content of at least 0.1 percent by weight. Nitrogen contents of more than 0.3 percent by weight cannot be introduced into the steel in a cost-effective manner. The nitrogen content should therefore be between 0.1 and 0.3, preferably between 0.1 and 0.15, percent by weight.
• a steel A according to the invention weighing about 10 kg, was melted in a vacuum melting furnace under 1 bar of nitrogen, homogenized and forged into bars. After solution annealing at 1150 ° C, the steel was cooled in moving air and then tempered at 780 ° C for about 4 hours. Appropriately dimensioned bars were forged from commercially available, tempered comparison steels B (steel according to the German standard designation X20CrMoV 12 1) and C (steel according to the name of a Japanese manufacturer). The chemical compositions of steels A, B and C are given in the table below.
• the mechanical properties of these steels and the results from creep and oxidation tests can be found in the table below.
• the creep resistance was determined on prestressed test specimens. The pretension just absorbed by the test specimens at 600 ° C after 1000 h served as a measure of the creep resistance.
• the oxidation resistance of the individual alloys was determined from the change in weight of plate-shaped test specimens which were exposed to air at 650 ° C. for 1000 hours. stolen A B C.
• a proportion of 0.001 to 2 percent by weight of copper in the steel according to the invention also has a favorable effect, since copper suppresses the formation of ⁇ -ferrite without greatly reducing the A c1 temperature.
• copper improves the mechanical properties in the heat-affected zone of weld seams. With copper contents of more than 2 percent by weight, elemental copper is excreted at the grain boundaries. Therefore, the copper content should not exceed 2 percent by weight.
• the steel according to the invention has an essentially ⁇ -ferrite-free structure made from a martensite tempered in a tempering process.
• This structure and the result Properties such as creep resistance and oxidation resistance at temperatures of 600 ° C and strength and toughness at room temperature are guaranteed with certainty if the elements contained therein chromium (Cr), molybdenum (Mo), tungsten (W), vanadium (V ), Niobium (Nb), silicon (Si), nickel (Ni), cobalt (Co), manganese (Mn), nitrogen (N), carbon (C) and possibly provided copper (Cu) meet the following inequality (element content in weight percent): (Cr + 1.5 Mo + 1.5 W + 2.3 V + 1.75 Nb + 0.48 Si - Ni - Co - 0.3 Cu -0.1 Mn - 18 N - 30 C) ⁇ 10 It is therefore advisable to limit the components of the steel according to the invention accordingly.
• a change in the structure combined with reduced creep resistance and embrittlement due to the formation of a Laves phase can be avoided in the steel according to the invention if the elements iron (Fe), chromium (Cr), molybdenum (Mo), tungsten (W ), Cobalt (Co), nickel (Ni), vanadium (V) and possibly provided copper (Cu) the following inequality (element content in atomic percent): (0.858 Fe + 1.142 Cr + 1.55 Mo + 1.655 W + 0.777 Co + 0.717 Ni + O, 615 Cu + 1.543 V) ⁇ 89.5 or in a particularly advantageous manner the inequality: (0.858 Fe + 1.142 Cr + 1.55 Mo + 1.655 W + 0.777 Co + 0.717 Ni + 0.615 Cu + 1.543 V) ⁇ 89.0 fulfill.

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Citations (8)

• 1994
  • 1994-05-06 US US08/239,413 patent/US5415706A/en not_active Expired - Lifetime
  • 1994-05-11 DE DE59409428T patent/DE59409428D1/de not_active Expired - Fee Related
  • 1994-05-11 EP EP94107344A patent/EP0626463B1/fr not_active Expired - Lifetime
  • 1994-05-24 JP JP10999194A patent/JP3422561B2/ja not_active Expired - Fee Related
  • 1994-05-27 CN CN94106160A patent/CN1037361C/zh not_active Expired - Fee Related

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