JP2000510904A - Martensite-austenitic steel - Google Patents

Abstract

  (57) [Summary] Mainly 8-15% chromium, 0.5-2.5% molybdenum, up to 2% tungsten, 4-10% cobalt, 0.5-6% nickel, 2.5-8% manganese, 0.1-1 vanadium A martensitic-austenitic steel consisting of 0.0%, 0.05-0.25% nitrogen, 0.2% carbon (as measured by weight), the balance being iron and impurities. The martensitic-austenitic steel can be used as a material for rotors or rotors of gas turbines, in particular at temperatures above 450 ° C. The adjustment of the two-phase martensite-austenite structure is performed by heat treatment of the solution-treated and quenched steel structure in a temperature range of 550 ° C to 650 ° C.

Description

Description: FIELD OF THE INVENTION The present invention relates to the use of martensitic-austenitic steel as a creep-loaded construction material for gas turbines, in particular for gas turbines. BACKGROUND OF THE INVENTION The use of temperable chrome steel as material for use in thermal power plants, in particular as material for rotors or rotor blades, is at the state of the art. Chrome steels that can be tempered compared to nickel-based superalloys are distinguished by good non-destructive testability. Furthermore, they have a relatively low coefficient of thermal expansion and high thermal conductivity, which increases their resistance to thermal fatigue. However, at temperatures above 450 ° C., this tempered steel does not reach the desired requirements in terms of heat resistance, creep resistance and toughness. A temperable martensitic steel with the name X12CrNiMo12 is known. In addition to iron, this steel contains C 0.10 to 0.14%, Si 0.10 to 0.40%, Mn 0.5 to 0.9%, Cr 11 to 12%, Mo 1.3 to 1.8%, V0 0.2-0.35% and N-0.02-0.05% and normal impurities. This steel has a relatively high hot yield point and a relatively high creep resistance in the temperature range below 450 ° C. However, at temperatures above 450 ° C., this high temperature yield point and creep properties are inadequate. In addition, the steel has a considerable tendency to embrittle at higher temperatures. From EP-A-481377, by weight, 10 to 17% chromium, up to 4% molybdenum, up to 4% cobalt, up to 8% nickel, up to 10% manganese, up to 1.0% vanadium, up to 0.3% nitrogen. High-strength martensitic-austenitic steels with a composition of up to 0.15% carbon, up to 6% silicon, up to 4% copper and the balance iron and impurities are known. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a martensitic-austenitic steel for use in a gas turbine rotor or rotor having sufficient heat resistance and toughness in the temperature range of 450 ° C to a minimum of 550 ° C. As an issue. According to the invention, this is achieved by the features of claim 1. At the heart of the present invention is the regulation of a very fine two-phase microstructure consisting of tempered martensite and thermodynamically stable austenite. This steel mainly contains 8 to 15% chromium, 4 to 10% cobalt, 2.5 to 8% manganese, 0.5 to 6% nickel, 0.5 to 2.5% molybdenum, 2% tungsten and 0% vanadium. 0.1-0.5%, carbon 0.05-0.2%, nitrogen 0.05-0.5%. 25%, with the balance being iron and impurities resulting from normal melting. The advantages of the present invention are, in particular, that the steel composed of a double structure has a high hot yield point and a high creep resistance even at temperatures higher than 450 ° C. and has a low tendency to embrittle due to its high structural stability. is there. Further advantageous embodiments are evident from the dependent claims. Embodiments of the Invention The steels developed for use according to the invention are mainly composed of 8 to 15% of chromium (measured as% by weight), 4 to 10% of cobalt, 2.5 to 8% of manganese, 0.5 of nickel. Up to 6%, molybdenum 0.5 to 2.5%, tungsten up to 2%, containing vanadium 0.1 to 1.0%, carbon 0.05 to 0.2% and nitrogen 0.05 to 0.25% And can be produced by casting or powder metallurgy. Known and used in industry, 9-12% chromium steels have increased their heat resistance and creep resistance through the action of stabilizing very fine precipitates on the dislocation network formed in the tempered martensite. Achieve. In steels developed for use in the present invention, the face-centered cubic, which is based on this fine precipitation plus its low self-dispersibility and contributes to an increase in heat resistance and creep resistance through the formation of phase boundaries A phase (austenite) is introduced. Martensitic steels with austenite as a second phase for creep improving properties are known in the industry. However, compared to the steel according to the invention, it differs in that its austenitic part is not thermodynamically stable and therefore exhibits a high sensitivity to thermal embrittlement. The following are particularly preferred amounts and reasons for each element for the selected alloy range of the alloy according to the invention. Oxidation resistance is increased by the chromium dissolved in the mixed crystal. Through the formation of hexagonal chromium nitride, Cr can also contribute to improved creep resistance. To achieve this, a minimum -Cr-content of 8% by weight is required. However, this chromium content should not exceed 15% by weight. Otherwise, δ-ferrite, which lowers the toughness and the high-temperature heat resistance, is formed. Thus, a suitable chromium range is about 8 to 15% by weight, preferably 9 to 12% by weight, especially about 10% by weight. In the steel of the present invention, manganese particularly effectively increases the solubility of nitrogen in the austenitic region. In order to dissolve the stable nitrides of hexagonal (Cr, V) ₂ N and cubic (V, Cr) N during this solution treatment, a minimum -Mn content of 2.5% by weight is required. Is desirable. During this tempering process, manganese is concentrated in the formed austenite, which crucially reduces its martensite-onset temperature, ie manganese stabilizes the austenite. For this reason, the manganese content should be at least 2.5% by weight. However, the Mn content does not exceed 8% by weight, since beyond which the alloy is not completely austenitic. Thus, a suitable manganese range is about 2.5-8% by weight, preferably 3.5-6.5% by weight, especially about 5% by weight. Nickel increases the tenacity in martensitic chromium steel, for example, because it effectively reduces the δ-ferrite content. Like manganese, nickel stabilizes the austenitic phase in the duplex structure. For this reason, the alloy should contain at least 0.5% by weight of nickel. At nickel contents higher than 6% by weight, the A _{C3 -point} is significantly reduced. Thus, a suitable nickel range is about 0.5-6% by weight, preferably 2-5% by weight, especially about 3.7% by weight. Cobalt increases the nickel equivalent to the extent that the alloy from the melt hardens to austenitic. This avoids nitrogen effusion and associated pore formation. Therefore, the Co content should be at least 4% by weight. However, in order for this alloy to still have a sufficiently high _AC3 temperature, the Co content must not exceed 10% by weight. Thus, a suitable range for cobalt is about 4 to 10% by weight, preferably 5 to 8% by weight, especially about 6% by weight. Since molybdenum promotes tempering stability and heat resistance, a minimum content of 0.2% by weight should be maintained. The Mo content should not exceed 2.0% by weight, since otherwise a dense Laves phase can form. Thus, a preferred range for molybdenum is 0.5-2.5% by weight, preferably 1.0-2.0% by weight, especially about 1.5% by weight. Tungsten acts similarly to molybdenum. Due to the danger of Laves phase formation, a content of 2% by weight should not be exceeded. Accordingly, the tungsten content is in the range of 0 to 2% by weight, preferably less than 1% by weight. Vanadium, together with nitrogen and a small percentage of chromium, forms a coherent and partially coherent cubic nitride rich dispersion, which considerably determines heat resistance and creep resistance. At least 0.1% by weight of V should be included in the alloy. However, V should not exceed 1.0% by weight because vanadium promotes the tendency for δ-ferrite formation. Preference is given to vanadium contents in the range from 0.15 to 0.65% by weight. Nitrogen, when dissolved in an atomic state, promotes martensitic conversion without diffusion from the austenitic phase upon cooling. Thereby, refining possibility is secured. In addition, this forms nitrides already described with V and Cr and possibly with Nb, Ti and Ta. Therefore, nitrogen should be alloyed in near stoichiometric amounts. The upper limit of 0.25% by weight must not be exceeded, since N-efugeon can occur during curing. Therefore, the N-content should be in the range of about 0.05 to 0.25% by weight, preferably in the range of 0.07 to 0.15% by weight. Carbon, together with chromium, promotes the formation of carbides in the form of Cr ₂₃ C ₆ . This carbide, based on its stoichiometry, removes more chromium from the matrix than Cr ₂ N formed in the steel according to the invention. For this reason, the carbon content should not exceed 0.2% by weight, preferably 0.1% by weight. Niobium, titanium, zirconium and tantalum are alloying elements that can form an MX-type special nitride together with vanadium. Their effect is based primarily on increasing the stability of V (N, C) -precipitation, even in small amounts of them. However, if the content is too high, the stability of the nitrides is too high and they cannot be dissolved during the solution treatment. For this reason, the total content of these elements should be limited to 0.5% by weight. Boron increases the coarsening resistance of the precipitate. Since it has a tendency to segregate, its content should be limited to 0.02% by weight. Of the impurities from the production which usually occur, the elements such as phosphorus, sulfur, antimony, tin and arsenic should not exceed the contents given in the following table. This is necessary to avoid embrittlement during tempering. The steel according to the invention has a martensite-austenite structure obtained by a tempering process. This tempering step consists of a solution treatment, hardening and subsequent tempering. This solution treatment should be carried out in a temperature range of 1050 ° C. ≦ T ≦ 1250 ° C., preferably 1100 ° C. ≦ T ≦ 1230 ° C., especially 1200 ° C., in order to dissolve all nitrides in the form of VN. is there. Through the tempering treatment, the austenite content of the martensite phase and the degree of hardening are adjusted. In order to adjust the desired austenite content of 15-45%, the tempering temperature should be selected in the range of 550 ° C ≦ T ≦ 650 ° C, preferably 580 ° C ≦ T ≦ 630 ° C. EXAMPLES The following particularly advantageous embodiment is described in greater detail by way of an example of such an alloy, referred to as alloy 817. The composition of the alloy 817 is described in Tables 1 and 2. In this case, Table 2 shows the maximum possible impurity content. After solution treatment and quenching, the alloy was tempered at 600 ° C. for 4 hours. After this heat treatment, the structure was a two-phase martensite-austenite with a phase range dimension of 200-300 nm and an austenite content of about 37%. The properties of the alloy 817 according to the invention are compared in Table 3 with the alloy X12CrNiMo12 described above. According to Table 3, alloy 817 is superior to alloy X12CrNiMo12 in overall good properties. High heat resistance and creep resistance enable use as gas turbine rotor or blade materials at high temperatures up to 550 ° C. Table 1 Table 2 Table 3

────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Rainer Steins             Switzerland Zurich Seegnessch             Torase 3

Claims (1)

[Claims] 1. 8-15% chromium, 0.5-2.5% molybdenum, measured by weight%, tongue Up to 2% stainless, 4-10% cobalt, 0.5-6% nickel, 2.5-manganese 8%, vanadium 0.1-1.0%, nitrogen 0.05-0.25%, carbon up to 0.2% The rest contains iron together with impurities, optionally still up to 0.02% boron And / or optionally the sum of the elements niobium, titanium, zirconium and tantalum A martensitic-austenitic steel containing up to 0.5%. 2. Chromium has a range of 9 to 12% and / or molybdenum has a range of 1.0 to 2.0%. And / or cobalt is in the range of 5-8% and / or nickel is in the range of 2-5% and / or Or manganese in the range of 3.5-6.5% and / or tungsten 1.0% and / or Or vanadium in the range of 0.15 to 0.65% and / or nitrogen in the range of 0.07 to 0.15. % Carbon and / or up to 0.1% carbon. Martensite-austenitic steel. 3. 3. A heat-resistant and creep-resistant material in a thermal power plant. Use of a martensitic-austenitic steel according to claim 1. 4. Solution treated and quenched steel alloy In a temperature range of 550 ° C. to 650 ° C., and a volume ratio of 20 to 50% of austenite. The method according to claim 1, wherein tempering is performed so as to generate a texture having the structure. A heat treatment method for obtaining the described martensite-austenite steel. 5. Solution treatment of the steel alloy within a temperature range of 1050 ° C ≦ T ≦ 1250 ° C. Item 5. The method according to Item 4.