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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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

Definitions

• This invention relates to steam turbine rotor materials for use in thermal electric power generation.
• High-temperature rotor materials for use in steam turbine plants for thermal electric powder generation include CrMoV steel and 12Cr steel. Of these, the use of CrMoV steel is restricted to plants having a steam temperature up to 566°C because of its limited high-temperature strength.
• rotor materials based on 12Cr steel e.g., those disclosed in Japanese Patent Provisional Publication Nos. 60-165359 and 62-103345
• the steam temperature exceeds 600°C such rotor materials based on 12Cr steel have insufficient high-temperature strength and can hardly be used for steam turbine rotors.
• the present invention comprises (1) a steam turbine rotor material for high-temperature applications consisting essentially of, on a weight percentage basis, 0.05 to 0.13% carbon, 0.005 to 0.1% silicon, 0.01 to 0.5% manganese, 9 to 12% chromium, 0.1 to 0.3% vanadium, a total of 0.01 to 0.15% niobium and/or tantalum, 0.01 to 0.1% nitrogen, 0.05 to 0.5% molybdenum, 1.5 to 3% tungsten, 1 to 4% cobalt, and the balance being iron and incidental impurities, and (2) a steam turbine rotor material for high-temperature applications as described above in (1) wherein 0.001 to 0.03% by weight of boron is substituted for a part of the iron.
• the first class of steam turbine rotor materials for high-temperature applications in accordance with the present invention have been developed by using 12Cr steel as the basic material and adding carefully selected alloying elements thereto for the purpose of improving high-temperature strength, and can provide novel steam turbine rotor materials for high-temperature applications having high-temperature characteristics.
• About 0.5% Ni is present in conventional turbine rotor materials based on 12Cr steel.
• Ni is an element which essentially causes a reduction in creep rupture strength, this element is used in the aforesaid rotor materials because it has the beneficial effects of inhibiting the formation of d-ferrite in controlling the matrix structure and of improving toughness.
• the rotor materials of the present invention are characterized in that, with preferential consideration for the securement of high creep rupture strength, Ni is completely eliminated except for a fraction present as an incidental impurity and, at the same time, Co having a powerful inhibitory effect on d-ferrite similarly to Ni is positively added. Moreover, high toughness is secured by minimizing the contents of Si and Mn which exert an adverse influence on toughness.
• C together with N, forms carbonitrides and thereby contributes to the improvement of creep rupture strength.
• the content of C should be in the range of 0.05 to 0.13%. The preferred range is from 0.09 to 0.11%.
• Si is an element which is effective as a deoxidizer but embrittles the matrix. Since the rotor materials of the present invention are prepared according to the vacuum carbon deoxidation process, Si should be added in a minimum amount required for steel making, i.e., in the range of 0.005 to 0.1%. The preferred range is from 0.005 to 0.05%.
• Mn is an element which acts as a deoxidizer and is also useful in the prevention of hot cracking during forging. Moreover, Mn has the effect of inhibiting the formation of d-ferrite. However, the addition of Mn will cause a corresponding reduction in creep rupture strength. Furthermore, since Mn is an element which essentially promotes the embrittlement of iron and steel, the present invention has chosen a maximum Mn content of 0.5% while attaching importance to the securement of high creep rupture strength. In particular, if the Mn content is limited to 0.15% or less, the creep rupture strength is further improved. If desired, therefore, Mn must be added in an amount limited to 0.15% or less.
• the minimum content of Mn has been set at 0.01% because the attainment of less than 0.01% necessitates the careful selection of steel used as the raw material and the employment of an unduly rigorous refining process, resulting in an increased cost.
• the preferred content range of Mn is from 0.01 to 0.15%.
• Cr form a carbide and thereby contributes to the improvement of creep rupture strength. Moreover, Cr dissolves in the matrix to improve oxidation resistance and also contributes to the improvement of creep rupture strength by strengthening the matrix itself. If its content is less than 9%, no sufficient effect will be produced, while if its content is greater than 12%, d-ferrite will tend to be formed, resulting in a reduction in strength and toughness. Accordingly, the content of Cr should be in the range of 9 to 12%. The preferred range is from 10.5 to 11.5%.
• V forms a carbonitride and thereby improves creep rupture strength. If its content is less than 0.1%, no sufficient effect will be produced. On the other hand, if its content is greater than 0.3%, the creep rupture strength will conversely be reduced. Accordingly, the content of V should be in the range of 0.1 to 0.3%.
• Nb and/or Ta form carbonitrides and thereby contribute the improvement of high-temperature strength. Moreover, they cause finer carbide (M 23 C 6 ) to precipitate at high temperatures and thereby contribute to the improvement of long-time creep rupture strength. If the total content of these elements is less than 0.01%, no beneficial effect will be produced, while if it is greater than 0.15%, the carbonitrides of Nb and/or Ta formed in the manufacture of steel ingots will fail to dissolve fully in the matrix during heat treatment (solution treatment) and will coarsen during use to cause a reduction in long-time creep rupture strength. Accordingly, the total content of Nb and/or Ta should be in the range of 0.01 to 0.15%.
• N together with C and alloying elements, form carbonitrides and thereby contributes to the improvement of high-temperature strength. If its content is less than 0.01%, sufficient creep rupture strength will not be achieved because no sufficient amount of carbonitrides cannot be formed. If its content is greater than 0.1%, the carbonitrides will aggregate to form coarse grains after the lapse of a long time and, therefore, sufficient creep rupture strength cannot be achieved. Accordingly, the content of N should be in the range of 0.01 to 0.1%. The preferred range is from 0.02 to 0.05%.
• Mo together with W, dissolves in the matrix and thereby improves creep rupture strength. If Mo is added alone, it may be used in an amount of about 1.5%. However, where W is also added as is the case with the present invention, W is more effective in improving high-temperature strength. Moreover, if Mo and W are added in unduly large amounts, d-ferrite will be formed to cause a reduction in creep rupture strength. Accordingly, with consideration for the balance with the content of W, the content of Mo should be in the range of 0.05 to 0.5%.
• W together with Mo, dissolves in the matrix and thereby improves creep rupture strength.
• W is an element exhibiting a more powerful strengthening effect as a result of solid solution.
• the content of W should be in the range of 1.5 to 3%.
• Co dissolves in the matrix to strengthen the matrix itself and, at the same time, inhibit the formation of d-ferrite. Accordingly, if Co is added, strengthening elements (e.g., Mo and W) having a strong tendency to form ferrite can be added in larger amounts than in cases where no Co is added. As a result, high creep rupture strength can be achieved.
• strengthening elements e.g., Mo and W
• Ni is a commonly used element which likewise has the effect of inhibiting the formation of d-ferrite, an increase in Ni content results in correspondingly reduced creep rupture strength. In conventional turbine rotor materials based on 12Cr steel, about 0.5% of Ni is used for the purpose of inhibiting the formation of d-ferrite and securing sufficient toughness.
• the present invention is characterized in that Ni is not particularly added with importance attached to the securement of high creep rupture strength.
• the formation of d-ferrite is fully inhibited by the addition of Co.
• the beneficial effect of Co addition manifests itself when its content is 1% or greater, but the addition of more than 4% Co will promote the precipitation of a carbide and thereby cause a reduction in long-time creep rupture strength.
• the content of Co should be in the range of 1 to 4%. The preferred range is from 2.5 to 3.5%.
• the second class of steam turbine rotor materials for high-temperature applications in accordance with the present invention are the same as the above-described first class of steam turbine rotor materials for high-temperature applications, except that 0.001 to 0.03% by weight of B is substituted for a part of Fe.
• B is substituted for a part of Fe.
• C, Si, Mn, Cr, Mo, W, V, Nb and/or Ta, Co and N they are the same as described above for the first class of steam turbine rotor materials for high-temperature applications and are hence omitted. Consequently, an explanation for B that is a new alloying element is given below.
• B has the effect of enhancing grain boundary strength. Consequently, it contributes to the improvement of creep rupture strength. However, if B is added in unduly large amounts, poor hot workability and low toughness will result. Accordingly, the lower limit should be a minimum value of 0.001% at which the amount of B added can be practically controlled, and the upper limit should be a value of 0.03% at which no adverse effect will be produced.
• the preferred content range of B is from 0.01 to 0.02%.
• Example 1 (First class of steam turbine rotor materials for high-temperature applications)
• Sample Nos. 1-9 correspond to inventive materials and sample Nos. 10-18 to comparative materials. All materials were prepared by melting in a 50 kg vacuum high-frequency furnace and then forged at a temperature of 1,200°C. Prior to various tests, these materials were heat-treated by hardening them under conditions which simulate the central part of an oil-quenched rotor having a drum diameter of 1,200 mm, and then tempering them at a temperature determined so as to give a 0.2% yield strength of about 63-67 kgf/mm 2 .
• Example 2 (Second class of steam turbine rotor materials for high-temperature applications)
• Sample Nos. 19-21 correspond to inventive materials and sample Nos. 22 and 23 to comparative materials.
• Sample Nos. 19, 20 and 22 are materials prepared by adding varying amounts of B to a base composition substantially equal to the composition of sample No. 8 used in the above-described Example 1.
• Sample Nos. 21 and 23 are materials prepared by adding varying amounts of B to a base composition substantially equal to the composition of sample No. 6 used in the above-described Example 1.
• the base compositions of sample Nos. 19, 20 and 22 were slightly different from the composition of sample No. 8, and the base compositions of sample Nos. 21 and 23 were slightly different from the composition of sample No. 6.
• Example No. 20 The estimated rupture strength of sample No. 20 was 12.7 kgf/mm 2 , indicating that the inventive material is also excellent in long-time creep rupture strength as compared with the base material (sample No. 8).
• the foregoing facts suggest that the creep rupture strength of the turbine rotor materials shown in Example 1 can further be improved by substituting B for a part of Fe in the content range shown in Example 2.
• the steam turbine rotor materials for high-temperature applications in accordance with the present invention have excellent high-temperature strength and are hence useful as high-temperature steam turbine rotor materials for use in hypercritical-pressure electrical power plants having a steam temperature higher than 600°C. It may be expected that the present invention is useful in further raising the operating temperature of the current hypercritical-pressure electrical power plants to afford a saving of fossil fuels and, at the same time, reduce the amount of carbon dioxide evolved.

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Citations (5)

• 1995
  • 1995-07-17 JP JP17976995A patent/JP3310825B2/ja not_active Expired - Fee Related
• 1996
  • 1996-02-29 EP EP96103067A patent/EP0754774A1/fr not_active Withdrawn

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