EP0754774A1

EP0754774A1 - Steam turbine rotor materials for high-temperature applications

Info

Publication number: EP0754774A1
Application number: EP96103067A
Authority: EP
Inventors: Masatomo c/o Mitsubishi Jukogyo K.K. Kamada; Akitsugu c/o Mitsubishi Jukogyo K.K. Fujita; Ikujirou Kitagawa; Katsuo Kaku
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1995-07-17
Filing date: 1996-02-29
Publication date: 1997-01-22
Also published as: JPH0931600A; JP3310825B2

Abstract

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, 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.

Description

BACKGROUND OF THE INVENTION

Field of the invention

This invention relates to steam turbine rotor materials for use in thermal electric power generation.

Description of the related art

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. On the other hand, rotor materials based on 12Cr steel (e.g., those disclosed in Japanese Patent Provisional Publication Nos. 60-165359 and 62-103345) have more excellent high-temperature strength than CrMoV steel and can hence be used in plants having a steam temperature up to 600°C. However, if 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.
Accordingly, it is an object of the present invention to provide 12Cr steel-based steam turbine rotor materials for high-temperature applications which have excellent high-temperature strength and can be used at steam temperatures higher than 600°C.

SUMMARY OF THE INVENTION

As a result of intensive investigations, the present inventors have now discovered the following excellent turbine rotor materials for high-temperature applications.
That is, 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.

On the first class of steam turbine rotor materials for high-temperature applications

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. Although 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. In contrast, 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.
The reasons for content restrictions in the first class of steam turbine rotor materials for high-temperature applications in accordance with the present invention are described below. In the following description, percentages are by weight.

Carbon

C, together with N, forms carbonitrides and thereby contributes to the improvement of creep rupture strength. However, if its content is less than 0.05%, no sufficient effect will be produced, while if its content is greater than 0.13%, the carbonitrides will aggregate during use to form coarse grains, resulting in a reduction in long-time high-temperature strength. Accordingly, 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%.

Silicon

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%.

Manganese

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. On the other hand, 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. Thus, the preferred content range of Mn is from 0.01 to 0.15%.

Chromium

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%.

Vanadium

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%.

Niobium and/or Tantalum

Nb and/or Ta form carbonitrides and thereby contribute the improvement of high-temperature strength. Moreover, they cause finer carbide (M₂₃C₆) 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%.

Nitrogen

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%.

Molybdenum

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%.

Tungsten

As described above, W, together with Mo, dissolves in the matrix and thereby improves creep rupture strength. As compared with Mo, W is an element exhibiting a more powerful strengthening effect as a result of solid solution. However, if W is added in an unduly large amount, d-ferrite and a large quantity of Laves phase will be formed to cause a reduction in creep rupture strength. Accordingly, with consideration for the balance with the content of Mo, the content of W should be in the range of 1.5 to 3%.

Cobalt

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. Although 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. In contrast, the present invention is characterized in that Ni is not particularly added with importance attached to the securement of high creep rupture strength. However, 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. Moreover, since Co itself is an expensive material, the addition of a large amount of Co will result in an increased cost. Accordingly, the content of Co should be in the range of 1 to 4%. The preferred range is from 2.5 to 3.5%.

On the second class of steam turbine rotor materials for high-temperature applications

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. The reasons for content restrictions are described below. However, with respect to 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.

Boron

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%.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to demonstrate the effects of the present invention, the first and second classes of steam turbine rotor materials for high-temperature applications in accordance with the present invention are more specifically explained with reference to the following examples.

Example 1 (First class of steam turbine rotor materials for high-temperature applications)

(Constitution of Example 1)

The chemical compositions of the materials used for tests are shown in Table 1. 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/mm².

(Effects of Example 1)

The mechanical properties and creep rupture characteristics of the inventive materials and the comparative materials are shown in Table 2. All of the inventive materials exhibit a Charpy impact value of as high as 9.0 kgf-m or greater (as tested at ordinary temperatures). This indicates that a sufficiently high impact value can be secured in spite of the elimination of Ni. When attention is paid to the creep rupture time measured at 650°C under a load of 15 kgf/mm², it can be seen that the inventive materials exhibit a markedly increased rupture time as compared with the comparative materials. Subsequently, with respect to sample No. 8 having the longest 650°C-15 kgf/mm² creep rupture time of all the inventive materials and sample No. 14 having the longest 650°C-15 kgf/mm² creep rupture time of all the comparative materials, additional creep rupture tests were carried out at 650°C and 700°C under various stresses. On the basis of the data obtained from these tests, the 10⁵ hour creep rupture strength at 650°C of each sample was estimated. The estimated rupture strength was 12.1 kgf/mm² for sample No. 8 and 8.2 kgf/mm² for sample No. 14, indicating that the inventive material is also very excellent in long-time creep rupture strength. The foregoing facts suggest that compositional design based on the elimination of Ni and the positive addition of Co is effective in improving creep rupture strength.

Example 2 (Second class of steam turbine rotor materials for high-temperature applications)

(Constitution of Example 2)

The chemical compositions of the materials used for tests are shown in Table 3. 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 65-67 kgf/mm². 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.
Strictly speaking, 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. However, it is practically difficult to control such a degree of variation in base composition. Accordingly, it may safely be said that the materials of sample Nos. 19, 20 and 22 were prepared on the basis of sample No. 8, and the materials of sample Nos. 21 and 23 were prepared on the basis of sample No. 6.

(Effects of Example 2)

The mechanical properties and creep rupture characteristics of the inventive materials and the comparative materials are shown in Table 4. All of the inventive materials exhibit a Charpy impact value of as high as 10.0 kgf-m or greater (as tested at ordinary temperatures) and compare favorably with the base materials (sample Nos. 6 and 8). This indicates that, at least in the content range employed for the inventive materials, the addition of B exerts no adverse influence on the impact value.
When attention is paid to the creep rupture time measured at 650°C under a load of 15 kgf/mm², it can be seen that the inventive materials exhibit an increased rupture time as compared with the base materials (sample Nos. 6 and 8). In contrast, the comparative materials (sample Nos. 22 and 23) having a higher B content exhibit a decreased rupture time as compared with the base materials (sample Nos. 6 and 8). Subsequently, with respect to sample No. 20 having the longest 650°C-15 kgf/mm² creep rupture time of all the inventive materials, additional creep rupture tests were carried out at 650°C and 700°C under various stresses. On the basis of the data obtained from these tests, the 10⁵ hour creep rupture strength at 650°C of each sample was estimated. The estimated rupture strength of sample No. 20 was 12.7 kgf/mm², 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.

Claims

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.
A steam turbine rotor material for high-temperature applications as claimed in claim 1 wherein 0.001 to 0.03% by weight of boron is substituted for a part of the iron.