JPH0234724A - Manufacture of turbine rotor - Google Patents

Abstract

PURPOSE:To manufacture the title turbine rotor having improved reliability in strength by forming a rotor stock with an alloy steel having specific compsn. and successively executing annealing, heating, cooling and tempering thereto under specific conditions. CONSTITUTION:An alloy steel contg., by weight, 0.1 to 0.4% C, 2.0 to 6.0% Ni, 0.5 to 3.5 % Cr, 0.5 to 2.0% Mo, 0.05 to 0.4% V, <=1.0% Mn, <=0.004% P, <=0.35% Si, <=0.01% Sn, <=0.008% As, <=0.005% Sb and the balance Fe with inevitable impurities is melted and cast to form a rotor stock by forging. The rotor stock is then annealed in the temp. range of 850 to 1000 deg.C, is thereafter heated again in the temp. range of 800 to 950 deg.C, is cooled at 400 deg.C/hr cooling speed and is furthermore tempered in the temp. range of 550 to 700 deg.C. By this method, the turbine rotor provided with high creep rupture strength in the place exposed to high temp. vapor and with high tensile strength and toughness in the place exposed to relatively low temp. vapor can be obtd.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Field of Application) The present invention provides a material that has excellent creep-loveure strength at high temperatures and excellent tensile strength and toughness at relatively low temperatures. The present invention relates to a method for manufacturing a turbine rotor having such characteristics.

(Prior Art) In a large-scale prime mover 1, such as a steam turbine or a gas turbine, the working fluid such as steam is used at a different temperature in each location with respect to one rotor (rotating shaft). A mixture of different compositions that can withstand high temperatures may be used. The rotor material in this case is ASTM
-Cr-Mo- specified in A470 (Class 80)
V steel is used, and on the low temperature side ASTM-A470 (
Ni-Cr-MO-v steel containing 2.5% or more of Ni by weight as defined in Classes 2 to 7) is used.

Even if a single rotor is manufactured by mixing such different materials, it may not be sufficient for actual application.

That is, although Cr-MO·V steel has excellent creep rupture strength at high temperatures, it is insufficient in terms of tensile strength and toughness at low temperatures.

For this reason, in the parts of one rotor that are exposed to relatively low-temperature steam, the height of the turbine blades installed in the final stage must be lowered due to the risk of ductile and brittle fractures. There is a disadvantage that it is not possible to obtain high output.

On the other hand, although Ni-Cr-MO-v steel has excellent tensile strength and toughness at low temperatures, it is insufficient in terms of creep rupture strength at high temperatures. For this reason, parts of a rotor that are exposed to high-temperature steam (350°C) are too sensitive to embrittlement, so the brakes are applied to the high temperature of the steam, resulting in a lack of durability and reliability during long-term operation. be.

12Cr# is a material that combines the above two excellent points.
Various types of W have been proposed, but their economical effects are lost due to their excessive cost.

(Problems to be Solved by the Invention) As described above, in a conventional steam turbine rotor, the parts of the rotor exposed to high-temperature steam have excellent creep rupture strength, while the parts exposed to relatively low-temperature steam In view of the problem that 1) does not simultaneously have contradictory attributes such as excellent tensile strength and toughness, this invention provides the following:
It is an object of the present invention to provide a steam turbine rotor that simultaneously has the above-mentioned contradictory attributes in one rotor.

[Structure of the invention]

(Means and effects for solving the 8 problems) In line with the above object, the present invention provides 0.1 to 0.4% of C and 0.4% of N at a weight ratio of 1.
i2.0-6.0%, Cr0. S~3.5%, M.

0.5-2.0%, V0.05-0.4%, Mn1.0
% or less, P0.004% or less, Si0.35% or less, S
n0.01% or less, As0.008% or less, Sb0.0
05% or less, the balance being Fe and incidental impurities, is molten N, cast, forged to form a rotor body, annealed in a temperature range of 850 to 1000°C, and then re-cast. Heating at a temperature range of 800-950°C, further cooling at a cooling rate of 400°C/hour, and then cooling at a temperature of 550-750°C.
The structure is such that tempering is performed in a temperature range of 00°C.

The composition of the alloy steel constituting the steam turbine rotor according to the present invention and the reasons for limiting the composition ratio thereof will be explained below, and the reasons for limiting the heat treatment temperature range and cooling rate will also be explained.

Note that the numbers for elements are weight ratios.

c: 0. i~0.4% C is an element that improves hardenability, contributes to increasing tensile strength and yield strength, and is also necessary for forming carbides. If the amount is less than 0.1%, an undesirable ferrite phase will be formed, making it impossible to obtain the necessary tensile strength and yield strength, or
, if it exceeds 4%, the toughness decreases, so 0.1 to 0.4
%.

Ni: 2.0-610% Ni is an element that improves hardenability and improves strength and toughness at low temperatures, but if it is less than 2.0%, the effect is not sufficient, and if it is 6.0% or more, If the amount is added, the high temperature strength tends to decrease and embrittlement tends to be promoted.
~6.0%. Preferably it is 3.0 to 4.5%.

Cr: 0.5-3.5% Cr is an element that improves hardenability and tensile strength, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 3.5%, the 0.5~ because high temperature strength decreases
It is set at 3.5%.

Mo: 0.5-2.0% MO is an element necessary to improve hardenability, improve high-temperature strength, and prevent temper brittleness, but 0.
．． If it is less than 5%, the effect will not be sufficient, and if it exceeds 2.0%, the toughness will decrease, so the content should be 0.5 to 2.0%.

v: 0. os ~ 0.4% ■ is an element necessary to improve high temperature strength, but 0
．． If it is less than 0.05%, the effect is not sufficient, and if it is less than 0.4%.
If it exceeds 0.05 to 0.4%, the toughness will decrease. 6 Mn:L, 0% or less
/S≧20 as a guideline. With current smelting technology, 0.
Addition of 1% or less is sufficient. However, Mn is P,
Since S has the effect of promoting embrittlement of steel when it exists together with elements such as Sn, As, and Sb, it should be reduced by methods such as ladle refining or reduced to 0.0 as a stable compound.
It is preferably 5% or less.

SL: 0.35% or less Si is added as a deoxidizing agent during dissolution,
If Si is added in excess, the toughness will decrease, and if Si is present together with elements such as P, Sn, As, or Sb, it tends to promote embrittlement of steel. For this reason, SL should be suppressed to 0.35% or less, and preferably suppressed to 0.05% or less by performing vacuum carbon deoxidation etc. 6 P: 0.004% or less, Sn: 0.01 % or less, As: 0.008% or less, sb: 0. oos
% or less P, Sn, As, and Sb are impurity elements that are unavoidably present in steel, and cause segregation on the grain boundaries of steel during heat treatment and the like, which causes a decrease in toughness. Therefore, from the viewpoint of preventing a decrease in embrittlement, it is preferable to suppress it to the above-mentioned value or less.

Next, the reason for limiting the heat treatment temperature of alloy steel having the above composition will be explained in detail.

Conventionally, the cooling method during quenching of Nil:r-No/V steel was as follows:
Water cooling or water spray cooling was used.

This was to suppress grain boundary segregation of impurity elements in the steel that occurs during cooling, prevent a decrease in toughness, and obtain the desired tensile strength and yield strength.

However, experiments have revealed that when conventional cooling methods are used, fine carbides such as v4C3 are rarely precipitated in steel, and therefore sufficient creep rupture strength cannot be ensured. For this reason, the inventor found that it is preferable to perform air cooling or fan cooling as a cooling method for precipitating fine carbides in the steel after heat treatment. That is, when using conventional cooling methods, a ferrite phase is formed in the steel, which is unfavorable in terms of material strength, but when air cooling or fan cooling is used, fine carbides are formed in the steel. The cooling rate in this case is
400°C/hour is preferred. Cooling rate 400℃/hour
When the temperature exceeds 1, the structure of the steel becomes close to martensite, and carbide precipitation is not sufficiently observed.

The quenching temperature is usually in the temperature range used to make N'i-cr-Mo-v steel, which austenitizes the steel cable body after annealing and further solid-solves carbides into the steel body. In order to prevent the crystal grains from becoming coarse, the temperature should be in the range of 850 to 950'C.
Prior to the quenching process, the steel cable body undergoes a forging process, but in this case, the steel cable body is tempered in order to make the composition uniform.

The temperature range for this tempering is 850°C to 1000°C.

As for the aging treatment after quenching the above-mentioned steel cable, a tempering temperature range of 550 to 700° C. is preferable in order to further improve the obtained material properties, that is, toughness, tensile strength, and yield strength.

If the above composition is configured and the temperature range of heat treatment and cooling method after heat treatment are selected as described above, the parts of one rotor exposed to high temperature steam will have excellent creep rupture strength, and the parts exposed to relatively low temperature steam will have excellent creep rupture strength. Excellent effects on tensile strength and toughness can be expected in areas where

(Example) A method for manufacturing a turbine rotor according to the present invention will be described in detail.

Table 1 lists the composition of the turbine rotor and the heat treatment procedure, and was created by comparing it with conventional comparative examples 1, 2.3. The compositions of Example 1 to Comparative Example 3 were melted in a vacuum melting furnace, deoxidized and vacuum agglomerated, and then used as samples for obtaining various properties of the materials. In this case, Example 1 to Comparative Example 2 were made using a 50 kg steel ingot, and Example 5 and Comparative Example 3 were made by simulating the trunk diameter of the actual machine, and the steel ingot was 40 tons. It is large enough.

(Hereinafter, blank space) The compositions of Example 1 to Comparative Example 3 are as shown in Table 2. In this table, FATT means ductile-brittle fracture surface transition temperature. Moreover, Table 3 shows the creep rupture test results.

(Margins below) In Tables 2 and 3, in Examples 1 to 5, the surface layer of the rotor body during quenching was cooled at a cooling rate of 400° C./hour. Further, in Example 5, the center of the rotor body is cooled at a cooling rate of 50° C./hour. As a result, it will be understood that Example 5 is much better than Comparative Example 3 in terms of tensile strength, impact strength, and creep rupture strength.

(Leaving space below) Table 4 Table 4 evaluates the susceptibility to long-term embrittlement.The samples were accelerated to embrittlement by a heat treatment called the step cool method shown in Figure 1, and then an impact test was conducted. It is something. In this table, SC stands for step-cool method, and data before and after SC is expressed. Also, ΔFATT
is expressed by subtracting the values before and after SC, and the positive and larger value of ΔFATT means higher susceptibility to embrittlement.

Looking at the results in Table 4, it will be understood that although the values from Example 1 to Example 5 vary widely, they are still significantly higher than the conventional creep rupture strength. In Comparative Example 2, the composition was the same as that according to the present invention, and the cooling method during quenching was a conventional water spray method, but although the susceptibility to embrittlement was reduced, Table 3. As far as we can see, the creep rupture strength has not improved.

〔Effect of the invention〕

As explained above, in the method for manufacturing a turbine rotor according to the present invention, the creep rupture strength is increased and embrittlement is reduced in the parts exposed to high-temperature steam, and the parts exposed to relatively low-temperature steam are The rotor is designed to have high tensile strength and toughness, and even a single rotor can fulfill mutually contradictory functions, further increasing reliability in terms of strength when applied to actual machines. It is expected that this will improve in terms of performance.

4,

[Brief explanation of the drawing]

Figure 1 is called the step cool method. Adds embrittlement FIG. 3 is a schematic diagram showing a heat treatment process for speeding up the process.

Claims (1)

[Claims] Weight ratio: C0.1-0.4%, Ni2.0-6.0%
, Cr0.5-3.5%, Mo0.5-2.0%, V0
．． 05-0.4%, Mn 1.0% or less, P 0.004%
Below, Si0.35% or less, Sn0.01% or less, As
After melting and casting an alloy steel consisting of 0.008% or less Sb, 0.005% or less Sb, and the balance Fe and incidental impurities, it is forged to form a rotor body.
After annealing in the temperature range of 000℃, annealing is performed again at 800~950℃.
A method for manufacturing a turbine rotor, which comprises heating in a temperature range of 550°C to 700°C, further cooling at a cooling rate of 400°C/hour, and then tempering in a temperature range of 550 to 700°C.