JPH01230723A - Manufacture of turbine rotor - Google Patents

Abstract

PURPOSE:To manufacture a stock of turbine rotor shape excellent in mechanical strength at high and low temps. and reduced in embrittlement by specifying the conditions in the heat treatment, such as heating, annealing, hardening, and tempering, of an Fe-base alloy having a specific composition. CONSTITUTION:An Fe-base alloy having a composition containing, by weight, 0.10-0.35% C, <=0.35% Si, <=1.0% Mn, 0.3-2.5% Ni, 1.5-3.5% Cr, 0.3-1.5% Mo, 0.05-0.30% V, and 0.1-2.0% W is heated up to a temp. in the range of 1000-1150 deg.C to undergo annealing and further heated at 900-970 deg.C to undergo hardening, followed by tempering at 600-700 deg.C. By this method, the stock of turbine rotor shape excellent in strength and toughness at low temp. and also excellent in creep rupture strength at high temp. and reduced in embrittlement can be manufactured.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a turbine rotor, which has particularly excellent creep-rubber strength at high temperatures and excellent tensile strength and toughness at relatively low temperatures. The present invention relates to a method of manufacturing a steam turbine rotor.

(Prior Art) Steam turbines generally use rotors made of different materials depending on the steam conditions used.

For example, in large steam turbines, A
C has excellent creep rupture strength at high temperatures as specified in STM-A470 (class 8).
rMoV steel is used, and ASTM-A470 (C1as
2.5% or more as specified in s2-7)
NiCrMoVfv4 with i is used. Currently, rotors made of different materials corresponding to these steam conditions are mechanically joined to form a steam turbine.

On the other hand, in relatively small steam turbines, a high-low pressure integrated rotor is usually used, which is composed of a single rotor made of the same material from the high-pressure side to the low-pressure side.

(Problems to be Solved by the Invention) By the way, conventional steam turbine rotors have the following problems. That is, in conventional large steam turbines, since the turbine is constructed by joining a plurality of rotors made of different materials, there is a problem in that the manufacturing process becomes complicated.

In addition, CrMoVfV4 and NiCrMoV steels are usually used as materials for high- and low-pressure integrated rotors used in small steam turbines, but the conventionally used CrMoV steels have sufficient tensile strength. Since this is not satisfactory, there is a problem in that the size of the blade that can be attached to the low-pressure final stage is limited.

Furthermore, although conventional NiCrMoV steel has excellent tensile strength, it lacks creep rupture strength, and
There is a problem in that embrittlement tends to progress in a temperature range of 0° C. or higher.

For this reason, conventional rotor materials have significant limitations when attempting to improve the efficiency of a steam turbine by using high-temperature steam and installing a long, low-pressure final stage.

In addition, 12 is already available as a rotor material with excellent creep rupture strength and toughness, as well as excellent tensile strength at low temperatures.
Crft4 has been developed, but since 12Cr steel is expensive, there is a problem in that using it as a rotor material increases manufacturing costs.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the problems of the conventional techniques described above, to have high tensile strength and toughness under relatively low-temperature steam conditions, and high creep rupture strength under high-temperature conditions. An object of the present invention is to provide a method for manufacturing a steam turbine rotor with less embrittlement.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention provides, in terms of weight ratio, C0
, 10-0.35%, Si 0.35% or less, Mn 1
．． 096 or less, Ni0.3-2.5%, Cr1.5
~3.5%, MO0.3~1.5%, V0.05~0.
30%, W0.1-2.0%, further Nb0 if desired
．． 01-0.15%, Ta0.01-0.15%, N0
．． A turbine rotor-shaped element body made of an Fe-based alloy containing 0.01 to 0.10% or B0.002 to 0.015% or more is heated to a temperature in the range of 1000 to 1150°C. 4900~970℃ after annealing
Quenched by heating to a temperature in the range of 600 to 70
It is characterized by tempering in a temperature range of 0°C.

(Function) By applying this manufacturing method, it has high tensile strength and toughness under relatively low temperature steam conditions, and
Achieved high leap rupture strength and low embrittlement even under high temperature conditions.

Next, the reason for limiting the above temperature range will be explained.

Annealing: 1000 to 1150°C The purpose of annealing is to dissolve coarse carbides into the steel matrix and to eliminate the non-uniformity of the structure caused by forging. To achieve this objective, it is necessary to maintain a temperature of 1000° C. or more for about one hour per inch of wall thickness of the rotor body to uniformly heat the entire rotor body. However, if heated to a temperature higher than 1150° C., the austenite crystal grains will become coarser, making it difficult to sufficiently recover the toughness even after the subsequent quenching and tempering steps. Therefore, 1000-115
The temperature range was set at 0°C, and slow cooling was performed when the above objective was achieved.

Quenching = 900 to 970℃ During quenching, it is necessary to austenite the base of this steel and dissolve carbides in the base, but since it has already been annealed, the temperature is slightly lower than that during annealing. It is appropriate to heat it to

In experiments, it was confirmed that at temperatures below 900°C, solid solution of carbides is not sufficient, resulting in a decrease in Labcha strength. Moreover, it was confirmed that when the temperature was higher than 970°C, coarsening of crystal grains was observed, which caused a decrease in toughness. Therefore, the temperature is set at 900 to 970°C. Note that, as in the case of annealing, the holding time after heating is preferably about 1 hour per inch of wall thickness of the rotor body, so that the temperature of the turbine rotor body is as uniform as possible.

Regarding the quenching cooling rate, air cooling or, if necessary,
Can be oil cooled or water cooled (water spray atomization).

Tempering 2600-700℃ Tempering is a process performed to restore toughness and ductility to quench-hardened materials, but in the case of this steel type, sufficient toughness cannot be obtained below 600℃, and above 700℃ On the contrary, it softens and its tensile strength at room temperature decreases. Therefore, the above temperature range is 600 to 700°C. The heating and holding time is set to an optimum time that provides a sufficient balance between strength, toughness, and ductility.

Next, the composition of the turbine rotor steel suitable for the above manufacturing method and the reasons for limiting the composition ratio thereof will be explained.

Note that the numbers are weight ratios.

C: 0.10-0.35% Carbon is an element that contributes to improving hardenability and increasing tensile strength, but in the present invention, it dissolves in the steel base at high temperatures and forms an austenite structure. , is also necessary to form carbides. If the composition ratio is less than 0.10%, undesirable ferrite will be produced, and if it exceeds 0.35%, the toughness will decrease. %.

Si: 0.35% or less Silicon is added as a deoxidizing agent during melting, but when added in large amounts, a portion of it remains in the steel as an oxide, adversely affecting toughness. Therefore, the amount added should be 0.35% or less, but preferably 0.10% or less in order to suppress aging embrittlement as much as possible.

Mn: 1.0% or less Manganese is added as a desulfurizing agent during melting, but an addition amount of 1.0% or less is sufficient to obtain its effect. In addition, if the raw materials are carefully selected or a process such as outside-furnace smelting is carried out to suppress sulfur to an extremely low level, the desulfurization effect of manganese becomes unnecessary, so manganese should be kept at 0.10% or less. preferable.

Ni: 0.3 to 2.5% Nickel is an element effective in increasing the hardenability of steel and increasing its tensile strength and toughness. It is necessary to add 3% or more. However, if more than 2.5% is added,
On the contrary, it tends to reduce creep rupture strength and promote embrittlement, so it is set at 0.3 to 2.5%, but preferably 1.
It is 0 to 2.0%.

Cr: 1.5 to 3.5% Chromium is an effective element for increasing the hardenability of steel and increasing its tensile strength, and in order to obtain the tensile strength necessary for a turbine rotor, chromium must be added at a concentration of 1.5%. It is necessary to add the above amount. However, if added in excess of 3.5%, the journal characteristics of the turbine rotor will deteriorate, so the content should be 1.5 to 3.5%, preferably 2.0 to 3.0%.

Mo: 0.3 to 1.5% Molybdenum is an effective element for increasing the hardenability, tensile strength, and creep rupture strength of steel. It is necessary to add 3% or more. However, if it is added in excess of 1.5%, the toughness will decrease, so 0. 3 to 1.5%, preferably 0.7 to 1.5%
It is 1.2%.

V: 0.05-0.30% Vanadium is an effective element for increasing the hardenability and creep rupture strength of steel, and is also effective for achieving refinement of crystal grains. In order to demonstrate its effect, 0,
It is necessary to add 0.05% or more. However, if it is added in excess of 0.30%, the toughness will decrease.
30%, preferably 0.10 to 0.25%.

W: 0.1-2.0% Tungsten is an element necessary for improving high-temperature strength through solid solution strengthening, but if it is less than 0.1%, the effect is not sufficient, and if it exceeds 2.0%. Since this lowers creep rupture strength and toughness, the content is set at 0.1 to 2.0%, but preferably 0.3 to 1.0%.

Furthermore, by adding Nb, Ta, N, and B in addition to the above components, tensile strength, toughness, and creep rupture strength can be further improved. The reason for limiting the composition ratio of each alloying element will be explained below.

Nb: 0.01 to 0.15% Niobium is an element that is effective in refining crystal grains, and in order to exhibit its effect, it is necessary to add 0.01% or more. However, if it is added in excess of 0.15%, coarse carbides will be formed and the toughness will be reduced.
01 to 0.15%, preferably 0.03 to 0
．． It is 10%.

Ta: 0.01 to 0.1596 Tantalum is an element that functions in the same way as niobium, and has a value of 0.
．． 0.01 to 0.15%, preferably 0.03 to 0.0
．． It is 10%.

N: 0.01-0.10% Nitrogen is an effective element for increasing tensile strength and creep rupture strength, and is added in an amount of 0.01-0.10%, preferably 0.03-0.10%. 07%. If it is less than 0.01%, the effect will be poor, while if it exceeds 0.10%, the toughness will decrease, which is not preferable.

B: 0.002-0.015% Boron is an element necessary to improve hardenability and creep rupture strength, and in order to obtain this effect, it is necessary to add 0.002% or more. However, on the contrary, 0
．． If added in excess of 0.015%, forging cracks are likely to occur during rotor manufacturing, so the content should be within this range, but preferably 0.003 to 0.010%.

Note that incidental impurities other than Fe that are not included above may be included. For example, PlS etc.
Although the amount cannot be removed by ordinary metallurgical means, it is desirable that it be as small as possible.

(Examples) The present invention will be more clearly understood from the experimental results of Examples and Comparative Examples shown below.

The samples to be heat-treated are made by melting raw materials blended to have the alloy composition shown in Table 1 in an electric arc furnace, deoxidized, vacuum-formed, and then forged into a cylindrical shape to be used as a turbine rotor. It was created as a model of the shape element.

This turbine rotor-shaped element is formed to have a diameter of 600 mm and a length of 1.000 mm, and is sufficiently large to be used as a model of a part of an actual turbine rotor-shaped element. The model of the turbine rotor-shaped element body was subjected to heat treatments 1 to 2 as shown in Table 1.

The heat treatments I to ① are shown in FIG. 1, where FC indicates furnace cooling and AC indicates air cooling.

The combinations of alloy compositions and heat treatments listed in Table 1 are examples of the present invention and comparative examples for reference.

That is, Examples 1 to 7 in Table 1 show that the composition materials for turbine rotors suitable for the manufacturing method of the present invention include 1000 to 1000.
A series of heat treatments were performed: annealing at a temperature of 1150°C, quenching at a temperature of 900-970°C, and tempering at a temperature of 600-700°C.

On the other hand, Comparative Example 1 is a simple high-temperature CrMoV steel with a composition different from that of the present invention, which was subjected to conventional heat treatment at a higher quenching temperature than that of the present invention. be. In Comparative Example 2, a material having the same composition as Comparative Example 1 was subjected to heat treatment (2) that can be included in the present invention. Similarly, Comparative Example 3 is a simple low-temperature N i CrMoV steel with a composition different from that of the present invention, and was subjected to conventional heat treatment (1) at a lower quenching temperature than that of the present invention. It is something. Further, in Comparative Example 4, similar to Comparative Example 3, the composition material was subjected to heat treatment (1) that can be included in the present invention.

Test pieces were cut out from the model of the turbine rotor-shaped body that had been heat-treated in this way, and subjected to a tensile test, an impact test, a creep rupture test, and an embrittlement test (step cool method).

Table 2 shows the results of the tensile test and Charpy impact test at room temperature.

From this table, Examples 1 to 7 according to the present invention are compared to Comparative Examples 1 to 7.
It is understood that the material exhibits tensile strength, proof stress, elongation, and area of area equivalent to or greater than the conventional material shown in No. 4, and has sufficient mechanical properties at relatively low temperatures. In particular, Comparative Example 1
．． Compared to Comparative Example 3, the strength is significantly improved, and the FATT (ductile-brittle fracture transition temperature) is also low, close to that of Comparative Example 3 as a low-pressure turbine rotor material. In Comparative Example 4, the high-temperature heat treatment according to the present invention was applied to the conventional low-pressure turbine rotor composition material, so the ductility and toughness were extremely reduced.

Table 3 shows the test temperature: 600℃, load stress: 14kg f
/ mtA, 17 kg f /m1r (7) The results of two types of leaf rupture tests are shown.

In this table, Examples 1 to 7 according to the present invention all exhibit excellent creep rupture strength, and have comparable characteristics compared to Comparative Example 1, which is a conventional high-temperature turbine rotor material. . Moreover, compared with other Comparative Examples 2 to 4, the creep rupture strength is extremely excellent. Furthermore, the results of elongation and reduction of area, which indicate creep ductility, are sufficiently satisfactory.

Table 4 evaluates the susceptibility to long-term embrittlement, and shows the results of an impact test after each test piece was subjected to accelerated embrittlement using a heat treatment called the step cool method shown in Figure 2. ing.

/ Table 4 In this table, ΔFATT indicates the difference in FATT before and after accelerated embrittlement by the step cool method, and is an index indicating the amount of embrittlement. That is, the smaller this ΔFATT, the less likely it is to become brittle.

Examples 1 to 7 according to the present invention have a small amount of embrittlement ΔFATT and a low FATT, so that they can be used for a long period of time under the same environment as the conventional one.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, it is possible to obtain a turbine rotor-shaped element having excellent strength and toughness at low temperatures, excellent creep rupture strength at high temperatures, and less embrittlement.

A turbine rotor formed from this element can be used as one with long blades on the low-pressure stage side, and can also be used in a high-temperature steam environment.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a heat treatment process, and FIG. 2 is a schematic diagram showing a heat treatment process for performing accelerated embrittlement. 1-V...type of heat treatment, FC...furnace cooling, AC...
・Air cooling. Figure 1

Claims (1)

[Claims] 1. By weight: C 0.10-0.35%, Si 0.
35% or less, Mn 1.0% or less, Ni 0.3-2.
5%, Cr 1.5-3.5%, Mo 0.3-1.5
%, V 0.05-0.30%, W 0.1-2.0%
A turbine rotor-shaped element made of a Fe-based alloy containing: is heated to a temperature in the range of 1000 to 1150°C to anneal it, then heated to a temperature in the range of 900 to 970°C for quenching, and then further heated to a temperature in the range of 600 to 970°C to quench it. A method for manufacturing a turbine rotor, comprising tempering in a temperature range of 700°C. 2. The turbine rotor-shaped element further contains Nb 0.01-0.15% and Ta 0.01-0.1% by weight.
5%, N 0.01-0.10%, B 0.002-0
．． 2. The method for manufacturing a turbine rotor according to claim 1, wherein at least one of 0.015% and 0.015% is contained.