JPH10226837A - Heat resistant steel for gas turbine disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the increase of fatigue strength, the increase of high temp. strength and the improvement of ductility for a gas turbine disk material by composing it of a compsn. contg. specified weight ratios of Ti, Nb, Cr, Fe, Al and C, in which the contents of C and N are prescribed by a specified inequality, and the balance Ni with inevitable impurities. SOLUTION: The componental compsn. of a heat resistance steel for a gas turbine disk is composed of the one contg., by weight, 1.0 to 2.0% Ti, 1.5 to 3.1% Nb, 10 to 20% Cr, 25 to 45% Fe and 0.5 to 2.0% Al, in which the contents of C and N are prescribed by an inequality, and the balance Ni with inevitable impurities. In the case the refining of the crystal grains is attained by increasing the content of C and increasing the content of Nb (C, N) in the Ni-Fe-Cr-Nb-Ti superalloy, deterioration in its strength caused by the reduction of Ni3 Nb (γ" phases) occurs, and Al is added thereto to prevent it. The increase of the temp. of a gas turbine disk and its high-reliability and made possible, and the increase of its combustion temp. and the increase of its scale, i.e., the improvement of its thermal efficiency are realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel heat-resistant steel, and more particularly to a heat-resistant steel used for a large forged product such as a gas turbine disk having a high combustion temperature and high efficiency.

[0002]

2. Description of the Related Art Industrial gas turbine disks are large and subject to high stresses such as centrifugal force and thermal stress.
Manufacturability and high strength are required for gas turbine disk materials. In conventional gas turbines, steel materials such as 12Cr steel and CrMoV steel excellent in these characteristics have been used. In recent years, in order to improve thermal efficiency, the combustion temperature of a gas turbine has risen, and the working temperature of a gas turbine disk has tended to rise accordingly. Since the strength of conventional steel materials significantly decreases at high temperatures, Ni-based alloys having excellent high-temperature strength have been used in gas turbines having high combustion temperatures. In particular, a Ni-Fe-Cr-Nb-Ti superalloy (IN706) is being widely used as an industrial gas turbine disk because a large forged product can be manufactured.

[0003]

SUMMARY OF THE INVENTION Conventional Ni-Fe-C
The r-Nb-Ti superalloy IN706 is made of Ni ₃ Nb (γ ″
Phase) Although it is a reinforced type alloy and excellent in high-temperature strength, there is a problem that ductility and fatigue strength are extremely low and reliability is low as compared with conventional steel materials. To improve ductility and fatigue strength, refinement of crystal grains is effective. Ni-Fe-Cr-Nb
Nb (C, N) precipitated in the -Ti superalloy has an effect of suppressing crystal grain coarsening during forging or heat treatment by a pinning effect and making crystal grains fine. For this reason, the C content is increased and niobium carbonitride (Nb (C, N))
By increasing the amount, the crystal grains are refined, but Nb in the parent phase is taken up by Nb (C, N) and reduced, so that Ni ₃ Nb (γ ″ phase) as a strengthening precipitate decreases and the strength increases. Also, Nb (C, C,
Although N) is increased, Nb is a segregating element, and it is known that an increase in Nb causes freckle defects particularly in large-sized parts.

[0004] If a process of sintering alloy powder and forming by isostatic pressing or the like is used, there is no need to consider workability and segregation, and a higher strength material can be used. Parts are also difficult to manufacture in terms of equipment.

[0005]

As described above, Ni-Fe
-Cr-Nb-Ti increases the amount of C with respect to superalloys IN706 Nb (C, N) amounts to suppress coarsening of crystal grains during the manufacturing process by increasing the case of working to grain refinement, Ni ₃ Nb (γ ″ phase) reduction in strength causes a problem.A number of studies have been conducted on this problem, and Al, which has been a trace addition element in the past, has been positively added, and the conventional gas turbine blade material and the like have been added. It has been found that this strength reduction can be compensated for by precipitating a large amount of the γ 'phase (Ni ₃ Al), which is a strengthening phase, that is, a γ ″ phase strengthened alloy, which is a conventional Nb compound, but an Al compound, γ相 By using a phase-strengthened alloy, the amount of C can be increased without reducing strength.
That is, the amount of Nb (C, N) can be increased, and the crystal grains are refined. Refinement of crystal grains by Nb (C, N) dispersion improves ductility and fatigue strength, and precipitation strengthening by γ 'phase enables strength maintenance or improvement.

Since the steel of the present invention can be manufactured by a conventional melting and refining process and hot forging, a large part can be manufactured at low cost by the present invention.

Hereinafter, the reasons for limiting the range of the components of the present invention will be described. The addition of C is important for increasing the amount of dispersion of Nb (C, N) and making the crystal grains fine. However, if added excessively, coarse Nb (C, N) at the time of solidification causes crystallization from the liquid phase and lowers ductility, so the amount of addition is desirably 0.2 (% by weight) or less. Further, when N is excessive, more coarse TiN is crystallized than in the liquid phase, so that the content is preferably set to 0.01 (% by weight) or less. In addition, an appropriate amount of Nb (C,
In order to precipitate N), the following conditions must be satisfied.

[0008]

0.15 ≦ [Nb (% by weight)] × [C (% by weight) + [N (% by weight)] ≦ 0.60 (Expression 2) To maintain the strength, the Al content is 0.75. (% By weight) or more, but if the added amount exceeds 2 (% by weight), ductility and forgeability deteriorate. By lowering the amount of Nb, a larger forged product can be manufactured. However, Nb is slightly dissolved in the γ 'phase, which is the strengthening phase, and strengthens. Further, as described above, it is an important element for forming Nb (C, N). From these balances, the Nb content is 1.0 (% by weight) or more and 3.1% or more.
(% By weight) or less. If Ti is too small, the γ 'phase used as a strengthening phase in the present invention will not be precipitated because Ti is a γ' phase stabilizing element, and if too large, a large amount of Ni ₃ Ti, which is a harmful phase, will be precipitated. (% By weight) or more and 2.0 (% by weight) or less,
1.5 to 2.0 (% by weight) is most preferable in terms of strength. In addition, excessive addition of Cr, which is an element for improving corrosion resistance, precipitates a harmful phase such as a δ phase and a σ phase.
(% By weight) and not more than 20 (% by weight).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION 50 kg of each sample having the composition shown in Table 1 was melted and kneaded, and a rod-shaped sample having a diameter of 200 mm was formed by hot forging at 982 ° C.

[0010]

[Table 1]

The test number 706A is a conventional disk material IN
706HC is equivalent to 706, and the C of the conventional material is increased. Hereinafter, 500A to 500D are materials of the present invention, and 500E and 500F are comparative materials.

After this sample was solution-treated at 1050 ° C. for 2 hours,
Aging treatment was performed at 720 ° C. for 2 hours and further at 650 ° C. for 8 hours. From this sample, a tensile test specimen having a parallel part diameter of 6 mm and a fatigue test having a parallel part diameter of 8 mm were taken and subjected to a tensile test, a fatigue test, and a structure observation (optical microscope, transmission electron microscope). Table 2 shows the results of the structure observation.

[0013]

[Table 2]

As a result of observation with an optical microscope, the parameter MC (M
In a sample having a large C = [Nb (% by weight)] × [C (% by weight)] + [N (% by weight)], the dispersion amount of Nb (C, N) is large, and as a result, the crystal grains are fine. there were. However, in the samples of test numbers 500E and 500F, coarse Nb (C, N) which seems to be crystallized from the liquid phase was frequently observed. As a result of transmission electron microscopy, the main precipitate of the conventional material was the γ 'phase, whereas the main precipitate of the invention material was the γ' phase.

Table 3 shows the results of the tensile test.

[0016]

[Table 3]

706HC with increased C compared to conventional material
Although the ductility is improved, the strength is remarkably reduced due to the decrease in the γ ″ phase. Inventive materials 500A to 5A which are refined by Nb (C, N) and strengthened by the γ ′ phase
In the case of 00D, elongation and drawing are greatly improved while maintaining almost the same strength as the conventional material. FIG. 3 shows the relationship between the parameter MC and the elongation. The parameter MC in which an appropriate amount of Nb (C, N) is precipitated is 0.15 to 0.6.
In the range, good elongation is shown. Coarse Nb (C,
The elongation sharply decreases when the parameter MC where N) is precipitated is 0.6 or more. Even if the MC parameter is in an appropriate range, the test number 706HC has an Al content of 0.75 to
The strength is low because 2.0 is not within the appropriate range.

FIG. 4 shows the results of a high-temperature tensile test of the conventional material of the invention material. The invention material shows a small decrease in strength particularly at high temperatures.
This is a characteristic of the γ 'phase strengthened alloy. FIG. 4 shows the results of the fatigue test. The inventive material 500C refined by Nb (C, N) has greatly improved fatigue characteristics over the conventional material.

[0019]

According to the present invention, it is possible to increase the fatigue strength, the high-temperature strength, and the ductility of the gas turbine disk material, to increase the temperature and reliability of the gas turbine disk, to increase the combustion temperature of the gas turbine, and to increase the size of the gas turbine disk. That is, improvement in thermal efficiency can be realized.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram of tensile strength of a conventional material and an inventive material at room temperature.

FIG. 2 is a characteristic diagram of drawing and elongation of a conventional material and an inventive material at room temperature.

FIG. 3 is a characteristic diagram of a relationship between a parameter MC and elongation.

FIG. 4 is a characteristic diagram of a high cycle fatigue test result of the invention material and the conventional material.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeyoshi Nakamura 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Hiroshi Fukui 7-1 Omikamachi, Hitachi City, Ibaraki Prefecture No. 1 Inside the Hitachi Research Laboratory, Hitachi, Ltd.

Claims (2)

[Claims] 1. The method according to claim 1, wherein Ti. 1.0 to 2.0, Nb. 1.
5-3.1, Cr. 10 to 20, Fe. 25-45, A
l. 0.5 to 2.0, a heat-resistant steel for gas turbine disks, characterized in that the C content and the N content are defined by the following formula, and the balance consists of Ni and unavoidable impurities. 0.15 ≦ [Nb (% by weight)] × ([C (% by weight)] + [N (% by weight)]) ≦ 0.60 (Equation 1) 2. The heat-resistant steel for gas turbine disks according to claim 1, wherein the steel is formed by hot forging and precipitation strengthened by a Ni ₃ Al phase.