JP3279949B2 - Precipitation strengthened superalloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a precipitation-hardened superalloy used for gas turbine disks for power generation, and more particularly to
The present invention relates to a precipitation strengthened superalloy suitable for large members.

[0002]

2. Description of the Related Art Precipitation-strengthened Ni-base superalloys have high structural stability from very low temperatures to high temperatures and good mechanical properties, and therefore are widely used in various generator members and aircraft members. Have been. Gamma prime phase in a matrix of austenite in such superalloys (hereinafter referred to as gamma) (Ll ₂ structure FCC-like, referred to hereinafter as gamma ') or / and gamma double prime phase (BCT like the D
Strengthening of the alloy by consistently and finely precipitating a so-called O ₂₂ structure, hereinafter referred to as γ ″) or / and a co-precipitation phase of γ ′ and γ ″ (hereinafter referred to as γ′−γ ″). Therefore, the strengthening ability of the alloy is γ ′
Alternatively, it is determined by the amount, size, etc. of γ ″. As such a material, a superalloy called Inconel 718 alloy (trademark of Incoalloys) has been widely used.

In recent years, gas turbine members for power generation have been increasing in size with an increase in power generation efficiency. As in this example,
The demand for superalloy products larger than before has been increasing. However, it has become clear that there are various problems in producing a large superalloy product using Inconel 718 alloy. That is, such a member is generally manufactured through processes such as ingot production, forging, heat treatment, and machining, but when a large superalloy product is manufactured using Inconel 718 alloy, the ingot production is performed. It is not practically applicable due to various problems such as segregation at the time of welding, hot workability at the time of forging, temperature uniformity at the time of heat treatment, and workability at the time of machining. Was.

Accordingly, a new superalloy called Inconel 706 alloy (trademark of Incoalloys Inc.) was developed. This alloy has reduced segregation as compared with Inconel 718 alloy by improving the composition, and is excellent in hot workability and machinability. Also, Inconel 71
Compared with the 8 alloy, the expensive Ni is reduced by about 10% and the inexpensive Fe is increased by that amount, so that it is a preferable improvement from the viewpoint of price, which is particularly important for large members. In fact, it has been industrially confirmed that it is suitable for manufacturing large members.

[0005]

However, recently,
The demand for larger size has been increasing. The demand for larger components is increasing, and the current Inconel 7
In some cases, it has been practically difficult to apply even the 06 alloy. In particular, a major problem relates to the production of large steel ingots, and is a problem caused by component segregation. For example, it may cause cracking during forging in a post-process or fail to obtain predetermined mechanical properties, which is a major problem in manufacturing.

[0006] From such a viewpoint, by further improving the composition of the Inconel 706 alloy, an alloy having similar mechanical properties and excellent in manufacturability of a large member has been developed. For example, in JP-A-5-214494,
An alloy is disclosed in which the amount of added Ti and Nb is reduced to facilitate the production of large ingots. However, although this alloy is excellent in manufacturability, it has a drawback that the mechanical properties are inferior to those of the conventional alloy because the content of the precipitation strengthening element is reduced. Similarly, JP-A-7-11376 and JP-A-7-332035 disclose similar alloys, but their mechanical properties are inferior to conventional alloys due to the small amount of Nb added.

On the other hand, JP-A-3-97823 and JP-A-4
No. 341538 discloses an alloy in which the amount of Nb added is equal to or greater than that of a conventional alloy, with emphasis on mechanical properties. However, in such a case, on the contrary, the manufacturability is inferior to the conventional alloy, and it is substantially difficult to manufacture a large-sized member.

As described above, although many proposals have been made for precipitation-strengthened superalloys for large-sized members, they have the same mechanical properties as conventional alloys, and are capable of manufacturing large-sized members. At present, there is no alloy yet. The present invention has been made in view of such circumstances, and has as its object to provide a precipitation-strengthened superalloy having mechanical properties similar to those of Inconel 706 alloy and excellent in manufacturability of large members. Is to do.

[0009]

In order to achieve this object, the present inventors have conducted an evaluation test on mechanical properties and large-parts manufacturability using many test materials based on Inconel 706 alloy. went. As a result, Inconel 70
The present inventors have found a novel alloy composition which can provide a precipitation-strengthened superalloy having the same mechanical properties as the alloy No. 6 and excellent in productivity of large-sized members, and have reached the present invention.

That is, the first invention of the present invention is:
By weight%, Ni: 35 to 45%, Cr: 15 to 25%,
Al: 1.0% or less, Ti: 2.0 to 4.0%, Nb:
A precipitation-strengthened superalloy containing 1.5 to 3.0%, with the balance being substantially iron, excluding unavoidable impurities. The second invention provides, in addition to the above components, B: 0.00
The present invention is a precipitation-strengthened superalloy characterized by containing 1 to 0.020%, and the third invention further comprises Mo: 0.05 to 2.0% and W: 0 in addition to the first or second invention. .1 to 4.0
% Of one or more kinds of precipitation-strengthened superalloys.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the limited ranges of the components in the present invention will be described in detail.

Ni stabilizes γ which is a matrix of the alloy, and combines with Al, Ti and Nb to form γ ′ and γ ″.
Is an important element that forms In order to exert its effect, the addition amount must be at least 35% or more. However, since Ni is an expensive metal, an excessive amount of addition causes an increase in the price of the entire alloy, and becomes more expensive than conventional alloys. Therefore, the addition amount is limited to the range of 35 to 45%. For the same reason, it is desirable to set the lower limit to 40% and the upper limit to 44%.

[0013] Cr is an element indispensable for improving the oxidation resistance, corrosion resistance and strength of the alloy. At the same time, it is an indispensable element for maintaining non-magnetism. In order to exhibit these effects, the addition amount must be at least 15% or more. However, an excessive amount of addition impairs the stability of the matrix γ, precipitates a TCP phase such as the σ phase, and adversely affects the ductility. Therefore, the added amount is 15 to 2
Limited to the range of 0% . The lower limit is set to 16 for the same reason.
It is desirable to exceed %.

Al combines with Ni to precipitate γ ', which contributes to strengthening of the alloy. However, in this alloy system, Ti more effectively contributes to γ 'formation than Al . Addition of too much Al results in coarsening of γ ', and concomitantly lowers ductility. Therefore, the amount of addition is limited to the range of 1.0% or less. For the same reason, the upper limit is more preferably set to 0.6%.

[0015] Ti performs the most important action among the strengthening precipitation elements. Ti, like Al, combines with Ni to precipitate γ 'and contributes to strengthening of the alloy. At the same time we have Nb
Has been found to be able to replace Ti at the Nb site of γ ″ precipitated by bonding with Ni, since Ti alone can play most of the strengthening ability of this alloy. , Too much addition is eta (HCP
like DO ₂₄ structure, hereafter referred to as η)
Decreases ductility. Therefore, the amount of addition is 2.0 to
Limited to the range of 4.0%. For the same reason, it is desirable to set the lower limit to 2.5% and the upper limit to 3.0%.

In another sense, Nb is a strengthening precipitation element that must be added. This is because Nb combines with Ni and precipitates γ ″. Since the matching strain with γ, which is a factor of strengthening, is larger in γ ″ than in γ ′, γ ″ is more effective. Accordingly, the alloy can be strengthened, so that the more Nb is added, the more effectively the alloy is strengthened. It has been found that an intermetallic compound, for example, a TCP phase called LAVES has been precipitated, and this precipitation of the TCP phase may cause cracking at the time of forging as described above or may not obtain predetermined mechanical properties. Because Nb is the heaviest element among the strengthening elements, it tends to stay at the lower part of the liquid phase during the production of the steel ingot, that is, during the solidification, and as a result, the most segregation occurs. This is because these elements are easy to perform. To reduce Nb
The smaller the addition amount of Nb, the better, in other words, the smaller the addition amount of Nb, in order to produce a large steel ingot. Thus, Nb has completely opposite effects on the two requirements of producing a large ingot and maintaining mechanical properties.

However, as described above, the present inventors have found that once γ ″ is formed, Ti
Was found to be replaceable. Therefore, by adding the minimum amount of Nb that forms γ ″ and simultaneously adding the amount of Ti to be replaced in advance, the segregation can be reduced and large-sized steel ingots can be manufactured without deteriorating mechanical properties. In view of the above, the amount of Nb added is limited to the range of 1.5 to 2.5% , and as apparent from the above description, the amount of Nb added is 1.5% . If the amount is less than γ ″, the mechanical properties are degraded because γ ″ is not formed. Conversely, if the addition amount exceeds 2.5% , segregation is remarkably promoted, which is unsuitable for the production of large ingots. . The same is desirable to 1.7% Vietnam the lower limit because further be desirable to 2.0% lower limit.

Since B segregates at grain boundaries and contributes to high-temperature characteristics, B is desirably added to the alloy of the present invention. Also, η
It is an effective element because it delays the transformation to Ti, particularly when the amount of Ti added is high (for example, 3.0% or more). However, too much addition tends to form borides, which may lead to grain boundary embrittlement. Therefore, even if it is added, its amount is limited to the range of 0.001 to 0.020%.

Mo and W are effective as a so-called solid solution strengthening element for forming a solid solution in γ to strengthen the matrix. Therefore, it is desirable to add Mo or W to the alloy of the present invention. At the same time, it delays transformation to η similarly to B, and is therefore an effective element particularly when the amount of Ti added is high (for example, 2.8% or more).
However, since Mo and W are also heavy elements like Nb,
Excessive addition promotes segregation and adversely affects large ingot production. Therefore, even when Mo is added, Mo: 0.05
To 2.0%, W: 0.1 to 4.0% or less. For the same reason as above, it is desirable to set the lower limit of Mo to 0.5% and the upper limit to less than 1.5%. For the same reason as above, it is desirable to set the lower limit of W to 1% and the upper limit to 3%.

[0020]

Embodiments of the present invention will be described below in detail. A 50 kg steel ingot obtained by melting a test material having the composition shown in Table 1 by a vacuum induction furnace was subjected to diffusion heat treatment, and then hot forged into a sheet material having a thickness of 30 mm and a width of 120 mm. A test piece taken from this plate material was subjected to a solution treatment at 980 ° C. for 3 hours, followed by a two-stage aging treatment at 720 ° C. for 8 hours and subsequently at 620 ° C. for 8 hours.

[0021]

[Table 1]

Each of the heat-treated test pieces was subjected to a tensile test at room temperature. Fig. 1 shows the tensile strength obtained from the results.
Shown in As is clear from FIG. 1, it is clear that the alloy of the present invention has almost the same or higher strength as the conventional alloy. It is also clear that the comparative material is inferior in strength to the conventional alloy and the alloy of the present invention.

Similarly, a test material was sampled from the test material, and a sample was produced by unidirectional solidification from the bottom surface by a unidirectional solidification device. A composition analysis was performed at each position of the solidified sample, and a component change rate per unit length was determined for each component.
Here, when the maximum width of the component change rate of the comparative material is 100, the maximum width of the component change rate of each test material is defined as the segregation tendency. That is, it means that the smaller the value of the segregation tendency is, the less the segregation in the actual steel ingot occurs. FIG. 2 shows the segregation tendency thus determined. It is clear that the alloy of the present invention has a smaller segregation tendency than the conventional alloy.

[0024]

As is apparent from the above description, according to the present invention, there is provided a precipitation-strengthened superalloy having the same mechanical properties as Inconel 706 alloy and having excellent manufacturability of large members. can do. Therefore, it is industrially extremely useful because it is a precipitation-strengthened superalloy particularly suitable for large-sized members such as gas turbine disks for power generation.

[Brief description of the drawings]

FIG. 1 shows the tensile strength of each test material obtained from the tensile test result of a test piece.

FIG. 2 shows segregation strength of each test material obtained from a unidirectional solidification test.

────────────────────────────────────────────────── ─── Continued on the front page (72) Mikio Kusuhashi, Inventor: 4, Chazu-cho, Muroran City, Hokkaido Inside Nihon Steel Works, Ltd. (56) References JP-A-61-147836 (JP, A) JP-A-56-20148 ( JP, A) JP-A-46-1005 (JP, A) JP-A-60-46343 (JP, A) JP-B-44-27138 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , (DB name) C22C 19/00-19/07 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. Ni: 35 to 45% by weight, Cr:
15-20% , Al: 1.0% or less, Ti: 2.0-
4.0%, Nb: 1.5 to 2.5% , the balance being substantially iron except for inevitable impurities. 2. The composition further comprises, by weight: B:
2. The precipitation-strengthened superalloy according to claim 1, comprising 0.001 to 0.020%. 3. The composition further comprises:
The precipitation-strengthened superalloy according to claim 1, comprising one or two kinds of o: 0.05 to 2.0% and W: 0.1 to 4.0%.