JPS61243143A - Superplastic co alloy and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a novel superplastic Co alloy by precipitating lump and granular carbide having a specified grain size in the matrix of a Co alloy having a specified grain size. CONSTITUTION:The composition of a molten Co alloy is composed of, by weight, 15-40% Cr, 0.15-1% C and >40% Co. The molten Co alloy is solidified at >=10<2>K/sec cooling rate to regulate the secondary dendrite arm spacing to <=10mum. The resulting Co alloy is aged at a prescribed temp. to precipitate carbide on the surface. The composition of the molten Co alloy may be composed of, by weight, 0.15-1% C, 15-40% Cr, 3-15% W and/or Mo, <1% B, 0-20% Ni, 0-1% Nb, 0-1% Zr, 0-1% Ta, 0-3% Ti, 0-3% Al and the balance Co.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a novel Co-based superplastic alloy and a method for producing the same.

[Background of the invention]

Carbide precipitation strengthened Co-based superalloy, jet engine,
It is used for desks or nozzles in gas turbines, etc., and cannot be forged due to its high strength, so it is manufactured into a product shape by precision casting.

However, it is known that even in superalloys that are difficult to process, if the grain size is reduced, superplastic deformation occurs and ductility improves. In particular, it is clear that r/precipitation-strengthened superalloys exhibit superplasticity.

Cast superalloys are manufactured using powder metallurgy, as grains cannot be refined using conventional melting methods. The smaller the grain size, the better the properties against superplastic deformation, but in conventional powder metallurgy methods, it is difficult to rapidly cool the molten metal, making it impossible to refine the powder. It is difficult to reduce the particle size to 10 μm or less.

Furthermore, it is known that sufficient superplasticity cannot be obtained in a low temperature range simply by reducing the grain size.

Cobalt-based alloys containing a large amount of Cr and reinforced with carbides have excellent corrosion resistance and high-temperature strength, and are therefore used as high-temperature structural materials. Chromium contained in such a cobalt-based alloy is an element that improves corrosion resistance, and on the other hand, C combines with chromium to form an element that improves corrosion resistance.
At the same time, chromium and carbon form eutectic carbides during solidification.

The eutectic carbide with the ferrite is the cause of the occurrence of shift, which is a base of stress concentration. Furthermore, since these eutectic carbides are distributed in an acute radial pattern, they are the main cause of extremely deteriorating properties such as ductility, toughness, and thermal fatigue. Therefore, it is a carbide that is extremely harmful to superplastic properties.

When producing cobalt-based superalloy powder by conventional powder metallurgy, the powder particle size is usually about 100 μm, and the crystal grain size within the powder grain is several μm.

The cobalt-based superalloy powder produced in this way is solidified using hot isostatic pressing and formed into a product of a predetermined shape, but the temperature is usually around 1100C, so the crystal grain size in the powder is changed during processing. Due to the recrystallization phenomenon, crystal grains grow to a crystal grain size of 20 μm or more, and conventional cobalt-based alloys have a problem in that superplasticity does not occur.

[Purpose of the invention]

An object of the present invention is to provide a cobalt-based superplastic alloy that can be plastically worked even in a low temperature range and a method for producing the same.

[Summary of the invention]

The first aspect of the present invention is characterized in that massive and granular carbides having a grain size of 0.5 to 10 μm or less are precipitated on a base of a cobalt-based alloy having a crystal grain size of 10 μm or less.

A second aspect of the present invention is to solidify a molten cobalt-based alloy that forms carbides by aging treatment at a cooling rate of 10"K/sec or more so that the secondary dendrite arm spacing is 10 μm or less, and the cobalt-based alloy is After aging treatment at a specified temperature,
This is a method for producing a cobalt-based superplastic alloy, characterized by precipitating carbides with a grain size of 0.5 to 10 μm.

The composition of the cobalt-based alloy that is the object of the present invention is as follows: C+0.15 to 1%, Cr: 15 to 40t in weight ratio
s and the remainder Co, and if the components are further limited, the weight ratio is C+0.15 to 1.

Cr r 15-40%, W and or MO: 3-15L
B+ 1% or less, Ni+ 0-20%, Nb
: 0~1.0%, Zr+0~1.0'1
6. Ti r O ~ 1.0 chi.

It consists of Ti+O~3%, Az+0~3%, and the balance Co. In particular, carbides precipitated in the matrix of cobalt-based alloys mainly contain Cr! , C6, and preferably contains 15 to 40% of chromium and 0.15 to 1% of carbon in the base.

The reason why the cobalt-based superalloy, which is the subject of the present invention, is limited to the above composition range will be described below.

C is an element that strengthens the base of the cobalt-based alloy, and has a content of 0.
If C is less than 151, high strength cannot be obtained, while if it exceeds l-, it is unfavorable from the viewpoint of weldability and embrittlement.In particular, C is 0.
A range of 2 to 0.4% is preferred.

Cr is an element that improves the corrosion resistance of the matrix of cobalt-based alloys, and its effect is small at 15- or less, while at 40-
If the chromium content exceeds 1, the toughness will deteriorate.
A range of 5 to 40% is preferred.

W and Mo are elements necessary to prevent coarsening of carbides and improve high-temperature creep strength, and 3%
Below that, the effect is small, and when it exceeds 15%, W
, Mo is produced, so W and Mo are limited to a range of 3 to 15%.

B is an element that, when added in a small amount, changes the crystal grain boundary structure into a ductile structure and is useful for improving the toughness of the material, and is preferably 1% or less. Particularly desirable is 0.001 to 0.1 inch.

Ni is an effective element for improving material strength, and from the viewpoint of high toughness, it is preferably 20 or less.

Nb, Zr, and Ti are elements that form fine secondary carbides with carbon and improve material strength, and are preferably 1 Ti or less. In particular, 0,1 to O, S* is good.

These can be used alone or in combination of two or more, such as Ti+Nb, Ti+Nb+Zr, Ti+'ra, T
There are combinations such as i+Ta+Zr.

Tt is an element that is necessary as a deoxidizing agent 2 and improves the strength of many materials by combining with nickel and precipitation strengthening,
3 inches or less is preferable.

T1 is an element that forms fine carbides and improves material strength by combining with Ni like aluminum, and is
．． 1 to 0.81 is preferred.

Co is a basic component of the cobalt-based superplastic alloy of the present invention, and is preferably 40 or more, particularly preferably 60 or more, for solid solution strengthening.

Table 1 shows examples of typical cobalt-based alloys, which have compositions that correspond to the alloys of the present invention.

The cobalt-based superplastic alloy of the present invention has the above-mentioned composition, has a crystal fine grain size of 10 μm or less, and contains massive and spherical carbide precipitates with a carbide grain size of 0.5 to 10 μm or less. This is an essential condition. The superplastic phenomenon does not occur when the molten metal solidifies, and it is necessary to adjust the size of the carbide to 0.5 to 10 μm by heat treatment or the like.

In general, superplasticity is not exhibited when the crystal grain size is larger than 10 μm, but on the other hand, even if the crystal grain size is small, carbide
．． In the case of a small diameter of 5 μm or less, superplasticity is not exhibited. on the other hand,
When the size of carbides becomes larger than 10 μm, superplasticity is no longer exhibited. This is because when carbides become coarser than 10 μm, segregation of carbides occurs within the alloy, resulting in a non-carbide state in some parts. Generally, carbides are preferably uniformly dispersed in the alloy. This is because the base crystal grains of the alloy are prevented from becoming coarse by uniformly dispersing them within the alloy.If the carbides exceed 10 μm, there will be areas where no carbides exist, and the crystal grains will become coarser there. The grain size is 10
When the thickness exceeds μm, superplasticity is no longer exhibited. From the above, it is preferable that the grain size of the carbide precipitated in the base of the cobalt-based alloy is 0.5 μm to 10 μm or less. In this way, the superplastic phenomenon is caused by the interaction between the size of carbides and the grain size.

Cobalt-based superplastic alloy value of the present invention The crystal grain size of the cobalt-based alloy base is 10 μm or less, and the carbide size is t-
o, s ~ 10 μm.

Next, a method for manufacturing a cobalt-based superplastic alloy will be described.

This method of producing a cobalt-based superplastic alloy essentially prevents the formation of harmful eutectic carbides by rapidly cooling and solidifying a molten cobalt-based alloy, and the crystal grain size is t-10.
It is characterized by adjusting the particle size of the carbide to 0.5 to 10 μm or less by heat treatment. The cooling rate during rapid solidification from this molten metal is 10”K.
/Bee or more, and the distance between the second and second arm during solidification corresponds to 10 μm or less, thereby obtaining a product with high superplasticity. In particular, it is preferable that the interval between the second punto arms is 1 μm or less. A specific method for cooling and solidifying the molten cobalt-based alloy at a cooling rate of 10" K/sec or more involves pouring the molten metal onto the surface of a roll rotating at high speed and applying l O' K/sec to the surface of the roll.
There is a method of rapidly solidifying at a speed of Sec or more.

If the molten metal is solidified at a cooling rate of IO'/see or higher in this manner, a crystal grain size of 10 μm or less can be obtained. Furthermore, the present invention is to age a cobalt-based alloy obtained by rapid solidification at a predetermined temperature to precipitate carbides of 0.5 to 10 μm in a state where the crystal grain size is 1071 m or less. Note that the roll has a polished surface, and the molten metal is poured into the surface. Furthermore, the roll is preferably formed from a metal with high thermal conductivity. Carbide particle size t
- The aging temperature for even adjustment is preferably in the range of 600 to 1000C. The grain size of the carbide can be adjusted to 0.5 to 10 μm by adjusting these temperature ranges and times. The atmosphere during this aging treatment is preferably an inert atmosphere. In this way, the cobalt-based alloy obtained by the method of the present invention is made by precipitating massive and granular carbides with a grain size of 0.5 to 10 μm or less on an alloy matrix with a crystal grain size of 10 μm or less. This is a characteristic feature.

[Embodiments of the invention]

The present invention will be explained in detail below.

Table 2 shows the chemical composition (weight qk) of test materials of the alloy of the present invention.

For the alloys shown in Table 2, 1500 tons of molten metal was poured directly between rotating rolls and solidified to produce a ribbon.

The shape of the obtained thin body has a length of 2000 ws-, @8~1
0 Wms thickness 70~xoo#m. In this case, the cooling rate of the molten metal from molten metal to solidification is 5X
It took 7 seconds for 10' to 5X10'.

FIG. 1 is a transmission electron micrograph showing the metal structure of No. 11 cobalt-based alloy that was poured and solidified.

As shown in the figure, the crystallinity of the cobalt-based alloy of the present invention is approximately 1 μm and 10 μm or less.

FIG. 2 is a high-magnification transmission electron micrograph of FIG. 1.

As can be seen from the figure, not only does eutectic carbide not form, but the formation of carbide itself is suppressed. This is because the examples of the present invention were solidified to 5X10' to 5XIO' at a cooling rate of 7 seconds, which simultaneously controls the grain size of the carbides within a predetermined range during subsequent heat treatment. This is an important part.

No. 3 is a photograph of the microscopic structure of a cobalt-based alloy melted and cast at a conventional slow cooling rate. The formation of is observed. The formation of this carbide has a superplastic property that is inherently harmful, and its size is 1
It is much larger than 0 μm, and it is clear that the conventional method does not essentially satisfy the carbide shape exhibiting superplastic performance.

FIG. 4(A) shows the cobalt-based alloy of the present invention aooc1
h Metal structure of the aged alloy, (B) shows the same alloy at 1000
FIG. 2 is a metallographic micrograph of a C1h aged alloy. (
In A), there is a small black Cr1IC at the grain boundary triple point.
In tLX (B), precipitation of Cr1IC6 was observed, and since aging was performed at a high m degree of 1000t:', Cr1IC6 has grown significantly. What is important here is that, as shown in Figure 2, the Co-based superplastic alloy of the present invention suppresses the formation of carbides in the solidified +11 state, and the size of the carbides can be freely controlled by subsequent heat treatment. It is something that can be controlled. And Sara K10
Even in FIG. 4(B), which was aged for 1 hour at 0OC, the carbide diameter was approximately 0.2 μm, which did not increase rapidly and the size could be freely controlled. On the other hand, in the conventional cobalt alloy as shown in FIG. 3, it is essentially impossible to control the size of carbides as shown in the present invention.

In the above, it has been clarified that the cobalt-based alloy produced by the production method of the present invention is excellent in controlling the carbide size, and this indicates that the cooling rate in the production method of the present invention is 102%.
This is because it is K/81IC or higher. The secondary dendrite arm spacing of the cobalt-based superplastic alloy according to the present invention is about 0.5-1 μm. An example of the 8EM observation image is shown in the 5th section.
As shown in the figure. Note that this secondary dendrite arm interval t
-The cooling rate from molten metal to solidification can be predicted by measuring, but in the case of the present invention, 5 X l G
'~5 X I O"K/8eC. As the cooling rate increases, the crystal grains tend to become finer, but
The alloy of the present invention also has fine crystal grains of about 1 μm.

When a superplastic phenomenon is caused in the Co-based ultra-vague alloy of the present invention, the superplastic phenomenon does not occur in the state at the time of production. In order to generate carbides, it is necessary to precipitate carbides in the alloy and make the size of the carbide t O,S μm or more and 10 μm or less.

FIG. 6 is a diagram showing the results of a tensile test at 900C and 950C for an alloy obtained by rapidly solidifying a molten Co-based alloy of the present invention. The sample was examined using a thin strip 70μ thick and 20W wide, stretched to 70mm. Superplasticity is the deformation stress O
In the equation α) that holds true between and strain rate, m
is said to occur frequently when α3 or more.

m=に＃・・・−・・・・・・・・・・・・
(1) Note that K is a material constant and m is a strain rate sensitivity index.

In the figure, the slope corresponds to m. When this m is 0.3 or more, superplasticity is exhibited. In Figure 6, the dotted line shows the results of the as-rolled product.
When performing a tensile test with C, m is as small as 0.16, and
Even at 950C, m is 0.28 and does not exhibit superplasticity. On the other hand, in the case where the ribbon was aged at 100Oc x 6h (solid line), carbide was grown to a thickness of 1 μm or more before the tensile test. The test result at 950C is m=
0.37 and 0.3 or more, indicating superplasticity. 9 more
The one of 00C is also m=0.24 and natte as rolle
The material approaches a state where it exhibits greater superplasticity than the d material. The inventor was able to clarify that this difference in the occurrence of superplastic phenomena is caused by the size of the carbide.

FIG. 7 shows the structure of FIG. 6 after the test at 950C. (A) as roll which did not show superplasticity
d, but the carbides are as small as 0.1 μm. (B
) has been heat treated at 1000C x 6h in advance, but the carbide has a thickness of 1 μm as shown by the Crtroll arrow.
It becomes larger than above, and m becomes 0.37, indicating superplasticity. As has become clear from the above explanation, the size of the carbide holds an important key to the generation of superplasticity, and is the most important part of the present invention. Generally, superplasticity is not exhibited unless the crystal grain size is small, 10 μm or less, but even if the crystal grain size is small, if the carbide is small, 0.5 μm or less, superplasticity is not exhibited as shown in Figure 7. do not have. In such cases, heat treatment should be performed in advance to reduce the carbide to a thickness of 0.5 to 10 μm.
If the size is adjusted to , superplasticity will be expressed. In addition, in the case of powder with small crystal grains, when solidified under certain conditions, carbides may grow to the size shown in the present invention and exhibit superplasticity. However, in this case, heat treatment for carbide adjustment was performed at the same time as solidification, and the size of the carbides was made to be 0.5 to 10 μm as shown in the present invention, so it exhibits superplasticity, and essentially corresponds to the content of the present invention. On the other hand, if the thickness of the carbide is increased to 10 μm or more, superplasticity will not be exhibited. This is because when carbides become coarser than 10 μm, segregation occurs in the distribution of carbides within the alloy, resulting in a non-carbide state in some parts. It is generally advantageous for the carbides to be uniformly distributed within the alloy. This is because the alloy base crystal grains are prevented from becoming coarser by uniformly distributing them within the alloy, and the carbides are 10 μm thick.
If it exceeds the range, carbide-absent areas occur, where the crystal grains become coarse and have a diameter of 10 μm or more, and as a result, superplasticity is no longer exhibited. From the above, the size of the carbide is 0.5μ
The crystal grain size of the alloy base is preferably 10 μm or more and 10 μm or less.
A value of less than μm is advantageous. Furthermore, it can be seen from the above description that the carbide size and the alloy matrix grain size are interrelated, and these factors also closely affect the method of manufacturing the alloy. An embodiment of the present invention has been described above based on a test material produced by a roll method, but the essence of the present invention is to reduce the grain size of the alloy base to 10/Jm or less and to reduce the size of carbides to 0. It is obvious that the essence of the present invention falls within the essence of the present invention, as long as the thickness is 5 to 10 .mu.m, and any manufacturing method that satisfies this requirement is acceptable.

〔Effect of the invention〕

As described above, the present invention provides a Co-based superplastic alloy that exhibits superplasticity even in a low temperature range, has an elongation rate of 701 or more, and is capable of producing complex-shaped objects by plastic working such as forging. can do.

[Brief explanation of drawings]

Fig. M1 is a transmission electron micrograph showing the microstructure of the cobalt-based superplastic alloy of the present invention, Fig. 2 is a high-magnification transmission electron micrograph of Fig. 1, and Fig. 3 is a micrograph of the conventional cobalt-based alloy. Photographs, Fig. 4 (A) and (B) are 600Cxlh and 1000CXlh of cobalt-based alloys of the present invention.
Fig. 5 is a scanning electron micrograph showing an example of an 8EM observation image of the cobalt-based superplastic alloy of the present invention, and Fig. 6 is a photomicrograph showing the aged metal structure of the cobalt-based superplastic alloy at 900C and 950C. A diagram showing the tensile test results, Figure 7 shows the Co after the test at 950C in Figures 6 (A) and (B).
A metallographic diagram of the base alloy @Patent applicant Director of the Agency of Industrial Science and Technology Todoro Labor Federation 1μm・σ, 3), t, vL) 143 eyes! ρμ Ishi-1111” (A (5-a/3P wide procedure amendment form) 1986, month, lb, day

Claims (1)

[Claims] 1. Co characterized by being formed by precipitating massive and granular carbides with a grain size of 0.5 to 10 μm or less on a cobalt-based alloy base with a crystal grain size of 10 μm or less. Base superplastic alloy. 2. In claim 1, the cobalt-based alloy has a weight ratio of Cr: 15 to 40% and C: 0.15 to 1.
% and 40% or more of Co. 3. In claim 1, the cobalt-based alloy has a weight ratio of C: 0.15 to 1% and Cr: 15 to 40.
%, W and/or molybdenum: 3 to 15%, B: 1% or less, Ni: 0 to 20%, Nb: 0 to 1.0%, Zr: 0 to
1.0%, Ta: 0-1.0%, Ti: 0-3%, Al
:0 to 3%, and the balance is Co.
O-based superplastic alloy. 4. In claims 1 to 3, the cobalt-based alloy has a cooling rate of 10^ from molten metal to solidification.
A Co-based superplastic alloy characterized in that it has a velocity of 2K/sec or more. 5. The molten Co-based alloy that forms carbides through aging treatment is solidified at a cooling rate of 10^2 K/sec or more to make the secondary dendrant arm gap 10 μm or less, and the Co-based alloy is aged at a predetermined temperature. A method for producing a Co-based superplastic alloy, which comprises precipitating carbides having a grain size of 0.5 to 10 μm.