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(11) | EP 0 460 678 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Nickel-based heat-resistant alloy for dies |
| (57) Disclosed herein is a nickel-based heat-resistant alloy for dies containing: 0.01-0.5
wt% Zr, 0.04-0.2 wt% Mn, 0.04-2.5 wt% Si, at least one selected from the group consisting
of 3.0-8.5 wt% Al, 1.7-4.8 wt% Ti and 0.87-2.5 wt% Nb, and one or both of 13-25.0
wt% Mo and 6.7-13 wt% W, and having 30-88% by volume of the y phase, 12-60% by volume
of the γ' phase and 2.5-11% by volume of the α phase, the balance being essentially
Ni. The alloy may further contain at least one selected from the group consisting
of 0.03-2 wt% Hf, 0.3-3 wt% Ta and 3-10 wt% Cr. The alloy has excellent strength,
ductility and oxidation resistance, and is excellent in high-temperature strength
at 1000°C or above. Therefore, the nickel-based heat-resistant alloy for dies exhibits
high performance as a material for dies to be used for forging at high temperatures. |
BACKGROUND OF THE INVENTION
(1) Field of the Invention
[0001] This invention relates to nickel-based heat-resistant alloys for dies, and more particularly to a nickel-based heat-resistant alloy for dies which is usable, for instance, in isothermal forging of heat-resistant alloys, namely, in forging heat-resistant alloys in dies heated at high temperatures.
(2) Description of the Prior Art
[0002] It is well known that Ni-based superalloys are formed into turbine disks or the like by the superplastic working process. Dies for isothermal forging used in the process are subjected to severer conditions than are dies for ordinary hot forging.
[0003] That is to say, in ordinary forging, the dies are always cooled in use for forging a heated work. Therefore, even if the actual forging temperature is 1000° C or above, the dies are not brought to such a high temperature.
[0004] In isothermal forging, on the other hand, the dies must be maintained at the same temperature (constant temperature) as that for the work, and therefore cannot be cooled as in the ordinary forging. Thus, the dies are inevitably brought to exactly the same high temperature as the actual forging temperature of 1000 to 1150°C. In addition, due to the slow working rate in the isothermal forging, the dies used in the forging are kept in the high-temperature condition for a long time, which is about 10 hours per batch in actual forging cut.
[0005] As a material for the dies required to have sufficient high-temperature strength for enduring the high temperatures for the long time as mentioned above, therefore, there has hitherto been used an Mo-based alloy (TZM).
[0006] However, the Mo-based alloy is poor in oxidation resistance and the Mo oxide formed on the surface of the alloy is evaporated at about 600 C, so that the dies made of the Mo-based alloy is gradually lost during isothermal forging. In practice, therefore, the forging by use of Mo-based alloy dies has been carried out in a vacuum chamber or in an inert gas atmosphere.
[0007] Accordingly, isothermal forging by use of Mo-based alloy dies have had the disadvantages of complicated equipment, low operability and high expenses for the equipment including the dies.
[0008] On the other hand, general heat-resistant alloys have good oxidation resistance and can be used in the atmospheric air. The general heat-resistant alloys, however, are insufficient in high-temperature compressive strength at temperatures of 1000° C or above, so that the alloys, when adopted as a material for isothermal forging dies, must be used at temperatures below 1000° C.
[0009] In consideration of the above, improved Ni-based heat-resistant alloys have been disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-221542 (1985) and 62-50429 (1985), in which an element such as Al, Mo and W is added for improving high-temperature compressive strength and a rare earth element such as Y is added for attaining an improved oxidation resistance.
[0010] These Ni-based alloys, however, are yet unsatisfactory in oxidation resistance and are impracticable, as will be shown by comparison of examples below.
[0011] Alloys prepared by adding Zr to the above Ni-based alloys in order to improve the oxidation resistance have been proposed in Japanese Patent Application No. 1-140551 (1989), filed on June 2, 1989, by the present inventors.
[0012] However, it has been found that though the Ni-based alloys improved by the Zr addition show excellent properties on a laboratory basis, an attempt to produce the alloys through melting and casting on a practical operating scale produces serious problems as follows.
[0013] Because of the large amount of Mo, W or the like added and many aluminum oxide grains being likely formed due to the presence of a large amount of Al, these Ni-based alloys upon melting have such a high viscosity that, in the casting process for forming an ingot, the melt shows poor fluidity and low packability into the mold, resulting in high possibility of porosity produced in the ingot.
[0014] The presence of the porosity in the ingot of the Ni-based alloy means that the dies made from the ingot may be cracked starting from the porosity, during the use thereof. The possibility of cracking is fatal to the dies.
SUMMARY OF THE INVENTION
[0015] This invention contemplates overcoming the aforementioned problems associated with the heat-resistant alloys which have been proposed as a material for dies for isothermal forging to be used at high temperatures.
[0016] It is accordingly an object of this invention to provide a nickel-based heat-resistant alloy for dies which has excellent oxidation resistance and high-temperature compressive strength and is capable of being formed into dies free of microporosity and of possibility of cracking in use thereof.
[0017] According to a first aspect of this invention, there is provided a nickel-based heat-resistant alloy for dies containing:
0.01-0.5 wt% Zr, 0.04-0.2 wt% Mn, 0.04-2.5 wt% Si,
at least one selected from the group consisting of 3.0-8.5 wt% Al, 1.7-4.8 wt% Ti and 0.87-2.5 wt% Nb, and
one or both of 13-25.0 wt% Mo and 6.7-13 wt% W, and having 30-88% by volume of the γ phase, 12-60% by volume of the γ' phase and 2.5-11 % by volume of the α phase, the balance being essentially Ni.
[0018] According to a second aspect of this invention, there is provided a nickel-based heat-resistant alloy for dies containing:
0.01-0.5 wt% Zr, 0.04-0.2 wt% Mn, 0.04-2.5 wt% Si,
at least one selected from the group consisting of 3.0-8.5 wt% Al, 1.7-4.8 wt% Ti and 0.87-2.5 wt% Nb, one or both of 13-25.0 wt% Mo and 6.7-13 wt% W, and
at least one selected from the group consisting of 0.03-2 wt% Hf, 0.3-3 wt% Ta and 3-10 wt% Cr, and having 30-88% by volume of the γ phase, 12-60% by volume of the γ' phase and 2.5-11 % by volume of the a phase, the balance being essentially Ni.
[0019] The nickel-based heat-resistant alloys for dies according to this invention have excellent strength, ductility and oxidation resistance, and are excellent in high-temperature strength at 1000°C or above. Therefore, the nickel-based heat-resistant alloys for dies exhibit high performance as a material for dies to be used for forging at high temperatures.
[0020] The above and other objects, features and advantages of this invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
Figure 1 is a diagram illustrating high-temperature fatigue characteristics of materials under test.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0022] The nickel-based heat-resistant alloys for dies according to this invention will now be explained more in detail with reference to some preferred embodiments thereof.
[0023] First, the constituents of the nickel-based heat-resistant alloys for dies of this invention and the proportions of the constituents will be explained.
[0024] AI is an element which increases the strength of the alloy by forming the y' phase (Ni3Al) capable of imparting high-temperature strength and which improves greatly the oxidation resistance at high temperatures of 800 C or above by forming a dense Al2O3 film on the surface of the alloy. When the aluminum content is less than 3.0 wt%, the Al2O3 film for improving the oxidation resistance is little formed When the AI content exceeds 8.5 wt%, on the other hand, the y' phase is formed in excess, grains of phase (NiAl) are coarsened, and a eutectic reaction occurs, to lower the compressive deformation stress of the alloy and worsen the castability. Therefore, the AI content is set in the range from 3.0 to 8.5 wt%.
[0025] The y' phase can be strengthened not only by addition of AI but also by addition of Ti or Nb. It is thus possible to increase the high-temperature strength of the alloy by substituting Ti or Nb for part or the whole of AI in the alloy. In that case, Ti is added in an amount of from 1.7 to 4.8 wt.%, and Nb in an amount of from 0.87 to 2.5 wt%. When the Ti content is less than 1.7 wt% or the Nb content is less than 0.87 wt%, the effect of strengthening the γ' phase is slight. When the Ti content is more than 4.8 wt% or the Nb content is more than 2.5 wt%, on the other hand, there may result the problem of formation of an excess of the γ' phase or coarsening of β phase grains. Accordingly, the Ti content is from 1.7 to 4.8 wt%, and the Nb content is from 0.87 to 2.5 wt%.
[0026] Each of Mo and W serves for solution strengthening of the γ phase of the matrix and, at the same time, for formation of an Mo- or W-enriched a phase, thereby enhancing the high-temperature strength of the alloy. In other words, the y' phase is softened at a temperature of 950° C or above, and, in order to maintain the high-temperature strength of the alloy after the softening, the a phase rich in either of Mo and W (which are high-melting metals) should be dispersed in the alloy in a proportion of 2.5 to 11 % by volume. If the volume percentage of the a phase is less than 2.5%, the high-temperature strength will be low, whereas if the volume percentage exceeds 11%, a eutectic with coarsened a phase will crystallize to lower the compressive deformation stress of the alloy. In order to control the proportion of the a phase to within the range from 2.5 to 11 % by volume, therefore, the Mo content is set in the range from 13 to 25.0 wt%, or, alternatively, part or the whole of the Mo in the alloy is replaced with 6.7 to 13 wt% of W. When both of the Mo and W contents are outside the respective ranges, it is impossible to obtain the a phase in the aforementioned volume percentage.
[0027] In the case of substituting Ti or Nb for AI or substituting W for Mo, the substitutive element may be used in an amount (atom%) equal to that of the Al or Mo thus replaced, whereby the volume percentage of the y' phase or α phase can be easily controlled to within the desired range.
[0028] Zr is an element for improving the adhesion of the Al2O3 film formed on the surface of the alloy and for attaining a remarkably enhanced oxidation resistance at high temperatures. When the Zr content is less than 0.01 wt%, the improving effect on oxidation resistance is slight, whereas a Zr content of more than 0.5 wt% leads to a lowered melting point. The Zr content is therefore set in the range from 0.01 to 0.5 wt%.
[0029] Mn and Si, which are characteristic elements according to this invention, improve the fluidity of the molten alloy through different actions thereof.
[0030] On the one hand, Mn is an element having a deoxidizing effect, and serves to remove solid oxides of AI or the like from the molten alloy, thereby increasing the fluidity of the molten alloy and preventing the formation of porosity in the resulting ingot. When the Mn content is less than 0.04 wt%, this effect is weak, whereas use of more than 0.2 wt% of Mo results in poor oxidation resistance. Therefore, the Mn content is from 0.04 to 0.2 wt%.
[0031] On the other hand, Si improves the fluidity of the molten alloy by the presence of the element itself, and also has a deoxidizing effect on the molten alloy. Similarly to Zr, furthermore, Si improves the adhesion of the Al2O3 film formed on the surface of the alloy, thereby enhancing the high-temperature oxidation resistance of the alloy. When the Si content is less than 0.04 wt%, the above effects are produced only slightly, whereas an addition of more than 2.5 wt% of Si reduces toughness and may result in embrittlement. Thus, the amount of Si to be added is from 0.04 to 2.5 wt%.
[0032] Hf improves markedly the high-temperature oxidation resistance of the alloy where the Al2O3 film is formed on the surface of the alloy. When the Hf content is less than 0.03 wt%, the improving effect on the oxidation resistance is slight, whereas Hf contents of more than 2 wt% cause a lowering in the melting point of the alloy. The Hf content is therefore from 0.03 to 2 wt%.
[0033] Ta is an element which improves high-temperature oxidation resistance. When the Ta content is less than 0.3 wt%, however, this effect is feeble. When the Ta content exceeds 3 wt% on the other hand, the alloy has a poorer high-temperature strength. Therefore, the Ta content is from 0.3 to 3 wt%.
[0034] Cr is an element for improving both oxidation resistance and ductility. The Cr content of less than 3 wt% does not produce such an effect, whereas Cr contents of more than 10 wt% cause precipitation of the γ phase with the result of a lowered ductility. The Cr content is therefore from 3 to 10 wt%.
[0035] The volume percentages of the y phase, γ' phase and a phase, which constitute the microstructure of the nickel-based heat-resistant alloy for dies of this invention, will now be explained.
[0036] The conventional nickel-based heat-resistant alloys, with their microstructures composed of the γ phase and the -y' phase, have the problem that the y' phase is softened at a temperature of 950 °C or above. On the other hand, the nickel-based heat-resistant alloy for dies according to this invention has both the y' phase and the a phase formed in the γ phase matrix so as to solve the problem of softening. According to this invention, the proportions of the y phase, y' phase and α phase in the alloy are set in the ranges of from 30 to 88%, from 12 to 60%, and from 2.5 to 11 % by volume, respectively, whereby an excellent high-temperature strength is obtainable.
[0037] The γ' phase and the α phase, if present singly, require a high stress for deformation. From this point of view, the proportion of the y' phase should be not less than 12% by volume, and the proportion of the a phase not less than 2.5% by volume. When the proportion of the y' phase is more than 60% by volume or the proportion of the α phase is more than 11% by volume, a coarse eutectic will crystallize to give a lowered strength. In connection with the proportions of the y' phase and the a phase, the proportion of the y phase is set in the range from 30 to 80% by volume.
EMBODIMENTS
[0038] The nickel-based heat-resistant alloys for dies according to this invention will now be explained more in detail below with reference to examples thereof and comparative examples.
Embodiment 1
[0039] Nickel alloys having the compositions as set forth in Table 1 below were melted and cast according to the usual process.
[0040] The alloys thus cast were subjected to high-temperature tensile tests at temperatures of 1050 C and 1150°C.
[0041] Also, the alloys were subjected to oxidation resistance tests, in which a cycle of maintaining each alloy sample at 1200°C for 1 hour and then forcibly cooling the alloy was repeated 10 times, followed by measurement of the weight loss by corrosion to evaluate the oxidation resistance.
[0042] The results of the high-temperature tensile tests and the oxidation resistance tests are shown in Table 2.
[0043] It is evident from Table 2 that the prior art materials No. 1 (heat-resistant steel), No. 2 (Ni-based heat-resistant alloy), No. 3 (TZM) and No. 4 and comparative material Nos. 6 to 17 have problems as to high-temperature strength, ductility, oxidation resistance or melting point.
[0044] The prior art material No. 1, which is a Co-containing heat-resistant steel, is insufficient in high-temperature strength. The prior art material No. 2, which is an Ni-based heat-resistant alloy having a y-y' two-phase structure and being an excellent material for aircraft engines, has an unsatisfactory strength at high temperatures such as 1050 C. Because of containing neither Si nor Mn, the prior art material No. 2 shows poor fluidity upon melting, and is liable to yield an ingot with porosity therein.
[0045] The prior art material No. 3, which is an Mo-based alloy called "TZM", has such an oxidation resistance problem that the material may be gradually lost through evaporation during its use as dies for forging.
[0046] The prior art materials No. 4 and No. 5, due to the absence of Zr therein, have a drawback as to intergranular strength, and are low in ductility.
[0047] The comparative material 6, equivalent to the prior art material according to Japanese Patent Application Laid-Open (KOKAI) No. 62-50429 (1987), has a problem concerning oxidation resistance, due to the low Si content.
[0048] The comparative material No. 7 has a lowered melting point of 1225°C, as a result of an excessively high Zr content.
[0049] The comparative material No. 8 is poor in oxidation resistance and ductility, due to a low Zr content.
[0050] The comparative material No. 9 has a lowered melting point of 1225°C, as a result of an excessively high Zr content.
[0051] The comparative material No. 10 has a lowered melting point, because of an excessively high Hf content.
[0054] The comparative material No. 13 is unsatisfactory in strength, due to an insufficient Mo content.
[0055] The comparative material No. 14 has a lowered ductility, because of an excessively high Mo content.
[0056] The comparative material No. 15 is unsatisfactory in strength, due to an insufficient AI content.
[0058] The comparative material No. 17 has a lowered ductility, due to an excessively high W content.
[0059] In contrast to the above, the material Nos. 18 to 24, which are nickel-based heat-resistant alloys for dies according to this invention, are excellent in strength, ductility and oxidation resistance, as shown in Table 2. These excellent properties prove that the alloys according to this invention have high performance as nickel-based heat-resistant alloys for dies to be used for forging at high temperatures.
Embodiment 2
[0060] The nickel alloys No. 2 and No. 4 according to the prior art and the nickel alloy No. 19 of this invention, as shown in Table 1, were melted and then cast to form ingots in the shape of a die with 300 mm diameter and 300 mm height by the usual process.
[0061] Test pieces were prepared from the ingots, and were each served to a high-temperature fatigue test at 1100° C. The test results are shown in Figure 1.
[0062] It is clearly seen from Figure 1 that the material No. 19 according to this invention (marked with 0) is superior in high-temperature fatigue strength to the prior art material No. 2 (marked with o) and the prior art material No. 4 (marked with Δ). It proves that the nickel-based alloys according to this invention is excellent in high-temperature fatigue strength and has high performance as a nickel-based heat-resistant alloys for dies to be used for forging at high temperatures.
1. A nickel-based heat-resistant alloy for dies containing:
0.01-0.5 wt% Zr, 0.04-0.2 wt% Mn, 0.04-2.5 wt% Si,
at least one selected from the group consisting of 3.0-8.5 wt% Al, 1.7-4.8 wt% Ti and 0.87-2.5 wt% Nb, and
one or both of 13-25.0 wt% Mo and 6.7-13 wt% W, and having 30-88% by volume of the γ phase, 12-60% by volume of the γ' phase and 2.5-11% by volume of the a phase, the balance being essentially Ni.
2. A nickel-based heat-resistant alloy for dies containing:
0.01-0.5 wt% Zr, 0.04-0.2 wt% Mn, 0.04-2.5 wt% Si,
at least one selected from the group consisting of 3.0-8.5 wt% Al, 1.7-4.8 wt% Ti and 0.87-2.5 wt% Nb,
one or both of 13-25.0 wt% Mo and 6.7-13 wt% W, and
at least one selected from the group consisting of 0.03-2 wt% Hf, 0.3-3 wt% Ta and 3-10 wt% Cr, and having 30-88% by volume of the γ phase, 12-60% by volume of the γ' phase and 2.5-11% by volume of the α phase, the balance being essentially Ni.