WO2020129282A1

WO2020129282A1 - Ni‑BASED SUPER-HEAT-RESISTANT ALLOY

Info

Publication number: WO2020129282A1
Application number: PCT/JP2019/024698
Authority: WO
Inventors: 良佐々木; 伊達　正芳
Original assignee: 日立金属株式会社
Priority date: 2018-12-17
Filing date: 2019-06-21
Publication date: 2020-06-25
Also published as: JP6802991B2; JPWO2020129282A1

Abstract

Provided is an alloy which has a composition enabling lower cost than Mar-M246, and which has a substitutable creep rupture strength.　The present invention provides a Ni-based super-heat-resistant alloy comprising, in terms of mass%, 0.02-0.5% C, 7-12% Cr, 4-14% Co, 3-6.5% Al, 0.5-4% Mo, 7-14% W, 1.0-3.5% Ti, 0-0.7% Ta, 0.001-0.05% B, 0-0.02% Mg, and 0-0.1% Zr, the remainder comprising Ni and unavoidable impurities.

Description

Ni-base super heat resistant alloy

The present invention relates to a Ni-base super heat resistant alloy.

Since Ni-based super heat-resistant alloys are alloys that can have both excellent heat resistance and strength, they are used for members used in various high temperature environments. For example, a turbocharger is a device that increases the density of inhaled air by rotating a turbine wheel with exhaust gas and interlocking rotation with a compressor through a shaft. By installing a turbocharger, more oxygen than usual can be sent to the engine to improve energy efficiency. The turbine wheel, which is a component of the turbocharger, is a component that rotates at high speed of 100,000 rpm or more when it receives exhaust gas of over 1000°C, so it is essential that its material has excellent heat resistance. ..

The Ni-based super heat-resistant alloys such as Alloy 713C and Mar-M246 are typical materials used for the turbine wheel. Alloy 713C is a relatively inexpensive and widely used turbine wheel material. The material cost of Mar-M246 is high because it contains a large amount of relatively rare elements Co and W, and particularly rare elements Ta. However, since Mar-M246 has excellent creep rupture strength, it is possible to use high-temperature exhaust gas that 713C cannot cope with and to design more efficiently than when 713C is used. Further, there has been proposed a material that does not add expensive elements such as Hf, Ta, and Co, and realizes a creep rupture strength of 713C or more by adjusting the composition while maintaining a low price of about 713C. (See, for example, Patent Documents 1 and 2).

JP-A-11-131162 Japanese Patent Laid-Open No. 2000-169924

Mar-M246 boasts excellent creep rupture strength as a material for turbochargers, but its high material cost poses the greatest problem. The high material cost poses a major problem in mass production of general-purpose vehicles equipped with highly efficient turbochargers.
So far, alloys that are cheaper than Mar-M246 and have a creep rupture strength of 713 C or higher have been developed, but the high-efficiency turbocharger to which Mar-M246 is applied has a high exhaust gas temperature, so The developed material lacks creep rupture strength and cannot be said to be sufficient as an alternative material. As a substitute material for Mar-M246, it is considered that creep rupture life under conditions of 1000° C. and 180 MPa is required to be at least 50 hours or more.
For example, the Ni-base superheat-resistant alloy disclosed in Patent Document 1 described above does not contain Ta and Co and is inexpensive, but its creep rupture life under conditions of 1000° C. and 180 MPa is less than 35 hours. Further, the super heat-resistant alloy disclosed in Patent Document 2 described above achieves low price and low specific gravity by not including Ta and W, but has a creep rupture life of 25 hours under conditions of 1000° C. and 180 MPa. Less than As described above, since it is estimated that rare elements such as Co, W, and Ta play an important role in the creep rupture strength at high temperature, it has a lower cost and replaceable creep rupture strength than the Mar-M246. It is difficult to develop materials, and it cannot be said that such materials have been developed.
An object of the present invention is to provide an alloy having a composition capable of lowering cost than Mar-M246 and having a creep rupture strength capable of substituting Mar-M246.

The present inventors presume that rare elements such as Co, W, and Ta play an important role in the creep rupture strength at high temperature, so that the development of an alloy having low cost and excellent high temperature creep rupture strength has been investigated. I examined the problem of difficulty. Ta, which is a rare element among Co, W, and Ta, contributes to the improvement of creep rupture strength by forming a solid solution with carbides and the γ′ phase. Ta forms a solid solution with the carbide formed at a high temperature to strengthen the solid solution of the carbide and improve the grain boundary strength. Further, by forming a solid solution in the γ'phase formed at a temperature lower than that of the carbide, the γ'phase is solid-solution strengthened to improve the intragranular strength. However, the present inventor has sufficiently crystallized MC type carbides containing a large amount of W at the crystal grain boundaries without containing Ta, and further, the inside of the crystal grains is sufficiently solid-solution strengthened by Co, W, and Ti. The inventors have found that, if this is done, an excellent creep rupture strength can be exhibited at high temperatures, and have reached the present invention.
That is, in the present invention, C: 0.02 to 0.5%, Cr: 7 to 12%, Co: 4 to 14%, Al: 3.0 to 6.5%, Mo: 0. 5 to 4%, W: 7 to 14%, Ti: 1.0 to 5.0%, Ta: 0 to 0.7%, Mg: 0 to 0.02%, B: 0.001 to 0.05 %, Zr: 0 to 0.1%, the balance being a Ni-base superheat-resistant alloy composed of Ni and inevitable impurities.
The Ta is preferably not added, the Mg content is preferably 0.001 to 0.02%, and the Ti content is preferably 1.0 to 3.5%.

According to the present invention, it is possible to obtain a Ni-base superheat-resistant alloy having a composition capable of lowering cost than Mar-M246 and having high-temperature creep rupture strength capable of substituting for Mar-M246. Therefore, for example, it is possible to reduce the material cost of a highly efficient turbocharger using the same.

The alloy of the present invention No. It is an optical microscope image which shows the crystal grain boundary of 1 alloy as cast. The alloy of the present invention No. 3 is an optical microscope image showing crystal grain boundaries of 3 alloy as cast. Conventional example No. 4 is an optical microscope image showing grain boundaries of 4 alloy as cast.

As described above, an important feature of the present invention is that MC type carbide containing a large amount of W is sufficiently crystallized at the grain boundaries without depending on Ta which is a rare element, and By sufficiently solid-solution strengthening with Co, W, and Ti, we have realized a composition that enables cost reduction compared to Mar-M246 and that can be substituted and has a high creep rupture strength at high temperatures. is there.
The reason for defining each chemical composition range in the Ni-base superalloy according to the present invention is as follows. In addition, unless otherwise specified, it is described as mass %.

<Ta: 0 to 0.7%>
Ta is an element that contributes to the improvement of creep rupture strength by forming a solid solution with a carbide or a γ'phase. However, Ta is a rare element among the elements constituting the present invention, and the material cost increases as the amount of Ta increases. Further, in the present invention, even if Ta is not added, the creep rupture strength can be maintained at a high level by adjusting the addition amounts of other elements. Therefore, the upper limit of Ta can be allowed within the range of 0.7%. The preferable permissible amount of Ta is 0.5% or less, and more preferably, it does not substantially contain Ta (0%). The phrase "substantially free of Ta (0%)" means that the impurity level is unavoidable or lower. That is, Ta is preferably not added.
<C: 0.02 to 0.5%>
C forms carbides by combining with alloy elements. The carbide precipitated at the grain boundary increases the high temperature strength by suppressing the grain boundary slip at high temperatures, so 0.02% or more is required. However, if the C content is too large, a large amount of coarse carbide may crystallize, which may impair the ductility and corrosion resistance. Therefore, the upper limit is 0.5%. The preferable lower limit for obtaining the above-mentioned effect of C more reliably is 0.1%. A particularly preferable upper limit of C is 0.3%, and a further preferable upper limit is 0.2%.

<Cr: 7-12%>
Since Cr forms an oxide film having high adhesion on the surface of the alloy during heating at high temperature and enhances oxidation resistance, 7% or more is required. However, if the amount of Cr is too large, the structure becomes unstable, and a harmful phase such as a hard and brittle σ phase is formed and the creep rupture strength and ductility are deteriorated. Therefore, the upper limit is 12%. A preferable lower limit is 7.5% and a preferable upper limit is 10% in order to more reliably obtain the above-described effect of Cr.
<Co: 4-14%>
Since Co contributes to the improvement of creep rupture strength by forming a solid solution in the γ phase, 4% or more is required. However, if the amount of Co is too large, the structure becomes unstable and forms a harmful σ phase. Further, Co is one of the rare elements, and the material cost increases as the added amount increases, so the upper limit is 14%. The preferable lower limit for more reliably obtaining the above-mentioned effect of Co is 4.5%, the preferable upper limit is 11%, and more preferably 10.5%.

<Al: 3.0 to 6.5%>
Since Al strengthens the precipitation in the crystal grains by forming the γ'(Ni ₃ Al) phase, 3.0% or more is required. However, if the amount of Al is too large, coarse γ/γ′ eutectic will occur at the grain boundaries. Coarse γ/γ' eutectic contributes little to the creep rupture strength at high temperature, and solute elements are deprived of the eutectic, which lowers the γ'phase fraction in the grains. To be Therefore, in the present invention, the upper limit of Al is 6.5% in order to prevent excessive eutectic formation. A preferable lower limit for more reliably obtaining the above-described effect of Al is 4.0%, and further preferably 4.5%. The particularly preferable upper limit of Al is 6.0%, and more preferably 5.8%.
<Ti: 1.0 to 5.0%>
Ti strengthens the γ'phase by forming a solid solution in the γ'(Ni ₃ Al) phase. Further, since the solid solution temperature of the γ'phase is raised, it contributes to increase the γ'amount in the high temperature region, so 1.0% or more is required. However, if the amount of Ti is too large, the solid solution temperature of the γ'phase rises, and coarse γ/γ' eutectic occurs at the crystal grain boundaries, so the upper limit is 5.0%. A preferable lower limit for more reliably obtaining the effect of Ti described above is 1.4%, and particularly preferably 1.5%. The preferable upper limit of Ti is 4.0%, and considering the balance with other elements, the upper limit of Ti is preferably 3.5, more preferably 3.0%, and further preferably 2.7%.

<Mo: 0.5-4%>
Mo forms a solid solution in the γ phase and contributes to the improvement of creep rupture strength, so 0.5% or more is required. However, if the amount of Mo is too large, the oxidation resistance is adversely affected, so the upper limit is 4%. A preferable lower limit is 1.0% and a preferable upper limit is 3.5% in order to more reliably obtain the effect of Mo described above.
<W: 7-14%>
W contributes to the improvement of creep rupture strength by forming a solid solution with γ phase, γ′ phase and carbide. In particular, since the contribution of W having a small diffusion coefficient is large to the creep rupture strength at high temperature, 7% or more is required. However, if the amount of W is too large, the structure becomes unstable, and a harmful phase such as a hard and brittle σ phase is formed, resulting in a decrease in creep rupture strength and ductility. Furthermore, since W has a high specific gravity, the higher the amount of W, the higher the density, which is disadvantageous as a rotating body. Therefore, the upper limit of W is 14%. A particularly preferable lower limit for more reliably obtaining the above-mentioned effect of W is 7.5%, and more preferably 9%. The preferable upper limit of W is 13%, and more preferably 12.5%.

<Mg:0-0.02%>
Mg is a selective element and can be added if necessary. Mg is added as a desulfurizing agent which forms a compound with S which is an embrittlement phase forming element when the alloy is melted. Addition of an appropriate amount of Mg has the effect of suppressing the grain boundary segregation of S and improving hot workability. To ensure this effect, Mg needs to be 0.001% or more. However, if excessive Mg is added, a low melting point phase of Mg precipitates and the grain boundary strength decreases, so 0.02% is made the upper limit. A preferable lower limit for more reliably obtaining the above-mentioned effect of Mg is 0.0005%, more preferably 0.002%, and a preferable upper limit is 0.01%.
<B: 0.001 to 0.05%>
It is considered that B segregates at the grain boundaries because Ni has a great difference in atomic radius from Ni forming the γ phase, which is the parent phase, and suppresses grain boundary slip, which is beneficial for high temperature strength. To ensure this effect, B needs to be 0.001% or more. However, addition of a large amount deteriorates the oxidation resistance, so the upper limit is 0.05%.

<Zr: 0-0.1%>
Zr is a selective element and can be added if necessary. It is considered that Zr has a great difference in atomic radius from Ni forming the γ phase as a matrix phase, so that it segregates at the grain boundaries and forms carbides at the grain boundaries to strengthen the grain boundaries and is beneficial for high temperature strength. However, addition of a large amount deteriorates the oxidation resistance, so the upper limit is 0.1%. In order to more surely obtain the above-mentioned effect of Zr, the preferable lower limit of the content of Zr is 0.01%.
<Remainder: Ni and inevitable impurities>
The balance is substantially Ni, but contains impurities that are unavoidably mixed in during manufacturing. It should be noted that Ta that is positively added to the conventional alloy does not need to be positively added, and Nb is an impurity element in the present invention.

The invention is described in more detail in the following examples.
As an example of the present invention and a conventional example, 10 kg of the alloy shown in Table 1 was melted in a vacuum furnace and cast into a cast iron φ80 mm mold installed in the furnace to produce an ingot of each alloy. These alloys are intended for use in turbine wheels, which are components of turbochargers.
No. shown as a conventional example. Alloy 11 is Alloy 713C, which is known as a typical material for turbochargers. Alloy 713C is inexpensive as a material for a turbocharger because it does not contain Co and W, which are relatively rare elements, and Ta, which is a particularly rare element. On the other hand, Mar-M246 contains a large amount of Co and W and also contains Ta in an amount of 1.5%, so that it is expensive as a material for a turbocharger. The alloy of the present invention is the conventional alloy No. 12 is an alloy having a composition that does not contain Ta or has a lower content than that of Mar-M246, so that the cost can be lower than that of Mar-M246.

A test piece was prepared from an ingot of each alloy, and the creep rupture strength at high temperature was evaluated by a rupture test according to ASTM E139. The parallel part of the test piece was 6.4 mm. Table 2 shows the test results of creep rupture strength.
As the test results shown in Table 2, No. of the conventional example. The 11th alloy (Alloy 713C) had the shortest rupture life at 1000° C./180 MPa among the alloys shown in Table 1. No. No. 11 alloy had a short rupture life. Since the No. 11 alloy does not contain Co, W, and Ta, it has a weaker intragranular strength than the other alloys shown in Table 1, and MC carbides that crystallize at the grain boundaries do not contain Ta or W. This is because the grain boundary strength is also weak.
In addition, the same conventional example No. Among the alloys shown in Table 1, No. 12 alloy (Mar-M246) had the longest rupture life at 1000° C./180 MPa. This is because Mar-M246 has strong intragranular strength due to solid solution strengthening of Co, W, and Ta, and MC carbides containing Ta and W are sufficiently crystallized in the grain boundaries, so that grain boundaries at high temperatures are high. This is because the strength is also strong.
The alloy of the example of the present invention is No. Although the rupture life is slightly shorter than that of 12 alloy (Mar-M246), it exceeds the rupture life of 50 hours at 1000°C/180MPa, which is considered to be the minimum condition that can replace Mar-M246 as a highly efficient turbocharger material. Most of them are over 60 hours. It can be seen that some alloys have been obtained for 70 hours or more. This is because the alloys of the examples of the present invention do not contain Ta, but a large amount of W is dissolved in the MC carbide crystallized at the grain boundaries so as to compensate for the absence of Ta, so the grain boundary strength at high temperature is Mar. -Much higher than M246. Furthermore, since the alloy of the present invention contains a large amount of Ti, a large amount of Ti is solid-solved in the γ'phase, and therefore the intragranular strength at high temperature is sufficiently higher than that of Mar-M246, which can replace Mar-M246. It is something. Due to the above effects, it is considered that the alloy of the present invention example has a rupture life at 1000° C./180 MPa of more than 50 hours.
In addition, in the elongation and the drawing, the alloy of the present invention shows an elongation of 2.5% or more. Elongation equal to or higher than that of No. 12 alloy was obtained. Also, the diaphragm is No. Similar or better than the 12 alloys were obtained. Among them, there are alloys in which both elongation and drawing are 4.0% or more. It can replace the 12 alloy Mar-M246.

Figure 1 shows the No. It is an optical microscope image which shows the crystal grain boundary of 1 alloy. FIG. 2 is an optical microscope image showing the crystal grain boundaries of No. 3 alloy. Further, FIG. 12 is an optical microscope image showing a crystal grain boundary of Mar-M246 which is a 12 alloy. In FIG. 1, FIG. 2, and FIG. 3, a grain boundary exists in the center, and carbides existing along the grain boundary can be confirmed. In Table 3, No. No. 1 alloy, No. 3 alloy and No. 3 alloy. The component analysis value which analyzed the carbide|carbonized_material of 12 alloy by SEM-EDX is shown. No. Since No. 12 alloy contains Ta, a large amount of Ta is contained also in the carbide. No. Since alloy No. 1 and alloy No. 3 do not contain Ta, the carbide does not contain Ta either. In addition, No. No. 1 alloy and No. 3 alloy are No. It contains more W than the 12th alloy, and W is the most contained metal element. Further, the content of W is 40% by mass or more. Since W has a small diffusion coefficient like Ta, No. The carbides of the No. 1 alloy and the No. 3 alloy are also No. 1 alloys. It is stable at high temperatures as well as the carbide of 12 alloy. Therefore, the alloy of the example of the present invention, in which the grain boundary is strengthened by the carbide stable at high temperature, has sufficiently high grain boundary strength at high temperature and can replace Mar-M246. From the above results, it is understood that the alloy of the present invention can be sufficiently applied to the turbine wheel which is a component of the turbocharger. Of course, it can be applied to the components of the turbocharger other than the turbine wheel.

According to the present invention, it is possible to provide a material that can replace Mar-M246 as a highly efficient turbocharger material that has a composition that enables lower cost than that of Mar-M246, and thus can be installed in a general-purpose vehicle with high efficiency. Applicable as turbocharger material.

Claims

% By mass, C: 0.02 to 0.5%, Cr: 7 to 12%, Co: 4 to 14%, Al: 3.0 to 6.5%, Mo: 0.5 to 4%, W : 7 to 14%, Ti: 1.0 to 5.0%, Ta: 0 to 0.7%, Mg: 0 to 0.02%, B: 0.001 to 0.05%, Zr: 0 to A Ni-based superheat-resistant alloy, characterized in that 0.1% and the balance Ni and unavoidable impurities.
The Ni-based superheat-resistant alloy according to claim 1, wherein Ta is not added.
The Ni-base superheat-resistant alloy according to claim 1 or 2, wherein the content of Mg is 0.001 to 0.02%.
The Ni-based superheat-resistant alloy according to any one of claims 1 to 3, wherein the content of Ti is 1.0 to 3.5%.