KR100203379B1 - Ultra heat-resistance nickel alloy - Google Patents

Abstract

The present invention relates to a casting-based Ni-based super heat-resistant alloy that can be applied to core components of various devices that require high strength and oxidation resistance at high temperatures, including turbine blades for onshore power generation operating at ultra high temperatures of 1000 ° C or higher. It is related to Ni-based super heat-resistant alloy which is significantly improved creep fracture life.

Such a composition of the present invention is based on Ni 6-10%, W 8-13%, Mo 1-4%, Al 3-7%, Ti 0.5-3%, Nb 0.5-3% , Co 8-12%, Ta 3-7%, Hf 0.05-2.0%, Zr 0.05-2.0%, B 0.01-0.1%, C 0.01-0.5%.

The reason for limiting the components of the alloy of the present invention is that Cr is added to improve oxidation resistance, and W serves to suppress creep deformation of the alloy to improve creep rupture life, and Mo is dissolved in the base together with W. It is intended to improve creep rupture life. In addition, Al and Ti increase the high temperature strength of the alloy appropriately for the intended use, Nb is one of the characteristic addition elements of the present invention, has a grain boundary strengthening effect, Co is strengthening precipitation by solid solution strengthening and carbide formation In addition to improving the corrosion resistance at high temperatures, Ta enhances the solubility and precipitation strengthening effect and the creep rupture life. In spite of the addition of a small amount of Hf, the ductility by grain boundary strengthening and grain refinement of the alloy is improved, thereby greatly improving the creep quality.

Description

Nickel-Based Super Heat-resistant Alloys for Casting

The present invention relates to a nickel-based super heat-resistant alloy for casting excellent in high temperature strength, and particularly applicable to the core parts of various devices that require high strength and oxidation resistance at high temperatures, including turbine blades for onshore power generation operating at very high temperatures of 1000 ℃ or more. It relates to a nickel-based super heat-resistant alloy for casting.

In general, nickel-based super heat-resistant alloys are widely used in various parts requiring high temperature corrosion resistance and high mechanical strength, such as gas turbines for power generation, tube and pipe materials for jet engines, chemical plants, and heat exchanger materials for nuclear power plants. It is becoming.

This is because it is superior to other alloys because the properties directly related to productivity, such as workability and weldability, as well as excellent high temperature corrosion resistance and high temperature oxidation resistance of nickel-based super heat-resistant alloys, are much researched as gas turbine materials. The company has been investing heavily in R & D in Germany and France.

In addition, in various high temperature devices, the use temperature is increased to improve energy efficiency. To this end, efforts have been made to increase the high temperature strength of materials used in the core parts of high temperature devices. Among the alloys that are considered to have the highest break life are the MM 247 alloy from Martin Marietta, USA, and the TM 321 alloy, which has not been commercialized but was developed by the National Reserch Institute for Metals, Japan.

In general, casting of a nickel-based super heat-resistant alloy is mainly performed in a vacuum by a precision casting method. Nickel-based super heat-resistant alloys have the most complicated composition and are the most widely used high temperature components. The operating temperature is also close to the temperature corresponding to 0.9 times the melting point, which is higher than any metal material.

Nickel-based super heat-resistant alloys have received a great deal of attention as a material for aircraft engine blades between 1940 and 1965, and aimed to maintain a constant creep strength up to 5,000 hours at high temperatures. In the case of the material is required to maintain the strength up to 20,000 to 50,000 hours, especially in the case of industrial gas turbine is required to maintain the strength up to 100,000 hours. As the most suitable material for these needs, nickel-based super heat-resistant alloy is recognized as the least suitable alloy, and many studies have been conducted in various countries around the world.

In light of these circumstances, the present applicant has been a nickel-based super heat-resistant main alloy (Korea Patent No. 17099, 1984), which has excellent heat resistance and corrosion resistance, and a nickel-based super heat-resistant alloy for casting that does not contain cobalt (Co). No. 17100) and an alloy (Korean Patent No. 86502, 1995, Republic of Korea) superior to other nickel-based superheat-resistant alloys that have been developed so far. do.

Nickel-based super heat-resistant alloy according to the present invention is to significantly improve the creep rupture life, which is usually the most problematic in the main alloy, the present invention is to add a relatively large amount of Nb and Mo among the elements constituting the alloy, It is an object of the present invention to provide a nickel-based super heat-resistant alloy for casting having excellent high temperature strength, which is about twice as much as the existing strongest alloy by improving the creep rupture life at 1000 ° C. or higher by appropriately adjusting the alloying amount of other elements. The composition of the alloy according to the present invention is based on Ni 6-10%, W 8-13%, Mo 1-4%, Al 3-7%, Ti 0.5-3%, Nb 0.5-3 %, Co 8-12%, Ta 3-7%, Hf 0.05-2.0%, Zr 0.05-2.0%, B 0.01-0.1%, C 0.01-0.5% are contained.

The reason for component limitation of the nickel-based super heat-resistant alloy for casting by the above-mentioned this invention is demonstrated.

Chrome (Cr)

Cr is added up to 10% for the purpose of improving the oxidation resistance of the alloy, there is a fear that the oxidation resistance of the alloy is less than 6%.

Tungsten (W)

W is the heaviest metal among the metals, so the diffusion rate at high temperature is very slow, which greatly suppresses creep deformation of the alloy and improves the creep rupture life. When applied, there is a possibility of lowering energy efficiency, and when it is 8% or less, it is not sufficient to obtain the employment strengthening effect that is desired.

Molybdenum (Mo)

Mo is dissolved in the base together with W and plays the same role as W. Mo has been recognized as an element that deteriorates oxidation resistance, but in the case of the alloy of the present invention, rather than adding Mo by more than 1%, TM 321 Compared with the alloy and the MM 247 alloy added in a smaller amount than the alloy of the present invention at about 0.6%, excellent creep rupture life and high temperature tensile strength could be obtained.

Aluminum (Al) and Titanium (Ti)

Al and Ti precipitate the gamma prime precipitate (symbol: γ ', chemical formula: Ni ₃ (Al, Ti) from the base, and adjust the content of precipitated γ' by adjusting the ratio of Al to Ti. It is known that the ratio of Al to Ti can be changed to about 5: 1 under the assumption of maintaining a face-centered cubic lattice, which is a bonding structure of γ '.

However, when the Ti content is excessive, a compound having a dense hexagonal lattice form called an eta phase (Ni ₃ Ti) is formed to spoil the mechanical properties of the alloy.

Much of the strength of Ni-based super heat-resistant alloys is caused by 1γ. In general, the volume fraction of γ 'is determined by the ratio of Ti to Al and the content of Al + Ti, which is 35% for the main alloy. It is common to have the above volume fraction, and in the case of the alloy of the present invention, the volume fraction of γ 'is adjusted to 60%.

If the content of Al and Ti is too high, the high temperature strength of the alloy is increased, but the ductility of the alloy is greatly deteriorated and tends to be easily broken, so the content of Al and Ti is adjusted in the range of 3 to 7% and 0.5 to 3%, respectively.

Niobium (Nb)

Nb is one of the characteristic addition elements in the alloy of the present invention and is not added in MM 247 or TM 321.

In general, Nb contributes to the formation of a precipitate called 1γ; body centered tetragonal phase, and is also known as an element having a grain boundary strengthening effect as it is distributed in the grain boundary as a carbide of the form NbC.

This strengthening effect of Nb is fully revealed by the alloy Inconel 718, which is currently being used by Inco in the US. However, when excessively contained, harmful Ni3Nb intermetallic compounds are precipitated and cause cracking at break. In consideration of this point, the content of Nb was adjusted within 0.5 to 3%.

Cobalt (Co)

Co is an effective element to enhance the corrosion resistance at high temperature as well as the precipitation strengthening effect by solid solution strengthening and carbide formation. In particular, Co increases the solvation temperature of 1γ, which is the main precipitation phase of nickel-based superheat-resistant alloy, By being able to be used at higher temperatures serves to increase the high temperature strength.

However, since the price of Co is very high compared to other additive elements, and the induced radioactivity generated by Co at the time of excessive addition may be a problem, the addition range is set to 8 to 12%.

Tantalum (Ta)

Ta exhibits a solid solution strengthening effect by solid solution at the base of the alloy, and partly precipitates as carbide in the form of TaC to simultaneously exhibit precipitation strengthening. It was also partially dissolved in 1γ, contributing to improving creep rupture life of the alloy, and the effect was adjusted in the range of 3-7%, the most pronounced.

Hafnium (Hf)

Hf is an element having excellent ductility improvement effect due to grain boundary strengthening and grain refinement of the alloy despite the addition of a small amount, and improves creep properties, and a range of 0.05 to 2.0% is generally agreed.

For example, existing alloys containing Hf include B-1900, MM 200 + Hf, MM 247, and TRW-NASA VIA alloys, all of which have a higher grain boundary strength than 900 ° C in comparison with other main alloys. It is said to exhibit relatively good high temperature strength.

Zirconium (Zr) and Boron (B)

Most of Zr and B are distributed at grain boundaries and increase grain boundary strength by preventing slippage at high temperature deformation. However, when Zr and B are added in large amounts, they cause brittle fracture due to grain boundary segregation. It is necessary to adjust in the range of -0.1%.

The manufacturing method and general characteristics of the nickel-based super heat-resistant alloy for casting according to the present invention having the composition as described above will be more clearly understood through the following examples, but are not limited to these examples.

Example

In order to manufacture the nickel-based super heat-resistant alloy for casting, the size of the incoat for casting is 52 × 52 × 150 mm, and all of the raw metals for melting are pure metals having a purity of 99.5% or more except for B, a trace additive element. In the case of B, a mother alloy of Ni-15% B was used. As a vacuum melting furnace, a high frequency vacuum melting furnace having a capacity of 10 kV and 50 kV of ULVA Co., Ltd. was used, and a magnesia crucible for 171bs was used as a melting crucible. In order to remove water that may remain in the crucible before the start of melting of the candidate alloy, Ni, a base metal of the present alloy system, was melted and tapped.

The charging sequence of the raw metal is as follows.

Particularly low Ni and Cr and low specific gravity, graphite and high melting point metals that are not evenly distributed in the molten metal, and relatively large amounts of metal, that is, W, Mo, Co is charged, the degree of vacuum is lowered 10 ^-3 Dissolution started when it fell below torr. It took about 1 hour until the molten metal was formed, and the molten Ni and Cr were completely added to form the molten metal before forming the molten metal completely without forming a bridge. After charging with Nb and Hf, the mixture was maintained for about 15 minutes. Finally, the active elements Al, B, Ti were charged and maintained for about 5 minutes, and then injected into a preheated mold at a temperature of about 150 ° C. above the melting point of the alloy. . At this time, the temperature of the molten metal was measured using a high temperature thermometer. The candidate alloy made of ink after the primary melting was completed removed the release agent on the surface of the ingot for secondary melting for precision casting.

Precision casting was first made into a precision casting fine milling mold, and then re-dissolved in an argon atmosphere using an ingot manufactured in a vacuum microwave melting furnace to finally produce a cylindrical specimen having a diameter of 15 mm and a length of 125 mm.

The manufacturing process of the ceramic mold for precision casting is explained.

After making a cylindrical mold to make a wax pattern, a tree design is made. After the assembly of the tree is completed, the ceramic cup is vertically attached to the hot water section, and the refractory coating layer including zircon sand and snow tex is repeatedly applied eight times to make the thickness of the refractory coating layer about 9 mm. When the refractory coating is completed, the casting mold is finally completed by drying the mold, removing the wax in the mold, and firing the mold. The completed precision casting mold was baked at a temperature of 1100 ° C. for at least 24 hours to completely remove moisture that may be present in the mold, and then taken out just before tapping and injected. Before starting the melting of the ingot for precision casting, various impurities remaining on the surface of the ingot were removed using a shot peening apparatus using a steel ball with a diameter of 0.75 mm. Precision casting was carried out in the high purity argon atmosphere from ingot to charging, melting, and tapping. The injection temperature was adjusted to about 150 ° C higher than the melting point of the alloy. After the injection mold was cooled in the air, after about 2 hours after tapping, a vibrator and a short peening apparatus were used to remove the debris from the ceramic mold on the surface of the specimen, and then the cylindrical specimen was cut out.

The finally processed specimen was subjected to a creep rupture test in the air at a stress of 140 MPa at 1020 ° C. One of the important components of the creep rupture tester was to adopt a three-zone heating method in which the upper, middle, and lower subdivisions were temperature-controlled, thereby widening the temperature uniformity of the specimen. In order to compensate for the temperature between the furnace and the specimen, three thermocouples were welded directly to the specimen at 1 cm intervals around the center of the specimen to determine the difference between the temperature present on the controller of the furnace and the actual specimen. Measurements were made to correct the temperature of the actual specimen to be maintained at a temperature within 10 ° C to 2 ° C. During creep rupture testing, when the strain of the specimen is transmitted to the liner variable differential transduser (LVDT) through an extensometer, the signal from this is automatically received by a data acquisition system mounted on a personal computer. In addition, the deformation state of the specimen was continuously recorded. In addition, the temperature signal of the specimen was continuously accepted by the computer and recorded to increase the accuracy of the creep rupture test. IN713LC was used as a grip and bar material for creep rupture test, and manufactured by precision casting method.

In order to measure the high temperature tensile strength of the alloy was subjected to a high temperature tensile test at 900 ℃ using INSTRON (Model No. 1127). The thermocouple was attached directly to the center of the specimen for correct calibration of the test temperature, and then the controller setting temperature in the furnace was adjusted to adjust the actual temperature of the specimen to the temperature to be tested. The test atmosphere was air, and the temperature rising rate of the furnace was 15 ° C. per minute, and the test temperature was maintained after maintaining the test temperature for 15 minutes. At the end of the test immediately removed from the furnace and water cooled. The specimens used were INCONEL 713LC and the cross head speed was kept constant at 0.6% / min.

The chemical compositions of the inventive alloys A, B, C, and D prepared as described above are shown in Table 1 below, and the results of the creep rupture test are shown in Table 2 below.

Alloys A, B, C, and D of the present invention were compared with TM 321 ordinary main alloy, the strongest alloy developed by the Japan Institute of Metals, and MM 247 alloy, which is known to have the highest creep strength among commercially available cast alloys.

As can be seen in Table 2 below, the comparative alloy TM 321 was broken in 1057 hours at a temperature lower than the test temperature of this study and at a low stress of 1000 ° C.-118 MPa, and in the case of MM 247, 669 hours. It was found that only the creep fracture characteristics of the alloys A, B, C, D of the present invention are very excellent.

The results of comparing the tensile strength at 900 ° C., which is the scanning temperature of the alloy, are shown in Table 3, and it can be seen that the alloy A of the present invention exhibits the best tensile strength.

Claims (1)

Based on Ni as the base composition, Cr 6-10%, W 8-13%, Mo 1-4%, Al 3-7%, Ti 0.5-3%, Nb 0.5-3%, Co 8-12%, A cast nickel-based super heat-resistant alloy comprising Ta 3-7%, Hf 0.05-2.0%, Zr 0.05-2.0%, B 0.01-0.1%, and C 0.01-0.5%.