High-purity cobalt-based superalloy K40M / DZ40M and preparation technology thereof

Abstract

The application discloses a high-purity cobalt-based high-temperature alloy K40M / DZ40M and a preparation technology thereof, and belongs to the technical field of alloy design, metal smelting and casting. The alloy composition is as follows: C: 0.45-0.55, Cr: 24.5-26.4, Ni: 9.5-11.5, Al: 0.8-1.0, W: 7.5-8.2, Mo: 0.2-0.4, Ti: 0.1-0.3, B: 0.010-0.015, Zr: 0.1-0.3, Ta: 0.1-0.4, and Co: balance. During smelting, the stage-type charging and ultrahigh-temperature melt treatment process are used to promote the removal of gas elements, so that the pure smelting effect is achieved; and the low-temperature pouring process is used to ensure that the eutectic and carbide phases in the alloy are uniformly distributed. The alloy disclosed by the application has good organizational stability, excellent cold and hot fatigue resistance, oxidation resistance and heat corrosion resistance, and is suitable for turbine guide vanes serving below 1040 DEG C for a long time.

Description

Technical Field

[0001] This invention relates to the fields of alloy design, metal smelting and casting technology, and specifically to a high-purity cobalt-based superalloy K40M / DZ40M and its preparation technology. Background Technology

[0002] High-temperature alloys are metallic materials capable of long-term operation at temperatures above 600°C and under certain stress conditions. They possess excellent high-temperature strength, good resistance to oxidation and hot corrosion, and good fatigue performance and fracture toughness. These material characteristics make high-temperature alloys an irreplaceable key material in aero-engines. In the development of advanced engines worldwide, high-temperature alloys account for 40% to 60% of the total engine material usage. Among them, cobalt-based high-temperature alloys, with their strong structural stability, low notch sensitivity, and excellent weldability, are widely used in the manufacture of stationary structural components such as guide vanes.

[0003] The main gaseous impurity elements in high-temperature alloys are O, N, and S. Among them, O mainly exists in the form of micron-sized inclusions. These inclusions are the main initiation sites and propagation channels for fatigue cracks, significantly reducing the fatigue life of high-temperature alloys. For example, alumina inclusions severely reduce the creep, fatigue, and yield strength properties of high-temperature alloys. Studies have found that when the oxygen content is less than 50 ppm, the fracture life of high-temperature alloys is significantly improved.

[0004] Nitrogen (N) exists in high-temperature alloys in the form of high-melting-point nitrides or carbonitrides, such as TiN and Ti(C,N). These often become nuclei for carbide precipitation, forming large, bulky carbonitrides that encapsulate nitrides, blocking interdendritic channels and significantly affecting the feeding properties of the molten alloy. This results in microporous areas at carbide boundaries, increasing the probability and rate of fatigue crack initiation and propagation, severely impacting the alloy's high-temperature mechanical properties. Because high-temperature alloys contain a large amount of nitrogen-loving elements such as Ti, Nb, and Zr, even when nitrogen levels are reduced to below 15 ppm, the microporosity of high-temperature alloy castings remains high. Furthermore, nitride or carbonitride inclusions are generally distributed at grain boundaries or between dendrites, exhibiting poor adhesion to the alloy matrix. Under long-term high-temperature and stress conditions, they are highly susceptible to becoming crack initiation sites or pathways for rapid crack propagation.

[0005] Sulfur (S) readily segregates at grain boundaries, severely reducing the bonding strength of grain boundaries in high-temperature alloys. Increased sulfur content significantly reduces grain boundary energy, weakening the grain boundaries and severely impacting the alloy's creep rupture performance. S reacts with Ti, Zr, and Cr to form M2SC (γ-phase), which becomes a crack initiation site, significantly reducing the creep rupture life of high-temperature alloys. For the M17 alloy, reducing the O and S content from 20 ppm to below 10 ppm improves the material's high-temperature creep rupture performance by 90%. For the K424 cast nickel-based high-temperature alloy, reducing the sulfur content from 50 ppm to 10 ppm increases the creep rupture life (900℃, 314 MPa) from 40 hours to 160 hours; further reducing the S, O, and N content significantly increases the alloy's service life. Each hour of improvement in engine life translates to economic benefits of at least $5 million for aircraft currently in service.

[0006] Therefore, this invention proposes a simple, economical and practical process for preparing cobalt-based superalloys, which not only helps to improve the high-temperature performance of cobalt-based superalloys, but also provides a technical basis for the purification preparation of superalloys, and has important social and economic significance for the preparation and application of superalloys. Summary of the Invention

[0007] The purpose of this invention is to provide a high-purity cobalt-based superalloy K40M / DZ40M and its preparation technology. This method addresses the problem of unstable chemical composition and impurity content in the alloy. Through multiple verification experiments, the optimal content control points of each element in the alloy have been determined. This not only ensures that the alloy has excellent mechanical properties at these composition points, but also guarantees the stability of the alloy's chemical composition. Combined with specialized purification smelting technology, it is possible to produce high-quality cobalt-based superalloy master alloy ingots with stable chemical composition, excellent mechanical properties, and low impurity content.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A high-purity cobalt-based superalloy K40M / DZ40M, with the following chemical composition by weight percentage:

[0010] C: 0.45~0.55%, Cr: 24.5~26.4%, Ni: 9.5~11.5%, Al: 0.8~1.0%, W: 7.5~8.2%, Mo : 0.2~0.4%, Ti: 0.1~0.3%, B: 0.010~0.015%, Zr: 0.1~0.3%, Ta: 0.1~0.4%, balance Co.

[0011] The preparation technology of the high-purity cobalt-based superalloy K40M / DZ40M includes the following steps:

[0012] (1) Load the raw materials into the crucible from bottom to top in the order of Co (half), Cr, Ni, C, W, Ta, Co (half), and wrap the remaining raw materials such as Al, Ti, B, and Zr with aluminum foil for later use;

[0013] (2) Melt the raw materials in the crucible in the vacuum induction furnace. When the melt is completely clear, raise the temperature to 1550℃ for high-temperature refining. After the high-temperature refining is completed, add raw materials such as Al, Ti, B, and Zr wrapped in aluminum foil, and stir for 2 to 5 minutes while maintaining (1 / 4) Pe power. Then turn off the power and cool down.

[0014] (3) After the surface of the melt cools down to about 1410°C, the power is increased to about (1 / 2) Pe to heat the melt and the surface temperature of the melt is increased to about 1450°C for casting to obtain the master alloy ingot.

[0015] (4) The master alloy ingot is prepared into oriented columnar crystal or equiaxed crystal castings by vacuum casting.

[0016] In step (2) above, the raw material melting process is as follows: after the gas pressure in the vacuum induction furnace is lower than 40Pa, power is supplied, and (1 / 4)Pe, (1 / 2)Pe and Pe are used, where Pe is the rated power of the vacuum furnace induction heating, to carry out staged material melting. During this period, the gas pressure in the furnace should be kept below 40Pa. When the vacuum gauge reading is stable and lower than 40Pa, the material melting power can be increased. After the melt is completely cleared, high-temperature refining is carried out.

[0017] In step (3) above, when the alloy melt is poured, the ratio of the pouring port diameter to the ingot mold diameter is 1 / 6 to 1 / 4. Since the melting point of the alloy is about 1390℃, low-temperature pouring helps to avoid excessive growth of carbides, which would cause uneven distribution of elements such as W, Ta, Ti, and Mo.

[0018] In step (4) above, a directional columnar crystal casting (K40M) is prepared by directional solidification process. The process parameters for directional solidification are: refining at 1600℃ for 3 min, pouring at 1450~1480℃, upper zone temperature 1420~1460℃, lower zone temperature 1470~1500℃, standing for 2~5 min after pouring, and pulling speed 4~6mm / min.

[0019] In step (4) above, equiaxed crystal casting (DZ40M) is prepared by equiaxed crystal casting. The process parameters for equiaxed crystal casting are: refining at 1600℃ for 5 minutes, pouring at 1500~1550℃, preheating temperature of mold shell at 900℃, and sand cooling.

[0020] The prepared oriented columnar crystal castings and equiaxed crystal castings can achieve creep retardation properties of over 80h and 40h respectively under 980℃ / 83MPa conditions.

[0021] The advantages of this invention are as follows:

[0022] The purity of raw materials for this invention must reach 99% or higher. For Cr and Ti raw materials, the O content must not exceed 300 ppm, the N content 40 ppm, and the S content 60 ppm. During smelting, a staged material preparation and ultra-high temperature melt treatment process promotes the removal of gaseous elements, achieving a pure smelting effect. A low-temperature casting process ensures uniform distribution of eutectic and carbide phases in the alloy. The alloy prepared by this process has uniform composition, with a main element concentration difference ≤0.1% and a trace element concentration difference ≤0.2% between the upper and lower parts of the alloy ingot. It also has low gas content: O ≤10 ppm, N ≤10 ppm, and S ≤10 ppm, with a combined total ≤25 ppm. The cobalt-based high-temperature alloy involved in this invention exhibits good structural stability, excellent resistance to thermal fatigue, oxidation, and hot corrosion, making it suitable for turbine guide vanes operating at temperatures below 1040℃ for extended periods. Attached Figure Description

[0023] Figure 1 The smelting process curve of the master alloy in Example 1 is shown.

[0024] Figure 2 The smelting process curve of the master alloy in Example 2 is shown. Detailed Implementation

[0025] To further understand the present invention, the present invention is described below with reference to examples. However, the examples are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0026] Chemical composition (wt.%) of the high-purity cobalt-based superalloy K40M / DZ40M of this invention:

[0027] C: 0.45~0.55%, Cr: 24.5~26.4%, Ni: 9.5~11.5%, Al: 0.8~1.0%, W: 7.5~8.2%, Mo : 0.2~0.4%, Ti: 0.1~0.3%, B: 0.010~0.015%, Zr: 0.1~0.3%, Ta: 0.1~0.4%, balance Co.

[0028] The method for preparing high-purity cobalt-based superalloys proposed in this invention includes the following steps:

[0029] (1) The batching point shall be determined according to the actual burn-out of the smelting equipment, wherein the O content of Cr and Ti raw materials shall not exceed 300ppm, the N content shall not exceed 40ppm, and the S content shall not exceed 60ppm.

[0030] (2) Load the raw materials into the crucible from bottom to top in the order of Co(1 / 2), Cr, Ni, C, W, Ta, Co(1 / 2), and wrap the raw materials such as Al, Ti, B, and Zr with aluminum foil for later use;

[0031] (3) Close the vacuum furnace cover and evacuate the furnace. When the vacuum level inside the furnace reaches 40 Pa or above, turn on the power (1 / 4) Pe. At this time, the vacuum level will decrease to a certain extent. When the vacuum level recovers to 40 Pa or above again, increase the heating power to (1 / 2) Pe. When the vacuum level recovers to 40 Pa or above again, increase the heating power to Pe until the material is completely melted.

[0032] (4) Increase the melt temperature to 1550℃ and carry out high-temperature refining. The relationship between refining time and the quality of the smelted alloy is shown in Table 1. Select an appropriate refining time according to the quality of the smelted alloy. After refining, add Al, Ti, B and Zr, and stir for 2 to 5 minutes while maintaining (1 / 4) Pe power. Then turn off the power and cool down.

[0033] Table 1. Correspondence between high-temperature refining time and the quality of smelted alloys

[0034]

[0035] (5) After the surface temperature of the melt drops to about 1410°C, the power is increased to about (1 / 2) Pe to heat and stir the melt to about 1450°C before pouring. The ratio of the pouring nozzle diameter to the ingot mold diameter is about 1 / 6 to 1 / 4, which can obtain a high-purity master alloy ingot with uniform composition and low gas content.

[0036] (6) Prepare oriented columnar or equiaxed crystal castings according to requirements. The process parameters for oriented solidification are: refining at 1600℃ for 3 minutes, pouring at 1450~1480℃, upper zone temperature 1420~1460℃, lower zone temperature 1470~1500℃, standing for 2~5 minutes after pouring, and pulling rate 4~6mm / min. The process parameters for equiaxed crystal casting are: refining at 1600℃ for 5 minutes, pouring at 1500~1550℃, mold shell preheating temperature 900℃, and sand-buried cooling. Alloy castings with excellent high-temperature performance can be obtained.

[0037] Example 1:

[0038] A 25kg vacuum induction furnace was used for smelting the master alloy. The raw materials used included Cr with 230ppm O, 32ppm N, and 33ppm S; and Ti with 288ppm O, 19ppm N, and 56ppm S. The purity of other raw materials was not less than 99%. The smelting process curve is shown below. Figure 1 As shown.

[0039] The chemical composition of different parts of the prepared master alloy ingot is shown in Table 2.

[0040] Table 2 Chemical composition of different parts of the master alloy ingot, wt.%

[0041]

[0042] Equiaxed crystal test rods were prepared using a vacuum induction furnace. The preparation process was as follows: high-temperature refining at 1600℃ for 5 min, casting temperature at 1530℃, mold preheating temperature at 900℃, and cooling method was sand embedding. The crease life of the alloy at 980℃ / 83MPa is shown in Table 3.

[0043] Table 3. Durability of equiaxed crystal specimens

[0044]

[0045] Example 2:

[0046] A 200kg vacuum induction furnace was used for smelting the master alloy. The raw materials used included Cr with 262ppm O, 41ppm N, and 28ppm S; and Ti with 291ppm O, 21ppm N, and 43ppm S. The purity of other raw materials was not less than 99%. The smelting process curve is shown below. Figure 2 As shown in Table 4, the chemical composition of different parts of the prepared master alloy ingot is shown in Table 4.

[0047] Table 4 Chemical composition of different parts of the master alloy ingot, wt.%

[0048]

[0049] Directional columnar crystal specimens were prepared using a vacuum induction directional solidification furnace. The preparation process was as follows: refining at 1600℃ for 3 min, casting at 1470℃, upper zone temperature of 1440℃, lower zone temperature of 1480℃, standing for 4 min after casting, and pulling at a rate of 4 mm / min. The creep rupture life of the alloy at 980℃ / 83MPa is shown in Table 5.

[0050] Table 5. Durability of Oriented Columnar Crystal Test Rods

[0051]

[0052] It can be seen that the master alloy ingot prepared by this invention has a uniform chemical composition, low impurity content, and stable high-temperature creep performance.

Claims

1. A method for preparing a high-purity cobalt-based superalloy K40M / DZ40M, characterized in that: The chemical composition of this alloy, by weight percentage, is as follows: C: 0.45~0.55%, Cr: 24.5~26.4%, Ni: 9.5~11.5%, Al: 0.8~1.0%, W: 7.5~8.2%, Mo : 0.2~0.4%, Ti: 0.1~0.3%, B: 0.010~0.015%, Zr: 0.1~0.3%, Ta: 0.1~0.4%, balance of Co; The preparation method includes the following steps: (1) Load the raw materials into the crucible from bottom to top in the order of half Co, Cr, Ni, C, W, Ta and the other half Co, and wrap the remaining Al, Ti, B and Zr raw materials with aluminum foil for later use; (2) Melt the raw materials in the crucible in the vacuum induction furnace. When the melt is completely clear, raise the temperature to 1550℃ for high-temperature refining. After the high-temperature refining is completed, add Al, Ti, B and Zr raw materials wrapped in aluminum foil and stir for 2 to 5 minutes at 1 / 4 Pe power. Then turn off the power and cool down. The raw material melting process is as follows: power is supplied after the gas pressure inside the vacuum induction furnace is lower than 40Pa, using 1 / 4Pe, 1 / 2Pe and Pe, where Pe is the rated power of the vacuum furnace induction heating, to carry out staged material melting, during which the gas pressure inside the furnace should be kept below 40Pa; when the vacuum gauge reading is stable and lower than 40Pa, the material melting power is increased; after the melt is completely cleared, high-temperature refining is carried out. (3) After the surface of the melt cools down to 1410°C, the power is increased to 1 / 2Pe to heat the melt and the surface temperature of the melt is increased to 1450°C for casting to obtain the master alloy ingot; when casting the alloy melt, the ratio of the diameter of the pouring gate to the diameter of the ingot mold is 1 / 6 to 1 / 4; the melting point of the alloy is 1390°C. (4) The master alloy ingot is prepared into oriented columnar crystal or equiaxed crystal casting by vacuum casting; the oriented columnar crystal is prepared by directional solidification process, and the process parameters of directional solidification are: refining at 1600℃ for 3min, casting at 1450~1480℃, upper zone temperature 1420~1460℃, lower zone temperature 1470~1500℃, standing for 2~5min after casting, and pulling speed 4~6mm / min; The equiaxed crystal casting DZ40M was prepared by equiaxed crystal casting. The process parameters for equiaxed crystal casting were: refining at 1600℃ for 5 minutes, pouring at 1500~1550℃, preheating temperature of the mold shell at 900℃, and cooling by embedded sand.

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