A method for producing an iron-nickel-based cast alloy and an iron-nickel-based cast alloy

Abstract

This invention discloses a method for preparing a nickel-based casting alloy and the nickel-based casting alloy itself, belonging to the field of alloy material technology. It overcomes the shortcomings of existing nickel-based high-temperature alloy preparation processes, which cannot meet the requirements for preparing large castings. The method for preparing the nickel-based casting alloy includes: Step 1, adding raw materials to molten steel according to a specified ratio to obtain the target composition; in the molten steel, the mass percentage of nitrogen (N) is ≤0.02%, and the mass percentage of silicon (Si) is ≤0.3%; Step 2, pouring the molten steel into a sand mold under inert gas protection, followed by molding, sand removal, and cooling to obtain an ingot; the molding temperature is ≥700℃; Step 3, homogenizing the ingot at 1150-1220℃ for 30-90 hours; Step 4, solution treating at 950℃-1150℃ for 5-30 hours; Step 5, heat treating at 650℃-850℃ for 6-24 hours. This invention can realize the preparation of large-size castings for ultra-supercritical units at 650℃ and above, meeting the performance requirements of large castings while reducing product costs.

Description

Technical Field

[0001] This invention belongs to the field of alloy materials technology, specifically relating to a method for preparing an iron-nickel-based casting alloy and the iron-nickel-based casting alloy itself. Background Technology

[0002] Based on my country's existing resource structure, in order to ensure energy supply, improve resource utilization efficiency, and simultaneously reduce CO2 and NOx emissions, x and SO x Reducing the emissions of oxides and improving the parameters of coal-fired power units is a feasible technical approach. Compared with existing ultra-supercritical units, the main difficulty in improving parameters lies in the performance requirements of the service environment for the unit, especially for turbine materials of units with parameters of 650℃ and above. When high-temperature and high-pressure steam flows into the turbine, its outer shell and valve shell are high-temperature pressure vessels, which simultaneously withstand multiple tests such as internal pressure, high temperature and corrosion. During service, they must withstand the effects of multiple factors such as high-temperature creep, thermal fatigue, oxidation and high-temperature flue gas corrosion. For turbine casings and valve bodies, due to their enormous weight and complex shape, casting is mostly used. Foreign countries have developed and verified casting materials for turbines operating at 650℃ and above. Based on In625 and In617, foreign companies have trial-produced valve bodies and other castings to verify the feasibility of casting components. However, since In625 and In617 are solid solution-strengthened nickel-based alloys containing large amounts of nickel and cobalt, the price of these raw materials has risen significantly in the face of the explosive demand for nickel and cobalt from new energy sources such as batteries. This makes the cost unacceptable for ultra-large castings such as cylinders. Meanwhile, foreign companies have also conducted preliminary casting tests on precipitation-strengthened nickel-based alloys. For example, castings using Haynes 282 and In740H revealed obvious microcracks in the finished products.

[0003] Compared to austenitic heat-resistant steel, the new iron-nickel-based superalloy exhibits superior performance at high temperatures. Furthermore, compared to nickel-based materials with nickel and cobalt as the matrix, its cost is significantly reduced due to the absence of cobalt and the extensive use of iron to replace nickel. Its excellent performance and cost-effectiveness suggest it could meet the requirements for turbine components in 650℃ ultra-supercritical units. However, existing iron-nickel-based superalloys employ smelting processes such as vacuum arc remelting and electroslag remelting, resulting in high costs. Moreover, the tonnage required for castings in 650℃ units is far from sufficient, making it impossible to manufacture large-tonnage castings. Additionally, iron-nickel-based alloy casting is prone to various casting defects. Currently, there are no corresponding iron-nickel-based casting alloys or preparation methods domestically or internationally that can meet the requirements of 650℃ ultra-supercritical units. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the preparation process of iron-nickel-based high-temperature alloys in the prior art cannot meet the requirements of large casting preparation, thereby providing a preparation method of iron-nickel-based casting alloy and iron-nickel-based casting alloy.

[0005] To this end, the present invention provides the following technical solution.

[0006] A method for preparing an iron-nickel-based casting alloy includes the following steps:

[0007] Step 1: Add the raw materials according to the proportion, and then proceed with EAF smelting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition.

[0008] The molten steel contains ≤0.02% N by mass and ≤0.3% Si by mass.

[0009] Step 2: Pour molten steel into a sand mold under inert gas protection, remove the sand, and cool to obtain an ingot; the temperature of removing the sand is ≥700℃.

[0010] Step 3: Homogenize the ingot at 1150-1220℃ for 30-90 hours, then cool it to room temperature;

[0011] Step 4: Solution treat at 950℃-1150℃ for 5-30 hours, then cool to room temperature;

[0012] Step 5: Heat treat at 650℃~850℃ for 6-24 hours, then cool to room temperature.

[0013] Furthermore, the raw materials include Fe, Cr, C, Mo, W, Ti, Al, B, Zr, and Ni.

[0014] Furthermore, the composition of the iron-nickel-based casting alloy, by mass percentage, includes: Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, with the remainder being Ni.

[0015] Furthermore, in step 1, the vacuum level during the VOD refining process does not exceed 120 Pa.

[0016] Furthermore, in step 2, the pouring temperature is 1500℃~1580℃.

[0017] Furthermore, in step 3, the ingot is heated to the homogenization treatment temperature along with the furnace.

[0018] Furthermore, in step 3, the ingot is heated to 900℃~1100℃ in the furnace and held for 5~10 hours, and then the temperature is further increased to the homogenization treatment temperature.

[0019] Furthermore, in step 4, the ingot is heated to the solution treatment temperature along with the furnace.

[0020] Furthermore, in step 4, the ingot is heated to 550-650°C in the furnace and held for 5-10 hours, and then the temperature is further increased to the solution treatment temperature.

[0021] Furthermore, the heating rate in steps 3 and / or 4 is ≤50℃ / h.

[0022] The obtained iron-nickel-based casting alloy has a two-phase structure: the matrix is ​​a face-centered cubic austenite, the precipitate is a dispersed γ′ phase with a volume fraction of >10% and a size of 30-100 nm.

[0023] The iron-nickel-based casting alloy has a room temperature yield strength >300MPa and a deformation rate >30%.

[0024] Optionally, in step 4, the cooling after solution treatment is either water cooling or air cooling.

[0025] Optionally, in step 5, the cooling after heat treatment is air cooling.

[0026] The technical solution of this invention has the following advantages:

[0027] 1. The preparation method of the iron-nickel-based casting alloy provided by the present invention includes the following steps: Step 1: Add raw materials according to the proportion, and then perform EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition; wherein the mass percentage of N element in the molten steel is ≤0.02% and the mass percentage of Si element is ≤0.3%; Step 2: Pour the molten steel into a sand mold under inert gas protection, and then remove the sand, cool it to obtain an ingot; Step 3: Homogenize the ingot at 1150-1220℃ for 30-90h and cool it to room temperature; Step 4: Solution treat at 950℃-1150℃ for 5-30h and cool it to room temperature; Step 5: Heat treat at 650℃-850℃ for 6-24h and cool it to room temperature.

[0028] This invention employs LF+VOD combined with inert gas protection casting, which differs from the high cost and limited capacity of casting nickel-based alloys using vacuum self-consumption and electroslag remelting processes, which are often used in casting nickel-based alloys. These processes are far from meeting the tonnage requirements of castings for 650℃ units. By utilizing existing equipment and strictly controlling the N and Si content, this invention achieves the melting and casting of nickel-based alloy castings, and can achieve melting and casting of more than 20 tons. This can meet the tonnage requirements of large castings while reducing product costs, and the performance meets the requirements of 650℃ and above ultra-supercritical units.

[0029] Nickel-based alloys suffer from significant casting stress during the casting process, which has long hindered the engineering manufacturing of large-size castings. This invention effectively eliminates casting stress and prevents large-size castings from cracking and becoming unusable by combining rational pouring, mortise and tenure, giving the alloy a significant competitive advantage in 650℃ ultra-supercritical units. Specifically, it strictly controls nitrogen element to prevent the excessive precipitation of harmful phases and carbides, strictly controls silicon element to prevent a significant decrease in plasticity, and combines pouring temperature control to prevent the excessive precipitation of harmful phases during solidification, which can lead to casting defects. High-temperature mortise and tenure effectively prevents the precipitation of harmful σ phases during cooling, preventing significant casting cracks during alloy cooling. A rational homogenization heat treatment process further reduces casting stress, preventing the initiation and propagation of cracks caused by casting stress. Heat treatment leads to precipitation strengthening, effectively improving the alloy's high-temperature creep performance and meeting the service requirements of 650℃ and above units.

[0030] 2. The method for preparing the iron-nickel-based casting alloy provided by the present invention, wherein the composition of the iron-nickel-based casting alloy, by mass percentage, includes: Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, and the remainder is Ni.

[0031] This invention relates to an iron-nickel-based casting alloy containing a high mass fraction of Cr to enhance its resistance to steam corrosion. By controlling the addition of Ti and Al elements, a stable and uniformly distributed strengthening phase Ni3(Al,Ti) is formed in the austenite, effectively improving its strength and high-temperature creep resistance. The content of high-cost solid solution elements is reduced, further lowering the alloy's cost. A reasonable combination of processes enables castings without significant casting defects, while a reasonable heat treatment process controls the microstructure to ensure its performance. Attached Figure Description

[0032] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0033] Figure 1 This is a microstructure diagram of the iron-nickel-based casting alloy prepared in Example 1;

[0034] Figure 2The microstructure characteristics of the iron-nickel based casting alloy prepared in Example 1 after heat exposure at 650°C for 3000 h. Detailed Implementation

[0035] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0036] Where specific experimental steps or conditions are not specified in the examples, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0037] Example 1

[0038] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0039] Step 1: Add the raw materials to the electric furnace according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree is 90 Pa.

[0040] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1560℃. Then, remove the sand at 1000℃ and air-cool it to obtain an ingot.

[0041] Step 3: The ingot is heated to 950℃ in the furnace at a rate of 50℃ / h and held for 8 hours; then the temperature is increased to 1200℃ and homogenized for 40 hours, followed by air cooling to room temperature.

[0042] Step 4: Heat the ingot to 1020℃ in the furnace at a rate of 50℃ / h, perform solution treatment for 10h, and then air cool to room temperature.

[0043] Step 5: Heat treat at 650℃ for 6 hours, then air cool to room temperature.

[0044] The iron-nickel-based casting alloy in this embodiment has good casting performance, with a casting weight of about 45 tons, and no obvious casting cracks or surface cracks were produced.

[0045] Example 2

[0046] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0047] Step 1: Add the raw materials according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree is 60 Pa.

[0048] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1550℃. Then, at 1000℃, the mold is opened, the sand is removed, and the mold is cooled by air to obtain an ingot.

[0049] Step 3: The ingot is heated to 1220℃ in the furnace at a rate of 40℃ / h, homogenized for 30h, and then water-cooled to room temperature.

[0050] Step 4: The ingot is heated to 1050℃ in the furnace at a rate of 40℃ / h, and then solution treated for 15h before being water-cooled to room temperature.

[0051] Step 5: Heat treat at 650℃ for 6 hours, then air cool to room temperature.

[0052] Example 3

[0053] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0054] Step 1: Add the raw materials according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree is 120 Pa.

[0055] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1580℃. Then, at 900℃, the mold is opened, the sand is removed, and the mold is cooled by air to obtain an ingot.

[0056] Step 3: The ingot is heated to 1190℃ in the furnace at a rate of 50℃ / h, homogenized for 60h, and then air-cooled to room temperature.

[0057] Step 4: The ingot is heated to 1050℃ in the furnace at a rate of 50℃ / h, and then solution treated for 20h before being water-cooled to room temperature.

[0058] Step 5: Heat treat at 800℃ for 6 hours, then air cool to room temperature.

[0059] Example 4

[0060] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0061] Step 1: Add the raw materials according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree shall not exceed 70 Pa.

[0062] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1560℃. Then, remove the sand at 1000℃ and air-cool it to obtain an ingot.

[0063] Step 3: The ingot is heated to 1180℃ in the furnace at a rate of 40℃ / h, homogenized for 90h, and then water-cooled to room temperature.

[0064] Step 4: The ingot is heated to 1050℃ in the furnace at a rate of 40℃ / h, and then solution treated for 10h before being water-cooled to room temperature.

[0065] Step 5: Heat treat at 650℃ for 6 hours, then air cool to room temperature.

[0066] Example 5

[0067] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0068] Step 1: Add the raw materials according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree shall not exceed 70 Pa.

[0069] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1500℃. Then, at 850℃, the mold is opened, the sand is removed, and the mold is cooled by air to obtain an ingot.

[0070] Step 3: The ingot is heated to 1150℃ in the furnace at a rate of 30℃ / h, homogenized for 90h, and then water-cooled to room temperature.

[0071] Step 4: The ingot is heated to 1050℃ in the furnace at a rate of 30℃ / h, and then solution treated for 15h before being water-cooled to room temperature.

[0072] Step 5: Heat treat at 700℃ for 10 hours, then air cool to room temperature.

[0073] Example 6

[0074] This embodiment provides a method for preparing an iron-nickel-based casting alloy, the composition of which is shown in Table 1, and includes the following steps:

[0075] Step 1: Add the raw materials according to the formula, and proceed with EAF melting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. During the VOD refining process, the vacuum degree shall not exceed 60 Pa.

[0076] Step 2: Pour molten steel into a sand mold under inert gas protection at a pouring temperature of 1150℃. Then, at 900℃, the mold is opened, the sand is removed, and the mold is cooled by air to obtain an ingot.

[0077] Step 3: The ingot is heated to 900℃ in the furnace at a rate of 50℃ / h and held for 6h; then the temperature is increased to 1200℃ and homogenized for 40h, and then cooled to room temperature.

[0078] Step 4: The ingot is heated to 600℃ in the furnace at a rate of 50℃ / h and held for 10h; then the temperature is increased to 1020℃ at a rate of 50℃ / h and the solution is treated for 10h, followed by water cooling to room temperature.

[0079] Step 5: Heat treat at 650℃ for 20 hours, then air cool to room temperature.

[0080] Comparative Example 1

[0081] Comparative Example 1 is an existing nickel-based superalloy, In625 alloy, the composition of which is shown in Table 1.

[0082] Comparative Example 2

[0083] This comparative example provides a method for preparing an iron-nickel-based casting alloy, which is basically the same as that in Example 1, except that in this comparative example, the N content in the molten steel obtained in step 1 is 0.021% and the Si content is 0.36%.

[0084] Table 1. Composition percentages of the alloys in the examples and comparative examples (balance: Ni).

[0085] Fe Cr Mo W Ti Al C B Nb Co Zr P N Si Example 1 45 16 0.5 0.5 1.9 1.6 0.07 0.005 0.08 - 0.03 ≤0.01 ≤0.02 0.1 Example 2 40 18 0.5 0.4 2.1 1 0.1 0.007 0.05 - 0.02 ≤0.01 ≤0.02 0.2 Example 3 48 14 0.2 1 1.5 1.5 0.08 0.005 0.1 - 0.02 ≤0.01 ≤0.02 0.1 Example 4 44 16 1 0.3 1 1.8 0.08 0.005 0.05 - 0.02 ≤0.01 ≤0.02 0.3 Example 5 41 17 0.8 0.6 1.6 1.7 0.03 0.003 0.04 - 0.02 ≤0.01 ≤0.02 0.2 Example 6 46 18 0.8 0.4 1.3 1.4 0.05 0.005 0.04 - 0.02 ≤0.01 ≤0.02 0.3 Comparative Example 1 - 21.3 8.9 - 0.22 0.15 0.02 3.3 0.16 0.03 - - 0.17 Comparative Example 2 45 16 0.5 0.5 1.9 1.6 0.07 0.005 0.08 0.03 ≤0.01 0.021 0.36

[0086] Test case

[0087] Figure 1 The microstructure of the iron-nickel-based casting alloy prepared in Example 1 shows a small amount of primary carbides distributed within and at grain boundaries. Simultaneously, an ordered precipitate phase γ′(Ni3(Al,Ti)) is uniformly distributed within the grains, with a volume fraction of approximately 15% and a size distribution of 30-80 nm. Discontinuous secondary carbides M are precipitated at the grain boundaries. 23 C6.

[0088] Figure 2 The microstructure characteristics of the iron-nickel based casting alloy prepared in Example 1 after heat exposure at 650°C for 3000 h.

[0089] Table 2 Room temperature mechanical properties of the alloy

[0090] alloy Yield strength (MPa) Tensile strength (MPa) Elongation (%) Example 1 339 603 45 Comparative Example 1 281 527 44 Comparative Example 2 316 597 23

[0091] Table 3 High-Temperature Duration Strength

[0092] alloy temperature 220MPa(h) 180 MPa(h) Comparative Example 1 700℃ 1087h Comparative Example 2 700℃ 302h Example 1 700℃ ＞3000h Comparative Example 1 650℃ 3695h Comparative Example 2 650℃ 528h Example 1 650℃ ＞5000h

[0093] As shown in Tables 2 and 3, the mechanical properties and high-temperature creep strength of the iron-nickel-based casting alloy of this invention are significantly improved compared to the comparative example, exhibiting excellent strength and plasticity as well as excellent microstructural stability. The method of this invention, while meeting the tonnage requirements of large castings, reduces product costs and improves the mechanical properties and high-temperature creep strength of the products.

[0094] The iron-nickel-based high-temperature alloy of the present invention is suitable for high-temperature components of ultra-supercritical steam turbines at 650°C and above, such as cylinders and valve housings of 650°C ultra-supercritical units.

[0095] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method of producing an iron-nickel based cast alloy, characterized by, Includes the following steps: Step 1: Add the raw materials according to the proportion, and then proceed with EAF smelting → LF refining → VOD refining → LF refining to obtain molten steel with the target composition. In the molten steel, the mass percentage of nitrogen (N) is ≤0.02%, and the mass percentage of silicon (Si) is ≤0.3%. Step 2: Pour molten steel into a sand mold under inert gas protection, remove the sand, and cool to obtain an ingot; the temperature of removing the sand is ≥700℃. Step 3: Homogenize the ingot at 1150-1220℃ for 30-90 hours, then cool it to room temperature; Step 4: Solution treat at 950℃-1150℃ for 5-30 hours, then cool to room temperature; Step 5: Heat treat at 650℃~850℃ for 6-24 hours, then cool to room temperature; The composition of the iron-nickel-based casting alloy, by mass percentage, includes: Fe: 40%-48%, Cr: 14%-18%, Mo: 0.2%-1.0%, W: 0.3%-1.0%, 0.5%≤Mo+W≤1.4%, Ti: 1.0%-2.1%, Al: 1.0%-1.8%, Nb≤0.1%, Zr≤0.03%, C: 0.03%-0.1%, B: ≤0.007%, P: ≤0.01%, N≤0.02%, Si≤0.3%, with the remainder being Ni.

2. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 1, during the VOD refining process, the vacuum level does not exceed 120 Pa.

3. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 2, the pouring temperature is 1500℃~1580℃.

4. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 3, the ingot is heated to the homogenization treatment temperature along with the furnace.

5. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 3, the ingot is heated to 900℃~1100℃ in the furnace and held for 5~10 hours, and then the temperature is further increased to the homogenization treatment temperature.

6. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 4, the ingot is heated to the solution treatment temperature along with the furnace.

7. The method of producing an iron-nickel based cast alloy according to claim 1, characterized by, In step 4, the ingot is heated to 550-650℃ in the furnace and held for 5-10 hours, and then the temperature is further increased to the solution heat treatment temperature.

8. The method of producing an iron-nickel based cast alloy according to any one of claims 3 to 7, characterized in that, The heating rate in step 3 and / or step 4 is ≤50℃ / h.

9. An iron-nickel based cast alloy produced according to the method of any one of claims 1 to 8, characterized in that, The iron-nickel-based casting alloy has a two-phase structure: the matrix is ​​a face-centered cubic austenite, and the precipitated phase is a dispersed γ′ phase with a volume fraction of >10% and a size of 30~100nm. The iron-nickel-based casting alloy has a room temperature yield strength >300MPa and a deformation rate >30%.

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