Method for hindering growth of grain size of high-temperature alloy by using precipitated carbide phase

Abstract

The invention discloses a method for hindering grain size growth of a high-temperature alloy by using a precipitated carbide phase, and belongs to the technical field of preparation of high-temperature alloys. The high-temperature alloy comprises the following components in percentage by weight: less than or equal to 0.1% of C, less than or equal to 0.4% of Si, less than or equal to 0.5% of Mn, less than or equal to 0.01% of S, 11.0-14.0% of Cr, less than or equal to 0.02% of P, less than or equal to 0.35% of Al, 2.0-3.0% of Ti, 5.0-7.0% of Mo, less than or equal to 1.0% of Co, less than or equal to 0.5% of Cu, 40.0-45.0% of Ni, 0.01-0.02% of B and the balance of Fe. According to the method, a pure and uniform steel ingot is obtained by adopting vacuum induction furnace smelting and vacuum electroslag remelting, and a material with a uniform structure is obtained through optimized homogenization system, heating system, forging process, forging heating temperature and heat preservation measures in the forging process. And after heat treatment, the average grain size of the edge center of the grain size is kept at 4.5-5.5 levels. The high-temperature alloy material produced through the method is sufficient in margin, and the performance requirement of the high-temperature alloy of the steam turbine is met.

Description

Technical Field

[0001] The present invention relates to a preparation method for hindering the grain size growth of superalloys by precipitating carbide phases, specifically to a steam turbine material and its preparation method, belonging to the technical field of alloy material applications. Background Art

[0002] With the progress of technology, superalloys are increasingly widely used, so there are great requirements for the performance and structure of materials. The main technical requirements include chemical composition, mechanical properties, structure, and surface quality. The difficulty lies in the instability of the metallographic structure after heat treatment. Especially for some superalloys, the grain size level will have a significant impact on the service performance of the materials.

[0003] Steam turbine alloy materials are used to process high-temperature fasteners, load-bearing components, and high-temperature nuts and bolts for gas turbines. For the components in gas turbines, they not only bear high-temperature and high-load effects but also bear the cyclic effects of hot corrosion and torsional forces. Therefore, gas turbine materials should not only have high strength indexes but also good high-temperature creep properties, so there are extremely high requirements for the material performance and structure.

[0004] For high-solution superalloys, the grain size grows rapidly during heat treatment. Especially, there is an inconsistent phenomenon in the final forging temperature at the edge and the center. Therefore, it is a difficult point in the current processing to hinder the significant growth of grains as much as possible during heat treatment. Thus, the research and development of grain structure are of great significance.

[0005] However, in the existing methods, mainly trace grain boundary segregation elements (such as B, Zr, Hf, rare earth elements, etc.) are added to the alloy to inhibit grain boundary migration through the following mechanisms: filling grain boundary vacancies and reducing grain boundary energy; or forming stable grain boundary precipitation phases (such as borides). However, the disadvantages of such methods are as follows: excessive addition may form brittle phases (such as borides), reducing the toughness of the material. Element segregation may cause local corrosion or oxidation problems.

[0006] Therefore, there is an urgent need to provide a new method to hinder the grain size growth during the smelting process of superalloys and ensure its performance well. Summary of the Invention

[0007] The purpose of the present invention is to solve the above problems, thereby providing a preparation method for hindering the grain size growth of superalloys by precipitating carbide phases to meet the usage requirements in complex working environments.

[0008] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0009] A method for hindering the grain size growth of superalloys by precipitating carbide phases. The superalloy, by weight percentage, includes the following elemental compositions:

[0010] C: ≤0.1%, Si: ≤0.4%, Mn: ≤0.5%, S: ≤0.01%, Cr: 11.0 - 14.0%, P: ≤0.02%, Al: ≤0.35%, Ti: 2.35 - 3.0%, Mo: 5.0 - 7.0%, Co: ≤1.0%, Cu: ≤0.5%, Ni: 40.0 - 45.0%, B: 0.01 - 0.02%, the balance is Fe;

[0011] The method includes the following steps:

[0012] Step A: For furnace charging, nickel plates and pure iron are placed at the bottom, and returned materials, Fe-Mo or molybdenum bars are placed in the central high-temperature area. After breaking the vacuum, the furnace needs to be evacuated to <5 Pa and then charged again.

[0013] Step B: Prepare the alloy according to the above element composition ratio, and melt it in a 6T vacuum induction furnace. The melting temperature of the 6T furnace is 1410 - 1430 °C, the melting power of the 6T furnace ≤ 1200 KW. After all the raw materials are melted, measure the temperature, and continue to heat up at the same power until completely melted;

[0014] Step C: Keep the molten steel temperature in the 6T furnace in Step A at 1460 - 1480 °C for refining. After 40 minutes of refining, measure the air leakage rate, and then measure the air leakage rate every 10 minutes. When the air leakage rate ≤ 2 Pa / min and the difference between two air leakage rates ≤ 0.2 Pa / min, the refining ends.

[0015] Step D: After meeting the air leakage requirement, cut off the power and cool down to 1430 ± 10 °C, add Al → Ti in sequence, and stir for 10 - 20 minutes after adding;

[0016] Step E: Pouring steel: Take samples to analyze the composition, supplement the composition, add B-Fe, fill with argon, measure the temperature T = 1430 ± 10 °C and prepare for tapping. Cool the furnace for 40 minutes, break the vacuum, and send the demolded product for annealing while it is still hot;

[0017] Step F: Use the electrode bar in Step E as the electrode. After making the head flat and turning it smooth, place it in a protective atmosphere electroslag furnace for secondary remelting and purification to form an electroslag ingot;

[0018] Step G: Heat the electroslag ingot made in Step F to 1190 °C and hold for 48 h for homogenization treatment;

[0019] Step H: Heat the steel ingot in Step G to 1150 °C and hold for 4 h. Forge it into a finished product through two upsetting and two drawing processes using a press, and take measures such as cooling, annealing, and cladding, and then air cool it;

[0020] Step I. Perform post-forging heat treatment on the bar after forging in Step H: After forging, re-load the cold material into the furnace. After loading, hold at 800°C for 4 h, then continue heating to 1080°C and hold for 3.5 h. After air cooling to room temperature, load into the furnace and heat to 775°C, hold for 4 h, then air cool to room temperature. Then load into the furnace and raise the temperature to 715°C, hold for 24 h, and then air cool after discharging from the furnace to obtain the required material.

[0021] The grain size structure of the superalloy material obtained by the method of the present invention is as follows:

[0022] As a preferred solution, in Step B, adjust the content of each element during the melting process to make its weight ratio meet the design requirements.

[0023] As a preferred solution, in Step C, the air leakage rate must meet the refining requirements, and the refining is completed.

[0024] As a preferred solution, in Step D, after meeting the air leakage rate requirements, cut off the power and cool down, and then add Al and Ti according to the ratio.

[0025] As a preferred solution, in Step E, charge argon, measure the temperature T = 1430 ± 10°C and prepare for tapping.

[0026] As a preferred solution, in Step F, the composition of the protective atmosphere electroslag is: Al2O3:MgO:CaO:CaF2 = 20:5:10:65.

[0027] As a preferred solution, in Step G, the heating temperature is 1190°C and the holding time is 48 h.

[0028] As a preferred solution, in Step H, the forging process is: Under the condition of using heat preservation measures such as cooling forging, warming up, and cladding, perform rapid prototyping forging at 1070°C, 1090°C, 1110°C, 1130°C, and 1150°C for the key heats respectively. Finally, it is determined that forging with cladding and warming up at 1110°C for the key heat is the best.

[0029] As a preferred solution, in Step H, after loading into the furnace, hold at 800°C for 4 h, then continue heating to 1080°C and hold for 3.5 h, and then air cool to room temperature.

[0030] The beneficial effects of the present invention are as follows:

[0031] The method provided by the present invention for using the precipitated carbide phase to hinder the growth of grain size results in good surface quality for all batches of superalloy materials obtained. After heat treatment, the grain sizes at the edge and center are uniformly between 4.5 and 5.5 levels. Smelting and pouring are carried out in a vacuum environment, and the second-step refining must meet the air leakage rate requirements to promote the removal of gases and inclusions and form good surface quality. The second heat treatment is carried out in a heat treatment furnace. Cold materials are charged into the furnace and must be kept at the carbide precipitation temperature of 800 °C for an appropriate time. Detailed implementation manners

[0032] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] Example 1

[0034] For the superalloy described in this example, in terms of weight percentage, the composition and ratio requirements of each element are as shown in Table 1 below. The composition and ratio of each element selected in the example are shown in Table 2.

[0035] Table 1 Composition and ingredient requirements of each element (wt%)

[0036]

[0037] Table 2 Composition and ratio of each element selected in the example (wt%)

[0038] Element C Si Mn P S Cr Fe Sample 1 0.1 0.4 0.5 -- -- 11 Balance Sample 2 0.06 0.35 0.41 -- -- 12.5 Balance Sample 3 0.04 0.26 0.38 -- -- 14 Balance Element Ti Al Mo Ni Co B Cu Sample 1 2 0.35 5 40 1.0 0.01 0.5 Sample 2 2.55 0.23 6.8 42 0.8 0.015 -- Sample 3 3 0.29 7 45 0.6 0.02 --

[0039] Note: In the table, "--" indicates that the content of the corresponding component is as low as possible, and there is no need for special addition, only as an impurity component.

[0040] A method for using the precipitated carbide phase to hinder the growth of grain size. Taking sample 1 as an example, the specific method is as follows:

[0041] The method includes the following steps:

[0042] Step A: When charging the furnace, use nickel plates and pure iron as the bottom padding, and put the return materials, Fe-Mo or molybdenum bars in the central high-temperature area. After breaking the vacuum, the furnace needs to be evacuated to 5 Pa and then charged again.

[0043] Step B: Prepare the alloy according to the element composition ratio of sample 1 and melt it in a 6T vacuum induction furnace. The melting temperature of the 6T furnace is 1410 °C, and the melting power of the 6T furnace is 1200 KW. After all the raw materials are melted, measure the temperature and continue to heat up at the same power until completely melted.

[0044] Step C: Keep the molten steel temperature in the 6T furnace at 1460°C for refining. After 40 minutes of refining, measure the air leakage rate, and then measure the air leakage rate every 10 minutes. When the air leakage rate is as low as 2 Pa / min and the difference between two consecutive air leakage rates is within the range of 0.2 Pa / min, the refining is completed;

[0045] Step D: After meeting the air leakage requirement, cut off the power and cool down to 1430°C, then add Al→Ti in sequence. After adding, stir for 10 minutes;

[0046] Step E: Pouring steel: Take samples for composition analysis, supplement the composition, add B-Fe, fill with argon, measure the temperature T = 1430°C and prepare for tapping. Cool the furnace for 40 minutes, break the vacuum, and send the demolded billet for annealing while it is still hot;

[0047] Step F: Use the electrode rod in Step E as the electrode. After making the head flat and turning it smooth, place it in a protective atmosphere electroslag furnace for secondary remelting and purification to form an electroslag ingot;

[0048] Step G: Heat the electroslag ingot made in Step F to 1190°C and hold for 48 hours for homogenization treatment;

[0049] Step H: Heat the steel ingot in Step G to 1150°C, hold for 4 hours, and use a press to perform two upsetting and two drawing operations, and take measures such as cooling, annealing, and sheathing to forge it into a finished product, and then air cool it;

[0050] Step I: Perform post-forging heat treatment on the bar after forging in Step H: After forging, put the cold material back into the furnace. After putting it into the furnace, hold at 800°C for 4 hours and then continue to heat to 1080°C, hold for 3.5 hours, air cool to room temperature, then put it into the furnace and heat to 775°C, hold for 4 hours, then air cool to room temperature, put it into the furnace and raise the temperature to 715°C, hold for 24 hours, and then take it out of the furnace and air cool to obtain the required material.

[0051] The superalloy material prepared by the above method of the present invention is smelted under vacuum conditions, which can completely precipitate carbides, thus effectively preventing the growth of grain size. After testing 100 samples, it is found that the surface quality of all batches of superalloys prepared is good, and the grain size at the center and edge after heat treatment has reached a uniform level of 4.5.

[0052] Example 2

[0053] A method for using precipitated carbide phases to hinder the growth of grain size. Taking sample 2 as an example, the specific method is as follows:

[0054] The method includes the following steps:

[0055] Step A: The furnace loading requirements are that nickel plates and pure iron are placed at the bottom, and return materials, Fe-Mo or molybdenum bars are placed in the central high-temperature area. After breaking the vacuum, the furnace needs to be evacuated to 4.5 Pa and then charged;

[0056] Step B: Prepare an alloy according to the elemental composition ratio of Sample 2 and melt it in a 6T vacuum induction furnace. The melting temperature of the 6T furnace is 1420°C, and the melting power of the 6T furnace is 1180KW. After the raw materials are completely melted, measure the temperature and continue to heat up at the same power until completely melted;

[0057] Step C: Keep the molten steel temperature in the 6T furnace in Step A at 1470°C for refining. After 40 minutes of refining, measure the air leakage rate, and then measure the air leakage rate every 10 minutes. When the air leakage rate is 1.8 Pa / min and the difference between two air leakage rates is 0.18 Pa / min, the refining ends;

[0058] Step D: After meeting the air leakage requirement, cut off the power and cool down to 1420°C, add Al→Ti in sequence, and stir for 15 minutes after adding;

[0059] Step E: Pour the steel: Take samples to analyze the composition, supplement the composition, add B-Fe, fill with argon, measure the temperature T = 1420°C and prepare to tap the steel. Cool in the furnace for 40 minutes, break the vacuum, and demold and send it for annealing while it is still hot;

[0060] Step F: Use the electrode rod in Step E as the electrode. After making it flat and turning it smooth, place it in a protective atmosphere electroslag furnace for secondary remelting and purification to form an electroslag ingot;

[0061] Step G: Heat the electroslag ingot made in Step F to 1190°C and hold for 48 hours for homogenization treatment;

[0062] Step H: Heat the steel ingot in Step G to 1150°C, hold for 4 hours, and use a press to perform two upsetting and two drawing operations, and take measures such as cooling, annealing, and sheathing to forge it into a finished product, and then air cool it;

[0063] Step I: Perform post-forging heat treatment on the bar after forging in Step H: After forging, put the cold material back into the furnace. After putting it into the furnace, hold at 800°C for 4 hours and continue to heat to 1080°C, hold for 3.5 hours, air cool to room temperature, then put it into the furnace and heat to 775°C, hold for 4 hours, then air cool to room temperature, put it into the furnace and heat up to 715°C, hold for 24 hours, and take it out of the furnace and air cool to obtain the required material.

[0064] The superalloy material prepared by the above method of the present invention is smelted under vacuum conditions, which can completely precipitate carbides, thereby effectively preventing the growth of grain size. After testing 200 samples, it is found that the surface quality of all batches of superalloys prepared is good, and the grain sizes at the edge and center after heat treatment have reached a uniform grade of 5.5.

[0065] Example 3

[0066] A method for using precipitated carbide phases to hinder the growth of grain size. Taking Sample 3 as an example, the specific method is as follows:

[0067] The method includes the following steps:

[0068] Step A: The requirements for charging the furnace are to use nickel plates and pure iron as the bottom layer, and return materials, Fe-Mo or molybdenum bars are placed in the high-temperature central area. After breaking the vacuum, the furnace needs to be evacuated to 4.5 Pa and then charged;

[0069] Step B: Prepare the alloy according to the above element composition ratio, and melt it in a 6T vacuum induction furnace. The melting temperature of the 6T furnace is 1430 °C, and the melting power of the 6T furnace is 1150 KW. After the raw materials are completely melted, measure the temperature, and continue to heat up at the same power until completely melted;

[0070] Step C: Keep the molten steel temperature in the 6T furnace in Step A at 1480 °C for refining. After 40 minutes of refining, measure the leak rate, and then measure the leak rate every 10 minutes. If the leak rate is 1.2 Pa / min and the difference between two leak rates is within 0.2 Pa / min, the refining is completed;

[0071] Step D: After meeting the leak tightness requirements, cut off the power and cool down to 1440 °C, add Al→Ti in sequence, and stir for 20 minutes after adding;

[0072] Step E: Pouring steel: Take samples to analyze the composition, supplement the composition, add B-Fe, fill with argon, measure the temperature T = 1440 °C and prepare for tapping. Cool the furnace for 40 minutes, break the vacuum, and send the mold out for annealing while it is still hot;

[0073] Step F: Use the electrode bar in Step E as the electrode. After making the head flat and turning it smooth, place it in a protective atmosphere electroslag furnace for secondary remelting and purification to form an electroslag ingot;

[0074] Step G: Heat the electroslag ingot made in Step F to 1190 °C and hold for 48 h for homogenization treatment;

[0075] Step H: Heat the steel ingot in Step G to 1150 °C, hold for 4 h, and use a press to perform two upsetting and two drawing operations, and take measures such as cooling, baking, and cladding to forge it into a finished product, and then air-cool it;

[0076] Step I: Perform post-forging heat treatment on the bar after forging in Step H: After forging, re-load the cold material into the furnace. After loading into the furnace, hold at 800 °C for 4 h and continue to heat to 1080 °C, hold for 3.5 h, air-cool to room temperature, then load into the furnace and heat to 775 °C, hold for 4 h, then air-cool to room temperature, load into the furnace and heat up to 715 °C, hold for 24 h, and then take it out of the furnace and air-cool to obtain the required material.

[0077] The superalloy material prepared by the above method of the present invention is smelted under vacuum conditions, which can completely precipitate carbides, thereby effectively preventing the growth of grain size. After testing 500 samples, it is found that the surface quality of all batches of superalloys prepared is good, and the grain sizes at the edge and center after heat treatment have reached a uniform level of 5.0.

[0078] It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any aspect, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims concerned.

[0079] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other implementations that can be understood by those skilled in the art.

Claims

1. A method for hindering the grain size growth of superalloys by precipitating carbide phases, characterized in that, The superalloy, by weight percentage, comprises the following elemental composition: C: ≤0.1%, Si: ≤0.4%, Mn: ≤0.5%, S: ≤0.01%, Cr: 11.0 - 14.0%, P: ≤0.02%, Al: ≤0.35%, Ti: 2.0 - 3.0%, Mo: 5.0 - 7.0%, Co: ≤1.0%, Cu: ≤0.5%, Ni: 40.0 - 45.0%, B: 0.01 - 0.02%, and the balance is Fe; The method comprises the following steps: Step A: When charging the furnace, nickel plates and pure iron are used as the bottom padding. First, the return materials, Fe-Mo or molybdenum bars are placed in the central high-temperature area. After the vacuum is broken, the furnace is evacuated to <5 Pa, and then other raw materials are charged; Step B: Prepare the superalloy according to the designed elemental composition ratio, melt the raw materials into molten steel, the melting temperature is 1410 - 1430 °C, the melting power ≤1200 KW, and molten steel is obtained after complete melting; Step C: Keep the temperature of the molten steel in Step B at 1460 - 1480 °C for refining. After 40 minutes of refining, measure the air leakage rate, and then measure the air leakage rate every 10 minutes. When the air leakage rate ≤2 Pa / min and the difference between two air leakage rates ≤0.2 Pa / min, the refining ends; Step D: Cool down to 1430 ± 10 °C, add Al and Ti in sequence, and stir for 10 - 20 minutes after adding; Step E: Pouring steel: Sample and analyze the composition, supplement the composition, add B-Fe, fill with argon, and prepare to tap the steel when the temperature T = 1430 ± 10 °C. Cool in the furnace for 30 - 40 minutes, break the vacuum, demold and send it for annealing while it is still hot to prepare an electrode bar; Step F: Use the electrode bar in Step E as the electrode. After making the head flat and turning it smooth, place it in a protective atmosphere electroslag furnace for secondary remelting and purification to form an electroslag ingot; Step G: Carry out homogenization treatment on the electroslag ingot made in Step F; Step H: Heat the steel ingot made in Step G to 1150 °C, hold for 4 h, and use a press to forge it into a bar through two upsetting and two drawing processes, and then carry out temperature reduction, heating with a torch, and cladding measures, and air cool; Step I: Carry out post-forging heat treatment on the bar forged in Step H: After forging, charge it into the furnace with cold materials again. After charging, hold at 800 °C for 4 h, then continue to heat to 1080 °C, hold for 3.5 h. After the first air cooling to room temperature, charge it into the furnace and heat to 775 °C, hold for 4 h, then after the second air cooling to room temperature, charge it into the furnace and heat up to 715 °C, hold for 24 h, and then take it out of the furnace and air cool to obtain the required material.

2. The method according to claim 1, wherein: In Step B, the molten steel is melted in a 6T vacuum induction furnace. During the melting process of the raw materials, the temperature is measured, and the temperature is continuously increased while maintaining the same power until complete melting.

3. The method according to claim 1 or 2, characterized in that: In Step C, after the molten steel is heated to 1470 °C, refining is carried out, and the refining duration is determined according to the air leakage rate.

4. The method according to claim 1 or 2, characterized in that: In Step D, the amounts of Al and Ti added are based on the elemental ratio.

5. The method according to claim 1 or 2, characterized in that: In Step E, the amount of B-Fe added is based on the elemental ratio.

6. The method according to claim 1 or 2, characterized in that: In Step F, the composition of the electroslag, by weight ratio, is: Al2O3: MgO: CaO: CaF2 = 20:5:10:

65.

7. The method according to claim 1 or 2, characterized in that: In Step G, the temperature for homogenization treatment of the steel ingot is 1190 °C, and the holding time is 48 h.

8. The method according to claim 1 or 2, characterized in that: In step H, during the process of forging into bars, the key heating passes are subjected to rapid prototyping forging at 1070°C, 1090°C, 1110°C, 1130°C, and 1150°C respectively.

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