A method for accelerating aging of GH4169 high-temperature alloy

Abstract

The present invention belongs to the field of heat treatment of metal materials, and in particular relates to a method for accelerated aging of GH4169 high-temperature alloy. The method first heats the GH4169 high-temperature alloy to 970±10°C for aging for 0.5-1.5 hours; then the alloy is subjected to secondary aging at 850-950°C for 500-1500 hours, and finally forms an aging structure equivalent to that under actual working conditions at different periods. The accelerated aging method provided by the present invention can quickly obtain different degrees of aging structure of GH4169 high-temperature alloy, and can accurately, quickly and reliably simulate the aging structure of GH4169 high-temperature alloy in actual working conditions, effectively solving the technical problem in the prior art that the aging structure of different degrees in the entire use stage cannot be obtained due to the short service time of GH4169 high-temperature alloy, and thus cannot be effectively evaluated for aging level.

Description

Technical Field

[0001] The invention belongs to the field of heat treatment of metal materials, and in particular relates to a method for accelerating aging of a GH4169 high-temperature alloy. Background Art

[0002] GH4169 is a Ni-Cr-Fe-based precipitation-strengthened high-temperature alloy with nickel as the primary element. Under normal conditions, the maximum service temperature that nickel-based high-temperature alloys can withstand can even exceed 1100°C. At relatively high service temperatures, GH4169 alloy exhibits high yield strength, excellent fatigue resistance, radiation resistance, oxidation resistance, corrosion resistance, and long-term structural stability, which can ensure the reliable operation of components. As a domestically produced high-temperature material, the application temperature of GH4169 alloy has been increased from the original 650°C to around 1000°C today. Its excellent mechanical properties in high-temperature environments make it the preferred material for high-temperature heat-resistant components in nuclear power plants. Long-term service of GH4169 alloy in the 850°C environment of gas-cooled fast reactors will cause the material to undergo "thermal aging" and its mechanical properties will deteriorate.

[0003] The constituent phases of GH4169 nickel-based high-temperature alloy include matrix phase γ phase, main strengthening phase γ″ phase, auxiliary strengthening phase γ′ phase and equilibrium phase δ phase of γ″ phase. In addition, it also includes a small amount of carbides and nitrides, such as MC, M6C and M 23 C6, etc. The primary strengthening phase of GH4169 alloy is γ″ (Ni3Nb), and the secondary strengthening phase is γ' (Ni3(Al,Ti)). γ″ is a metastable phase that gradually transforms into a stable δ phase (Ni3Nb) with increasing aging time under high temperature conditions. The precipitation temperature of the δ phase ranges from 720 to 980°C, with a peak precipitation temperature of approximately 900°C. The dissolution starts at 980°C, and complete dissolution is not achieved until 1020°C.

[0004] Currently, GH4169 alloy has been widely used in my country's ultra-supercritical (ULC) thermal power plants. However, due to long-term use in high-temperature, high-pressure, and corrosive media conditions, its microstructure ages over time. This degradation can lead to performance degradation of the GH4169 alloy, resulting in failures such as cracking and tube bursts during operation. These failures not only disrupt power plant operations and cause significant economic losses, but can also result in fatalities and other tragedies, with devastating social impacts. Consequently, there is an urgent need to establish reliability guidelines to guide on-site testing and safety assessments of thermal power plants. However, due to the limited service life of GH4169 alloy in actual operating conditions, it is not possible to assess the aging microstructure of the material at different levels throughout its lifetime, hindering a more accurate aging assessment of the GH4169 alloy. Summary of the Invention

[0005] The purpose of the present invention is to provide a heat treatment method for accelerated aging of GH4169 alloy to obtain aged structures of different degrees in GH4169 alloy, so as to solve the technical problem in the prior art that due to the short service life of GH4169 alloy, aged structures of different degrees in the entire service stage cannot be obtained, and thus effective aging level assessment cannot be carried out.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for accelerating aging of GH4169 high-temperature alloy comprises the following steps:

[0008] (1) Heat the GH4169 high temperature alloy to 970±10℃ for aging for 0.5-1.5 hours;

[0009] (2) After step (1) is completed, the alloy is subjected to secondary aging at 850-950°C for 500-1500 hours, ultimately forming an aging structure equivalent to that in actual working conditions at different periods.

[0010] Step (1) Aging is performed at the maximum precipitation temperature of the δ phase (Ni3Nb) (970±10°C). At this temperature, the δ phase will precipitate in large quantities, and the matrix and carbides will also undergo accelerated aging. The purpose of this step is to make the alloy reach an aging state equivalent to that after long-term use in a short period of time. Step (2) At 850-950°C, the Ni3Nb-type δ phase will precipitate at the fastest rate, and the metastable phase γ″ will transform into the δ phase. The purpose of this step is to simulate the aging structure of the alloy at different stages of use.

[0011] Furthermore, the chemical composition range of the GH4169 high-temperature alloy is: C≤0.08; Cr 17.00-21.00; Ni 50.00-55.00; Co≤1.0; Mo 2.80-3.30; Al 0.20-0.80; Ti 0.65-1.15; Nb 4.75-5.50; Mg≤0.010; B≤0.006; Si≤0.35; Mn≤0.35; P≤0.015; S≤0.015; and Cu≤0.300.

[0012] Furthermore, the time of step (1) is 1 hour.

[0013] Furthermore, the temperature for secondary aging in step (2) is 900°C.

[0014] Furthermore, before step (1), the GH4169 high temperature alloy is subjected to a standard heat treatment process.

[0015] Furthermore, the standard heat treatment process includes the following steps:

[0016] (i) Heat GH4169 superalloy to 970 ± 10 °C, hold for 0.5-1.5 hours, and then cool in oil;

[0017] (ii) heating the alloy to 700-750°C and holding the temperature for 7-9 hours;

[0018] (iii) Cool the alloy at a rate of 55-60°C per hour to 600-650°C, hold at this temperature for another 7-9 hours, and finally cool in air.

[0019] Furthermore, the insulation time in step (i) is 1 hour.

[0020] Furthermore, in step (ii), the alloy is heated to 720° C. and kept at this temperature for 8 hours.

[0021] Furthermore, in step (iii), the alloy is cooled to 620°C at a rate of 55-60°C per hour and kept at this temperature for 8 hours.

[0022] The purpose of subjecting GH4169 superalloy to a standard heat treatment process is to provide a uniform and stable microstructure for subsequent accelerated aging. Step (i) GH4169 superalloy is heated to 970±10°C. This high-temperature treatment homogenizes the alloy's internal microstructure and eliminates any internal stresses that may have occurred during the production process. Oil cooling allows for rapid cooling, preventing unwanted structural changes during the cooling process. Step (ii) GH4169 superalloy is heated to 700-750°C to further optimize the alloy's microstructure and achieve a relatively stable state. Step (iii) GH4169 superalloy is further adjusted by slow cooling to achieve a more uniform and stable microstructure.

[0023] The design idea of ​​the present invention is:

[0024] The selection of the accelerated aging test temperature should be based on two thermodynamic considerations: First, ensuring that the volume fraction of the precipitated phase within the accelerated aging temperature range remains essentially unchanged compared to the actual operating conditions, to avoid re-dissolution of the precipitated phase due to excessively high temperatures, which would reduce the volume fraction and prevent the precipitation characteristics from being the same as under actual operating conditions. Second, since the δ phase has different transition precipitation temperatures, the most appropriate temperature should be selected during accelerated aging based on its precipitation characteristics.

[0025] Because GH4169 alloy contains approximately 5 wt.% Nb and less than 0.08 wt.% C, long-term aging produces two main precipitation types: carbides and δ phase (Ni3Nb), with the δ phase being the predominant. At 900°C, the Ni3Nb-type δ phase precipitates most rapidly, and the metastable γ″ phase transforms to δ.

[0026] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0027] The accelerated aging method provided by this invention can rapidly obtain various degrees of aged microstructure in GH4169 alloy, accurately and reliably simulating the aged microstructure of GH4169 alloy under actual operating conditions. This solves the existing technical problem of inability to obtain various degrees of aged microstructure throughout the entire service life of GH4169 alloy due to its short service life, thus preventing effective aging grade assessment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the Vickers hardness change curve of the GH4169 high-temperature alloy of Example 1 after accelerated aging. In the figure, the horizontal axis is the aging time (h); the vertical axis HV is the Vickers hardness.

[0029] Figures 2(a)-2(c) show the metallographic structures of GH4169 after accelerated aging and standard heat treatment. Figure 2(a) shows the standard heat treatment state, while Figures 2(b)-2(c) show the corresponding metallographic structures after accelerated aging at 900°C for 500h and 1134h.

[0030] Figures 3(a)-3(c) show the scanned images of the accelerated aging and standard heat treatment states. Figure 3(a) shows the standard heat treatment state of GH4169, while Figures 3(b)-3(c) show the corresponding scanned images of the accelerated aging at 900°C for 500h and 1134h.

[0031] Figure 4 This is the metallographic structure diagram of comparative example 1 after accelerated aging at 800°C for 1000 hours.

[0032] Figure 5 This is the metallographic structure diagram of comparative example 2 after accelerated aging at 1000℃ for 50 hours.

[0033] Figure 6 This is the hardness change curve of GH4169 high-temperature alloy under accelerated aging at 800℃, 900℃ and 1000℃. DETAILED DESCRIPTION

[0034] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment provides a method for accelerated aging of GH4169 high-temperature alloy. The material used is cut from the fastener material used in a supercritical generator set of a power plant. The method includes the following steps:

[0037] Before accelerated aging, the GH4169 high-temperature alloy is subjected to a standard heat treatment process, which includes the following steps:

[0038] (i) GH4169 superalloy was heated to 970 °C, held at this temperature for 1 h, and then cooled in oil;

[0039] (ii) heating the alloy to 720°C and holding the temperature for 8 hours;

[0040] (iii) The alloy was cooled at a rate of 58°C per hour to 620°C, held at this temperature for another 8 hours, and finally cooled in air.

[0041] After the standard heat treatment process, accelerated aging is carried out through the following steps:

[0042] (1) Heating the GH4169 high temperature alloy to 970℃ for aging for 1 hour;

[0043] (2) After step (1) is completed, the alloy is subjected to secondary aging at 900°C for 500-1500 hours, ultimately forming an aging structure equivalent to that in actual working conditions at different periods.

[0044] like Figure 1 Figure 2 shows the hardness changes of the GH4169 superalloy of this embodiment after standard heat treatment and accelerated aging. Under this treatment, the hardness changes in accordance with microstructural degradation, with a rapid decrease followed by a gradual leveling off. By 500 hours of aging, the Vickers hardness has dropped to approximately HV 267, and by 1134 hours, it has dropped to nearly HV 270.

[0045] Figures 2(a)-(c) show a comparison of the microstructure after accelerated aging simulation and the metallographic structure after standard heat treatment. Figure 2(a) shows the standard heat-treated state of GH4169, while Figures 2(b)-2(c) show the corresponding metallographic structures after accelerated aging at 900°C for 500h and 1134h. As can be seen from the figures, compared to Figure 2(a), Figures 2(b)-(c) exhibit very similar microstructures, with relatively little variation in grain size. The microstructures are primarily composed of δ phase, which is distributed throughout the matrix. At this stage, the microstructures are both experiencing δ phase precipitation and a gradual transition from the metastable γ″ phase to the δ phase.

[0046] Figures 3(a)-3(c) show scans of the microstructure after accelerated aging simulation and the standard heat treatment state. Figure 3(a) shows the standard heat treatment state for GH4169, while Figures 3(b)-3(c) show the corresponding scans after accelerated aging at 900°C for 500 h and 1134 h. As can be seen from the figures, the precipitates in the microstructure under standard heat treatment are primarily located at grain boundaries. The scans of the microstructure after accelerated aging, as shown in Figures 3(b) and 3(c), show that the precipitates are primarily δ phase. Based on this, Image Pro-Plus analysis software was used to analyze the precipitates obtained at different aging times. The Axis (minor) parameter represents the change in precipitate size. The average value of this parameter for the precipitates in Figure 3(b) is 0.782, while the average value for the precipitates in Figure 3(c) is 0.802. Comparison shows that increasing aging time significantly coarsens the δ phase.

[0047] Comparative Example 1

[0048] This comparative example provides a method for accelerated aging of GH4169 high-temperature alloy, which differs from Example 1 in that the temperature of the secondary aging in step (2) is 800°C.

[0049] like Figure 4 The figure shows the metallographic structure of the comparative example after accelerated aging at 800°C for 1000 hours. As can be seen from the figure, the grain size of the matrix austenite structure has not changed much, the amount of δ precipitated phase in the matrix is ​​relatively small, and the overall structure is not significantly different from the initial standard heat treatment state.

[0050] Comparative Example 2

[0051] This comparative example provides a method for accelerated aging of GH4169 high-temperature alloy, which differs from Example 1 in that the temperature of the secondary aging in step (2) is 1000°C.

[0052] like Figure 5 The figure shows the metallographic structure of the comparative example after accelerated aging at 1000°C for 50 hours. As can be seen from the figure, the austenite grains in the matrix have been significantly coarsened, and the precipitated phase has been completely dissolved into the matrix.

[0053] like Figure 6 The figure shows the hardness changes of GH4169 superalloy after accelerated aging at 800°C, 900°C, and 1000°C. As can be seen from the figure, at 1000°C, the precipitated phases in the alloy completely dissolve back, and the GH4169 superalloy reaches matrix strength after 50 hours of aging, with no obvious aging pattern. At 800°C, the material ages more slowly, with no significant hardness drop after 1000 hours of aging. Longer aging times are required to see structural degradation.

[0054] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for accelerated aging of GH4169 high-temperature alloy, comprising the following steps: (1) Heat the GH4169 high temperature alloy to 970±10℃ for aging for 0.5-1.5 hours; (2) After step (1) is completed, the alloy is subjected to secondary aging at 850-950°C for 500-1500 hours, ultimately forming an aging structure equivalent to that in actual working conditions at different periods.

2. The method according to claim 1, wherein: The chemical composition range of the GH4169 high-temperature alloy is: C≤0.08; Cr 17.00-21.00; Ni 50.00-55.00; Co≤1.0; Mo 2.80-3.30; Al 0.20-0.80; Ti 0.65-1.15; Nb 4.75-5.50; Mg≤0.010; B≤0.006; Si≤0.35; Mn≤0.35; P≤0.015; S≤0.015; and Cu≤0.

300.

3. The method according to claim 1, wherein: The time of step (1) is 1 hour.

4. The method according to claim 1, wherein: The temperature for secondary aging in step (2) is 900°C.

5. The method according to claim 1, wherein: Before step (1), the GH4169 high temperature alloy is subjected to a standard heat treatment process.

6. The method according to claim 5, characterized in that: The standard heat treatment process includes the following steps: (i) Heat GH4169 superalloy to 970 ± 10 °C, hold for 0.5-1.5 hours, and then cool in oil; (ii) heating the alloy to 700-750°C and holding the temperature for 7-9 hours; (iii) Cool the alloy at a rate of 55-60°C per hour to 600-650°C, hold at this temperature for another 7-9 hours, and finally cool in air.

7. The method according to claim 6, characterized in that: The insulation time in step (i) is 1 hour.

8. The method according to claim 6, wherein: Step (ii) The alloy was heated to 720°C and kept at this temperature for 8 hours.

9. The method according to claim 6, wherein: Step (iii) Cool the alloy to 620°C at a rate of 55-60°C per hour and keep at this temperature for 8 hours.

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