Aluminum-containing austenitic heat-resistant steel and its preparation method and use

Abstract

The present invention relates to the field of heat-resistant steel, and in particular to a novel aluminum-containing austenitic heat-resistant steel, a preparation method thereof, and uses thereof. The aluminum-containing austenitic heat-resistant steel is composed of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si, and Ti elements, wherein Ni is (19-31) wt.%, Cr is (13.5-14) wt.%, Mn is (0.8-1.0) wt.%, Mo is less than 0.03 wt.%, W is (1.8-2.2) wt.%, C is (0.055-0.065) wt.%, Nb is (0.55-0.65) wt.%, Al is (2.5-3.0) wt.%, Cu is (1.3-1.6) wt.%, Si is (0.1-0.15) wt.%, Ti is (0.01-0.015) wt.%, and the balance is Fe. After the Cu element is added to the aluminum-containing austenitic heat-resistant steel, the L12-Ni3Al phase is introduced, which enhances the precipitation strengthening effect of the aluminum-containing austenitic heat-resistant steel; after aging treatment, there are four precipitated phases, namely, NbC phase, L12-Ni3Al phase, M 23 C6 phase and B2‑NiAl phase, among which L12‑Ni3Al phase is dispersed in the matrix, which can effectively improve the high-temperature strength of the material and has excellent high-temperature oxidation performance, and can be used in key components of thermal power generating units.

Description

Technical Field

[0001] The present invention belongs to the field of heat-resistant steel, and in particular relates to an aluminum-containing austenitic heat-resistant steel and a preparation method and application thereof. Background Art

[0002] Environmental pollution is currently one of the greatest challenges facing humanity. Coal will remain the primary energy source for power generation for the foreseeable future, and reducing or eliminating CO2 emissions is a top priority for thermal power plants. Increasing the steam temperature and pressure of thermal power units can effectively improve power generation efficiency, reduce coal consumption, and thus reduce CO2 emissions. Therefore, the development of ultra-supercritical power generation technology with a pressure range of 700°C / 35 MPa or higher is urgent. A key constraint on the development of thermal power generation technology is the choice of materials for key components in thermal power generation units. Martensitic heat-resistant steel experiences significant deterioration in its structural stability and oxidation resistance at temperatures exceeding 650°C. Nickel-based alloys can meet service requirements, but they are expensive and have high economic costs. Austenitic heat-resistant steel, with its excellent oxidation and corrosion resistance and high-temperature endurance, holds broad application prospects in thermal power generation.

[0003] The excellent oxidation resistance of aluminum-containing austenitic heat-resistant steel (AFA steel) stems from the Al element in the matrix. At high temperatures, this element forms an Al2O3 oxide film, which provides superior protection compared to Cr2O3. However, how to further improve the creep properties of the material while maintaining excellent oxidation resistance has become a key concern.

[0004] The second phases that may appear in aluminum-containing austenitic heat-resistant steels include MC carbides, M 23 C6 phase, B2-NiAl phase, σ phase, Laves phase and L12-Ni3Al phase. Generally speaking, nano-sized MC-type carbides can provide the most effective strengthening effect, thereby obtaining high-temperature creep strength. MC-type carbides mainly refer to NbC, TiC, etc. In addition, nano-sized L12 phase can also effectively improve the high-temperature creep strength of aluminum-containing austenitic heat-resistant steel. L12 phase is a common precipitation phase in nickel-based alloys, and maintains a coherent relationship with the austenite matrix. If you want to obtain this phase in aluminum-containing austenitic heat-resistant steel, increasing the Ni content is the most effective means. At the same time, adjusting or adding other alloying elements to promote the precipitation of L12 phase is also a feasible method.

[0005] The original aluminum-containing austenitic heat-resistant steel mainly used nano-sized MC-type carbides as the main strengthening phase, and lacked the coherent strengthening mechanism similar to the γ / γ′ two-phase structure in nickel-based high-temperature alloys. Therefore, based on the current development status of aluminum-containing austenitic heat-resistant steel, the strength of the material can still be further improved, and there is still room for optimization of the material composition. Summary of the Invention

[0006] The purpose of the present invention is to provide an aluminum-containing austenitic heat-resistant steel (hereinafter referred to as 30Ni-AFA steel), and to propose a preparation method and use thereof. The provided 30Ni-AFA steel contains NbC phase, L12-Ni3Al phase, M 23 C6 phase and B2-NiAl phase, among which the average particle size of L12-Ni3Al phase only grows to 28.5 nm during aging for 480 h, and the coarsening rate is very slow, which is conducive to the improvement of the high-temperature strength of the material. It has excellent antioxidant and corrosion resistance as well as high-temperature endurance strength to meet the requirements of ultra-supercritical power generation technology in thermal power generating units above 700℃ / 35MPa.

[0007] According to the first aspect of the present invention, the present invention provides an aluminum-containing austenitic heat-resistant steel: composed of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si and Ti elements, with the following mass percentages: Ni is (29-31) wt.%, Cr is (13.5-14) wt.%, Mn is (0.8-1.0) wt.%, Mo<0.03 wt.%, W is (1.8-2.2) wt.%, C is (0.055 -0.065) wt.%, Nb is (0.55-0.65) wt.%, Al is (2.5-3.0) wt.%, Cu is (1.3-1.6) wt.%, Si is (0.1-0.15) wt.%, Ti is (0.01-0.015) wt.%, and the balance is Fe. The sum of the mass percentages of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si and Ti is 100wt.%.

[0008] The 30Ni-AFA steel provided by the manufacturer contains NbC phase, L12-Ni3Al phase, M 23 C6 phase and B2-NiAl phase, among which the average particle size of L12-Ni3Al phase only grows to 28.5nm during aging for 480h, and the coarsening rate is very slow, which is beneficial to the improvement of the high temperature strength of the material.

[0009] The 30Ni-AFA steel provided contains 12 elements, among which Al is a ferrite-forming element. To suppress the formation of δ-ferrite, Mn is added to adjust the content. For example, between 0.8% and 1% can make the matrix of 30Ni-AFA steel a single austenite. Adding too much can negatively affect the material's oxidation resistance. W is added to suppress the coarsening of the Laves phase while achieving a solid solution strengthening effect. This is because the Laves phase is composed of elements such as Fe, Mo, and W. At high temperatures, the diffusion rate of W is greater than that of Mo, which can effectively reduce the coarsening rate of the Laves phase. The L12-Ni3Al phase is the primary strengthening phase in nickel-based alloys. Adding an appropriate amount of Cu can promote the precipitation of the L12-Ni3Al phase in AFA steel, thereby reducing the Ni content and lowering the material cost. The Si content cannot be too high, otherwise it will reduce the material's high-temperature oxidation resistance. Adding a small amount of Si can help refine the size of the Laves phase. In addition, there is a small amount of Ti element in the material, which can stabilize the L12-Ni3Al phase.

[0010] Furthermore, the mass fraction of Ni is 29.74%, the mass fraction of Cr is 14.31%, the mass fraction of Mn is 1.05%, the mass fraction of Mo is 0.03%, the mass fraction of W is 1.84%, the mass fraction of C is 0.06%, the mass fraction of Nb is 0.59%, the mass fraction of Al is 2.81%, the mass fraction of Cu is 1.52%, the mass fraction of Si is 0.11%, the mass fraction of Ti is 0.01%, and the balance is Fe. The matrix of the cast 30Ni-AFA steel is a single austenite, and large and small NbC phases are randomly distributed in the matrix. After forging and aging treatment, L12-Ni3Al phase, Mn phase and NbC phase are precipitated in the 30Ni-AFA steel. 23 C6 phase, and B2-NiAl phase.

[0011] Furthermore, the forging temperature of the 30Ni-AFA steel is 1150°C, and the solution temperature is 1200°C, which is higher than the dissolution temperature of all precipitated phases in the 30Ni-AFA steel except NbC. The aging temperature is 700°C, which is the expected service temperature of the 30Ni-AFA steel. During the aging process, supersaturated elements will precipitate in the form of a second phase, thereby improving the strength of the material. After aging for 480 hours, the tensile strength of the 30Ni-AFA steel reaches 898MP and the yield strength is 554MPa. The oxidation temperature is 800°C, and after oxidation for 512 hours, the weight gain per unit area is 1.59mg / cm -2 .

[0012] In addition, according to a second aspect of the present invention, the present invention provides a method for preparing 30Ni-AFA steel, wherein the aluminum-containing austenitic heat-resistant steel is the 30Ni-AFA steel described in the first aspect of the present invention, and the preparation method is as follows:

[0013] a. The raw materials are mixed and melted to obtain as-cast 30Ni-AFA steel;

[0014] b. The ingot was then cast to obtain a cylindrical ingot of the 30Ni-AFA steel, which was naturally cooled to room temperature; the as-cast 30Ni-AFA steel was uniformly heated to 1150°C during 8 hours of heating, and after holding for 2 hours, the ingot was forged in four dimensions along the radial direction to obtain a forged steel plate of the 30Ni-AFA steel, which was naturally cooled to room temperature;

[0015] c The forged state 30Ni-AFA steel was solution treated in a vacuum environment at 1200 ℃ for 2h to obtain a solid solution state 30Ni-AFA steel;

[0016] d. The solid solution state 30Ni-AFA steel was aged in air at 700 ℃ for 480h to obtain aged state 30Ni-AFA steel;

[0017] e. The aged 30Ni-AFA steel was oxidized in dry air at 800°C.

[0018] According to the third aspect of the present invention, the present invention provides an application of 30Ni-AFA steel in the field of improving the strength of aluminum-containing austenitic heat-resistant steel, wherein the aluminum-containing austenitic heat-resistant steel is the 30Ni-AFA steel described in the first aspect of the present invention. The aged 30Ni-AFA steel product of the present invention is mainly suitable for key components in critical power generation boilers exceeding 700°C in thermal power plants.

[0019] Further, the component is a superheater or a reheater.

[0020] Compared with the existing technology, the technical solution of this application has the following beneficial effects:

[0021] 1. The present invention discloses an aluminum-containing austenitic heat-resistant steel and its preparation method and use. After adding Cu element, L12-Ni3Al phase is introduced, which enhances the precipitation strengthening effect of aluminum-containing austenitic heat-resistant steel. After aging treatment, there are four precipitated phases, NbC phase, L12-Ni3Al phase, M 23 C6 phase and B2-NiAl phase, among which L12-Ni3Al phase is dispersed in the matrix, which can effectively improve the high-temperature strength of the material. At the same time, it has excellent high-temperature oxidation performance and can be used in key components of thermal power generating units.

[0022] 2. The aluminum-containing austenitic heat-resistant steel and its preparation method and use demonstrated in the present invention can withstand high temperatures of up to 700°C and above, meeting the use requirements of critical power generation boilers exceeding 700°C in thermal power plants.

[0023] 3. The present invention demonstrates an aluminum-containing austenitic heat-resistant steel, its preparation method, and its use. During aging at 700°C for 12-480h, the average particle size of L12-Ni3Al grows from 9.4nm to 28.5nm at a very slow growth rate, which is beneficial to improving the high-temperature strength of the material.

[0024] 4. The aluminum-containing austenitic heat-resistant steel and its preparation method and use shown in the present invention has a weight gain per unit area of ​​only 1.59 mg / cm after oxidation in dry air at 800°C for 512 hours. -2 , proving that 30Ni-AFA steel has excellent oxidation resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The solidification path diagram of 30Ni-AFA steel and its local magnified diagram calculated by JMatPro;

[0026] Figure 2 SEM images of 30Ni-AFA steel after different aging times, where (a) is a low-magnification image after aging for 0 h, (b) is a high-magnification image after aging for 0 h, (c) is a image after aging for 2 h, (d) is a image after aging for 12 h, (e) is a image after aging for 24 h, (f) is a image after aging for 120 h, (g) is a image after aging for 480 h, and (h) is a image after aging for 960 h.

[0027] Figure 3 The recrystallization conditions of 30Ni-AFA steel, where (a) is the recrystallization distribution diagram and (b) is the recrystallization fraction diagram;

[0028] Figure 4 TEM bright field images of 30Ni-AFA steel, where (a) is the bright field image inside the grain after aging for 24 h, and (b) is the bright field image of the grain boundary after aging for 480 h.

[0029] Figure 5 TEM dark field images of 30Ni-AFA steel, where (a) is the dark field image after aging for 12 h, (b) is the dark field image after aging for 120 h, and (c) is the dark field image after aging for 480 h.

[0030] Figure 6 The average radius and volume fraction of the L12-Ni3Al phase after aging for different times, where (a) is the average radius of the L12-Ni3Al phase and (b) is the volume fraction of the L12-Ni3Al phase;

[0031] Figure 7 The room temperature tensile curves of 30Ni-AFA steel after aging, where (a) is the stress-strain curve and (b) is the yield strength curve;

[0032] Figure 8 is the Vickers hardness of 30Ni-AFA steel;

[0033] Figure 9 Mass gain of 30Ni-AFA steel in dry air at 800°C as a function of oxidation time. DETAILED DESCRIPTION

[0034] First of all, it should be noted that in the various embodiments described, identical components are provided with identical reference numerals or identical component names, wherein the disclosure contained throughout the entire description can be transferred to the same components having the same reference numerals or identical component names. Positional designations selected in the description, such as top, bottom, lateral, etc., also refer to the directly described and illustrated figures and are transferred to the new position in the event of a change in position.

[0035] Example 1: Material preparation

[0036] Disclosed are aluminum-containing austenitic heat-resistant steel, a preparation method thereof, and uses thereof. The steel is composed of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si, and Ti elements, wherein the mass fraction of Ni is 29%-31%, the mass fraction of Cr is 13.5%-14%, the mass fraction of Mn is 0.8%-1.0%, the mass fraction of Mo is less than 0.03%, the mass fraction of W is 1.8%-2.2%, the mass fraction of C is 0.055%-0.065%, the mass fraction of Nb is 0.55%-0.65%, the mass fraction of Al is 2.5%-3%, the mass fraction of Cu is 1.3%-1.6%, the mass fraction of Si is 0.1%-0.15%, the mass fraction of Ti is 0.01%-0.015%, and the balance is Fe. The sum of the mass percentages of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si, and Ti is 100%.

[0037] In this example, a vacuum induction melting furnace was used to prepare cylindrical ingots of 30Ni-AFA steel. To eliminate defects such as as-cast shrinkage produced during the casting process and to optimize the microstructure, the as-cast 30Ni-AFA steel was free-forged to obtain a forged steel plate with a forging ratio of 3.5:1. The precipitation behavior of the second phase in aluminum-containing austenitic heat-resistant steel was studied by solution treatment and aging. The actual composition of 30Ni-AFA steel is shown in Table 1. In Table 1, the iron element Bal. represents the rest of the components except for those in Table 1.

[0038] Table 1 Actual composition of 30Ni-AFA steel (wt.%)

[0039] Example 2: Thermodynamic calculations

[0040] like Figure 1 The calculated solidification path diagram for 30Ni-AFA steel and its enlarged detail are shown in Figure 2. Thermodynamic calculations predict the possible second phases in 30Ni-AFA steel. The composition of each element in 30Ni-AFA steel was input into JMatPro. The solidification path diagram shows that at 700°C, four second phases exist in 30Ni-AFA steel: NbC begins to appear before the liquid phase completely solidifies. Within the 1350°C-1000°C range, the mole fraction of NbC gradually increases with decreasing temperature. After temperatures below 1000°C, the mole fraction of NbC remains essentially constant. The Laves phase begins to appear at 850°C, and its mole fraction increases with decreasing temperature. After the B2-NiAl phase and σ phase appear, the mole fraction of the Laves phase decreases sharply. Both the B2-NiAl phase and the σ phase begin to precipitate at around 650°C, and their mole fractions increase with decreasing temperature.

[0041] It is worth noting that the "stainlesssteel" module in JMatPro does not have information about the L12-Ni3Al phase, and therefore cannot accurately calculate the solidification path of the steel. The B2-NiAl phase and the L12-Ni3Al phase are composed of the same elements. Within the precipitation temperature range of the B2-NiAl phase, the L12-Ni3Al phase is very likely to appear, and there is a competitive relationship between the two phases.

[0042] Example 3: Heat Treatment

[0043] Use an electric spark wire cutting machine to cut 30Ni-AFA steel plate into 7 strips with a size of 30mm×10mm×10mm along the length direction. When cutting, try to stay away from the surface of the steel plate and take samples from the inside of the steel plate.

[0044] (1) Solution treatment: In order to better obtain information on the precipitation of the second phase, solution treatment is used to dissolve as many alloy elements as possible in the matrix to obtain a supersaturated austenite solid solution, so that fine and dispersed second phases can be re-precipitated during the aging treatment. Six of the cut long strips of 30Ni-AFA steel samples were solution treated in a tubular vacuum furnace. The selected solution treatment temperature should be slightly lower than the solidus temperature of 30Ni-AFA steel and higher than the precipitation temperature of all second phases except NbC. The solution treatment system used in this experiment was to heat the sample to 1200℃ at a rate of 10℃ / min, keep it at this temperature for 2h, and then cool it in water.

[0045] (2) Aging treatment: To obtain information on the precipitation of various second phases, the 30Ni-AFA steel after solution treatment was aged at 700°C, the actual application temperature of 30Ni-AFA steel. The aging treatment was carried out in a box-type resistance furnace. The aging treatment system used in this experiment was to increase the temperature to 700°C at a rate of 10°C / min, and then age for 2 h, 12 h, 24 h, 120 h, 480 h, and 960 h, respectively, and then cool in air.

[0046] The aging time of samples without solution and aging treatment was recorded as 0 h.

[0047] Example 4: Scanning electron microscopy experiment

[0048] Samples with a size of 10 mm × 5 mm × 5 mm were cut from the 7 samples in Example 3 (i.e., samples with aging times of 0 h, 2 h, 12 h, 24 h, 120 h, 480 h, and 960 h) for SEM testing, wherein the surface to be observed was the side surface of the plate in Example 1, and the surface to be observed had a size of 5 mm × 5 mm.

[0049] First, polish the adjacent surfaces of the seven samples to be observed one by one with SiC sandpaper, removing any surface oxide layers. Polish until the entire surface reveals a metallic luster, ensuring a smooth surface and good conductivity. This will be attached to a copper block for subsequent scanning and photographing. The sandpaper used was 320 grit, 600 grit, 1000 grit, 1500 grit, and 2000 grit. Each polishing was performed in the same direction with even pressure. Austenitic steel is prone to scratching, so ensure the proper pressure. When no staggered scratches are visible, replace the sandpaper and rotate the polishing direction 90°. After polishing to 2000 grit, the sample surface is smooth, free of dirt and visible scratches. The next step is polishing the sample. This is done on a polishing machine. Apply W2.5 (4000 grit) polishing paste evenly to the polishing cloth. While rotating the polisher, hold the sample in hand and polish with moderate pressure. Polishing is complete when the sample surface is scratch-free and mirror-smooth. The polishing process lasts approximately 1-2 minutes. Before scanning electron microscopy observation, the sample needs to be corroded. The corrosive liquid is a mixed solution of HCl: HNO3: H2O = 3:1:3. The test surface is corroded for 30-60 seconds. After corrosion, the surface changes color slightly and loses its metallic luster. The corrosive liquid is washed away with alcohol and blown dry with cold air from a hair dryer. The grain boundary can be seen under the metallographic microscope, and the corrosion process is completed.

[0050] After corrosion, the sample should be subjected to SEM testing as soon as possible to prevent further surface oxidation. A copper block measuring 20mm × 20mm × 15mm was wrapped with conductive adhesive and the conductive surface of the sample was placed on the copper block, ensuring that the surface to be tested was at the same height as the top surface of the copper block. The focal length was adjusted to approximately 10mm, and the magnifications were 500×, 2000×, 5000×, and 10000×. The precipitated phases were distinguished using the energy dispersive spectrometer (EDS) provided by the SEM.

[0051] The experimental results are as follows:

[0052] like Figure 2 The following is the SEM picture of 30Ni-AFA steel. Figure 2 (a)-(h) are SEM images of 30Ni-AFA steel after aging for 0h low magnification, 0h high magnification, 2h, 12h, 24h, 120h, 480h, and 960h, respectively. Figure 2 (a) It can be seen that the 30Ni-AFA steel matrix is ​​a single austenite, which ensures good creep resistance. The presence of δ-ferrite would significantly deteriorate the material's properties. Precipitates, measured in micrometers, are randomly distributed throughout the matrix. These precipitates were identified as NbC by an energy dispersive spectrometer. NbC appeared before the liquid phase completely solidified, indicating that the NbC in the material was formed during the smelting process. Due to its high melting point, the forging temperature prevented its dissolution, preserving it. Furthermore, numerous small precipitates are dispersed within the grains. The energy dispersive spectrometer equipped with a scanning electron microscope cannot accurately capture the details of these small precipitates. Based on thermodynamic calculations, at a forging temperature of 1150°C, only NbC can exist, and it is speculated that these small precipitates are also NbC.

[0053] Throughout the aging process from 2 to 960 hours, the grains showed a slight increase in size, indicating that the 30Ni-AFA steel remained essentially stable at 700°C, while NbC still existed, irregularly distributed within the matrix. During aging for 2 to 24 hours, the grain boundary width increased, and the number of precipitates at the grain boundaries gradually increased with increasing aging time. After aging for 120 hours, the number of precipitates at the grain boundaries increased significantly, and precipitates other than NbC could be observed within the grains. With further aging time, the grain boundaries were gradually covered by precipitates, and by 960 hours, the grain boundaries were completely covered by precipitates. After the grain boundary precipitates approached saturation, precipitates began to precipitate within the grains. After aging for 960 hours, the largest number of precipitates was observed within the grains of the 30Ni-AFA steel.

[0054] Example 5: EBSD experiment

[0055] EBSD testing was performed on the as-forged scanning test sample from Example 4. The sample used for EBSD analysis was electrolytically etched and polished at room temperature using a 10% methanol solution of perchloric acid at 20 V for 10 seconds. After etching, the sample was quickly rinsed with running water and dried. The acceleration voltage during testing was 20 kV, and the safe working distance was 24 mm. The data obtained was processed using Channel-5™ software.

[0056] The experimental results are as follows:

[0057] Figure 3 Figures 2 and 3 show the recrystallization distribution and recrystallization fraction of as-forged 30Ni-AFA steel. The microstructure of the forged AFA steel consists of uniform equiaxed grains and a certain number of twins, with an average grain size of 12.2 μm. In the recrystallization distribution diagram, red represents deformed grains, blue represents recrystallized grains, and yellow represents recovered grains. 30Ni-AFA steel is primarily composed of deformed grains, accounting for 67.7%.

[0058] Example 6: Transmission electron microscopy experiment

[0059] TEM tests were performed on the samples aged for 12h, 24h, 120h, and 480h in Example 4. The test surface of the corresponding sample in Example 4 was removed using an electric spark wire cutting machine, and then a thin slice with a thickness of 0.5mm was cut. The oxide scale on the surface of the thin slice sample was manually ground off with 600-mesh SiC sandpaper. Then, in order to facilitate polishing and make the sample evenly stressed, the sample was glued to a 30mm×30mm×5mm glass sheet with glue and polished on 2000-mesh sandpaper. The polishing force should be light to avoid introducing stress. After the sample was ground to less than 50μm, it was soaked in acetone to separate the sample from the glass sheet. Finally, the sample was punched into a disc with a diameter of 3mm using a punching machine and placed in an ion bombardment device for thinning. The microstructure of the test sample was observed using a transmission electron microscope, and the energy dispersive spectrometer equipped on the transmission electron microscope was used to determine the composition of each precipitated phase in the steel. Selected area electron diffraction (SAED) was used to analyze the morphology and crystallographic properties of the sample, so as to accurately determine the type of precipitated phase.

[0060] The experimental results are as follows:

[0061] Figure 4 (a) is the microstructure of 30Ni-AFA steel after aging for 24h, in which the black round particles are identified as NbC by SAED, which is consistent with the Figure 2 This corresponds to the small-sized particles in (b), confirming the previous conjecture. Figure 4 (b) is the microstructure of 30Ni-AFA steel after aging for 480h. The precipitated phases at the grain boundaries and inside the grains are M 23C6 and B2-NiAl phases, M at the grain boundary 23 C6 is long and large in size, and square and small in size in the grains. The B2-NiAl phase at the grain boundary is blocky, and the B2-NiAl phase inside the grains is long and strip-shaped.

[0062] Figure 5 TEM dark field images of 30Ni-AFA steel after aging for 12, 120, and 480 hours reveal the presence of numerous nanosized precipitates in the matrix, which appear white and are identified as L12-Ni3Al phases by SAED and HRTEM. The matrix appears black, and the mismatch between the L12-Ni3Al phase and the matrix is ​​0.47%, indicating a coherent relationship. With increasing aging time, the average diameter of the L12-Ni3Al phase increases, while its number density decreases. The L12-Ni3Al phase maintains a coherent relationship with the matrix, demonstrating excellent thermal stability. This change in L12-Ni3Al morphology is attributed to minimizing the total free energy of the system, which includes interfacial energy and elastic strain energy. To achieve this minimum energy requirement, the L12-Ni3Al phase tends to coarsen at the expense of smaller particles, thereby reducing interfacial energy. This is known as Ostwald ripening.

[0063] The volume fraction and average radius of the L12-Ni3Al phase during the aging process are plotted on Figure 6 In the aging process of 480h, the average radius of the L12-Ni3Al phase gradually increases, and the volume fraction remains at a relatively stable state. According to the Ostwald ripening theory, the volume fraction of the second phase will gradually increase during the nucleation or growth stage, while the volume fraction of the second phase remains stable during the coarsening stage, which means that the L12-Ni3Al phase is already in the coarsening stage rather than the nucleation or growth stage. It should be noted that when calculating the volume fraction of the L12-Ni3Al phase, only the L12-Ni3Al phase was selected, and the influence of other precipitates on the volume fraction of the L12-Ni3Al phase was not considered.

[0064] So far, all the precipitated phases that appeared during the aging process have been confirmed, and no Figure 1 The Laves phase and Cu-rich phase appear in the solidification roadmap.

[0065] Example 7: Tensile test

[0066] Tensile properties tests were performed on the seven samples in Example 3. Tensile tests can measure the performance of a material under axial tensile load. Tensile properties such as the elastic limit, elongation, elastic modulus, proportional limit, area reduction, tensile strength, yield point, and yield strength of the material can be determined using data from the tensile test.

[0067] First, use an electric spark wire cutting machine to cut the 7 samples in Example 3 into bow-tie-shaped tensile specimens with a thickness slightly greater than 1.1 mm, a width of 7 mm, and a gauge length of 4 mm. This size is based on international standards and takes a proportional coefficient of 5.65. Three tensile specimens are cut out of each sample in Example 3 to ensure the accuracy of the experiment. Before performing the tensile test, the sample surface needs to be polished smooth to reduce the error of the experiment. The thickness of the polished sample is 1.1 mm. Subsequently, a tensile test is performed with a tensile rate of 0.1 mm / min.

[0068] The experimental results are as follows:

[0069] Figure 7 Figure 3 shows the room temperature stress-strain curve and yield strength curve of 30Ni-AFA steel after aging. The yield strength of the forged state is 419MPa. At this time, only NbC particles exist in the matrix. After solution treatment and aging for 2h, the yield strength of 30Ni-AFA steel decreases to 373MPa, which is related to the loss of NbC and the growth of grain size during the solution process. During the aging process of 2-480h, the amount of precipitated phase gradually increases, and L12-Ni3Al phase appears. The yield strength continuously increases to 554MPa. After aging for 960h, the yield strength decreases to 514MPa. This is because during the aging process of 480-960h, part of the L12-Ni3Al phase is transformed into B2-NiAl phase. The two are composed of the same elements, but the B2-NiAl phase has a more stable structure. The contribution of the newly formed B2-NiAl relative yield strength is not enough to compensate for the decrease in yield strength caused by the loss of the L12-Ni3Al phase. The yield strength of the material reaches its maximum after aging for 480h.

[0070] Example 8: Hardness test

[0071] A hardness test was performed on the seven scanning test samples in Example 4. The test load was 5 kgf, the holding time was 10 seconds, and the hardness values ​​at 10 different points on each sample were taken. The average value was taken and recorded as the final hardness to ensure the accuracy of the data.

[0072] The experimental results are as follows:

[0073] Figure 8 is the Vickers hardness of 30Ni-AFA steel. Similar to the change trend of yield strength, the hardness of 30Ni-AFA steel decreases after solution treatment and aging for 2h. During the aging process of 2-480h, the hardness gradually increases and decreases after aging for 960h. The maximum hardness occurs in the sample after aging for 480h, and its value is 247HV5.

[0074] Example 9: Oxidation Experiment

[0075] Cut a 10mm × 20mm × 20mm strip sample from the as-forged plate. Use 600-, 1500-, and 2000-grit SiC sandpaper to polish all six surfaces of the sample until the metallic luster is completely exposed. The sample is then placed in alcohol, cleaned in an ultrasonic cleaner, and blown dry with cold air from a blower before measuring the sample surface area. Heat-resistant cotton is placed on the bottom of the crucible to ensure that all surfaces of the sample are exposed to air during the experiment. The crucible with heat-resistant cotton is placed in an 800°C furnace for 30 minutes to remove moisture from the crucible and heat-resistant cotton. The furnace used for the oxidation experiment is the same box-type resistance furnace used for the aging experiment, ensuring air circulation. The temperature of the oxidation experiment is 800°C. After keeping warm, air-cool to room temperature, weigh the total mass of the crucible and heat-resistant cotton, put the sample into the crucible, weigh the total mass of the crucible, heat-resistant cotton and sample, and then put the three into an 800℃ furnace and keep warm for 2h. Take out the three, air-cool to room temperature and weigh the total mass of the three, and then put the three into an 800℃ furnace to continue oxidation. Record the oxidation weight gain of the three at 4h, 8h, 16h, 32h, 64h, 128h, 256h and 512h. Finally, subtract the mass of the crucible and heat-resistant cotton from the total mass of the three, which is the mass of the sample at each weighing.

[0076] The experimental results are as follows:

[0077] Figure 9 The mass gain of 30Ni-AFA steel in dry air at 800°C is a function of oxidation time. During the 64h oxidation time, due to the lack of protection of the oxide layer between the sample surface and oxygen, the sample is in direct contact with oxygen, and the unit area mass of the sample increases rapidly. As the oxidation time increases, a protective oxide layer gradually forms on the sample surface, and the unit mass growth rate of the sample slows down, tending to be stable after 256h of oxidation. In general, 30Ni-AFA steel has excellent oxidation resistance. After 512h of oxidation in dry air at 800°C, the unit area weight gain is only 1.59mg / cm -2 .

[0078] While the embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are illustrative and should not be construed as limiting the present invention. Any simple modifications, equivalent variations, and modifications to the above embodiments based on the technical essence of the present invention remain within the scope of the present invention.

[0079] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. An aluminum-containing austenitic heat-resistant steel, characterized in that: The aluminum-containing austenitic heat-resistant steel is composed of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si and Ti elements, with the mass percentage of Ni being 29-31wt.%, Cr being 13.5-14wt.%, Mn being 0.8-1.0wt.%, Mo<0.03wt.%, W being 1.8-2.2wt.%, C being 0.055-0.065wt.%, Nb being 0.55-0.65wt.%, Al being 2.5-3wt.%, Cu being 1.3-1.6wt.%, Si being 0.1-0.15wt.%, Ti being 0.01-0.015wt.%, and the balance being Fe, and the sum of the mass percentages of Fe, Ni, Cr, Mn, Mo, W, C, Nb, Al, Cu, Si and Ti being 100wt.%.

2. The aluminum-containing austenitic heat-resistant steel according to claim 1, characterized in that: The mass fraction of Ni is 29.74%, the mass fraction of Cr is 14.31%, the mass fraction of Mn is 1.05%, the mass fraction of Mo is 0.03%, the mass fraction of W is 1.84%, the mass fraction of C is 0.06%, the mass fraction of Nb is 0.59%, the mass fraction of Al is 2.81%, the mass fraction of Cu is 1.52%, the mass fraction of Si is 0.11%, the mass fraction of Ti is 0.01%, and the balance is Fe.

3. The method for preparing aluminum-containing austenitic heat-resistant steel according to any one of claims 1 to 2, characterized in that: The following steps are involved: a. Mixing and melting the raw materials to obtain a cast aluminum-containing austenitic heat-resistant steel; b. The cast aluminum-containing austenitic heat-resistant steel is forged at 1150°C to obtain a forged aluminum-containing austenitic heat-resistant steel; c. The forged aluminum-containing austenitic heat-resistant steel was solution treated in a vacuum environment at 1200 ℃ for 2h to obtain a solid solution aluminum-containing austenitic heat-resistant steel; d. The solid solution state aluminum-containing austenitic heat-resistant steel is aged in air at 700 ℃ for 480h to obtain an aged state aluminum-containing austenitic heat-resistant steel; e. The aged aluminum-containing austenitic heat-resistant steel is oxidized in dry air at 800°C.

4. The method for preparing aluminum-containing austenitic heat-resistant steel according to claim 3, characterized in that: The microstructure of the forged aluminum-containing austenitic heat-resistant steel is an austenite matrix and randomly distributed NbC particles. The aged aluminum-containing austenitic heat-resistant steel has four precipitation phases: NbC phase, L12-Ni3Al phase, M 23 C6 phase and B2-NiAl phase.

5. The method for preparing aluminum-containing austenitic heat-resistant steel according to claim 4, characterized in that: The L12-Ni3Al phase is dispersed in the aged aluminum-containing austenitic heat-resistant steel, and during the 480-h aging process, the average particle size thereof increases to 28.5 nm.

6. The method for preparing aluminum-containing austenitic heat-resistant steel according to claim 3, characterized in that: The maximum yield strength of the aged aluminum-containing austenitic heat-resistant steel is above 554 MPa, and the maximum tensile strength is above 898 MPa.

7. The method for preparing aluminum-containing austenitic heat-resistant steel according to claim 3, characterized in that: After the aged aluminum-containing austenitic heat-resistant steel is oxidized in dry air at 800°C for 512 hours, the unit area weight gain is 1.59 mg / cm -2 .

8. The use of the aluminum-containing austenitic heat-resistant steel according to any one of claims 1 or 2, characterized in that: The aluminum-containing austenitic heat-resistant steel is suitable for key components of critical power generation boilers exceeding 700° C. in thermal power plants.

9. The use of the aluminum-containing austenitic heat-resistant steel according to claim 8, characterized in that: The aluminum-containing austenitic heat-resistant steel is used in reheaters and superheaters in thermal power generating sets.

Images