Recovery heat treatment method for novel pre-deformed nickel-iron-based high-temperature alloy pipe

Abstract

The invention belongs to the technical field of high-temperature alloy, and particularly relates to a recovery heat treatment method of a novel pre-deformed nickel-iron-based high-temperature alloy pipe. The method comprises the following steps: carrying out 5%-35% room temperature plastic deformation on the alloy pipe; the pre-deformed alloy pipe is subjected to heat preservation at the temperature of 1080-1130 DEG C and then cooled to the room temperature; and secondary recovery heat treatment is conducted at the temperature of 960-1050 DEG C, heat preservation is conducted, the temperature is lowered to the room temperature, tissue damage is eliminated, and gamma'phase precipitation is induced. After the alloy pipe is subjected to restorative heat treatment, the recrystallization process can be caused under the driving of relatively high strain storage energy, so that the grain size is recovered to the normal size, and the comprehensive strength of the material is further improved; the Sigma CSL (Sigma < = 29) grain boundary proportion is stabilized at 30% or above, and the special grain boundary proportion returns to normal and is close to the original state; the high-proportion special grain boundary inhibits crack propagation, and compared with a pre-deformed alloy, the high-temperature durability of the alloy is remarkably improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature alloys, and particularly relates to a recovery heat treatment method of a pre-deformed novel nickel-iron-based high-temperature alloy pipe. BACKGROUND

[0002] Under the global trend of responding to climate change and promoting green and low-carbon energy transformation, countries around the world have made energy structure adjustment and energy saving and emission reduction as national strategic priorities. As the largest energy producer and consumer in the world, China is striving to build a clean, low-carbon, safe and efficient modern energy system. When the efficient coal-fired ultra-supercritical thermal power generating unit is in operation, it needs to work continuously under extreme conditions of ultra-high temperature and ultra-high pressure, which puts strict requirements on the performance of the core high-temperature components of the unit. In particular, the performance of the heat receiving surface alloy pipe directly relates to the safety, reliability and economy of the thermal power generating unit.

[0003] Generally, the microstructure of the high-temperature alloy material will be degraded during cold working deformation or pipe bending, for example, when the critical strain (>5%) is exceeded, dislocation pile-ups are easy to induce micro-cracks, but existing non-destructive monitoring technologies are difficult to evaluate the substructure damage, which has an adverse effect on the stability of the alloy structure and the long-term creep performance, and an effective means is urgently needed to repair the deformation damage of the high-temperature alloy. The microstructure repair method for the pre-deformed novel nickel-iron-based high-temperature alloy pipe through the staged heat treatment method provided by the present application has strong practical operability and low economic cost, and has very important significance for the application and development of the novel nickel-iron-based high-temperature alloy. SUMMARY

[0004] Therefore, the technical problem to be solved by the present application is to overcome the organizational defects generated in the process of cold working deformation or pipe bending of the high-temperature alloy material in the prior art, so as to provide a method for eliminating strain damage and restoring microstructure through a recovery heat treatment process.

[0005] To this end, the present application provides the following technical solution: A recovery heat treatment method for a pre-deformed novel nickel-iron-based high-temperature alloy pipe, comprising the following steps: (1) Pre-deformation: plastic deformation of the novel nickel-iron-based high-temperature alloy pipe at room temperature, with a strain of 5%-35%; (2) First step recovery heat treatment: heating the pre-deformed alloy pipe from room temperature to 1080-1130 DEG C; holding for a certain time, and cooling by air cooling; (3) Second step recovery heat treatment: secondary recovery heat treatment at 960-1050 DEG C, holding time of 60-90 min, and cooling by air cooling to eliminate organizational damage and induce gamma prime phase precipitation.

[0006] Preferably, the new nickel-iron-based superalloy pipe is composed of the following elements: Fe: 25.0%-30.0%, Cr: 19.0%-22.0%, Al: 1.8%-2.5%, Ti: 2.0%-2.5%, and the balance of Ni.

[0007] Preferably, the pre-strain in step (1) is preferably 15%-35%.

[0008] Preferably, in step (2), the heating rate is 150-200℃ / min.

[0009] Preferably, in step (2), the holding time is 15-45min.

[0010] Preferably, in step (2), the holding time of the recovery heat treatment is adjusted according to the pre-strain: when the pre-strain is 5%-15%, the holding time is 15-25min; when the pre-strain is 15%-35%, the temperature is not lower than 1100℃, and the holding time is 25-45min.

[0011] Preferably, after the first recovery heat treatment, the MC type carbide is distributed in the microstructure, the grain size of the alloy is 75-140μm, and the room temperature yield strength is not lower than 300MPa.

[0012] Preferably, in step (3), the heating rate is 150-200℃ / min.

[0013] Preferably, in step (3), the holding time is 60-90min.

[0014] A nickel-iron-based superalloy pipe after recovery heat treatment obtained by the above method, (1) the average grain size is 80-150μm; (2) the proportion of ∑CSL (∑≤29) grain boundaries is ≥30%; (3) the grain interior is uniformly distributed with γ' phase strengthening particles with a particle size of ≤30nm; (4) MC type carbide and M23C6 carbide exist at the grain boundaries; (5) the room temperature yield strength is not lower than 450MPa.

[0015] The technical scheme of the present application has the following advantages: Deformation may occur during cold working or pipe bending process, and pre-deformation may introduce dislocation network into the alloy, which is prone to cause dislocation pile-up and micro-cracks, and there is a large strain storage energy in the pre-deformation structure. After the restoration heat treatment, the recrystallization process may occur due to the driving of the high strain storage energy, so that even if high pre-strain occurs in the material, the grain size can still be restored to the normal size by the appropriate heat treatment system, which is not abnormal or slightly refined compared with the original grain size, and the comprehensive strength of the material is further improved. In addition, after the restoration heat treatment, the proportion of ∑CSL (∑≤29) grain boundaries is stable at more than 30%, and the proportion of special grain boundaries is restored to normal, close to the original state. The high proportion of special grain boundaries inhibits crack propagation, and the high-temperature durability of the alloy after pre-deformation is significantly improved. The dislocation pile-up and structural defects caused by pre-deformation can be fully recovered after restoration heat treatment, and the recrystallization kinetics can be controlled by strain energy storage, so as to realize the "structural damage repair-strengthening phase precipitation" synergistic optimization. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0017] Figure 1 is the alloy morphology after 6% deformation in Example 1; Figure 2 is the transmission microstructure morphology of the alloy after 6% deformation in Example 1; Figure 3 is the metallographic morphology of the alloy after restoration heat treatment in Example 1; Figure 4 is the grain boundary scanning microstructure morphology of the alloy after restoration heat treatment in Example 1; Figure 5 is the intragranular scanning microstructure morphology of the alloy after restoration heat treatment in Example 1; Figure 6 is the special grain boundary EBSD characterization result of the alloy after restoration heat treatment in Example 1; Figure 7 is the alloy morphology after 33% deformation in Example 2; Figure 8 is the transmission microstructure morphology of the alloy after 33% deformation in Example 2; Figure 9 is the metallographic morphology of the alloy after restoration heat treatment in Example 2; Figure 10is the grain boundary scanning microstructure of the alloy after recovery heat treatment in Example 2; Figure 11 is the intragranular scanning microstructure of the alloy after recovery heat treatment in Example 2; Figure 12 is the grain boundary scanning microstructure of the alloy after recovery heat treatment in Example 3; Figure 13 is the intragranular scanning microstructure of the alloy after recovery heat treatment in Example 3; Figure 14 is the microstructure of the alloy after 25% deformation but without recovery heat treatment in Comparative Example 1; Figure 15 is the transmission microstructure of the alloy after 25% deformation but without recovery heat treatment in Comparative Example 1. DETAILED DESCRIPTION

[0018] The following examples are provided to better enable those skilled in the art to further understand the application, and are not intended to limit the scope of the application or the content of the disclosure, and do not limit the scope of the application and the scope of protection, any person under the inspiration of the application or the combination of the application with other prior art features, any product same or similar to the application, falls within the scope of the application.

[0019] Example 1 A new type of nickel-iron-based superalloy tube was subjected to room temperature plastic deformation with a strain of 6%, and the pre-deformed alloy tube was heat treated at 1090°C for 20 min, with a heating rate of 180°C / min and a cooling rate of water quenching. Further recovery heat treatment was carried out at 980°C for 60 min, with a heating rate of 180°C / min and a cooling rate of air cooling. The microstructure of the alloy after 6% deformation is shown in Figure 1 After plastic deformation, many strain damages were formed inside the grains of the alloy; Figure 2 The transmission microstructure of the alloy after plastic deformation is shown in Figure 3 The microstructure of the alloy after recovery heat treatment is shown in Figure 4 The grain boundary scanning microstructure of the alloy after recovery heat treatment is shown in 23 C6 carbide, and the intragranular scanning microstructure is shown in Figure 5 The intragranular precipitated phase is γ' phase, and its size is measured to be 20 nm; the special grain boundary EBSD results of the alloy after recovery heat treatment are shown in Figure 6As shown, the ∑CSL (∑≤29) grain boundary ratio is 48%. The alloy after recovery heat treatment was subjected to room temperature tensile testing, and the yield strength was 509 MPa. Creep tests on the alloy after recovery heat treatment showed a creep rupture life of 6366 h at 700 ℃ and 215 MPa, demonstrating good long-term creep performance.

[0020] Example 2 The novel nickel-iron-based high-temperature alloy tube was subjected to room temperature plastic deformation with a strain of 33%. The pre-deformed alloy tube was held at 1130℃ for 40 min with a heating rate of 150℃ / min, and then cooled by water quenching. It was then further subjected to a secondary recovery heat treatment at 1030℃ for 75 min with a heating rate of 150℃ / min, and then cooled by air cooling. The alloy morphology after 33% deformation is as follows: Figure 7 As shown, after plastic deformation, many strain damages are formed inside the grains of the alloy. Figure 8 The image shows the transmission microstructure of the alloy after plastic deformation. It can be seen that there are many dislocation pile-ups at the grain boundaries. Figure 9 This is the metallographic morphology of the alloy after restorative heat treatment, with a grain size of 124 μm. Figure 10 This is a grain boundary scanning microstructure of the alloy after restorative heat treatment. A small amount of MC-type carbides and M-type carbides are present at the grain boundaries. 23 C6 carbide, intragranular scanning microstructure as follows: Figure 11 As shown, the intragranular precipitate is a γ′ phase, with a measured size of 22 nm. After restorative heat treatment, the EBSD results for the special grain boundaries of the alloy show that the ∑CSL (∑≤29) grain boundary ratio is 40%. A room temperature tensile test was performed on the alloy after restorative heat treatment, and the yield strength was 534 MPa. A creep rupture test was conducted on the alloy after restorative heat treatment, and the results showed that the creep rupture life at 700℃ and 215 MPa was 6930 h, demonstrating good long-term creep rupture performance.

[0021] Example 3 The novel nickel-iron-based high-temperature alloy tube was subjected to room temperature plastic deformation with a strain of 30%. The pre-deformed alloy tube was held at 1125℃ for 40 min with a heating rate of 180℃ / min, and then cooled by water quenching. It was then further subjected to a secondary recovery heat treatment at 1020℃ for 80 min with a heating rate of 180℃ / min, and then cooled by air cooling. The alloy, after undergoing a 30% deformation recovery heat treatment, was analyzed using a grain boundary scanning microstructure as follows: Figure 12 As shown, a small amount of MC-type carbides and M-type carbides exist at the grain boundaries. 23 C6 carbide, intragranular scanning microstructure as follows: Figure 13The intragranular precipitates are γ' phase, and the size of which is 20 nm. The yield strength of the alloy after recovery heat treatment is 522 MPa. The results of the stress rupture test of the alloy after recovery heat treatment show that the stress rupture life at 700 ℃, 215 MPa is 5877 h, which indicates that the alloy has good long-term stress rupture performance.

[0022] Comparative Example 1 The new nickel-iron-based superalloy tube is subjected to plastic deformation at room temperature, and the strain is 25%. The pre-deformed alloy tube is not subjected to any recovery heat treatment, Figure 14 is the microstructure morphology after 25% deformation but without recovery heat treatment, and many strain damages are formed in the grain interior; Figure 15 is the corresponding transmission microstructure morphology. The yield strength of the alloy without recovery heat treatment is 300 MPa. The results of the stress rupture test of the alloy without recovery heat treatment show that the stress rupture life at 700 ℃, 215 MPa is 1199 h, and the comprehensive performance is low.

[0023] Test Example The stress rupture performance of the alloys of the examples and the comparative examples is shown in Table 1.

[0024] Table 1. Stress rupture performance of the alloys of the examples and the comparative examples

Claims

1. A method of recovery heat treatment of a pre-deformed novel nickel-iron based superalloy tube, characterized in that, The method comprises the following steps: (1) pre-deformation: the new nickel-iron-based superalloy pipe is plastically deformed at room temperature, and the strain is 5%-35%; (2) first recovery heat treatment: the pre-deformed alloy pipe is heated from room temperature to 1080-1130℃; the temperature is kept for a certain time, and the pipe is cooled by air cooling; (3) second recovery heat treatment: the second recovery heat treatment is performed at 960-1050℃, the temperature is kept for 60-90min, and the pipe is cooled by air cooling, so as to eliminate the damage of the structure and induce the precipitation of γ' phase.

2. The method of claim 1, wherein, The composition of the new nickel-iron-based superalloy pipe is as follows: Fe: 25.0%-30.0%, Cr: 19.0%-22.0%, Al: 1.8%-2.5%, Ti: 2.0%-2.5%, and the balance is Ni.

3. The method of claim 1, wherein, The pre-strain in the step (1) is preferably 15%-35%.

4. The method of claim 1, wherein, In the step (2), the heating rate is 150-200℃ / min.

5. The method of claim 1, wherein, In the step (2), the temperature is kept for 15-45min.

6. The method of claim 5, wherein, The temperature keeping time of the recovery heat treatment in the step (2) is adjusted according to the pre-deformation: When the pre-deformation is 5%-15%, the temperature keeping time is 15-25min; When the pre-deformation is 15%-35%, the temperature is not lower than 1100℃, and the temperature keeping time is 25-45min.

7. The method of claim 1, wherein, After the first recovery heat treatment, MC type carbides are distributed in the structure, the grain size of the alloy is 75-140μm, and the yield strength at room temperature is not lower than 300MPa.

8. The method of claim 1, wherein, In the step (3), the heating rate is 150-200℃ / min.

9. The method of claim 1, wherein, In the step (3), the temperature is kept for 60-90min.

10. A nickel-iron-based superalloy pipe after recovery heat treatment obtained by the method of claims 1-9, characterized in that: (1) the average grain size is 80-150μm; (2) the proportion of ∑CSL (∑≤29) grain boundaries is ≥30%; (3) the γ' phase strengthening particles with a particle size of ≤30nm are uniformly distributed in the grain; (4) MC-type carbides and M 23 C6 carbides; (5) the yield strength at room temperature is not lower than 450MPa.

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