Cyclic heat treatment method for healing of holes at the interface of diffusion bonding of austenitic heat-resistant steel and ferritic heat-resistant steel

Abstract

The application discloses a cyclic heat treatment method for healing holes in an interface of diffusion connection of austenitic heat-resistant steel and ferritic heat-resistant steel. The joint with holes in the interface obtained through direct diffusion connection is put into a heat treatment furnace at 1000-1100 DEG C, and after heat preservation, the sample is taken out and air-cooled. Then the diffusion connection joint is put into the heat treatment furnace at 1000-1100 DEG C again, and after heat preservation, the sample is taken out and air-cooled. Then the sample is put into the heat treatment furnace again for heat preservation, taken out and air-cooled to room temperature. The heat treatment is cycled for more than 8 times. The cyclic heat treatment process realizes micron-level repair of holes in the interface of the solid-phase diffusion connection joint of the austenitic heat-resistant steel and the ferritic heat-resistant steel, optimizes microstructure, and improves mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of welding technology, and more specifically to post-weld heat treatment processes, involving a cyclic heat treatment process for healing pores at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel. Background Technology

[0002] The 600-1000MW ultra-supercritical (USC) units being built in my country have reached maximum steam temperatures of 610℃ and pressures of 31MPa. Heat-resistant steel is essential for these high-temperature sections in the superheater and reheater. Austenitic heat-resistant steel, due to its high Cr and Ni content, exhibits superior heat resistance, oxidation and corrosion resistance, excellent weldability, and high-temperature creep resistance. Consequently, considering cost, key performance characteristics, and operating conditions, austenitic heat-resistant stainless steel is considered a crucial candidate material. High-Cr ferritic heat-resistant steel, with its good strength and toughness, oxidation resistance, high-temperature corrosion resistance, high creep strength, hardenability suitable for operating conditions, excellent weldability, and low coefficient of thermal expansion, meets the requirements of modern thermal power plants and is used in the superheater and reheater heat exchanger tubes of USC unit boilers.

[0003] In the piping system of the USC unit boiler heating surface, a large number of dissimilar metal welded joints (DMWJs) inevitably exist, consisting of austenitic and ferritic heat-resistant steels. Due to the significant differences in their chemical composition, microstructure, mechanical properties, and physical characteristics, extensive domestic and international production experience has shown that the actual service life of DMWJs is far shorter than their design life. Early failure, especially at the interface, occurs because stress concentration occurs at the interface of joints formed by direct diffusion bonding of dissimilar heat-resistant steels, leading to the initiation and propagation of microcracks, threatening the safe operation of the power plant's generating units. Therefore, the healing of voids at the interface of DMWJs is one of the most important problems that thermal power plants both domestically and internationally urgently need to solve. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of joint structure and performance without cyclic healing treatment, and to provide a process method for achieving interface void healing of diffusion bonding. This method achieves interface void healing through high-temperature short-time heating, which not only shortens the time spent in the embrittlement temperature sensitive zone, but also takes into account the problem of cold cracking caused by excessively low temperature. Therefore, a reliable joint with intact bonding interface and good mechanical properties can be obtained in the end.

[0005] The technical objective of this invention is achieved through the following technical solution.

[0006] The cyclic heat treatment method for healing the interfacial voids of austenitic heat-resistant steel and ferritic heat-resistant steel diffusion bonding is carried out according to the following steps: The diffusion bonding joint of austenitic heat-resistant steel and ferritic heat-resistant steel with interfacial voids obtained by direct diffusion bonding is placed in a heat treatment furnace at 1000-1100℃ and held for 10-15 min. The sample is then removed, air-cooled for 10-15 min, and placed back into the heat treatment furnace for 10-15 min. The heat treatment operation is repeated at least 8 times. After removing the sample, it is air-cooled to room temperature of 20-25 degrees Celsius.

[0007] Furthermore, the austenitic heat-resistant steel used has the following composition by mass percentage: C 0.008%–0.15%, Cr 18%–20%, Ni 9%–15%, Mn 0.8%–1.6%, Si 0.1%–0.6%, Mo≤3.5%, Nb 0.2%–0.8%, Ti≤0.1%, Cu≤3%, N 0.013%–0.5%, S 0.0008%, P 0.024%, with the remainder being Fe. The ferritic heat-resistant steel used has the following composition and mass percentage: C 0.092%–0.141%, Cr 1.120%–8.870%, Mn 0.280%–0.557%, P 0.0049%–0.0170%, S 0.0015%–0.0050%, Si 0.092%–0.590%, Mo 0.572%–0.967%, V 0.003%–0.210%, N 0.0049%–0.0440%, Nb 0.003%–0.060%, Ni 0.170%–0.590%, with the remainder being Fe.

[0008] Furthermore, the heating rate of the heat treatment furnace is 5-20℃ / min, preferably 5-10℃ / min; the cooling rate of the heat treatment furnace is 5-30℃ / min, preferably 10-20℃ / min.

[0009] Furthermore, the heat treatment furnace is filled with an atmosphere of air, nitrogen, helium, or argon.

[0010] Moreover, the heat treatment process is repeated 8-10 times.

[0011] Furthermore, the heat treatment furnace temperature is 1050–1100℃, the holding time is 10–12 min, and the air cooling time is 10–12 min.

[0012] This invention provides a cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel. It employs direct diffusion bonding and designs a cyclic phase transformation heat treatment process to obtain a reliable joint with an intact bonding interface and excellent mechanical properties. This cyclic heat treatment method for healing voids at the diffusion bonding interface of austenitic heat-resistant steel and ferritic heat-resistant steel is time-efficient, produces a joint with uniform microstructure and composition compared to the base material, and increases hardness. Compared to diffusion-bonded joints without a healing process, this invention uses direct diffusion bonding of austenitic and ferritic heat-resistant steel followed by cyclic heat treatment, resulting in a shorter cyclic heat treatment time and a more uniform microstructure and superior mechanical properties. Attached Figure Description

[0013] Figure 1 The image shows the IPF (Intensity Per Flow) diagram of the diffusion connection joint in the comparative example of this invention, without undergoing cyclic heat treatment.

[0014] Figure 2 This is an IPF diagram of the diffusion connection joint in Embodiment 1 of the present invention after two cycles of heat treatment.

[0015] Figure 3 This is an IPF diagram of the diffusion connection joint in Embodiment 2 of the present invention after 4 cycles of heat treatment.

[0016] Figure 4 This is an IPF diagram of the diffusion connection joint in Embodiment 3 of the present invention after 8 cycles of heat treatment.

[0017] Figure 5 This is a scanning electron microscope image of the diffusion connection joint in the comparative example of the present invention before heat treatment.

[0018] Figure 6 The image shows a scanning electron microscope image of the diffusion connection joint after two cycles of heat treatment in Embodiment 1 of the present invention.

[0019] Figure 7 The image shows a scanning electron microscope (SEM) image of the diffusion connection joint after four cycles of heat treatment in Embodiment 2 of the present invention.

[0020] Figure 8 This is a scanning electron microscope image of the diffusion connection joint after eight cycles of heat treatment in Embodiment 3 of the present invention.

[0021] Figure 9 This is a statistical chart showing the hardness of the diffusion connection joint in this invention after undergoing different cycles of heat treatment. Detailed Implementation

[0022] The technical solution of this application will be further described below with reference to specific embodiments.

[0023] To achieve a reliable connection between austenitic heat-resistant steel and ferritic heat-resistant steel, vacuum diffusion bonding is employed, which offers advantages such as lower pressure required for connection and less workpiece deformation. However, direct diffusion bonding can lead to stress concentration at the interface, causing microcracks to initiate and propagate, threatening the safe operation of power plant generator units. Therefore, this invention proposes a process for healing interface voids in diffusion bonding. This method uses high-temperature short-time heating to heal interface voids, shortening the time spent in the embrittlement temperature-sensitive zone and addressing the issue of cold cracking caused by excessively low temperatures. Thus, a reliable joint with an intact interface and good mechanical properties is ultimately obtained. In the specific implementation, austenitic heat-resistant steel and ferritic heat-resistant steel are vacuum diffused bonded, and the joint undergoes a specific heat treatment process. The composition and mass percentage of the austenitic heat-resistant steel are as follows:

[0024] The composition and mass percentage of austenitic heat-resistant steel are as follows: C 0.008%~0.15%, Cr 18%~20%, Ni 9%~15%, Mn 0.8%~1.6%, Si 0.1%~0.6%, Mo≤3.5%, Nb 0.2%~0.8%, Ti≤0.1%, Cu≤3%, N 0.013%~0.5%, S 0.0008%, P 0.024%, with the remainder being Fe.

[0025] The composition and mass percentage of ferritic heat-resistant steel are as follows: C 0.092%~0.141%, Cr 1.120%~8.870%, Mn 0.280%~0.557%, P 0.0049%~0.0170%, S 0.0015%~0.0050%, Si 0.092%~0.590%, Mo 0.572%~0.967%, V 0.003%~0.210%, N 0.0049%~0.0440%, Nb 0.003%~0.060%, Ni 0.170%~0.590%, with the remainder being Fe.

[0026] Comparative Example 1

[0027] In the comparative example, austenitic heat-resistant steel and ferritic heat-resistant steel without post-weld heat treatment were directly welded by vacuum diffusion welding (the same applies below, see reference HUA Yu, et al, Microstructure Evolution and Mechanical Properties of Dissimilar Material Diffusion-Bonded Joint for HighCr Ferrite Heat-Resistant Steel and Austenitic Heat-Resistant Steel. ActaMetall Sin, 2022: p.141-154). Figure 1 The image shows the EBSD test results IPF (Inverse Pole Figure map) of the diffusion joint. The interface of the joint obtained by the direct diffusion connection method has continuous, elliptical holes. Figure 5 The image shows the SEM morphology of the joint after vacuum diffusion bonding. The tensile strength of the joint after vacuum diffusion bonding is 630.5 MPa (for tensile strength testing, see Chen, M., et al., Microstructure evolution and tensile behaviors of dissimilar TLP joint of austenitic steel and high-Crferritic steel. Materials Science and Engineering: A, 2023, 870: p. 144818, the same below).

[0028] Example 1

[0029] In this embodiment, austenitic heat-resistant steel and ferritic heat-resistant steel are joined by vacuum diffusion bonding. The post-weld cyclic heat treatment process for hole healing is as follows:

[0030] The obtained solid-phase diffusion joint was placed in a heat treatment furnace (air atmosphere) at 1050℃ and held for 10 minutes. The sample was then removed, air-cooled for 10 minutes, and then placed back into the heat treatment furnace (air atmosphere) and held for 10 minutes. The sample was then removed and air-cooled to room temperature of 20-25 degrees Celsius.

[0031] Figure 2 The image shows the EBSD test results IPF (Inverse Pole Figure map) of the diffusion joint after 1050℃ / 2 cycles of heat treatment. This heat treatment process optimizes the microstructure of the vacuum diffusion joint and improves its mechanical properties. Figure 6 The image shows the SEM morphology of the vacuum diffusion joint after two cycles of heat treatment at 1050℃. The tensile strength of the joint at this point is 671.5 MPa.

[0032] Example 2

[0033] In this embodiment, austenitic heat-resistant steel and ferritic heat-resistant steel are joined by vacuum diffusion bonding. The post-weld cyclic heat treatment process for hole healing is as follows:

[0034] The obtained solid-phase diffusion joint was placed in a heat treatment furnace (air atmosphere) at 1050℃ and held for 10 min. The sample was then removed, air-cooled for 10 min, and then placed back into the heat treatment furnace (air atmosphere) and held for 10 min. The heat treatment operation was repeated 4 times. The sample was then removed and air-cooled to room temperature.

[0035] Figure 3 The image shows the EBSD test results (IPF - Inverse Pole Figure map) of the diffusion joint after 4 cycles of heat treatment at 1050℃. The microstructure of the diffusion joint was further optimized after 4 cycles of heat treatment, the pores at the joint interface were basically healed, and the mechanical properties were improved. Figure 7 The image shows the SEM morphology of the vacuum diffusion joint after 1050℃ / 4 cycles of heat treatment. The tensile strength of the joint at this point is 742.5MPa.

[0036] Example 3

[0037] In this embodiment, austenitic heat-resistant steel and ferritic heat-resistant steel are joined by vacuum diffusion bonding. The post-weld cyclic heat treatment process for hole healing is as follows:

[0038] The obtained diffusion joint was placed in a heat treatment furnace (air atmosphere) at 1050℃ and held for 10 min. The sample was then removed, air-cooled for 10 min, and then placed back into the heat treatment furnace (air atmosphere) and held for 10 min. The heat treatment operation was repeated 8 times. The sample was then removed and air-cooled to room temperature.

[0039] Figure 4 The image shows the EBSD test results IPF (Inverse Pole Figure map) of the diffusion joint after 8 cycles of heat treatment at 1050℃. The diffusion joint has a uniform microstructure after 8 cycles of heat treatment, and all the pores at the joint weld have been closed. Figure 8 The image shows the SEM morphology of the vacuum diffusion joint after heat treatment at 1050℃ for 8 cycles. The tensile strength of the joint at this point is 755MPa.

[0040] The joints of the above comparative examples and embodiments were subjected to hardness tests, referring to Chen, M., et al., Microstructure evolution and tensile behaviors of dissimilar TLP joint of austenitic steel and high-Cr ferritic steel. Materials Science and Engineering: A, 2023, 870: p. 144818. Figure 9 As shown, the hardness statistics of the diffusion-bonded joint after different cycles of heat treatment in this invention are as follows: The diffusion-bonded joint obtained at 1050℃ and 7.5MPa uniaxial pressure, held for 15 minutes, exhibits a microhardness distribution in the joint connection area after different cycles of heat treatment. The vertical axis represents Vickers hardness, and the horizontal axis represents the distance to the weld center, with 0 representing the weld center. The ferrite side consists of δ-ferrite and lath martensite, resulting in a high average hardness value, while the austenite side is composed of single-phase austenite, leading to a low average hardness value. The 0th cycle represents the original welded joint without heat treatment, exhibiting the lowest average hardness value at the weld center. This is because the joint obtained through direct diffusion bonding has continuously distributed elliptical pores at the weld center connection, resulting in reduced hardness in the pore area. As can be seen from the figure above, as the number of cycles increases, not only does the hardness value of the base material on both sides gradually increase, but the hardness value at the center of the weld also gradually increases. This is because, during the continuous process of multiple cycles, the hole located at the center of the weld gradually heals and eventually disappears, and the hardness value reaches its maximum value when the number of cycles is eight.

[0041] Adjusting the process parameters according to the present invention, as tested, can achieve the healing of pores at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel, with the joint tensile strength averaging over 750 MPa. The present invention has been described above as exemplary. It should be noted that any simple modifications, alterations, or equivalent substitutions that can be made by those skilled in the art without creative effort, without departing from the core of the invention, fall within the protection scope of the present invention.

Claims

1. A cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel, characterized in that, The procedure is as follows: Place the diffusion-bonded joint between austenitic heat-resistant steel and ferritic heat-resistant steel (with interfacial pores obtained through direct diffusion bonding) into a heat treatment furnace at 1000–1100 °C and hold for 10–15 min. Remove the sample, air-cool for 10–15 min, then place it back into the heat treatment furnace and hold for 10–15 min. Repeat this heat treatment process at least 8 times. After removing the sample, air-cool it to room temperature (20–25 °C). The heating rate of the heat treatment furnace is 5–20 °C / min. The composition of the austenitic heat-resistant steel used, by mass percentage, is: C 0.008%–0.15%, Cr 18%–20%, Ni 9%–15%, Mn 0.8%–1.6%, Si 0.1%–0.6%, Mo≤3.5%, Nb 0.2%–0.8%, Ti≤0.1%, Cu≤3%, N The composition and mass percentage of the ferritic heat-resistant steel used are as follows: C 0.092%~0.141%, Cr 1.120%~8.870%, Mn 0.280%~0.557%, P 0.0049%~0.0170%, S 0.0015%~0.0050%, Si 0.092%~0.590%, Mo 0.572%~0.967%, V 0.003%~0.210%, N 0.0049%~0.0440%, Nb 0.003%~0.060%, Ni 0.170%~0.590%, with the remainder being Fe.

2. The cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel according to claim 1, characterized in that, The heating rate of the heat treatment furnace is 5-10℃ / min.

3. The cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel according to claim 1, characterized in that, The cooling rate of the heat treatment furnace is 10-20℃ / min.

4. The cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel according to claim 1, characterized in that, The heat treatment furnace is filled with an atmosphere of air, nitrogen, helium, or argon.

5. The cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel according to claim 1, characterized in that, Repeat the heat treatment process 8-10 times.

6. The cyclic heat treatment method for healing voids at the diffusion bonding interface between austenitic heat-resistant steel and ferritic heat-resistant steel according to claim 1, characterized in that, The heat treatment furnace temperature is 1050~1100℃, the holding time is 10-12min, and the air cooling time is 10-12min.

7. A diffusion joint between austenitic heat-resistant steel and ferritic heat-resistant steel obtained by the cyclic heat treatment method as described in any one of claims 1-6, characterized in that, The interface pores are completely healed, and the average tensile strength of the joint can reach over 750 MPa.

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