Low-stress gradient annealing process for X65 steel / Inconel 625 composite board

Abstract

The invention relates to a low-stress gradient annealing process of an X65 steel / Inconel 625 composite plate, belongs to the technical field of heat treatment of metal composite materials, and is suitable for structure regulation and interface strengthening of hot-rolled composite materials. The process sequentially comprises three stages, in the first stage, heat preservation is conducted at the temperature 20-50 DEG C below the recrystallization temperature of the X65 steel, the time t1 meets the condition that t1 = 2-4 min / mm, and the protective atmosphere is nitrogen and hydrogen mixed gas; in the second stage, heat preservation is conducted at the temperature 20-50 DEG C below the recrystallization temperature of the Inconel 625 nickel-based alloy, the time t2 is equal to (0.6-0.8) t1, and the protective atmosphere is vacuum; in the cooling stage, the cooling rate V2 is larger than or equal to 15 DEG C / min in the interval of 650-700 DEG C, in the other stages, cooling is conducted to 300 DEG C at the rate V1 smaller than or equal to 30 DEG C / h, and the protective atmosphere is high-purity argon. The interface bonding strength of the composite board can be remarkably improved, the residual stress is reduced, the structure stability is improved, and the high-reliability composite board is provided for the fields of petroleum and natural gas, chemical equipment, ocean engineering and the like.

Description

TECHNICAL FIELD

[0001] The application relates to a low-stress gradient annealing process for an X65 steel / Inconel 625 composite plate and belongs to the technical field of heat treatment of metal composite materials, and is suitable for microstructure regulation and interface strengthening of hot-rolled composite materials. BACKGROUND

[0002] A bimetal composite plate is widely used in the fields of petroleum and natural gas, chemical equipment and marine engineering due to its excellent performance of different materials. The composite structure of X65 steel and Inconel 625 nickel-based alloy combines high strength and high corrosion resistance, and has become a research and application hotspot. The initial composite of the composite plate is usually achieved by hot rolling, but due to the large difference between X65 and Inconel 625 in the thermal expansion coefficient, recrystallization behavior and element diffusion rate, residual stress concentration, microstructure discontinuity and brittle phase precipitation are easily formed at the interface, which reduces the bonding strength and service life. Annealing treatment is helpful to optimize the interface structure and release stress, however, the traditional annealing process is mostly single constant temperature annealing, which is difficult to simultaneously consider the stress release of the X65 layer and the microstructure regulation of the Inconel 625 layer. If the sensitive temperature range of 650-700 DEG C is stayed for too long, the Nb-rich brittle phase will be easily induced to precipitate, resulting in interface embrittlement.

[0003] The patent with the publication number CN104480261A proposes a spheroidizing annealing process for an anti-wear bimetal laminated composite material, which is aimed at high manganese steel and low carbon steel composite plates, and adopts single-stage spheroidizing annealing (680±60 DEG C for 8-14 h), only through single temperature to eliminate stress, without considering the thermal expansion difference of dissimilar materials. The patent with the publication number CN110791624A proposes an online annealing method for stainless steel composite plate strips, which is aimed at stainless steel and carbon steel composite plates, and adopts separate annealing and then welding to composite, the annealing temperature of the stainless steel is 850-900 DEG C, and the annealing temperature of the carbon steel is 650-700 DEG C, which cannot simultaneously eliminate the residual stress commonly existing after bimetal rolling, and it is difficult to control the interface brittle phase. The patent with the publication number CN108425081A proposes an online annealing method for copper-aluminum cast-rolled composite plate strips, which is aimed at copper-aluminum composite plates, and adopts medium-high temperature short-time online annealing (420-480 DEG C, 5-20 min), only through single temperature to control interface diffusion, and the material system is quite different from the steel-nickel-based alloy, and the problem of Nb-rich brittle phase precipitation cannot be solved.

[0004] Therefore, it is urgent to develop a multi-stage annealing process with controllable time-temperature path, to coordinate the thermal response behavior between the bimetals, to optimize the interface structure and stress state, and to improve the overall performance and reliability of the composite plate. SUMMARY

[0005] The purpose of the present invention is to provide a low-stress gradient annealing process for X65 steel / Inconel625 composite plates to solve the problems of large interface residual stress, discontinuous structure, and precipitation of brittle phases in the existing heat treatment process, thereby improving the interface bonding strength and the overall performance of the composite plates.

[0006] In order to achieve the above object, the technical solution of the present invention is: A low stress gradient annealing process for an X65 steel / Inconel 625 composite plate comprises the following steps: (1) The first stage of low temperature pre-annealing: keep the temperature at 20-50℃ below the recrystallization temperature of X65 steel, and the time t1 satisfies t1=2-4 min / mm; (2) Second stage high temperature annealing: keep warm at 20-50℃ below the recrystallization temperature of Inconel625 nickel-based alloy, time t2=(0.6-0.8) t1; (3) Cooling stage: The cooling rate V2 in the range of 650-700 °C is ≥15 °C / min to avoid the formation of brittle phase; in other stages, the cooling rate V1 is ≤30 °C / h to 300 °C to reduce the interfacial thermal stress; The protective atmosphere in the first stage is a nitrogen-hydrogen mixture, with H2 accounting for 3-8% by volume; the protective atmosphere in the second stage is vacuum, with a vacuum pressure of ≤5×10 –2 Pa; the protective atmosphere during the cooling stage is high-purity argon, O2≤ 10ppm.

[0007] In the low stress gradient annealing process of the X65 steel / Inconel625 composite plate, the temperature in the first stage is controlled at 610-630°C.

[0008] In the low stress gradient annealing process of the X65 steel / Inconel625 composite plate, the temperature of the second stage is controlled at 910-930°C.

[0009] In the low stress gradient annealing process of the X65 steel / Inconel625 composite plate, the cooling rate V2 in the range of 650-700°C is 15-30°C / min, and the cooling rate V1 in other stages above 300°C is 10-30°C / h.

[0010] The design concept of the present invention is: The existing technology uses a single constant temperature annealing process, which makes it difficult to balance the stress release of high-strength steel and the microstructure control of nickel-based alloys. In addition, the brittle phase is easily precipitated when the temperature stays in the sensitization zone of 650-700°C. In order to ensure that the bimetallic composite plate has good interface bonding strength, the core of the annealing process of the present invention lies in the use of staged precise temperature control and atmosphere management to synergistically optimize the microstructure and interface bonding quality of the two heterogeneous materials. Specifically, 1) Differentiated temperature control: In the first stage (610-630°C), the temperature is kept below the recrystallization temperature of X65 steel for a sufficient time (t1 = 2-4 min / mm) to fully eliminate its cold working stress without causing grain coarsening. In the second stage (910-930°C), the temperature is kept below the recrystallization temperature of Inconel625 for a shorter time (t2 = 0.6-0.8 t1) to promote interface element diffusion and metallurgical bonding, while inhibiting excessive grain growth of nickel-based alloy.

[0011] 2) Rapid cooling in critical areas: During the cooling stage, special emphasis is placed on rapidly passing through the sensitive temperature zone of 650-700°C for the precipitation of brittle phases in nickel-based alloys at a rate of ≥15°C / min to avoid the formation of brittle phases. Slow cooling (V1 ≤ 30°C / h) is used in other stages to reduce interfacial thermal stress.

[0012] 3) Stepped Atmosphere Protection: Different protective atmospheres are used at each stage (nitrogen-hydrogen mixture to prevent steel oxidation in the first stage, high vacuum to prevent alloying element oxidation and maintain interface purity in the second stage, and high-purity argon to prevent cooling oxidation in the third stage), ensuring the surface condition and interface quality of the material at each stage. The overall design, through the coordinated control of temperature, time, cooling rate, and atmosphere, effectively addresses key issues such as stress relief, structural stability, interface bonding, and embrittlement prevention caused by differences in the thermophysical properties of dissimilar materials, enabling the production of high-performance composite panels.

[0013] The advantages and beneficial effects of the present invention are: 1. The annealing process of the present invention addresses the problems of structural mismatch and interface embrittlement in the composite annealing of high-strength steel and nickel-based alloys through the coordinated design of temperature, time, atmosphere, and cooling. While improving the metallurgical bonding quality of the interface, it also ensures the stability of the bimetallic microstructure, with an interface bonding strength of ≥500 MPa and a residual stress of ≤38 MPa. This provides a high-efficiency, low-defect, and high-toughness manufacturing solution for high-performance composite plates.

[0014] 2. In view of the recrystallization characteristics of X65 steel and Inconel625, the present invention designs a stepped insulation method of "long time at medium and low temperature + short time at high temperature", which not only releases the stress of X65 steel but also avoids the coarsening of Inconel625 grains. In addition, forced rapid cooling is performed in the brittle phase sensitive area of ​​650-700℃ to directly inhibit the precipitation of Nb-rich phase.

[0015] 3. The annealing process of the present invention can significantly enhance the interfacial bonding strength of the composite plate, reduce residual stress and improve the structural stability, and is suitable for the preparation and application of high-reliability bimetallic structural materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the scanning electron microscope (SEM) structure near the interface after annealing in Example 1.

[0017] Figure 2 1 and 2 are X-ray diffraction (XRD) patterns after annealing of Examples 2 and 3. In the figure, curve 1 represents Example 2, and curve 2 represents Example 3.

[0018] Figure 3 This is the electron backscatter diffraction (EBSD) structure near the interface after annealing in Example 3. DETAILED DESCRIPTION

[0019] In the specific implementation process, three bimetallic composite plate embodiments and comparative examples with different thicknesses (20 mm, 40 mm, and 60 mm) all adopt the same hot rolling process. The annealing process of the present invention includes three stages in sequence: the first stage is a low-temperature holding pre-annealing at 610-630 ° C, with a time t1 = 2-4 min / mm, to release the residual stress of the X65 steel layer. The pre-annealing is carried out in a nitrogen-hydrogen mixed gas with a H2 volume fraction of 3-8% to prevent oxidation of the steel side; the second stage is a high-temperature holding annealing at 910-930 ° C, with t2 = (0.6-0.8) t1, to promote the recovery of Inconel625 alloy grains and the diffusion of interface elements. The high-temperature annealing is carried out at a vacuum pressure of ≤5×10 -2 Pa in a vacuum environment to prevent alloy oxidation and keep the interface pure; in the third stage, cooling is carried out to below 300°C at a rate of ≤30°C / h, and quickly passes through the sensitization range of 650-700°C at a rate of ≥15°C / min. During the cooling stage, high-purity argon with an O2 content of ≤10 ppm is used for protection to suppress the precipitation of Nb-rich brittle phase.

[0020] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are all commercially available unless otherwise specified.

[0021] Example 1 (plate thickness 20 mm) In this embodiment, a hot-rolled bimetallic composite plate with a thickness of 20 mm (including an 18 mm X65 steel layer and a 2 mm Inconel625 alloy layer) was subjected to a three-stage annealing treatment: first, in a nitrogen-hydrogen mixture with a H2 volume fraction of 3%, the temperature was kept at 620°C for 60 min; then, the temperature was kept at 5×10 -2 Under a vacuum environment of 1000 Pa, the annealed composite plate was held at 915°C for 42 minutes. Finally, under high-purity argon with an O2 content of 10 ppm, the plate was cooled at a rate of 25°C / h to below 300°C. The plate was then rapidly cooled at a rate of 20°C / min in the 700-650°C range. Once the temperature fell below 300°C, the plate was naturally cooled to room temperature. Testing of the annealed composite plate revealed an interfacial bond strength of 520 MPa and an interfacial residual stress of approximately 36 MPa. The interfacial transition layer was dense and uniform, with no apparent brittle phase precipitation, demonstrating excellent overall mechanical properties.

[0022] Example 2 (40 mm thick) In this example, the hot-rolled bimetallic clad plate with a thickness of 40 mm (X65 steel layer 36 mm, Inconel 625 alloy layer 4 mm) was subjected to three-stage annealing in turn: first, 620°C for 100 min in a nitrogen-hydrogen mixed gas with a H2 volume fraction of 5%; then, 915°C for 60 min in a vacuum environment with a vacuum air pressure of 3x10 -2 Pa; finally, cooling to below 300°C at a rate of 26 ℃ / h under the protection of high-purity argon gas with an O2 content of 8 ppm, and rapidly passing through the interval of 700-650 ℃ at a rate of 22 ℃ / min, and naturally cooling to room temperature below 300°C. After annealing, the clad plate was tested, with an interface bonding strength of 510 MPa, an interface residual stress of about 38 MPa, uniform interface bonding, stable structure, and excellent mechanical properties.

[0023] Example 3 (60 mm thick) In this example, the hot-rolled bimetallic clad plate with a thickness of 60 mm (X65 steel layer 54 mm, Inconel 625 alloy layer 6 mm) was subjected to three-stage annealing in turn: first, 620°C for 120 min in a nitrogen-hydrogen mixed gas with a H2 volume fraction of 8%; then, 915°C for 96 min in a vacuum environment with a vacuum air pressure of 1x10 -2 Pa; finally, cooling to below 300°C at a rate of 23 ℃ / h under the protection of high-purity argon gas with an O2 content of 6 ppm, and rapidly passing through the interval of 700-650 ℃ at a rate of 21 ℃ / min, and naturally cooling to room temperature below 300°C. After annealing, the clad plate was tested, with an interface bonding strength of 500 MPa, an interface residual stress of about 35 MPa, effective control of thick plate annealing, significant reduction of interface stress concentration, and improved overall composite performance.

[0024] Comparative Example 1 (without pre-annealing) The difference from Example 1 is that only 915°C for 70 min (20 mm plate) is performed, without the first stage. Result: bonding strength decreased to 402 MPa, residual stress increased to 112 MPa, leading to a tendency of micro-cracks in the X65 steel layer, and high interface defect rate.

[0025] Comparative Example 2 (insufficient pre-annealing) The difference from Example 2 is that the first stage is only 50 min for a 40 mm plate (lower than required). Result: bonding strength decreased to 380 MPa, residual stress increased to 120 MPa, leading to insufficient stress release.

[0026] Comparative Example 3 (without controlling cooling rate in the sensitization interval) The difference from Example 2 is that the cooling rate of the 40 mm plate at 700-650°C was not controlled. Result: The bonding strength dropped to 385 MPa, and the residual stress increased to 117 MPa, resulting in a large amount of δ phase precipitation and interface embrittlement.

[0027] Comparative Example 4 (High Temperature Annealing Time is Short) The difference from Example 1 is that the second stage for the 20 mm plate was only 15 min (less than the requirement). Result: The bond strength dropped to 376 MPa, and the residual stress increased to 102 MPa, resulting in unrelieved stress in the Inconel 625 layer and discontinuous microstructure.

[0028] Comparative Example 5 (high temperature annealing time is long) The difference from Example 1 is that the second stage of holding the 20 mm plate was 80 min (exceeding the limit). Result: The bonding strength dropped to 375 MPa, and the residual stress increased to 100 MPa, leading to the formation of element-enriched zones and δ / Laves brittle phases at the interface.

[0029] The detailed annealing process is shown in Table 1. The examples and comparative examples show that the core parameters of the three-stage annealing process have a significant synergistic effect: (1) The first stage (620°C): The holding time is designed to be 2-4 min / mm to fully release the interface stress and form a dense transition layer to avoid microstructure coarsening (the risk of insufficient verification in Example 2); (2) Second stage (915°C): The holding time is 60-80% of the first stage, ensuring complete recrystallization of the Inconel 625 layer and uniform diffusion of elements, and inhibiting the precipitation of brittle phases (Comparative Example 4 / 5 verifies that the time deviation leads to insufficient brittle phase or stress release); (3) Cooling stage: Forced rapid cooling (≥20℃ / min) in the range of 700-650℃ to avoid the brittle phase sensitive temperature zone and ensure the stability of the interface structure (Comparative Example 3 verifies that rapid cooling does not cause embrittlement).

[0030] Comprehensive effect: interface bonding strength ≥500 MPa, residual stress ≤38 MPa, significantly better than the control (strength reduction of 20-27%, stress increase of 65-200%).

[0031] Table 1 Serial number type Plate thickness First stage insulation (℃ / min) Second stage insulation (℃ / min) Interface bonding strength (MPa) Residual stress (MPa) 1 Example 1 20 620 / 60 915 / 42 520 36 2 Example 2 40 620 / 100 915 / 60 510 38 3 Example 3 60 620 / 120 915 / 96 500 35 4 Comparative Example 1 20 — 915 / 70 402 112 5 Comparative Example 2 40 620 / 50 915 / 30 380 120 6 Comparative Example 3 40 620 / 100 915 / 60 385 117 7 Comparative Example 4 20 620 / 60 915 / 15 376 102 8 Comparative Example 5 20 620 / 60 915 / 70 375 100 like Figure 1 As shown, from the SEM structure near the interface after annealing in Example 1, it can be seen that the composite interface is flat and has no obvious metallurgical defects.

[0032] like Figure 2 As shown, it can be seen from the XRD patterns of Examples 2 and 3 after annealing that no brittle phase is precipitated.

[0033] like Figure 3 As shown, the EBSD structure near the interface after annealing in Example 3 shows that the structure near the interface is in an obvious annealed diffusion state and the interface is well composited.

[0034] Implementation results demonstrate that the present invention utilizes a differentiated temperature-time design. The first stage of low-temperature pre-annealing involves a long hold at 20-50°C below the recrystallization temperature of X65 to fully release cold working stress. The second stage of high-temperature annealing involves a short hold at 20-50°C below the recrystallization temperature of Inconel 625 to promote interfacial diffusion and inhibit grain coarsening. Rapid cooling is employed in critical intervals, with a cooling rate of ≥15°C / min in the 650-700°C range to prevent brittle phase precipitation, while slower cooling at ≤30°C / h in other stages reduces interfacial thermal stress. Furthermore, a stepped atmosphere (nitrogen-hydrogen mixture / vacuum / argon) is employed for staged oxidation protection, enhancing the performance of X65 steel / Inconel 625 composite plates. This allows for the development of highly reliable composite plates for applications in the oil and gas, chemical equipment, and marine engineering sectors.

[0035] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A low stress gradient annealing process for X65 steel / Inconel625 composite plate, characterized in that: The following steps are involved: (1) The first stage of low temperature pre-annealing: keep the temperature at 20-50℃ below the recrystallization temperature of X65 steel, and the time t1 satisfies t1=2-4 min / mm; (2) Second stage high temperature annealing: keep warm at 20-50℃ below the recrystallization temperature of Inconel625 nickel-based alloy, time t2=(0.6-0.8) t1; (3) Cooling stage: The cooling rate V2 in the range of 650-700 °C is ≥15 °C / min to avoid the formation of brittle phase; in other stages, the cooling rate V1 is ≤30 °C / h to 300 °C to reduce the interfacial thermal stress; The protective atmosphere in the first stage is a nitrogen-hydrogen mixture, with H2 accounting for 3-8% by volume; the protective atmosphere in the second stage is vacuum, with a vacuum pressure of ≤5×10 –2 Pa; the protective atmosphere during the cooling stage is high-purity argon, O2≤ 10ppm.

2. The low stress gradient annealing process for X65 steel / Inconel625 composite plate according to claim 1, characterized in that: The temperature in the first stage is controlled at 610-630℃.

3. The low stress gradient annealing process for X65 steel / Inconel625 composite plate according to claim 1, characterized in that: The temperature in the second stage is controlled at 910-930℃.

4. The low stress gradient annealing process for X65 steel / Inconel625 composite plate according to claim 1, characterized in that: The cooling rate V2 in the range of 650-700°C is 15-30°C / min, and the cooling rate V1 in other stages above 300°C is 10-30°C / h.

Images