Low stress gradient annealing process for x65 steel / inconel 625 clad plate

Abstract

The present application relates to a kind of low stress gradient annealing process of X65 steel / Inconel625 composite plate, belong to the heat treatment technical field of metal composite material, it is applicable to the structure regulation and interface strengthening of hot-rolled composite material.The process includes three stages in turn: the first stage is kept at 20~50 ℃ below X65 steel recrystallization temperature, time t1 satisfies t1=2~4 min / mm, protective atmosphere is nitrogen hydrogen mixed gas;Second stage is kept at 20~50 ℃ below Inconel625 nickel-based alloy recrystallization temperature, time t2=(0.6~0.8) t1, protective atmosphere is vacuum;Cooling stage is cooled at the cooling rate V2≥15 ℃ / min in 650~700 ℃ interval, at other stages with rate V1≤30 ℃ / h cooling to 300 ℃, protective atmosphere is high-purity argon.The present application can significantly improve the interface bonding strength of composite plate, reduce residual stress and improve the stability of organization, provide high reliability composite plate for petroleum and natural gas, chemical equipment and marine engineering and other fields.

Description

Technical Field

[0001] This invention relates to a low-stress gradient annealing process for X65 steel / Inconel 625 composite plates, belonging to the field of heat treatment technology for metal composite materials, and is applicable to the microstructure control and interface strengthening of hot-rolled composite materials. Background Technology

[0002] Bimetallic composite plates, due to their combination of the superior properties of different materials, are widely used in oil and gas, chemical equipment, and marine engineering. The composite structure of X65 steel and Inconel 625 nickel-based alloy, combining high strength and high corrosion resistance, has become a research and application hotspot. These composite plates often achieve initial bonding through hot rolling. However, due to significant differences between X65 and Inconel 625 in terms of thermal expansion coefficients, recrystallization behavior, and element diffusion rates, residual stress concentration, microstructure discontinuity, and brittle phase precipitation easily form at the interface, reducing bonding strength and service life. Annealing helps optimize the interface structure and release stress; however, traditional annealing processes are mostly single isothermal annealing, making it difficult to simultaneously address stress release in the X65 layer and microstructure control in the Inconel 625 layer. Prolonged exposure to the sensitization temperature range of 650–700℃ can easily induce the precipitation of Nb-rich brittle phases, leading to interface embrittlement.

[0003] Patent CN104480261A discloses a spheroidizing annealing process for wear-resistant bimetallic laminated composite materials. For high-manganese steel and low-carbon steel composite plates, it employs single-stage spheroidizing annealing (680±60℃, holding for 8–14 hours), eliminating stress only through a single temperature and failing to consider the thermal expansion differences between the dissimilar materials. Patent CN110791624A discloses an online annealing method for stainless steel composite plates and strips. For stainless steel and carbon steel composite plates, it employs separate annealing followed by welding, with stainless steel annealing temperatures of 850–900℃ and carbon steel annealing temperatures of 650–700℃. This method cannot simultaneously eliminate the residual stresses shared after bimetallic rolling and is difficult to control the brittle phases at the interface. The patent with publication number CN108425081A proposes an online annealing method for copper-aluminum cast-rolled composite plates and strips. For copper-aluminum composite plates, it adopts medium-high temperature short-time online annealing (420~480℃, 5~20min). It only controls the interface diffusion through a single temperature, and the material system is very different from that of steel-nickel-based alloys, which cannot solve the problem of precipitation of Nb-rich brittle phase.

[0004] Therefore, there is an urgent need to develop a multi-stage annealing process with controllable time and temperature paths to coordinate the thermal response behavior between bimetals, optimize the interface structure and stress state, and thus improve the overall performance and reliability of the composite board. Summary of the Invention

[0005] The purpose of this invention is to provide a low-stress gradient annealing process for X65 steel / Inconel 625 composite plates to solve problems such as large residual stress at the interface, discontinuous microstructure, and precipitation of brittle phases in existing heat treatment processes, thereby improving the interfacial bonding strength and the overall performance of the composite plate.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A low-stress gradient annealing process for X65 steel / Inconel 625 composite plates includes the following steps:

[0008] (1) First stage of low temperature pre-annealing: hold at 20-50℃ below the recrystallization temperature of X65 steel for a time t1 = 2-4 min / mm;

[0009] (2) Second stage high-temperature annealing: Hold at 20-50°C below the recrystallization temperature of Inconel 625 nickel-based alloy for time t2=(0.6-0.8) t1;

[0010] (3) Cooling stage: The cooling rate V2 in the range of 650 to 700℃ is ≥15℃ / min to avoid the formation of brittle phase; in other stages, the cooling rate V1 is ≤30℃ / h to 300℃ to reduce interfacial thermal stress.

[0011] The first stage protective atmosphere is a nitrogen-hydrogen mixture, with H2 accounting for 3-8% by volume; the second stage protective atmosphere is a vacuum, with a vacuum pressure ≤5×10⁻⁶. –2 Pa; the protective atmosphere during the cooling stage is high-purity argon, with O2 ≤ 10ppm.

[0012] The low-stress gradient annealing process for the X65 steel / Inconel 625 composite plate involves controlling the temperature in the first stage at 610–630°C.

[0013] The low-stress gradient annealing process for the X65 steel / Inconel 625 composite plate involves controlling the temperature in the second stage at 910–930°C.

[0014] The low-stress gradient annealing process of the X65 steel / Inconel 625 composite plate has a cooling rate V2 of 15-30℃ / min in the 650-700℃ range, and a cooling rate V1 of 10-30℃ / h in other stages above 300℃.

[0015] The design concept of this invention is as follows:

[0016] Existing technologies employ a single isothermal annealing process, which struggles to simultaneously address stress release in high-strength steel and microstructure control in nickel-based alloys. Furthermore, brittle phases tend to precipitate during the sensitization zone of 650–700°C. To ensure good interfacial bonding strength in the bimetallic composite plate, the core of this invention's annealing process lies in employing staged, precise temperature control and atmosphere management to synergistically optimize the microstructure and interfacial bonding quality of the two dissimilar materials. Specifically:

[0017] 1) Differentiated temperature control: In the first stage (610~630℃), the temperature is held 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℃), the temperature is held below the recrystallization temperature of Inconel625 for a shorter time (t2=0.6~0.8 t1) to promote the diffusion of interfacial elements and metallurgical bonding, while inhibiting excessive grain growth of nickel-based alloys.

[0018] 2) Rapid cooling in critical zones: During the cooling stage, special emphasis is placed on rapidly passing through the sensitive temperature range of 650-700℃, where brittle phases of nickel-based alloys precipitate, at a rate of ≥15℃ / min to avoid the formation of brittle phases; other stages adopt slow cooling (V1≤30℃ / h) to reduce interfacial thermal stress.

[0019] 3) Stepped Atmosphere Protection: Different protective atmospheres are matched for each stage (the first stage is a nitrogen-hydrogen mixture to prevent steel oxidation, the second stage is a high vacuum to prevent alloy element oxidation and maintain interface purity, and the third stage is high-purity argon to prevent cooling oxidation), ensuring the surface condition and interface quality of the materials at each stage. The overall design effectively solves key problems such as stress relief, microstructure stability, interface bonding, and embrittlement inhibition caused by the differences in thermophysical properties of dissimilar materials through the coordinated control of temperature, time, cooling rate, and atmosphere, thus realizing the preparation of high-performance composite plates.

[0020] The advantages and beneficial effects of this invention are:

[0021] 1. The annealing process of this invention, through the coordinated design of temperature-time-atmosphere-cooling, addresses the problems of microstructure mismatch and interface embrittlement in the composite annealing of high-strength steel and nickel-based alloys. While improving the metallurgical bonding quality of the interface, it ensures the stability of the bimetallic microstructure, with an interface bonding strength ≥500 MPa and residual stress ≤38 MPa. This provides a high-efficiency, low-defect, and tough manufacturing solution for high-performance composite plates.

[0022] 2. In view of the recrystallization characteristics of X65 steel and Inconel 625, this invention designs a stepped heat preservation method of "medium and low temperature long time + high temperature short time", which not only releases the stress of X65 steel, but also avoids the coarsening of Inconel 625 grains; in addition, forced rapid cooling in the brittle phase sensitive area of ​​650-700℃ directly suppresses the precipitation of Nb-rich phase.

[0023] 3. The annealing process of this invention can significantly improve the interfacial bonding strength of composite plates, reduce residual stress and improve microstructure stability, and is suitable for the preparation and application of high-reliability bimetallic structural materials. Attached Figure Description

[0024] Figure 1 The tissue near the interface after annealing in Example 1 is a scanning electron microscope (SEM) image.

[0025] Figure 2 The figures show the X-ray diffraction (XRD) patterns after annealing in Examples 2 and 3. In the figures, curve 1 represents Example 2 and curve 2 represents Example 3.

[0026] Figure 3 The electron backscatter diffraction (EBSD) structure near the interface after annealing in Example 3. Detailed Implementation

[0027] In the specific implementation process, the three bimetallic composite plate examples and comparative examples with different thicknesses (20 mm, 40 mm, and 60 mm) all adopted the same hot rolling process. The annealing process of this invention includes three stages: the first stage is a medium-low temperature pre-annealing at 610–630℃ for 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 mixture with a H2 volume fraction of 3–8% to prevent oxidation of the steel side; the second stage is a high-temperature annealing at 910–930℃ for a time t2 = (0.6–0.8)·t1 to promote grain recovery and diffusion of interface elements in the Inconel 625 alloy. The high-temperature annealing is carried out under a vacuum pressure ≤5×10 -2 The process is carried out under a vacuum environment of Pa to prevent alloy oxidation and maintain interface purity; the third stage is cooled to below 300℃ at a rate of ≤30℃ / h, and rapidly passed through the sensitization range of 650~700℃ at a rate of ≥15℃ / min. The cooling stage is protected by high-purity argon gas with O2 content ≤10 ppm to suppress the precipitation of Nb-rich brittle phase.

[0028] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0029] Example 1 (plate thickness 20 mm)

[0030] In this embodiment, a hot-rolled bimetallic composite plate with a thickness of 20 mm (18 mm of X65 steel layer and 2 mm of Inconel 625 alloy layer) undergoes a three-stage annealing process: first, it is held at 620°C for 60 min in a nitrogen-hydrogen mixture with a H2 volume fraction of 3%; then, it is annealed under vacuum pressure of 5×10⁻⁶ mm.-2 Under a vacuum of 915℃ for 42 min, the furnace was then protected with high-purity argon gas (10 ppm O2) and cooled to below 300℃ at a rate of 25℃ / h. The furnace was then rapidly passed through the 700–650℃ range at a rate of 20℃ / min, and allowed to cool naturally to room temperature below 300℃. After annealing, the composite plate was tested and found to have an interfacial bonding strength of 520 MPa, an interfacial residual stress of approximately 36 MPa, a dense and uniform interfacial transition layer with no obvious brittle phase precipitation, and excellent overall mechanical properties.

[0031] Example 2 (plate thickness 40mm)

[0032] In this embodiment, a hot-rolled bimetallic composite plate with a thickness of 40 mm (36 mm of X65 steel layer and 4 mm of Inconel 625 alloy layer) undergoes a three-stage annealing process: first, it is held at 620°C for 100 min in a nitrogen-hydrogen mixture with a H2 volume fraction of 5%; then, it is annealed under vacuum pressure of 3×10⁻⁶ mm. -2 Under a vacuum of 915℃ for 60 min, the furnace was then protected with high-purity argon gas (8 ppm O2) and cooled to below 300℃ at a rate of 26℃ / h. The furnace was then rapidly passed through the 700–650℃ range at a rate of 22℃ / min, and allowed to cool naturally to room temperature below 300℃. After annealing, the composite plate was tested and found to have an interfacial bonding strength of 510 MPa, an interfacial residual stress of approximately 38 MPa, uniform interfacial bonding, stable microstructure, and excellent mechanical properties.

[0033] Example 3 (plate thickness 60mm)

[0034] In this embodiment, a hot-rolled bimetallic composite plate with a thickness of 60 mm (54 mm of X65 steel layer and 6 mm of Inconel 625 alloy layer) undergoes a three-stage annealing process: first, it is held at 620°C for 120 min in a nitrogen-hydrogen mixture with a H2 volume fraction of 8%; then, it is annealed under vacuum pressure of 1×10⁻⁶ mm. -2 Under a vacuum of 915°C for 96 min, the furnace was held at 915°C for 96 min. Finally, under the protection of high-purity argon gas with an O2 content of 6 ppm, the furnace was cooled to below 300°C at a rate of 23°C / h, and then rapidly passed through the furnace in the 700–650°C range at a rate of 21°C / min. Below 300°C, the furnace was allowed to cool naturally to room temperature. After annealing, the composite plate was tested and found to have an interfacial bonding strength of 500 MPa and an interfacial residual stress of approximately 35 MPa. This effectively controlled the annealing of thick plates, significantly reduced interfacial stress concentration, and improved the overall composite performance.

[0035] Comparative Example 1 (without pre-annealing)

[0036] The difference from Example 1 is that only a 915°C heat treatment for 70 min (20 mm plate) was performed, and the first stage was not executed. Result: The bond strength decreased to 402 MPa, the residual stress increased to 112 MPa, resulting in a tendency for microcracks in the X65 steel layer and a high interface defect rate.

[0037] Comparative Example 2 (Insufficient Pre-annealing)

[0038] Unlike Example 2, the first stage of the 40 mm plate lasted only 50 minutes (below the required time). Result: The bond strength decreased to 380 MPa, and the residual stress increased to 120 MPa, resulting in insufficient stress release.

[0039] Comparative Example 3 (Cooling rate within the sensitization range was not controlled)

[0040] The difference from Example 2 is that the cooling rate of the 40 mm plate was not controlled between 700 and 650°C. As a result, the bond strength decreased to 385 MPa, the residual stress increased to 117 MPa, leading to a large amount of δ phase precipitation and interface embrittlement.

[0041] Comparative Example 4 (short high-temperature annealing time)

[0042] Unlike Example 1, the second stage of the 20 mm plate lasted only 15 minutes (below the required time). Result: The bond strength decreased to 376 MPa, and the residual stress increased to 102 MPa, resulting in unreleased stress in the Inconel 625 layer and discontinuous microstructure.

[0043] Comparative Example 5 (long high-temperature annealing time)

[0044] The difference from Example 1 is that the second stage of heat treatment on the 20 mm plate was 80 min (exceeding the limit). Result: The bond strength decreased to 375 MPa, the residual stress increased to 100 MPa, resulting in the formation of element-enriched bands and δ / Laves brittle phases at the interface.

[0045] 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:

[0046] (1) First stage (620℃): The heat preservation time is designed to be 2-4 min / mm to fully release the interfacial stress and form a dense transition layer, avoiding the coarsening of the structure (the risk of insufficient verification in Comparative Example 2).

[0047] (2) Second stage (915℃): The holding time is 60-80% of the first stage to ensure complete recrystallization of the Inconel 625 layer and uniform diffusion of elements, and to inhibit the precipitation of brittle phase (comparative example 4 / 5 verified that the time deviation caused insufficient release of brittle phase or stress).

[0048] (3) Cooling stage: Forced rapid cooling in the range of 700~650℃ (≥20℃ / min) to avoid the sensitive temperature range of brittle phase and ensure the stability of the interface structure (Comparative Example 3 verifies that embrittlement occurs without rapid cooling).

[0049] Overall results: The interfacial bonding strength is ≥500 MPa and the residual stress is ≤38 MPa, which is significantly better than the comparative example (strength reduction of 20-27% and stress increase of 65-200%).

[0050] Table 1

[0051] Serial Number type Plate thickness First stage of heat preservation (°C / min) Second stage of heat preservation (°C / min) Interfacial bond 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

[0052] like Figure 1 As shown in the SEM microstructure near the interface after annealing in Example 1, the composite interface is flat and has no obvious metallurgical defects.

[0053] like Figure 2 As shown in the XRD patterns after annealing in Examples 2 and 3, no brittle phase was precipitated.

[0054] like Figure 3 As shown in the figure, the EBSD structure near the interface after annealing in Example 3 shows that the structure near the interface is in a clear annealed diffusion state, and the interface is well composited.

[0055] The results show that this invention employs a differentiated temperature-time design. In the first stage, low-temperature pre-annealing is held at 20–50°C below the X65 recrystallization temperature for an extended period to fully release cold working stress. In the second stage, high-temperature annealing is held at 20–50°C below the Inconel 625 recrystallization temperature for a short period to promote interfacial diffusion and inhibit grain coarsening. Rapid cooling is used in critical zones, with a rate of ≥15°C / min in the 650–700°C range to prevent brittle phase precipitation, while other stages use a slow cooling rate of ≤30°C / h to reduce interfacial thermal stress. Simultaneously, a stepped atmosphere (nitrogen-hydrogen mixture / vacuum / argon) is used for staged oxidation prevention, which enhances the performance of the X65 steel / Inconel 625 composite plate. Therefore, this invention can provide highly reliable composite plates for fields such as oil and gas, chemical equipment, and marine engineering.

[0056] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A low-stress gradient annealing process for X65 steel / Inconel 625 composite plates, characterized in that, Includes the following steps: (1) First stage of low temperature pre-annealing: hold at 20-50℃ below the recrystallization temperature of X65 steel for a time t1 = 2-4 min / mm; (2) Second stage high-temperature annealing: Hold at 20-50°C below the recrystallization temperature of Inconel 625 nickel-based alloy for time t2=(0.6-0.8) t1; (3) Cooling stage: The cooling rate V2 in the range of 650 to 700℃ is ≥15℃ / min to avoid the formation of brittle phase; in other stages, the cooling rate V1 is ≤30℃ / h to 300℃ to reduce interfacial thermal stress. The first stage protective atmosphere is a nitrogen-hydrogen mixture, with H2 accounting for 3-8% by volume; the second stage protective atmosphere is a vacuum, with a vacuum pressure ≤5×10⁻⁶. –2 Pa; the protective atmosphere during the cooling stage is high-purity argon, with O2 ≤ 10ppm.

2. The low-stress gradient annealing process for the X65 steel / Inconel 625 composite plate according to claim 1, characterized in that, The temperature for the first stage is controlled between 610 and 630℃.

3. The low-stress gradient annealing process for the X65 steel / Inconel 625 composite plate according to claim 1, characterized in that, The temperature in the second stage is controlled between 910 and 930℃.

4. The low-stress gradient annealing process for the X65 steel / Inconel 625 composite plate according to claim 1, characterized in that, The cooling rate V2 in the 650-700℃ range is 15-30℃ / min, and the cooling rate V1 in other stages above 300℃ is 10-30℃ / h.

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