Process for accelerating high-temperature structure stabilization of 9Cr heat-resistant steel weld

Abstract

The application discloses a process method for accelerating high-temperature structure stabilization of 9Cr heat-resistant steel welds, and belongs to the technical field of metal material heat treatment. For 9Cr heat-resistant steel welds which need to serve under long-term high-temperature conditions, a post-weld heat treatment process is adopted to heat the as-welded 9Cr heat-resistant steel welds to 720-780 DEG C, keep heat for 1-4 hours, cool down to below 200 DEG C in the furnace, take out of the furnace and air cool to room temperature, and then perform subsequent high-temperature long-term aging treatment. The process can make the structure of the 9Cr heat-resistant steel welds reach a stabilized state before aging treatment, and the performance changes little under subsequent long-term high-temperature service conditions, thereby guaranteeing the overall structural stability and safety of the components, avoiding direct use under the as-welded condition, and preventing the performance of the welds from being obviously deteriorated due to the structural change under long-term high-temperature conditions, and increasing the possibility of accidents due to the destruction of the overall structural stability.

Description

Technical Field

[0001] This invention relates to the field of material heat treatment technology, specifically to a process for accelerating the stabilization of the high-temperature microstructure of 9Cr heat-resistant steel welds. Background Technology

[0002] The development of new materials is one of the main bottlenecks restricting the development of advanced nuclear energy, which determines the feasibility, safety and economy of nuclear energy systems. 9Cr heat-resistant steel has become a widely used structural material for nuclear power components due to its excellent mechanical properties, good low activation and low thermal expansion rate.

[0003] As a structural material for nuclear power plants, 9Cr heat-resistant steel undergoes significant degradation in its mechanical properties during long-term service at high temperatures, resulting in both microstructure aging and mechanical deterioration. The evolution of the microstructure during creep determines this degradation. The degradation mechanism of 9Cr heat-resistant steel during long-term high-temperature service primarily involves changes in dislocation substructure and precipitates. After prolonged service, the dislocation density decreases significantly, with numerous dislocations canceling each other out, being absorbed by grain boundaries and subgrain boundaries, or entangled into dislocation walls that transform into subgrain boundaries. This promotes the breakage and recrystallization of lath martensite. Simultaneously, subgrains coarsen, gradually transforming from an initial lath-like structure to an equiaxed structure. Furthermore, precipitates play a crucial role in the high-temperature creep strength of 9Cr heat-resistant steel. Over time, some precipitates coarsen, while others dissolve and transform into other thermodynamically more stable coarse precipitates, or even consume solid solution strengthening elements in the matrix to precipitate new phases. These changes in the material's microstructure and properties can compromise the overall structural stability of components, increasing the likelihood of accidents. The weld seams of 9Cr heat-resistant steel also have these problems. It is necessary to stabilize the microstructure of the 9Cr heat-resistant steel weld seams before use to ensure that the performance changes are small under long-term high-temperature service conditions and to ensure the overall structural stability of the component. Summary of the Invention

[0004] To avoid the problems of significant performance deterioration caused by the direct use of 9Cr heat-resistant steel welds under as-welded conditions, resulting in changes in the weld microstructure under long-term high-temperature conditions, which could undermine the overall structural stability and increase the possibility of accidents, the present invention aims to provide a process for accelerating the high-temperature microstructure stabilization of 9Cr heat-resistant steel welds. This process can stabilize the microstructure of 9Cr heat-resistant steel welds before aging treatment, resulting in minimal performance changes under subsequent long-term high-temperature service conditions, thus ensuring the overall structural stability and safety of the component.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A process for accelerating the stabilization of the high-temperature microstructure of 9Cr heat-resistant steel welds is disclosed. This process involves heat-treating 9Cr heat-resistant steel welds that are required to serve under long-term high-temperature conditions. The heat treatment process is as follows: after welding, the as-welded 9Cr heat-resistant steel weld is heated to 720-780℃ in a heat treatment furnace and held at that temperature for 1-4 hours.

[0007] During the heat treatment process, the weld seams of the 9Cr heat-resistant steel are heated in the furnace at a rate greater than or equal to 1℃ / min. After holding at that temperature, the weld seams are cooled in the furnace to below 200℃ before being air-cooled to room temperature. This process stabilizes the microstructure of the 9Cr heat-resistant steel weld seams, resulting in minimal changes in their properties under subsequent long-term high-temperature service conditions.

[0008] The mechanical properties of the 9Cr heat-resistant steel weld after heat treatment are as follows: tensile strength ≥600MPa, yield strength ≥500MPa, and elongation ≥17% after aging at 550℃ for 3000h.

[0009] The present invention has the following advantages:

[0010] 1. This invention starts with the process of controlling the high-temperature microstructure stabilization of 9Cr heat-resistant steel welds, so that the microstructure of 9Cr heat-resistant steel welds reaches a stable state before high-temperature service. Therefore, under subsequent long-term high-temperature service conditions, its performance changes are small, ensuring the overall structural stability of the component and the safety of long-term use.

[0011] 2. All heat treatment process parameters used in this invention can be applied in industrial applications. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the high-temperature microstructure stabilization process for the weld seam of 9Cr heat-resistant steel according to the present invention.

[0013] Figure 2 This is the weld structure in the weld state.

[0014] Figure 3 This refers to the weld microstructure after stabilization treatment.

[0015] Figure 4 This refers to the weld microstructure after long-term aging following stabilization treatment.

[0016] Figure 5 This is the weld microstructure in the long-term aged state directly after welding. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to specific embodiments, but this is not intended to limit the present invention in any way.

[0018] The heat treatment equipment used in this invention has no special requirements; a common resistance furnace is sufficient. The chemical composition of the 9Cr heat-resistant steel weld seam used in this invention is shown in Table 1. A schematic diagram of the process is shown below. Figure 1 As shown, the microstructure of the weld in the as-welded state is as follows: Figure 2 As shown in Table 2, the tensile properties of the weld in the as-welded state are as follows.

[0019] Table 1. Chemical composition (mass fraction, %) of 9Cr heat-resistant steel welds

[0020] C Si Mn P S Nb V Cr Ni Mo N Fe 1.0 1.1 1.2 <0.005 0.003 0.08 0.20 9.0 1.0 0.8 0.04 margin

[0021] Table 2. Results of tensile property tests on weld seams in the as-welded state.

[0022] Weld condition Yield strength, MPa Tensile strength, MPa Elongation, % Welded 1050 1275 12.5

[0023] Example 1:

[0024] The stabilization process is as follows: the weld of 9Cr heat-resistant steel is rapidly heated to 750℃ at a rate of 5℃ / min, held at that temperature for 2.5 hours, furnace cooled to 200℃, and then air cooled. The weld microstructure is shown in the figure. Figure 3 The tensile properties of the weld are shown in Table 3. After stabilization treatment, the weld was subjected to long-term aging at 550℃ for 3000℃. The weld microstructure is shown in Table 3. Figure 4 Performance test data are shown in Table 4.

[0025] Comparative Example 1:

[0026] The weld in its as-welded state was directly subjected to long-term aging at 550℃ for 3000℃. The weld microstructure is shown in the figure. Figure 5 Performance test data are shown in Table 4.

[0027] Table 3. Results of tensile property tests on welds after stabilization treatment.

[0028]

[0029]

[0030] Table 4. Test results of weld tensile properties after long-term aging for the Examples and Comparative Examples.

[0031]

[0032] From Example 1, Comparative Example 1, Tables 2-4 and Figures 2-5It can be seen that, using the process method designed in this invention to accelerate the stabilization of the high-temperature microstructure of 9Cr heat-resistant steel welds, Example 1 shows that compared with the as-welded microstructure, after stabilization treatment, obvious precipitates appear in the microstructure. Compared with the tensile properties of the as-welded state, the yield and tensile strengths are significantly reduced, while the elongation is significantly increased. The microstructure and tensile properties of the weld after long-term aging are not significantly different from those after stabilization treatment. This indicates that after stabilization treatment, the microstructure and properties can remain stable after long-term high-temperature aging, and the tensile properties after aging meet the requirements of tensile strength ≥600MPa, yield strength ≥500MPa, and elongation ≥17%. Comparative Example 1 shows that after direct long-term high-temperature aging treatment after welding, the weld microstructure and properties change significantly compared with the as-welded state, which is detrimental to the overall stability of the component during long-term high-temperature service, and the elongation after aging is lower than the required value.

Claims

1. A process for accelerating the stabilization of the high-temperature microstructure of welds in 9Cr heat-resistant steel, characterized in that: This process involves heat treatment of the weld seams of 9Cr heat-resistant steel that needs to be used under long-term high-temperature conditions. The heat treatment process is as follows: after welding, the weld of the 9Cr heat-resistant steel in the welded state is heated to 720-750℃ in a heat treatment furnace at a heating rate of 5℃ / min; the temperature is held for 2.5-4 hours; after the holding time is completed, the temperature is cooled to below 200℃ in the furnace and then removed from the furnace and air-cooled to room temperature. The chemical composition of the 9Cr heat-resistant steel by mass percentage is as follows: C 1.0%, Si 1.1%, Mn 1.2%, Nb 0.08%, V 0.20%, Cr 9.0%, Ni 1.0%, Mo 0.8%, N 0.04%, P<0.005%, S 0.003%, with Fe as the balance.

2. The process for accelerating the stabilization of high-temperature microstructure of 9Cr heat-resistant steel welds according to claim 1, characterized in that: The mechanical properties of the 9Cr heat-resistant steel weld after heat treatment are as follows: tensile strength ≥600MPa, yield strength ≥500MPa, and elongation ≥17% after aging at 550℃ for 3000h.

Images