Gradient design method for prolonging high-temperature endurance life of heat-resistant steel / nickel-based alloy dissimilar joint

Abstract

The invention provides a gradient design method for prolonging the high-temperature endurance life of a heat-resistant steel / nickel-based alloy dissimilar joint. The invention accidentally finds that in the design of the gradient transition layer of the heat-resistant steel and the nickel-based alloy, by adopting 5% or 10% component gradient, the interface with the mass ratio of 85 / 15 to 80 / 20 or 90 / 10 to 80 / 20 of the heat-resistant steel and the nickel-based alloy is preferentially cracked, and the harmful interface is easy to form strain concentration and becomes a preferential position for crack initiation and expansion due to obvious hardness change. Therefore, by creatively adopting the 20% stepped component gradient design, on one hand, slow transition of components of dissimilar interfaces is achieved, and the structure property difference between dissimilar materials is reduced; and on the other hand, a strain concentration area on a harmful interface is actively avoided and eliminated, the heat-resistant steel / nickel-based alloy component with excellent high-temperature endurance life is prepared, and the life of the heat-resistant steel / nickel-based alloy component is nearly doubled compared with that of a traditional gradient joint component.

Description

Technical Field

[0001] This invention belongs to the field of dissimilar component technology and relates to a gradient design method for improving the high-temperature creep life of heat-resistant steel / nickel-based alloy dissimilar joints, thereby achieving high-quality connection of heat-resistant steel and nickel-based alloy dissimilar joints. Background Technology

[0002] Dissimilar metal joining of heat-resistant martensitic steel and nickel-based alloys is an essential choice for manufacturing key high-temperature components such as rotors for 700°C-class advanced ultra-supercritical (A-USC) generator sets. Heat-resistant martensitic steel exhibits excellent high-temperature strength and creep resistance below 600°C, while nickel-based alloys are suitable for high-temperature environments above 700°C. However, significant differences exist in their physical and chemical properties. Direct welding leads to oxide notches, stress concentration, and a non-uniform microstructure containing brittle martensite at the joint, making it a weak point in the entire component under high-temperature service. To mitigate this problem, functionally graded materials are typically used as transition layers. Achieving a performance gradient through continuous changes in composition and microstructure has become an effective method. Currently, laser-directed energy deposition (LDED) technology, due to its high controllability of composition, has become an effective means of preparing such gradient transition layers. However, despite employing gradient transition schemes, fracture is still inevitable in high-temperature creep tests of steel-nickel joints with gradient transition layers, severely limiting the service life and reliability of the joints and making it difficult to meet the stringent requirements of advanced energy equipment for long service life and high safety of key components.

[0003] Therefore, the key to achieving high-performance, long-life heat-resistant steel / nickel-based alloy gradient joints lies in how to fundamentally avoid the brittle fracture problem at the steel-nickel interface through innovative composition and structural design. Summary of the Invention

[0004] To address the shortcomings of the prior art, this invention provides a gradient design method for improving the high-temperature creep life of heat-resistant steel / nickel-based alloy dissimilar joints, which is mainly applied to the high-performance manufacturing of key dissimilar components in advanced ultra-supercritical power plants, nuclear power and other energy equipment.

[0005] Preliminary research for this invention unexpectedly revealed that in the design of gradient transition layers between heat-resistant steel and nickel-based alloys, a 5% composition gradient leads to preferential cracking at the interfaces of 85 / 15 and 80 / 20 mass ratios of heat-resistant steel / nickel-based alloy. These interfaces, due to significant hardness variations, are prone to strain concentration, becoming preferred sites for crack initiation and propagation. Similarly, a 10% composition gradient also results in cracking at the interfaces of 90 / 10 and 80 / 20 mass ratios of heat-resistant steel / nickel-based alloy due to hardness variations. Essentially, this is because when the steel content exceeds 85%, a considerable amount of martensite still exists within the transition layer, which readily forms strain concentration at the interface with the predominantly austenitic structure at 80% composition due to uneven deformation. Therefore, by creatively adopting a 20% step-gradient composition design, this invention achieves a slow transition of composition at the dissimilar interface, reducing the differences in microstructure and properties between dissimilar materials. On the other hand, it actively avoids and eliminates strain concentration areas at the 85 / 15 and 80 / 20 interfaces and the 90 / 10 and 80 / 20 interfaces, thereby producing a heat-resistant steel / nickel-based alloy dissimilar joint with excellent high-temperature creep life.

[0006] The objective of this invention can be achieved through the following methods: In a first aspect, the present invention provides a gradient design method for improving the high-temperature creep life of heat-resistant steel / nickel-based alloy dissimilar joints, comprising the following steps: Step 1: Using laser-directed energy deposition technology, heat-resistant steel powder is deposited on the surface of a heat-resistant steel substrate to prepare the underlayer; Step 2: According to the preset 20% composition gradient, heat-resistant steel powder and nickel-based alloy powder are mixed sequentially to prepare functionally graded material powder; Step 3: Following the order of decreasing heat-resistant steel powder content, functionally graded material powders are sequentially deposited onto the surface of the substrate using laser-directed energy deposition technology to prepare functionally graded layers; Step 4: Continue to deposit nickel-based alloy powder on the surface of the functional gradient layer to prepare the top layer, thus obtaining the heat-resistant steel / nickel-based alloy gradient component.

[0007] In one embodiment of the present invention, in step 1, the heat-resistant steel comprises martensitic steel. In some embodiments, the martensitic steel is 9Cr martensitic steel.

[0008] In one embodiment of the present invention, in step 1, the heat-resistant steel substrate is a pre-treated heat-resistant steel substrate; the pre-treatment includes: machining and polishing the heat-resistant steel substrate, and then ultrasonically cleaning it with anhydrous ethanol, thus obtaining the substrate.

[0009] In one embodiment of the present invention, in step 1 or step 2, the particle size of the heat-resistant steel powder is 45-105 μm.

[0010] In one embodiment of the present invention, in step 1 or step 3, the parameters for laser-directed energy deposition include: laser power 1.2-1.5 kW, scanning speed 600-800 mm / min, powder feed rate 6-7 g / min, and spot diameter 2 mm. Preferably, the parameters for laser-directed energy deposition are: laser power 1.3-1.4 kW, scanning speed 650-750 mm / min, powder feed rate 6.5-6.8 g / min, spot diameter 2 mm, and overlap rate 40%.

[0011] In one embodiment of the present invention, in step 1, the thickness of the base layer is 1.8-2.2 mm.

[0012] In one embodiment of the present invention, in step 2, the preset 20% composition gradient path is as follows: 80wt.% heat-resistant steel and 20wt.% nickel-based alloy, 60wt.% heat-resistant steel and 40wt.% nickel-based alloy, 40wt.% heat-resistant steel and 60wt.% nickel-based alloy, and 20wt.% heat-resistant steel and 80wt.% nickel-based alloy. The present invention, through its unique 20% stepped composition design, avoids the composition range from 85wt.% heat-resistant steel / 15wt.% nickel-based alloy to 80wt.% heat-resistant steel / 20wt.% nickel-based alloy, and also avoids the composition range from 90wt.% heat-resistant steel / 10wt.% nickel-based alloy to 80wt.% heat-resistant steel / 20wt.% nickel-based alloy.

[0013] In one embodiment of the present invention, in step 2, the particle size of the nickel-based alloy powder is 45-105 μm, and the nickel-based alloy includes Inconel 625 nickel-based alloy.

[0014] In one embodiment of the present invention, in step 3, the interlayer residence time during laser-directed energy deposition is 30-45 s, and the cooling rate is 800-1000 K / s. The laser-directed energy deposition is performed under an argon protective atmosphere with an oxygen content below 100 ppm. By controlling the interlayer residence time and introducing cooling gas, rapid cooling is achieved, inhibiting the precipitation of elements dissolved in the matrix and the formation of chain-like carbides, thus reducing the solid solution strengthening effect.

[0015] In one embodiment of the present invention, in step 3, the total thickness of the functional gradient layer is 3.0-3.5 mm, and the thickness of each layer is 0.6-1.0 mm.

[0016] In one embodiment of the present invention, in step 4, the thickness of the top layer is 1.8-2.2 mm.

[0017] Secondly, the present invention provides a heat-resistant steel / nickel-based alloy gradient component prepared by the gradient design method described above.

[0018] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention unexpectedly discovered that in the design of a gradient transition layer between heat-resistant steel and nickel-based alloys, using a 5% composition gradient leads to preferential cracking at the interfaces of 85 / 15 and 80 / 20 mass ratios of heat-resistant steel / nickel-based alloy. These harmful interfaces, due to significant hardness changes, are prone to strain concentration, becoming preferred sites for crack initiation and propagation. Similarly, using a 10% composition gradient results in cracking at the interfaces of 90 / 10 and 80 / 20 mass ratios of heat-resistant steel / nickel-based alloy due to hardness changes. Essentially, this is because when the steel content is above 85%, a considerable amount of martensite still exists in the transition layer. This martensite is highly susceptible to strain concentration at the interface with the predominantly austenitic structure at 80% composition due to uneven deformation, thus affecting the high-temperature creep life of the component. Therefore, by creatively adopting a 20% step-gradient composition design, this invention achieves a slow transition of composition at the dissimilar interface, reducing the differences in microstructure and properties between dissimilar materials. On the other hand, it actively avoids and eliminates strain concentration areas at the 85 / 15 and 80 / 20 interfaces and the 90 / 10 and 80 / 20 interfaces, thereby producing a heat-resistant steel / nickel-based alloy dissimilar joint with excellent high-temperature creep life.

[0019] 2. This invention utilizes a unique 20% stepped composition design to eliminate the risk of hardness collapse and early fracture caused by drastic changes in microstructure and precipitation of harmful phases in the harmful interface region. It solves the fundamental weakness of dissimilar material gradient transition layer preparation, significantly improving the high-temperature long-term service capability of the component. Its high-temperature creep life reaches 1500 hours (620℃ / 220 MPa test conditions), which is more than 4 times longer than that of traditional steel-nickel welded joints (345 hours), and nearly 2 times longer than that of traditional gradient joint components (5% and 10% gradient). Simultaneously, the failure mode of the joint is fundamentally improved; the fracture location successfully shifts from the steel-nickel interface to the heat-affected zone of the heat-resistant steel, and the fracture morphology changes from intergranular brittle fracture to ductile fracture dominated by dimples. This indicates that this invention not only significantly extends the service life but also significantly improves the safety margin and fracture reliability of the component.

[0020] 3. This invention optimizes the rapid cooling process (800-1000K / s), promoting the uniform precipitation of the reinforcing phase as fine, dispersed nanoparticles, avoiding harmful continuous chain phases, and significantly improving the microstructure. Simultaneously, the clearly defined 20% stepped process window significantly reduces the number of deposition layers while ensuring performance, significantly improving manufacturing efficiency and economy, demonstrating outstanding advantages for industrial applications. Attached Figure Description

[0021] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is the design diagram for the 20% component gradient transition in Example 1; Figure 2 This is a schematic diagram of the laser-directed energy deposition (L-DED) process in Example 1; Figure 3 The image shows the microhardness distribution of the endurance fracture specimens from Example 1, Comparative Example 1, and Comparative Example 2. Figure 4 Fracture location diagrams of the durable samples from Example 1, Comparative Example 1, Comparative Example 2, and conventional arc welding. Figure 5 The bar chart shows the high-temperature creep life comparison of Example 1 (20% gradient), Comparative Example 1 (conventional gradient component, 5% gradient), Comparative Example 2 (conventional gradient component, 10% gradient), and conventional steel-nickel arc welded sample under 620°C / 220MPa conditions. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The following examples are implemented under the premise of the technical solution of the present invention, providing detailed implementation methods and specific operating procedures, which will help those skilled in the art to further understand the present invention. It should be noted that the scope of protection of the present invention is not limited to the following embodiments; any adjustments and improvements made under the concept of the present invention are all within the scope of protection of the present invention.

[0023] Example 1 In this implementation case, the test materials were 9Cr martensitic heat-resistant steel powder and Inconel 625 nickel-based alloy powder prepared by gas atomization method, both with a particle size of 45-105μm.

[0024] Step 1: Substrate Pretreatment: Standard 9Cr martensitic heat-resistant steel is selected as the substrate. Its surface is machined to ensure flatness, then polished sequentially using sandpaper ranging from 180# to 2000#. Finally, it is ultrasonically cleaned with anhydrous ethanol to thoroughly remove oil and impurities. The treated substrate is placed in the working chamber of the laser-directed energy deposition equipment and preheated to 180°C.

[0025] Step 2: Preparation of the underlayer: Atomized 9Cr martensitic heat-resistant steel powder with a particle size of 45-105 μm is fed into the powder feeder to prepare the underlayer on the pretreated substrate surface. The process parameters used are: laser power 1.3 kW, scanning speed 650 mm / min, powder feed rate 6.5 g / min, spot diameter 2 mm, overlap rate 40%, and argon gas protection (oxygen content <100 ppm) is introduced into the working chamber. The single-layer deposition thickness is controlled at 0.65 mm, with natural cooling between layers for 2 minutes. A total of 3 layers are deposited, with a total underlayer thickness of 2.0 mm.

[0026] Step 3: Depositing the Functionally Graded Layer: On the surface of the base layer, continue depositing the 20% stepped functionally graded layer as described in this invention. Its composition follows a predetermined path: 80 wt.% 9Cr martensitic heat-resistant / 20 wt.% Inconel 625 nickel-based alloy → 60 / 40 → 40 / 60 → 20 / 80, thus precisely avoiding the harmful component range of 70 / 30 to 60 / 40. The optimized process parameters used in this step are: laser power 1.35 kW, scanning speed 675 mm / min, powder feed rate 6.7 g / min, and interlayer rotation angle 67°. By controlling the interlayer dwell time to 40 seconds and supplementing with forced argon purging, the cooling rate of the molten pool is stably maintained at 800-1000 K / s. The nominal thickness of each layer is 0.8 mm, and the total thickness of the functionally graded region is 3.2 mm. The design of the 20% stepped functionally graded layer is as follows... Figure 1 As shown.

[0027] Step 4, Deposit the top layer: On top of the gradient layer, continue to deposit pure Inconel 625 nickel-based alloy material to the target thickness of 2 mm. The process parameters are fine-tuned as follows: laser power 1.4 kW, scanning speed 700 mm / min, and powder feed rate 7.0 g / min.

[0028] The schematic diagram of the laser-directed energy deposition (L-DED) process is shown below. Figure 2 As shown.

[0029] Comparative Example 1 The preparation method of this comparative example is basically the same as that of Example 1, except that in step 3, the 20% stepped functionally graded layer is replaced with a 5% stepped functionally graded layer, and its composition is as follows according to the preset path: 95wt.% 9Cr martensitic heat-resistant steel / 5wt.% Inconel 625 nickel-based alloy → 90 / 10 → 85 / 15 → 80 / 20 → 75 / 25 → 70 / 30 → 65 / 35 → 60 / 40 → 55 / 45 → 50 / 50 → 45 / 55 → 40 / 60 → 35 / 65 → 30 / 70 → 25 / 75 → 20 / 80 → 15 / 85 → 10 / 90 → 5 / 95.

[0030] Comparative Example 2 The preparation method of this comparative example is basically the same as that of Example 1, except that in step 3, the 20% stepped functionally graded layer is replaced with a 10% stepped functionally graded layer, and its composition is as follows according to the preset path: 90wt.% 9Cr martensitic heat-resistant steel / 10wt.% Inconel 625 nickel-based alloy → 80 / 20 → 70 / 30 → 60 / 40 → 50 / 50 → 40 / 60 → 30 / 70 → 20 / 80 → 10 / 90.

[0031] like Figure 3 As shown, it can be seen that compared with Comparative Examples 1 and 2, the microhardness difference in Example 1 is smaller, and its hardness peak is more due to the necking deformation strengthening of the heat-resistant steel.

[0032] like Figure 4 As shown, the different failure locations corresponding to creep-induced failure joints of conventional arc welding, Comparative Example 1, Comparative Example 2 and Example 1 are illustrated. It can be seen that the failure mode of the joint in Example 1 is fundamentally improved. The fracture location is successfully transferred from the steel-nickel interface or the transition interlayer interface to the heat-resistant steel base material. The fracture morphology is also changed from brittle fracture along the interface to ductile fracture dominated by dimples.

[0033] like Figure 5 As shown, the samples of Example 1, Comparative Examples 1-2, and traditional steel-nickel arc welded samples were tested according to the GB / T2039-2012 "Metallic Materials Uniaxial Tensile Creep Test Method". The service life of the joints at 620℃ / 220 MPa was tested. It can be seen that the present invention, through its original 20% stepped composition design, fundamentally skips the harmful component ranges of 85 / 15 and 80 / 20 and 90 / 10 and 80 / 20 that are difficult to avoid in traditional processes. This significantly improves the high-temperature creep life of 9Cr martensitic heat-resistant steel / Inconel 625 nickel-based alloy gradient components and has excellent high-temperature long-term service capability.

[0034] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A gradient design method for improving the high-temperature creep life of heat-resistant steel / nickel-based alloy dissimilar joints, characterized in that, Includes the following steps: Step 1: Using laser-directed energy deposition technology, heat-resistant steel powder is deposited on the surface of a heat-resistant steel substrate to prepare the underlayer; Step 2: According to the preset 20% composition gradient, heat-resistant steel powder and nickel-based alloy powder are mixed sequentially to prepare functionally graded material powder; Step 3: Following the order of decreasing heat-resistant steel powder content, functionally graded material powders are sequentially deposited onto the surface of the substrate using laser-directed energy deposition technology to prepare functionally graded layers; Step 4: Continue to deposit nickel-based alloy powder on the surface of the functional gradient layer to prepare the top layer, thus obtaining the heat-resistant steel / nickel-based alloy gradient component.

2. The gradient design method according to claim 1, characterized in that, In step 1, the particle size of the heat-resistant steel powder is 45-105 μm, and the heat-resistant steel includes martensitic steel.

3. The gradient design method according to claim 1, characterized in that, In step 1, the heat-resistant steel substrate is a pre-treated heat-resistant steel substrate; the pre-treatment includes: machining and polishing the heat-resistant steel substrate, and then ultrasonically cleaning it with anhydrous ethanol.

4. The gradient design method according to claim 1, characterized in that, In step 1, the thickness of the base layer is 1.8-2.2 mm.

5. The gradient design method according to claim 1, characterized in that, In step 2, the particle size of the nickel-based alloy powder is 45-105 μm, and the nickel-based alloy includes Inconel 625 nickel-based alloy.

6. The gradient design method according to claim 1, characterized in that, In step 2, the preset 20% composition gradient path is as follows: 80wt.% heat-resistant steel and 20wt.% nickel-based alloy, 60wt.% heat-resistant steel and 40wt.% nickel-based alloy, 40wt.% heat-resistant steel and 60wt.% nickel-based alloy, and 20wt.% heat-resistant steel and 80wt.% nickel-based alloy.

7. The gradient design method according to claim 1, characterized in that, In step 1 or step 3, the parameters for laser-directed energy deposition include: laser power 1.2-1.5 kW, scanning speed 600-800 mm / min, powder feed rate 6-7 g / min, and spot diameter 2 mm.

8. The gradient design method according to claim 1, characterized in that, In step 3, during laser-directed energy deposition, the interlayer dwell time is 30-45 s, and the cooling rate is 800-1000 K / s.

9. The gradient design method according to claim 1, characterized in that, In step 3, the total thickness of the functionally graded layers is 3.0-3.5 mm, and the thickness of each layer is 0.6-1.0 mm; And / or, in step 4, the thickness of the top layer is 1.8-2.2 mm.

10. A heat-resistant steel / nickel-based alloy gradient component prepared by the gradient design method according to any one of claims 1-9.

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