A post-weld composite heat treatment method for a laser welded joint of a dissimilar steel containing a Ni interlayer

Abstract

The application discloses a post-welding composite heat treatment method for a laser-welded joint of dissimilar steel containing a Ni interlayer. In view of problems of uneven interface structure of the dissimilar steel welded joint, large element diffusion gradient and insufficient performance matching, a composite heat treatment system is provided, which comprises 1050 DEG C solid solution treatment for 1 hour and then air cooling, and then 480 DEG C aging treatment for 2 hours. The heat treatment can promote the redistribution of interface elements and realize the dispersion strengthening of Cu-rich phase. Experimental results show that under the condition of 0.02mm Ni single-layer interlayer, the comprehensive performance of the welded joint after the heat treatment is better than that of the Ni / Cu composite interlayer structure. The application significantly improves the uniformity of the welded joint structure and the matching of the mechanical properties, and is suitable for the fields of nuclear power and high-end pressure-bearing equipment manufacturing.

Description

Technical Field

[0001] This invention relates to the field of microstructure control and post-weld heat treatment technology for dissimilar steel welded joints, specifically to a post-weld composite heat treatment method for laser-welded dissimilar steel joints containing a Ni intermediate layer, and particularly to a method for synergistic control of interface microstructure and performance matching through solution treatment and aging. Background Technology

[0002] 05Cr17Ni4Cu4Nb precipitation-hardening stainless steel possesses high strength, good corrosion resistance, and excellent comprehensive mechanical properties, while HR-2 heat-resistant steel exhibits good high-temperature structural stability and creep resistance. Reliable joining of precipitation-hardening stainless steel and heat-resistant steel can simultaneously leverage the advantages of both materials in terms of room-temperature strength, corrosion resistance, and high-temperature stability, making it widely used in nuclear power equipment, high-temperature pressure-bearing components, and energy and power plants. With the increasing integration and lightweighting of nuclear power equipment structures, higher requirements are placed on the structural uniformity and performance matching of dissimilar steel joints. However, dissimilar welding of 05Cr17Ni4Cu4Nb steel and HR-2 steel still faces many key technical challenges, mainly in the following aspects:

[0003] (1) The two materials differ significantly in chemical composition and strengthening mechanism. Precipitation hardening stainless steel relies on Cu enriched phase precipitation for strengthening, while HR-2 steel mainly relies on solid solution strengthening and high temperature stable structure. The two materials have different microstructure transformation behaviors under welding thermal cycling, which can easily lead to discontinuous interface structure and performance mismatch.

[0004] (2) The two materials have different coefficients of thermal expansion and thermal conductivity. During the rapid thermal cycle of laser welding, residual stress concentration or even local deformation is easily generated.

[0005] (3) Under rapid solidification conditions, the weld and interface area may form a compositional segregation zone or a sudden change in hardness. When an obvious compositional gradient is formed in the interface area, it will lead to unstable mechanical properties.

[0006] (4) Although the abrupt change in interface composition can be mitigated to some extent by setting Ni or Ni / Cu intermediate layers, the thickness and structure of the intermediate layer have a significant impact on the interface diffusion behavior. If not properly controlled, local softening zones or excessively wide diffusion zones may be formed during heat treatment.

[0007] A literature review of existing technologies revealed a large body of research on interface control between dissimilar steels. Some studies have employed thickening the Ni transition layer to improve the continuity of interfacial composition; however, results show that an excessively thick interfacial layer leads to an increase in the width of the interfacial diffusion zone, causing localized softening during post-weld heat treatment and hindering the synergistic optimization of strength and toughness. Other studies have used a Ni / Cu composite interfacial structure to mitigate abrupt compositional changes through multi-element transitions; however, the interfacial microstructure becomes more complex due to the inter-diffusion interactions of multiple components, potentially leading to the formation of new unstable phases and increasing the difficulty of microstructure control.

[0008] Other studies have focused on controlling weld microstructure by optimizing welding parameters or altering heat input methods. While these methods have improved the uniformity of the weld microstructure to some extent, they have not fundamentally addressed the issues of post-weld microstructure evolution and the re-precipitation behavior of precipitation-hardening phases. For precipitation-hardening stainless steel, welding thermal cycling disrupts the original distribution of strengthening phases. Without a proper post-weld heat treatment regime, joint performance is difficult to recover and may even decline further.

[0009] It is evident that existing technologies primarily focus on improving interface properties by adjusting the intermediate layer structure or welding parameters, but generally neglect the coupling relationship between the intermediate layer structure and the post-weld heat treatment regime. In particular, under the condition of ultrathin Ni monolayer structure, a systematic method for achieving microstructure continuity and performance matching optimization by synergistically controlling the diffusion of interfacial elements and precipitation-enhancing phase precipitation behavior through solid solution and aging has not yet been publicly reported.

[0010] Therefore, from the perspective of synergistic regulation of interfacial element diffusion behavior and precipitation strengthening mechanism, establishing a post-weld composite heat treatment method suitable for laser-welded joints of dissimilar steels containing Ni intermediate layer 05Cr17Ni4Cu4Nb / HR-2 is of great significance for breaking through the key technical bottleneck of interfacial microstructure and performance matching of dissimilar steels. Summary of the Invention

[0011] The purpose of this invention is to effectively control the microstructure continuity and performance matching of the interface region of the laser-welded dissimilar metal joint between 05Cr17Ni4Cu4Nb precipitation-hardening stainless steel with a Ni interlayer and HR-2 heat-resistant steel. A post-weld composite heat treatment method for this dissimilar steel joint is proposed. Addressing the main technical challenges in dissimilar steel welding, such as significant abrupt changes in interface composition, inhomogeneous weld microstructure, and residual stress concentration, this invention achieves redispersion of interface elements and microstructure homogenization during the high-temperature solution treatment stage, and dispersed precipitation strengthening phases during the subsequent aging stage, through reasonable setting of the Ni interlayer thickness and combined with a synergistic control process of solution treatment and aging. During solution treatment, the weld and heat-affected zone are fully austenitized, promoting the redistribution of elements such as Ni, Fe, and Cr in the interface region and weakening the original composition gradient abrupt change zone. After air cooling, a fine lath martensite microstructure is formed, providing a uniform matrix for subsequent strengthening. During the aging stage, nanoscale Cu-enriched phases and Nb carbonitride strengthening phases precipitate in the supersaturated martensite, restoring and further enhancing the joint strength. Under the synergistic effect of solution treatment and aging, the interface region no longer forms a distinct hardness abrupt layer, but instead exhibits a gradual transition characteristic. By controlling the Ni interlayer thickness within the range of 0.02 mm, the interface buffering effect is ensured while avoiding excessively wide diffusion zones and local softening caused by an excessively thick interlayer. This approach differs from the technical route of simply improving interface performance by thickening the interlayer or setting a Ni / Cu composite transition layer. In terms of process, this method achieves microstructure control through optimizing the post-weld heat treatment regime, simplifying the design process of complex multi-layer interlayers. In terms of microstructure, by controlling element diffusion behavior and precipitated phase morphology, a continuous and uniform microstructure is formed in the interface transition zone. In terms of performance, a synergistic improvement in strength and toughness is achieved, significantly reducing the interface hardness gradient. In engineering applications, material and assembly costs are reduced, and the reliability of the joint in service is improved. This method provides a systematic solution for microstructure control of laser-welded dissimilar steel joints containing Ni interlayers, and has important guiding significance for overcoming the key technical bottlenecks in matching the interface performance of dissimilar steels.

[0012] A post-weld composite heat treatment method for laser-welded dissimilar steel joints containing a Ni interlayer is disclosed. By setting a reasonable Ni interlayer thickness and employing a synergistic control process of solution treatment and aging, the method achieves uniform diffusion of interfacial elements and dispersed precipitation of strengthening phases, thereby improving the continuity of the joint microstructure and the matching of its properties. The specific steps are as follows:

[0013] (1) Select 05Cr17Ni4Cu4Nb stainless steel base plate and HR-2 heat-resistant steel base plate, and set a single Ni intermediate layer with a thickness of 0.01mm to 0.05mm between the two base plates, preferably with a thickness of 0.02mm; before welding, mechanically grind the base plate and Ni intermediate layer to remove the oxide film, and clean and dry them with acetone for later use.

[0014] (2) The above-mentioned assembly test plate was butt welded using high energy density laser welding. The welding parameters were controlled to make the weld form a rapid solidification structure. After the welding was completed, a dissimilar steel welded joint containing a Ni intermediate layer was obtained.

[0015] (3) Place the welded joint in a heat treatment furnace and heat it to 1040℃~1060℃ for solution treatment. The heat treatment time is 0.5h~2h, so that the weld and heat-affected zone are completely austenitized and the interface elements are re-diffused.

[0016] (4) After solution treatment, cool to room temperature using air cooling;

[0017] (5) The sample was then heated to 470℃~500℃ for aging treatment, and the holding time was 1h~3h, so that nano-sized Cu enriched phase and Nb(C, N) strengthening phase were precipitated in the supersaturated martensite;

[0018] (6) After aging, air cool to room temperature to complete the entire post-weld composite heat treatment process.

[0019] In some preferred embodiments, the solution temperature is 1050°C and the holding time is 1 hour; the aging temperature is 480°C and the holding time is 2 hours; the Ni intermediate layer thickness is preferably 0.02 mm.

[0020] This invention differs from existing methods that improve the interfacial properties of dissimilar steels by thickening the intermediate layer or setting a Ni / Cu composite transition layer. Instead, it achieves synergistic regulation of interfacial element diffusion redistribution and precipitation strengthening by optimizing the post-weld heat treatment regime. Without increasing the complexity of the transition layer structure, it achieves uniformity of joint microstructure and improved performance matching. This provides a simple, stable, and engineering-approved new method for optimizing the microstructure and properties of laser-welded dissimilar steel joints with Ni-containing intermediate layers. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the laser welding joint structure of dissimilar steels containing a Ni single-layer intermediate layer according to the present invention; wherein, 1. 05Cr17Ni4Cu4Nb steel; 2. pure Ar gas; 3. laser beam; 4. HR-2 steel; 5. Ni intermediate layer.

[0022] Figure 2 These are comparison images of the microstructure before and after heat treatment; among them,

[0023] (a) 0.02mm Ni joint in weld condition;

[0024] (b) 0.02mm Ni connector after heat treatment;

[0025] (c) Welded 0.1mm Ni + 0.02mm Cu joint;

[0026] (d) 0.1mm Ni + 0.02mm Cu joint after heat treatment.

[0027] Figure 3 This is a comparison chart of hardness before and after heat treatment; among them,

[0028] (a) Distance from the center of the weld;

[0029] (b) Distance from the weld surface.

[0030] Figure 4 This is a comparison chart of tensile properties before and after heat treatment; among them,

[0031] (a) Stress-strain curve;

[0032] (b) Tensile strength + elongation.

[0033] Figure 5 This is a comparison chart of impact performance before and after heat treatment;

[0034] Figure 6 These are comparison images of the fracture morphology before and after heat treatment; among them

[0035] (a) 0.02mm Ni joint in weld condition;

[0036] (b) 0.02mm Ni connector after heat treatment;

[0037] (c) Welded 0.1mm Ni + 0.02mm Cu joint;

[0038] (d) 0.1mm Ni + 0.02mm Cu joint after heat treatment. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0040] Example 1:

[0041] See appendix Figure 1 A single Ni transition layer with an intermediate layer thickness of 0.02 mm was selected, and 05Cr17Ni4Cu4Nb steel was joined to HR-2 steel using laser welding. After welding, the joint sample was placed in a protective atmosphere furnace, heated to 1050℃ and held for 1 hour to fully austenitize the weld area and heat-affected zone, promoting the diffusion of interfacial elements and homogenization of the microstructure. It was then air-cooled to room temperature to form a refined martensitic microstructure. Afterwards, an aging treatment was performed at 480℃ for 2 hours to disperse the Cu-enriched phase and Nb-strengthening phase, followed by final air cooling.

[0042] Metallographic observation showed that the interface transition zone was continuous and uniform, with no obvious abrupt changes in composition. The hardness distribution exhibited a gentle gradient, and the microstructure in the weld area was refined without any obvious softening zone. The impact fracture surface was dominated by dimple morphology, and the overall strength and impact toughness were well matched. The results indicate that, under the condition of a 0.02 mm Ni interlayer, superior microstructural stability and mechanical properties can be obtained after controlling the heat treatment regime of this invention.

[0043] Example 2:

[0044] A composite interlayer structure consisting of 0.1 mm Ni and 0.02 mm Cu was selected for laser welding. After welding, a solution treatment of 1050℃ for 1 hour and air cooling were performed, followed by an aging treatment of 480℃ for 2 hours.

[0045] Microstructural analysis revealed a certain degree of elemental redistribution in the composite intermediate layer region during the solution treatment stage. However, due to the relatively thick Ni layer, the interfacial diffusion zone was relatively wide. After aging treatment, the precipitated phase distribution was more uniform, but localized softening of the microstructure was observed in the composite layer region. Compared to Example 1, although the joint strength was improved, the hardness gradient was relatively large, the interfacial buffer zone was too wide, and the overall strength-toughness match was slightly lower.

[0046] The results show that, under the same heat treatment regime, although the composite intermediate layer can improve the interfacial bonding state, it does not exhibit superior overall performance compared to the thin Ni monolayer structure.

[0047] Comparative example:

[0048] Welding joints with a 0.02 mm Ni single-layer interlayer and a 0.1 mm Ni + 0.02 mm Cu composite interlayer were selected. No solution treatment or aging treatment was performed after welding.

[0049] Microstructural observation revealed significant heterogeneity in the weld and heat-affected zone, with localized martensite coarsening and abrupt compositional gradient changes at the interfaces. Hardness testing showed marked hardness fluctuations in the weld area. The impact fracture surface exhibited a mixture of quasi-cleavage and dimples, indicating insufficient matching of the as-welded microstructure.

[0050] Compared with Examples 1 and 2, the uniformity of the microstructure and the comprehensive mechanical properties of the joints that were not treated with solution treatment at 1050℃ and aging at 480℃ were significantly reduced, indicating that the heat treatment regime described in this invention plays a key role in controlling the performance of dissimilar steel welded joints with Ni-containing intermediate layers.

[0051] In summary, solution treatment at 1050 ℃ for 1 hour followed by aging at 480 ℃ for 2 hours can effectively regulate the microstructure and properties of dissimilar steel welded joints, but its effect is closely related to the intermediate layer system. For 0.02 mm Ni joints, this heat treatment process achieves a synergistic improvement in strength and toughness; however, for 0.1 mm Ni + 0.02 mm Cu joints, while significantly improving strength, it introduces a noticeable loss of toughness. These results provide experimental basis for the rational selection and performance optimization of post-weld heat treatment processes for dissimilar steel welded joints.

[0052] The above description is merely a preferred embodiment of the present invention, but the present invention should not be limited to the content disclosed in this embodiment. Therefore, any equivalent or modified versions made without departing from the spirit of the present invention fall within the scope of protection of the present invention.

Claims

1. A post-weld composite heat treatment method for laser-welded dissimilar steel joints containing a Ni interlayer, characterized in that, Includes the following steps: S1. A single Ni intermediate layer is set between the 05Cr17Ni4Cu4Nb stainless steel base material and the HR-2 heat-resistant steel base material and laser butt welding is completed. S2. Perform solution treatment and subsequent aging treatment on the welded joint to achieve uniform diffusion of interfacial elements and dispersed precipitation of precipitated strengthening phases.

2. The post-weld composite heat treatment method according to claim 1, characterized in that, Step S1 specifically includes: S11. Select 05Cr17Ni4Cu4Nb stainless steel base plate and HR-2 heat-resistant steel base plate, mechanically grind the base material and Ni intermediate layer to remove the oxide film, and clean and dry them for later use. S12. After setting a single Ni interlayer between the two base materials, clamp them together and complete the butt welding using laser welding.

3. The post-weld composite heat treatment method according to claim 2, characterized in that, The thickness of the Ni monolayer intermediate layer is 0.01 mm to 0.05 mm.

4. The post-weld composite heat treatment method according to claim 3, characterized in that, The thickness of the Ni monolayer intermediate layer is 0.02 mm.

5. The post-weld composite heat treatment method according to claim 1, characterized in that, Step S2 specifically includes: S21. Heat the welded joint to 1040℃~1060℃ for solution treatment and hold for 0.5h~2h. S22. After solution treatment, cool to room temperature using air cooling. S23. Then heat to 470℃~500℃ for aging treatment, and hold for 1h~3h. S24. After the aging period is over, air cool to room temperature.

6. The post-weld composite heat treatment method according to claim 5, characterized in that, In step S21, the solution temperature is 1050℃ and the holding time is 1h.

7. The post-weld composite heat treatment method according to claim 5, characterized in that, In step S23, the aging temperature is 480℃ and the holding time is 2h.

8. The post-weld composite heat treatment method according to claim 1, characterized in that, The solution treatment is used to promote the re-diffusion of Ni, Fe and Cr elements in the interface region and eliminate abrupt changes in composition gradient. The aging treatment is used to induce the precipitation of Cu-enriched phase and Nb(C, N) strengthening phase in the supersaturated martensite, so as to achieve uniformity of joint structure and optimization of performance matching.

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