B-type sleeve in-service repair welding method based on phase change stress compensation and application of B-type sleeve in-service repair welding method

Abstract

The invention belongs to the technical field of in-service repair of pressure pipelines, particularly discloses a B-type sleeve ring fillet weld welding method based on phase change stress compensation and application of the B-type sleeve ring fillet weld welding method, and aims to solve the problem of cold cracks caused by residual stress concentration at weld toes. Through specific alloy element ratio and control, the special welding wire with excellent cold crack resistance can be prepared through the design method. Comparative analysis results show that in the deposited metal welded by adopting the low-temperature phase change welding wire and the matched welding process, the martensite structure content (55-60%) is slightly higher than that of the deposited metal welded by using the welding wire of a control group; the level of welding residual stress generated by the special welding wire in a welding line area (especially a welding site area) is obviously lower than that of a control group welding wire, and the risk of generating cold cracks at the weld toe of the B-type sleeve repair ring fillet welding line is obviously reduced in cooperation with optimization of a welding line structure. And an effective technical solution is provided for improving the welding quality and the service reliability of B-type sleeve repair.

Description

Technical Field

[0001] This invention relates to the field of in-service repair technology for pressure pipelines, specifically to a welding method for B-type sleeve circumferential fillet welds based on phase transformation stress compensation, which is used to solve the problem of cold cracking caused by residual stress concentration at the weld toe. Background Technology

[0002] With the strategic adjustment of my country's energy structure and the large-scale construction of long-distance oil and gas pipelines, X80 high-strength steel, due to its excellent tensile strength (≥550MPa), low-temperature toughness (impact energy ≥100J at -30℃), and corrosion resistance, has become the core pipe material for major national projects such as the West-East Gas Pipeline and the China-Russia natural gas pipeline. However, during the long-term service of oil and gas pipelines, the circumferential weld area has become a high-risk area for failure due to geometric discontinuities and concentration of residual welding stress. According to statistics, circumferential weld failure accounts for more than 32% of pipeline accidents, mainly manifested as cold cracking, lack of fusion, and stress corrosion cracking, seriously threatening the pressure-bearing integrity of the pipeline.

[0003] To ensure the continuity and economy of energy transmission, in-service pipeline repair technology has become a key method for pipeline maintenance. Among these, the Type B sleeve repair technology, through the formation of a composite pressure-bearing structure between the external sleeve and the pipeline, can effectively disperse stress in the defective area of ​​the circumferential weld. It is suitable for repairing defects with a crack depth <0.8t (t is the wall thickness) or a dent depth of 4%–6%D (D is the pipe diameter). However, this technology requires circumferential fillet welds on the surface of the pressurized pipeline, and the high strength (yield strength ≥555MPa) and high carbon equivalent (CE ≥0.23) of X80 steel pose a significant challenge to its weldability. 1. High sensitivity to cold cracking: The heat-affected zone (HAZ) of weld is prone to forming hardened martensite structure, and the medium transported in the pipeline (such as moisture) introduces diffusible hydrogen (≥5mL / 100g), forming a ternary coupling effect of "stress-hydrogen embrittlement-hardened structure" at the weld toe of the fillet weld, which significantly increases the tendency to cold cracking. 2. Residual stress concentration: The residual stress peak at the weld toe of the circumferential fillet weld repaired by the type B sleeve can reach 520MPa (close to 1.5 times the yield strength of the X80 base material), far exceeding the critical value specified by API1104 standard (≤400MPa). 3. Risk of burn-through due to thin walls: Urban gas pipelines have relatively thin walls (usually 6-8mm), and improper control of welding heat input can easily lead to burn-through accidents.

[0004] Existing prevention and control measures (such as preheating and ultra-low hydrogen welding materials) can partially alleviate cold cracking, but they have limitations: 1. Construction costs increase by 30% when the preheating temperature is ≥100℃, and it is difficult to preheat large-diameter pipes evenly. 2. The toughness (impact energy at -20℃ ≤40J) of traditional low alloy welding materials is severely mismatched with that of X80 base material (≥100J), causing the repair joint to become a weak area in terms of toughness; 3. The lack of coordinated optimization of "welding process-microstructure control-stress redistribution" makes it difficult to fundamentally suppress the initiation and propagation of cold cracks.

[0005] Therefore, developing a cold-crack resistant welding technology for the repair of type B sleeves for X80 pipelines, which achieves high reliability and economical repair results by precisely controlling the microstructure of the circumferential fillet weld and introducing compressive residual stress, is of great significance and practical value for improving the performance and development of online pipeline repair structures in my country's petrochemical and other industrial sectors. Summary of the Invention

[0006] This invention aims to address the problem that during the repair of type B sleeves, the weld toe area of ​​the circumferential fillet weld is prone to cold cracking due to the concentration of residual tensile stress, leading to structural sealing failure. Existing welding materials are insufficient to reduce the residual stress in this area. To overcome these shortcomings, this invention provides an in-service repair welding technology for type B sleeves based on phase transformation stress compensation.

[0007] To achieve the above objectives, the present invention includes the following technical solutions: A welding method for in-service repair of type B sleeves based on phase transformation stress compensation includes the following steps: 1) Low-temperature phase transformation welding wire is provided, which comprises the following components by weight percentage: Ni: 6%~10%, Cr: 8%~12%, Ti: 0.012%~0.020%, Mn: 1.4%~2.2%, Si: 0.2%~0.4%, C: 0.06%~0.08%, P≤0.01%, S≤0.01%, with the balance being Fe and unavoidable impurities; 2): Using 70-90%Ar + 10-30%CO2 as the shielding gas, circumferential fillet welds are performed using gas metal arc welding (GMAW). 3): Control the interlayer temperature to 200℃~250℃; 4): Control the heat input to 8–12 kJ / cm; 5): Control the post-weld cooling rate to 5-10℃ / s.

[0008] Furthermore, in the above steps, in step 1), the welding wire also contains Mo, the content of which does not exceed 0.2% of the total mass of the welding wire.

[0009] Furthermore, the above steps include a preheating step before step 3), with a preheating temperature of 100℃~250℃.

[0010] Furthermore, in the above steps, the welding method controls the Cr / Ni ratio to be around 1.25, so that the martensite content in the weld metal is 55% to 60% and the retained austenite content is 40% to 45%.

[0011] Furthermore, in the above steps, the welding method forms a residual compressive stress field in the 0-4mm region of the weld toe.

[0012] Furthermore, in the above steps, the weld metal obtained by the welding method has a tensile strength of not less than 1000 MPa, an elongation of not less than 9%, and a service life of not less than 15 years for the repaired joint.

[0013] The present invention also discloses the application of the above welding method in the pressurized repair of the circumferential fillet weld of the type B sleeve of X80 steel high-pressure natural gas pipeline.

[0014] The present invention also discloses the application of the above welding method in the welding of circumferential fillet welds during in-service repair of offshore platforms or subsea pipelines.

[0015] The present invention also discloses a quality control method for verifying the above welding method, comprising the following steps: Step S1: Identification and Control of Key Alloying Elements In some embodiments, by analyzing the chemical composition and cold cracking susceptibility of X80 steel base material, carbon, manganese, silicon, chromium, and nickel are identified as key alloying elements in low-temperature phase transformation welding wire. Specifically, manganese is present at 1.40%–2.20%, and silicon at 0.2%–0.4%, to ensure good microstructural stability and crack resistance in the weld. Step S2: Simulation and optimization of phase structure and mechanical properties In some embodiments, simulations are performed using Jmatpro thermodynamic calculation software. By comparing the phase composition of traditional X80 steel and optimized low-temperature phase transformation welding wire under equilibrium conditions, the effects of the contents of elements such as C, Mn, Cr, and Ni on the martensitic transformation temperature (Ms point), alloy phase transformation behavior, and room temperature stable phase composition (especially the content of retained austenite) are systematically evaluated, providing theoretical basis and parameter support for the design of low-temperature phase transformation welding wire. Step S3: Co-design of welding process performance In some embodiments, titanium (Ti: 0.012–0.020%) is added to refine the grains to improve droplet transfer, and the content of sulfur (S≤0.01%) and phosphorus (P≤0.01%) is limited to suppress spatter. At the same time, Mo is added to improve the toughness of the weld metal of the low-temperature phase transformation welding wire. Step S4: Regulation of Residual Austenite Stability In some embodiments, by adjusting the nickel (Ni: 6–10%) to optimize the stability and transformation of austenite, the phase transformation path during cooling is controlled to prevent welding residual stress concentration and brittle fracture caused by excessive martensite formation. Step S5: Regulation of carbide precipitation and strengthening of grain boundaries Literature indicates that higher Cr content significantly promotes M 23 The precipitation of C6-type carbides, which tend to segregate at grain boundaries, poses a risk of intergranular embrittlement and crack initiation. Therefore, the Cr content in low-temperature phase transformation welding wire should be appropriately controlled. Controlled carbide precipitation is beneficial for maintaining microstructure continuity and weld toughness. Thus, in some embodiments, the Cr content is limited to the range of 8–12% to balance corrosion resistance and brittleness resistance requirements, ensuring that the weld has good high-temperature stability and fracture toughness. Step S6: Regulation of Martensitic Phase Transformation Behavior In some embodiments, by controlling the synergistic relationship between the martensitic transformation temperature (Ms point) and the timing of the welding thermal stress, the Ms point of the low-temperature phase transformation welding wire is adjusted, so that the martensite completes the transformation before the peak stress of the welding thermal cycle occurs, thereby inducing "phase transformation induced compressive stress" in the weld toe region and significantly reducing the peak tensile stress. Step S7: Adjustment of the austenite-martensite duplex microstructure ratio The stability of austenite is not only related to the Ni content, but also closely related to the precise control of the Cr / Ni ratio. The low Cr / Ni ratio in the control group welding wire resulted in excessively stable austenite, making it difficult to transform into martensite during the welding cooling process and lacking a phase transformation strengthening mechanism. However, in some embodiments, the LTT welding wire maintained a reasonable Cr / Ni ratio (approximately 1.25), retaining an appropriate amount of stable austenite (approximately 15%) while not hindering martensite transformation, resulting in significant microstructural strengthening. Step S8: Stress measurement in the weld toe area In some embodiments, by measuring residual stress, it was found that the low-temperature phase change welding wire exhibits obvious residual compressive stress in the region 0-4 mm from the weld toe, which then gradually turns into tensile stress, showing a "near-compression and far-tension" transverse residual stress distribution; while the control group welding wire lacks this gradient, and the weld toe is the residual tensile stress zone, which is prone to becoming the crack initiation point. Step S9: Mechanical Performance Evaluation In some embodiments, according to the tensile test results, the low-temperature phase transformation welding wire, thanks to its dense martensite network and appropriate amount of retained austenite, exhibits an excellent combination of mechanical properties: a tensile strength of 1058 MPa. In contrast, the control group welding wire lacks a martensite strengthening mechanism, and its tensile strength drops sharply to 633 MPa, which is far inferior to that of the low-temperature phase transformation welding wire. Step S10: Quantitative analysis of phase composition and tissue observation In some embodiments, XRD pattern analysis revealed that the control group welding wire had the strongest austenite peak and almost no obvious martensite signal, indicating a softer microstructure. In contrast, the low-temperature phase transformation welding wire had a moderate austenite content and stability (moderate austenite peak in XRD), and the martensite peak (110) was clearly distinguishable, indicating that its two-phase ratio distribution was reasonable. In terms of microstructure, it exhibited a mixed microstructure dominated by lath martensite with fine residual austenite embedded in it, which had good crack resistance.

[0016] The present invention also discloses a low-temperature phase transformation welding wire (LTT welding wire) for phase transformation stress compensation welding, which is composed of the following components by weight percentage: C: 0.06%~0.08%, Si: 0.2%~0.4%, Mn: 1.4%~2.2%, Cr: 8%–12%, Ni: 6%~10%, Mo: ≤0.2%, Ti: 0.012%~0.020%, P: ≤0.01%, S: ≤0.01%, The balance consists of Fe and unavoidable impurities; Preparation process: Using pure steel strip as the outer sheath material, the following components are weighed by weight percentage: C: 0.06%~0.10%, Si: 0.2%~0.4%, Mn: 1.4%~2.2%, Cr: 8%~12%, Ni: 6%~10%, Mo≤0.2%, Ti: 0.012%~0.020%, P≤0.01%, S≤0.01%, and the balance Fe. After the steel strip is cut, degreased and dried, the flux core is prepared, dried and mixed, and then rolled and drawn into a flux core welding wire with a diameter of 1.2mm.

[0017] When the welding wire is used for welding, compressive residual stress is introduced in the weld toe region by controlling the phase transformation behavior of the weld metal.

[0018] Compared with the prior art, the present invention has the following outstanding advantages: The in-service repair welding technology for type B sleeves based on phase transformation stress compensation provided by this invention solves the core problem of cold cracking induced by residual tensile stress concentration at the weld toe of the circumferential fillet weld in traditional repair processes through the synergistic effect of low-temperature phase transformation welding materials and precise thermal cycle control.

[0019] The specific beneficial effects are as follows: (1) Significant effect of residual stress control. A compressive residual stress field was successfully introduced in the weld toe area of ​​the circumferential fillet weld, realizing the transformation from tensile residual stress to compressive residual stress compared with the traditional welding process.

[0020] (2) The microstructure optimization is clear. By adding Mn-Cr-Ni composite, the austenite-martensite phase transformation temperature of the weld microstructure is reduced to below 200℃, and the phase transformation volume expansion is used to compensate for the shrinkage stress of the weld.

[0021] (3) Improved process adaptability. Under the conditions of 80%Ar+20%CO2 shielding gas and interpass temperature of 200-250℃, the width of the weld heat-affected zone is reduced by 40%. The precise matching of heat input (8~12kJ / cm) and cooling rate (5~10℃ / s) makes this technology suitable for pressurized conditions.

[0022] (4) Significant engineering application value. The failure cycle of the sleeve repair ring weld joint is extended from an average of 5 years to more than 15 years, significantly reducing the risk of unplanned shutdown of oil and gas pipelines. Therefore, this technical solution has important practical significance in improving the quality of sleeve repair for in-service oil and gas pipelines, and can ensure the safety and stability of the pipeline system in long-term operation. Attached Figure Description

[0023] Figure 1 Comparison of the metallographic structure of the weld metal of the welding wire (a) of the present invention and the control group welding wire (b); Figure 2 Phase distribution diagrams of weld metal from the welding wire (a) of the present invention and the control group welding wire (b); Figure 3 TEM microstructure of weld metal from welding wire (a) of the present invention and welding wire (b) of the control group; Figure 4 The dislocation morphology of the weld metal of the welding wire (a) of the present invention and the control group welding wire (b); Figure 5 SEM images of the tensile fracture surfaces of the weld metal of the welding wire (a) of the present invention and the control group welding wire (b); Figure 6 A comparison diagram of the mechanical properties of the welding wire of this invention and the control group welding wire; Figure 7 XRD diffraction patterns of the welding wire of the present invention and the control group welding wire; Figure 8 The longitudinal stress distribution along the width of the plate is shown in the welding wire of the present invention and the control group welding wire. Detailed Implementation

[0024] Based on the design principles of low-temperature phase transformation welding wire, the selection of the deposited metal alloy system mainly considers the influence of elements on the martensitic transformation initiation temperature, Ms. The Ms point refers to the critical temperature at which austenite transforms into martensite during the cooling process of steel. Lowering this temperature helps reduce the risk of cold cracking during welding. Therefore, the selection and proportioning of alloying elements are crucial to the performance of the welded joint. Many scholars have studied the effect of alloying elements in lowering the Ms point, resulting in the following formula:

[0025] The formulas show that the order of influence of alloying elements on reducing the Ms point is: C, Mn, Cr, Ni, Mo, and Si. Among these alloying elements, chromium (Cr) and nickel (Ni) play the main roles in reducing the Ms point. The addition of chromium can improve the corrosion resistance of welded joints and effectively lower the martensitic transformation temperature (Ms point), thereby reducing the risk of cold cracking. Nickel, on the other hand, enhances the toughness and impact strength of the material, resulting in better stability of welded joints under extreme conditions. The presence of nickel can effectively improve the crack resistance of welded joints, especially under conditions of drastic temperature changes. Furthermore, using JmatPro thermodynamic calculations, the effects of elements such as Mn, Cr, and Ni on the phase transformation behavior and room-temperature stable phase composition of alloy steels were systematically evaluated. The results show that the synergistic regulation of Cr and Ni content has a significant effect on austenite stability and second-phase precipitation: Cr tends to stabilize ferrite and promote MC carbide precipitation, which helps improve corrosion resistance and thermal stability, but excessive Cr can easily lead to weld embrittlement; Ni significantly expands the austenite phase region, improves the stability of room temperature residual austenite, and enhances weld toughness and crack resistance. Therefore, considering the performance requirements of the welding wire composition, the chromium and nickel contents in the low-temperature phase transformation welding wire design of this invention are set to 8%–12% and 6%–10%, respectively. This composition range can effectively reduce the Ms point of the weld metal, thereby reducing the residual stress and cold cracking risk caused by temperature changes during welding, while also taking into account the toughness of the weld joint and improving the stability and reliability of sleeve circumferential fillet weld repair.

[0026] Based on the above principle analysis and related experiments, a low-temperature phase transformation welding wire suitable for use in the welding repair of X80 pipeline steel was developed. In addition to the conventional carbon, silicon, manganese, phosphorus, and sulfur elements, the low-temperature phase transformation welding wire uses a variety of alloying elements, with a Cr-Ni alloy as the alloy system. The wire diameter is 1.2 mm, and MAG welding with an 80% Ar + 20% CO2 shielding gas is used for surfacing.

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1 In-service repair welding of circumferential fillet welds on X80 pipeline steel type B sleeves based on phase transformation stress compensation This embodiment details the specific application process and effect verification of the welding method and the special low-temperature phase change welding wire (LTT welding wire) described in this invention.

[0029] 1. Preparation of experimental materials Base material: X80 grade pipeline steel from a certain batch is used. Its specific chemical composition (weight percentage, wt%) is shown in the table below. Its yield strength is ≥555MPa and tensile strength is ≥625MPa, which meets the requirements of API5L standard.

[0030] Table 1: Chemical composition of X80 pipeline steel base material (wt.%) ; Welding materials: A comparative experiment was conducted using the low-temperature phase change welding wire (LTT welding wire) designed according to this invention and another control group of welding wires. The diameter of both welding wires was 1.2 mm, and their detailed chemical composition is shown in the table below.

[0031] Table 2: Comparison of chemical composition of welding wire used in the experiment (wt.%) ; 2. Welding process The circumferential fillet weld of the type B sleeve was performed using gas metal arc welding (MAG) with a gas shield of 80% Ar + 20% CO, under simulated pressurized conditions. Specific welding process parameters are shown in the table below. Table 3: Welding process parameters for CO2 gas shielded welding ; The key process control points in this embodiment are: Interlayer temperature: The interlayer temperature is strictly controlled within the range of 200-250℃ by monitoring with an infrared thermometer.

[0032] Heat input calculation: Based on the current, voltage and welding speed parameters in the table above, the heat input in this example is approximately 8-12 kJ / cm, which meets the requirements of this invention.

[0033] Cooling rate control: After welding, natural cooling supplemented by intermittent air cooling is used to control the cooling rate within the ideal range of 5-10℃ / s.

[0034] 3. Performance Testing After welding is completed, the welded joints formed by the two types of welding wires are cut and sampled for residual stress testing, mechanical property testing and microstructure analysis.

[0035] (1) Mechanical property testing Tensile tests were performed on the weld metal, and the results are shown in the table below: Table 4: Comparison of Mechanical Properties of Weld Metal ; Results Analysis: As shown in Table 4, the weld metal obtained using the LTT welding wire of this invention has a tensile strength (1058 MPa) that significantly exceeds that of the control group welding wire (633 MPa), representing an increase of approximately 67%. This indicates that the present invention achieves a significant phase transformation strengthening effect through composition and process design. Although the elongation of the LTT welding wire (9.5%) is slightly lower than that of the control group welding wire (11.2%), its stress-strain curve exhibits continuous plastic deformation characteristics without any signs of brittle fracture, indicating that it maintains sufficient toughness even under ultra-high strength.

[0036] (2) Microscopic tissue observation Observation of the weld metal using metallographic microscopy and TEM revealed the following: The control group welding wire: The microstructure is mainly composed of ferrite and a large amount of retained austenite, with fewer and larger martensite laths and a lower dislocation density.

[0037] LTT welding wire: The microstructure is dominated by fine lath martensite (approximately 55.7%), with retained austenite (approximately 44.3%) uniformly distributed in thin films between the martensite laths. TEM reveals a high density of dislocations within the martensite laths, which is the main reason for its ultra-high strength. This dual-phase microstructure, dominated by martensite and supplemented by retained austenite, combines high strength with high crack resistance.

[0038] (3) Residual stress test The longitudinal residual stress distribution in the weld toe region was measured using X-ray diffraction. The results show that: Control group welding wire: The weld toe is the peak area of ​​residual tensile stress, with a high stress value, which is easy to become a crack initiation source.

[0039] LTT welding wire exhibits significant residual compressive stress (approximately -200 MPa) in the critical area 0-4 mm from the weld toe, which gradually transitions to tensile stress. This "near-compression, far-tension" stress distribution characteristic benefits from the volume expansion caused by the martensitic phase transformation, effectively offsetting the welding tensile stress and fundamentally suppressing the generation of cold cracks.

[0040] 4. Further analysis of results: according to Figure 1 The metallographic images clearly show the differences in microstructure between the low-temperature phase transformation welding wire and the conventional welding wire. The image on the left shows the microstructure of the weld metal from the control group wire, primarily revealing ferrite and retained austenite phases. Ferrite (indicated by the yellow arrow) exhibits a uniform morphology, possessing low strength and good plastic deformation capacity, enabling it to effectively absorb stress and reduce crack formation during welding. Retained austenite (indicated by the white arrow) displays irregular morphologies, typically present in regions that have not fully transformed into martensite during weld cooling. The presence of retained austenite generally helps improve toughness and alleviates weld joint brittleness to some extent, but it may also reduce strength and transform at high temperatures, leading to changes in the weld joint's performance. The image on the right shows the microstructure of the low-temperature phase transformation weld metal of this invention, revealing the characteristic lath-like martensite phase (indicated by the red arrow). Further combined with... Figure 2 The phase distribution diagram shows that the microstructure of LTT welding wire is mainly composed of body-centered cubic martensite (bcc), accounting for as high as 55.7%, while the proportion of face-centered cubic austenite (fcc) is 44.3%. This microstructure is closely related to its relatively low nickel content. Chromium and nickel are austenite stabilizing elements, especially nickel, which can significantly improve the thermodynamic stability of the fcc phase in iron-based alloys. However, in LTT welding wire, due to the low nickel content, the austenite stabilization ability is weak, which leads to the material easily undergoing martensitic transformation during welding cooling, i.e., transforming from high-temperature austenite to low-temperature martensite, forming a martensitic microstructure dominated by bcc. Figure 3 and Figure 4 TEM tissue observation results and Figure 2The phase distribution analysis results are consistent with those of the control group: the ratio of martensite to retained austenite in the LTT welding wire is relatively balanced (55.7% martensite, 44.3% austenite). TEM shows uniformly distributed martensite laths with significant dislocation accumulation within the laths, exhibiting high-density dislocation characteristics, consistent with the conclusion that the synergistic effect of grain refinement and dislocation strengthening contributes to its highest strength. In contrast, the control group welding wire shows more retained austenite regions under TEM, with fewer and larger martensite laths and sparser dislocation distribution, confirming its weakened dislocation strengthening and grain refinement strengthening effects and increased elongation. Martensite has high hardness and strength, and is typically found in high-strength materials, with a usually dense distribution in the structure. This structure significantly improves the overall mechanical properties of the weld metal, especially exhibiting better resistance to deformation under high loads and impacts.

[0041] Tensile tests were performed on the weld metal of the low-temperature phase change welding wire and the control group welding wire. The tensile fracture morphology is as follows: Figure 5 As shown, the stress-strain curve is as follows: Figure 6 As shown in Table 3, the mechanical properties are as follows. Table 3 shows that the tensile strength of the weld metal from the control group welding wire was 633 MPa, while the tensile strength of the weld metal from the low-temperature phase change welding wire of this invention was significantly improved, reaching 1058 MPa, an increase of approximately 67%. This improvement indicates that the new welding wire has a significant advantage in enhancing the strength of the weld metal, especially for structures subjected to high loads and long-term use, exhibiting stronger tensile strength. Furthermore, from... Figure 2 The stress-strain curves show that the deformation behavior of the two weld metals differs significantly during the stress process.

[0042] Combination Figure 5 From the fracture morphology, the LTT welding wire fracture exhibits deep and uniformly distributed dimples, with abundant dimples and clear boundaries. The overall morphology is dominated by a typical micropore aggregation mechanism. However, under high magnification, it was found that although micropores induced by small inclusions exist in the dimples, no obvious plastic deformation zone is formed, indicating that the material's plastic slip is limited before fracture. This is consistent with its mechanical performance of "high strength and low elongation," that is, the martensitic structure leads to an imbalance between strength and plasticity, limiting the full development of plastic deformation. In contrast, the control group welding wire has a more uniform dimple distribution, and the size and depth of the dimples are less different, exhibiting typical ductile fracture characteristics. The high Cr / Ni ratio promotes the stabilization of austenite, making the deformation process dominated by austenite slip, and the growth and aggregation of micropores are more complete, significantly improving the elongation. Although the phase transformation strengthening effect is weakened, resulting in a decrease in strength, the overall deformation mechanism is dominated by toughness.

[0043] Figure 6The stress-strain curve of medium- and low-temperature phase transformation welding wire rises rapidly within a relatively small strain range, exhibiting a higher elastic modulus and a higher yield point, followed by a stable plastic deformation zone, ultimately reaching an extremely high stress peak. In contrast, Figure 6 The stress-strain curve of the control group welding wire had a lower slope and the peak stress was much lower than that of the low temperature phase transformation welding wire, indicating that its tensile strength was relatively weak and the plastic deformation area during the stress process was smaller, making it prone to premature failure.

[0044] At the same time, combined Figure 6 As shown in Table 3, although the elongation of the low-temperature phase change welding wire was 9.5%, which was 15.1% lower than that of the control group (11.2%), indicating that while the strength of the material was improved, its ductility was sacrificed, but its stress-strain curve still maintained continuous plastic deformation characteristics before fracture, without any signs of sudden necking or brittle fracture. This shows that despite the reduced elongation, the low-temperature phase change welding wire can still maintain a certain deformation capacity and toughness under ultra-high strength, and has a certain fault tolerance space, avoiding the risk caused by brittle fracture.

[0045] Figure 7 Further verification revealed a significant difference in austenite and martensite content between the LTT welding wire and the control group welding wire, the fundamental reason being the difference in their chromium and nickel content. Ni, as a strong austenite stabilizing element, can reduce the driving force for the transformation of austenite to martensite; while Cr, as a ferrite forming element, weakens the thermal stability of austenite when present in excess. The changes in the content of both affect the stability of austenite: compared to the control group welding wire, the LTT welding wire has a lower Ni content. The lower Ni content, combined with a certain amount of Cr, significantly weakens the stability of austenite, leading to a rapid martensitic transformation of a large amount of austenite during the welding cooling stage. The intensity of the (110) diffraction peak of martensite is the most prominent among the two, while the austenite, due to the large amount of transformation into martensite, has a relatively small amount of residual austenite, and its characteristic diffraction peak (111) intensity is weak, with the microstructure dominated by martensite; in the control group welding wire, the high Ni content gives austenite extremely strong stability. During the welding cooling process, the thermodynamic conditions for the transformation of austenite into martensite are difficult to meet, so the degree of transformation of austenite into martensite is low, which makes its characteristic diffraction peaks of austenite (such as (111), (200) and other crystal planes) not only complete in number, but also have high diffraction intensity, indicating that austenite accounts for a large proportion in the microstructure, while the (110) diffraction peak of martensite is relatively weak, and the microstructure is dominated by austenite.

[0046] from Figure 8It can be seen that the LTT joint exhibits a certain degree of residual compressive stress characteristics in the range of 0 mm to 4 mm from the weld toe, especially in the vicinity of the weld toe. The LTT welding wire has a lower Cr and Ni content and a relatively higher Ms point, which allows the martensitic transformation to occur at a higher temperature and be more complete. Therefore, it can effectively offset the tensile stress formed by welding thermal stress at the weld toe, and instead form obvious residual compressive stress. As the distance from the weld toe increases, the compressive stress gradually weakens, and transforms into a residual tensile stress peak near 5 mm, showing a typical "compression before tension" characteristic. In contrast, the control group welding wire with a higher alloy content shows a significant decrease in the Ms point due to the significant increase in Cr and Ni. The phase transformation is delayed until the end of cooling, causing the martensite to transform mainly at a lower temperature, missing the optimal compensation opportunity. In addition, the higher Cr content leads to enhanced microstructure stability, further weakening the phase transformation-driven compressive stress. The weld toe approaches a tensile stress state, showing the significant inhibitory effect of high alloying on phase transformation-induced compressive stress. Therefore, appropriate Cr and Ni content is more conducive to the formation of effective residual compressive stress at the weld toe, which may improve the joint performance; while high alloy design suppresses martensitic phase transformation and weakens the compressive stress effect brought about by phase transformation expansion, thus forming tensile stress at the weld toe.

[0047] in conclusion: This embodiment fully verifies the effectiveness of the low-temperature phase change welding wire and its matching welding process described in this invention. Through specific composition control and process parameters (interpass temperature 200-250℃, heat input 8-12kJ / cm, cooling rate 5-10℃ / s), a dual-phase microstructure with good strength and toughness matching was successfully obtained in the weld, and a beneficial residual compressive stress field was introduced at the weld toe, which significantly improved the tensile strength and cold crack resistance of the welded joint, fully achieving the intended purpose of this invention.

[0048] The above are merely a few preferred embodiments of the present invention, described in a relatively specific and detailed manner, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A welding method for in-service repair of type B sleeves based on phase transformation stress compensation, characterized in that, Includes the following steps: 1) Low-temperature phase transformation welding wire is provided, which comprises the following components by weight percentage: Ni: 6%~10%, Cr: 8%~12%, Ti: 0.012%~0.020%, Mn: 1.4%~2.2%, Si: 0.2%~0.4%, C: 0.06%~0.08%, P≤0.01%, S≤0.01%, with the balance being Fe and unavoidable impurities; 2): Using 70-90%Ar + 10-30%CO2 as the shielding gas, circumferential fillet welds are performed using gas metal arc welding (GMAW). 3): Control the interlayer temperature to 200℃~250℃; 4): Control the heat input to 8–12 kJ / cm; 5): Control the post-weld cooling rate to 5-10℃ / s.

2. The method according to claim 1, characterized in that, In step 1), the welding wire also contains Mo, the content of which does not exceed 0.2% of the total mass of the welding wire.

3. The method according to claim 1, characterized in that, A preheating step is included before step 3), with a preheating temperature of 100℃~250℃.

4. The method according to claim 1, characterized in that, The welding method controls the Cr / Ni mass ratio to be around 1.25, so that the martensite content in the weld metal is 55% to 60% and the retained austenite content is 40% to 45%.

5. The method according to claim 1, characterized in that, The welding method creates a residual compressive stress field in the 0-4mm region of the weld toe.

6. The method according to claim 1, characterized in that, The weld metal obtained by the welding method has a tensile strength of not less than 1000 MPa, an elongation of not less than 9%, and a service life of not less than 15 years for the repaired joint.

7. The application of the method as described in claim 1 in the pressurized repair of the circumferential fillet weld of the in-service B-type sleeve of X80 steel high-pressure natural gas pipeline.

8. The application of the method as described in claim 1 in the welding of circumferential fillet welds during in-service repair of offshore platforms or subsea pipelines.

9. A quality control method for verifying the welding method as described in any one of claims 1-8, comprising the following steps: S1: Identification and Control of Key Alloying Elements The key alloying elements and their content ranges in low-temperature phase transformation welding wire were determined, including, by weight percentage: C：0.05%～0.10%， Mn: 1.40%~2.20%, Si: 0.2%~0.4%, Cr：8%～12%， Ni: 6%~10%, P≤0.01%， S≤0.01%， The balance consists of Fe and unavoidable impurities; S2: Simulation and Optimization of Phase Structure and Mechanical Properties The influence of the content of the alloying elements on the martensitic transformation temperature Ms point, phase composition and residual austenite content was analyzed by thermodynamic software simulation, and the welding wire composition was optimized. S3: Co-design of welding process performance Add Ti (0.012%–0.020%) to the welding wire to refine the grains, control the S and P contents to not exceed 0.01%, and add Mo, with a content not exceeding 0.2%. S4: Stability regulation of retained austenite The stability and transformation of austenite can be adjusted by controlling the Ni content between 6% and 10%. S5: Carbide precipitation regulation and grain boundary strengthening Control the Cr content between 8% and 12% to inhibit M. 23 Excessive precipitation of C6-type carbides at grain boundaries; S6: Regulation of Martensitic Phase Transformation Behavior The Ms point of the welding wire is controlled so that it completes the martensitic transformation before the peak of welding thermal stress occurs; S7: Regulation of the ratio of austenite to martensite duplex microstructure The Cr / Ni mass ratio is controlled at around 1.25 to ensure that the martensite content in the weld metal is 55%–60% and the retained austenite content is 40%–45%. S8: Stress Measurement and Verification in Weld Toe Area The presence of compressive residual stress in the 0–4 mm region of the weld toe was verified by residual stress testing. S9: Mechanical Performance Evaluation Tensile tests were performed on the welded joints to ensure that their tensile strength was not less than 1000 MPa; S10: Quantitative analysis of phase composition and microstructure observation The ratio and distribution of martensite and austenite in the weld were verified by XRD or metallographic analysis.

10. A low-temperature phase transformation welding wire for phase transformation stress compensation welding, characterized in that, It consists of the following components by weight percentage: C：0.06%～0.08%， Si: 0.2%~0.4%, Mn: 1.4%~2.2%, Cr：8%～12%， Ni: 6%~10%, Mo: ≤0.2%, Ti: 0.012%~0.020%, P：≤0.01%， S：≤0.01%， The balance consists of Fe and unavoidable impurities; When the welding wire is used for welding, compressive residual stress is introduced in the weld toe region by controlling the phase transformation behavior of the weld metal.

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