An efficient welding method

Abstract

The present application relates to a kind of high-efficiency welding methods, belong to welding process technical field, solve the existing welding process when welding high-strength material easily produce hot crack, porosity, coarse grain and other defects and the strength and low temperature toughness of weld metal are insufficient etc.Provide a kind of high-efficiency welding method, the welding method adopts groove welding process, the direction of adjacent weld layer is opposite, the direction of the same weld layer is same;The welding method adopts a kind of low-carbon low-alloy welding wire, each component in the welding wire is as follows according to mass percentage: C 0.04~0.08%, Si 0.30~0.50%, Mn 1.6~2.0%, Cr 0.15~0.35%, Ni 3.5~4.0%, Cu 0.008~0.01%, Mo 0.007~0.01%, V 0.0005~0.001%, Ti 0.02~0.07%, Al 0.01~0.018%.The high-efficiency welding method is applicable to the welding of P690QL2 steel, significantly improve the mechanical properties and low temperature toughness of welded joint.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and in particular to a high-efficiency welding method. Background Technology

[0002] With the increasing global demand for liquefied carbon dioxide (COD) storage and transportation, cryogenic high-pressure storage tanks have become critical equipment. However, the materials currently used for COD storage tanks are mostly steels with relatively low yield strength, such as 07MnNiMoVDR (yield strength 490MPa) and 15MnNbR (yield strength 370MPa). While these materials can meet the requirements of ordinary cryogenic storage tanks, they often exhibit shortcomings under the extreme high-pressure and low-temperature conditions of COD storage tanks, such as limited strength and unsatisfactory impact resistance, making it difficult to achieve optimal tank load-bearing capacity and safety.

[0003] To achieve higher storage capacity per unit volume in storage tanks, the selection of higher-strength materials has become an inevitable trend. P690QL2, a 690MPa grade high-strength cryogenic steel, is one of the ideal materials for manufacturing liquefied carbon dioxide storage tanks due to its excellent cryogenic toughness and high strength. Using P690QL2 steel can effectively reduce the thickness of the tank walls and decrease structural weight, thereby improving storage efficiency and economic benefits without increasing costs. Furthermore, the use of this high-strength material provides a higher safety margin for the tank under high-pressure conditions.

[0004] However, the welding process of P690QL2 steel presents significant technical challenges. Due to its high strength, the material is prone to defects such as hot cracking, porosity, and coarse grain formation during welding. These defects affect the toughness and mechanical properties of the weld joint, particularly its impact resistance at low temperatures. Existing processes produce weld metals with poor low-temperature toughness and impact resistance, making it difficult to meet the specific requirements of liquefied carbon dioxide storage tanks. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide an efficient welding method to solve at least one of the following problems in existing welding processes when used with high-strength materials (especially P690QL2 steel): hot cracking, porosity, coarse grain formation, and poor low-temperature toughness and impact resistance of the weld metal.

[0006] This invention provides a high-efficiency welding method, wherein the welding method adopts a bevel welding process, the weld bead direction of adjacent weld layers is opposite, and the weld bead direction of the same weld layer is the same;

[0007] The welding method employs a novel low-carbon, low-alloy welding wire, the components of which, by mass percentage, are: C 0.04–0.08%, Si 0.30–0.50%, Mn 1.6–2.0%, Cr 0.15–0.35%, Ni 3.5–4.0%, Cu 0.008–0.01%, Mo 0.007–0.01%, V 0.0005–0.001%, Ti 0.02–0.07%, and Al 0.01–0.018%.

[0008] The welding method includes the following steps:

[0009] S1: Cut the bevel according to the material and specifications of the workpiece to be welded;

[0010] S2: Gas metal arc welding (MAG) is used for layer-by-layer stacking. After each layer is completed, the welding direction is adjusted. The weld directions of adjacent layers are opposite, while the weld directions of the same layer are the same.

[0011] S3: After welding is completed, post-weld treatment is performed.

[0012] Specifically, the workpiece to be welded is preheated before welding at a temperature of 100-120°C; the interpass temperature is controlled at 100-120°C during welding.

[0013] Specifically, the protective gas composition for MAG welding in step S2 is 95% argon + 5% carbon dioxide (by volume).

[0014] Specifically, the flow rate of the protective gas is controlled at 18–22 L / min.

[0015] Specifically, the specific process parameters for step S2 are: welding current of 310-340A, welding voltage of 31-33V, welding speed of 40-44cm / min, and heat input controlled at 14-16KJ / cm.

[0016] Specifically, a heat preservation and slow cooling method is used for post-weld treatment.

[0017] Specifically, the welding method is applicable to the welding of P690QL2 steel.

[0018] Specifically, the weld metal microstructure formed by the welding method consists of ferrite and bainite; the ratio (volume ratio) of the two is bainite:ferrite = 0.80~0.85:0.15~0.20, and the grain size is 1.70~1.80μm.

[0019] Furthermore, the welded joint formed by the welding method has a yield strength ≥792MPa, a tensile strength ≥894MPa, an elongation ≥13.5%; an average energy absorbed in the -60℃ impact test ≥130.4J; and a joint hardness of 315~367HV.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] 1. The welding method provided by this invention is less prone to cracking during the welding process. During the welding process, the welding direction is adjusted after each weld bead is completed to avoid the concentration and cumulative effect of thermal stress caused by continuous welding, thereby effectively reducing the risk of welding cracks.

[0022] 2. The present invention uses a new type of low-carbon low-alloy welding wire for welding, and the resulting weld has good compatibility with high-strength steel (especially P690QL2 steel) and excellent mechanical properties and low-temperature toughness.

[0023] The novel low-carbon, low-alloy welding wire used in this invention exhibits compatibility with P690QL2 steel primarily in the following aspects:

[0024] Compositional Compatibility: The chemical composition of the welding wire is highly compatible with that of P690QL2 steel. For example, the high Ni and Mo content in the welding wire forms a good bond with the corresponding elements in the base metal, improving the bonding strength between the weld and the base metal interface and reducing interface defects caused by chemical incompatibility. Matching Coefficients of Thermal Expansion: The thermal expansion coefficients of the welding wire and P690QL2 steel are close, which can reduce residual stress during cooling after welding and lower the risk of cracking in the weld area. Matching Mechanical Properties: P690QL2 steel has a yield strength (Rp0.2) ≥ 690 MPa and a tensile strength (Rm) of 770–940 MPa. The joint formed by the welding method described in this invention has a yield strength (≥ 792 MPa) and tensile strength (≥ 894 MPa) that match the strength of the base metal, thus ensuring the load-bearing capacity of the weld area in actual use.

[0025] The weld metal has excellent mechanical properties:

[0026] The role of Ni (Ni): With a Ni content of 3.5%-4.0%, it forms fine and uniform acicular ferrite and bainite structures in the weld, improving the strength and low-temperature toughness of the weld metal. Ni also reduces grain boundary segregation, enhancing the weld's impact resistance. The role of Mo (Molybdenum): With a Mo content of 0.007%-0.01%, it effectively inhibits grain growth in the weld structure, improving toughness at low temperatures while enhancing the weld's wear resistance and corrosion resistance. The role of Ti (Titanium): With a Ti content of 0.02%-0.07%, it refines the weld grains, forming a uniform fine-grained structure, improving the joint's mechanical properties and toughness. The role of Mn (Manganese): With a Mn content of 1.6%-2.0%, it effectively improves the strength and toughness of the weld metal and, by combining with S (sulfur) to form MnS inclusions, reduces the weld's susceptibility to hot cracking. The role of Al (aluminum): With an Al content of 0.01%-0.018%, it has the effect of deoxidation and grain refinement, improving the toughness and impact resistance of the weld.

[0027] In summary, the welding wire design and process control in this invention ensure a high degree of compatibility between the weld and P690QL2 steel, resulting in a high-strength, high-toughness welded joint that meets the requirements of liquefied carbon dioxide storage tanks under extreme high-pressure and low-temperature environments. The welded joint formed by the welding method has a yield strength ≥792MPa, tensile strength ≥894MPa, elongation ≥13.5%; the average absorbed energy in the -60℃ impact test is ≥130.4J, and the joint hardness is 315~367HV.

[0028] 3. The welding method provided by this invention innovatively uses a shielding gas of 95% argon + 5% carbon dioxide instead of the traditional 80% argon + 20% carbon dioxide. This gas ratio significantly reduces the formation of porosity and inclusions by increasing the penetration depth and stability of the welding zone, improving the microstructure of the weld metal, and giving the welded joint higher strength and low-temperature toughness.

[0029] 4. The welding method provided by the present invention uses a slow cooling method to control the cooling rate after welding. Compared with post-weld heat treatment, the present invention can prevent the weld structure from becoming brittle and ensure the excellent toughness of the welded joint under low temperature conditions.

[0030] 5. The welding method provided by this invention is simple in process, low in cost, and produces welded joints with excellent comprehensive performance, making it suitable for large-scale promotion.

[0031] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0032] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0033] Figure 1 Here are metallographic images of the weld metal in Example 1;

[0034] Figure 2 Metallographic photographs of the weld metal in Comparative Example 1;

[0035] Figure 3 The image shows the weld morphology in Example 1. Detailed Implementation

[0036] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0037] Based on extensive research and theoretical analysis, the following problems have been identified with the existing welding process:

[0038] (1) Insufficient strength of welded joint: The high strength characteristics of P690QL2 steel make it prone to cracks and defects during welding, resulting in the tensile strength and yield strength of the joint failing to meet the standards of the base material.

[0039] (2) Poor low-temperature toughness: Traditional protective gases (such as 80% Ar + 20% CO2) tend to cause coarsening of weld metal grains, and the low-temperature impact resistance of welded joints is not up to standard, making it difficult to meet the requirements of liquefied carbon dioxide storage tanks in low-temperature environments.

[0040] (3) Stress concentration problem: The existing welding process lacks optimization design for welding direction and sequence, which leads to stress concentration during welding and easily induces cracks.

[0041] This invention provides a highly efficient welding method employing a bevel welding process. Adjacent weld layers have opposite weld bead directions, while weld beads within the same weld layer have the same direction. By altering the welding direction of adjacent weld layers, stress concentration during welding is reduced, preventing crack formation and improving the joint's strength and toughness. During welding, maintaining the same weld bead direction within the same layer ensures uniform heat input, reducing the risk of cracking due to thermal stress concentration. It also optimizes molten pool fluidity, reducing defects such as porosity and slag inclusions, and improving weld formation quality and microstructure uniformity. Furthermore, this method makes the mechanical properties of the weld metal more stable, avoiding stress concentration and grain inhomogeneity caused by direction switching, thereby enhancing the strength and toughness of the welded joint. This operation facilitates stability control in manual welding and also helps automated welding equipment improve welding efficiency and quality, making it particularly suitable for welding high-strength, low-temperature steels.

[0042] The welding method employs a novel low-carbon, low-alloy welding wire, the components of which, by mass percentage, are: C 0.04–0.08%, Si 0.30–0.50%, Mn 1.6–2.0%, Cr 0.15–0.35%, Ni 3.5–4.0%, Cu 0.008–0.01%, Mo 0.007–0.01%, V 0.0005–0.001%, Ti 0.02–0.07%, and Al 0.01–0.018%.

[0043] Preferably, the components in the welding wire, by mass percentage, are: C 0.04–0.06%, Si 0.35–0.45%, Mn 1.75–1.85%, Cr 0.25–0.30%, Ni 3.6–3.8%, Cu 0.008–0.01%, Mo 0.007–0.01%, V 0.0005–0.001%, Ti 0.04–0.06%, and Al 0.013–0.015%.

[0044] More preferably, C: 0.051%, Si: 0.39%, Mn: 1.81%, Cr: 0.28%, Ni: 3.77%, Cu: 0.0097%, Mo: 0.0092%, V: 0.0006%, Ti: 0.055%, Al: 0.013%.

[0045] C (carbon, 0.04–0.08%): Carbon is an important element for enhancing weld strength. With an appropriate carbon content, trace amounts of carbides can be formed in the weld metal, enhancing its hardness and strength. Controlling the carbon content within this range can ensure the strength of the weld while avoiding brittleness and cracking tendency caused by excessive carbon content.

[0046] Si (0.30–0.50%): As a deoxidizer, silicon can effectively remove oxygen from the molten pool during welding, improving the purity of the weld metal. Furthermore, an appropriate amount of silicon helps to enhance the tensile strength and ductility of the weld.

[0047] Mn (1.6–2.0%): Manganese has excellent desulfurization properties, forming MnS inclusions with sulfur, reducing the embrittlement effect of sulfur on weld metal, and significantly lowering the risk of weld hot cracking. High manganese content can also improve the strength and toughness of the weld, especially at low temperatures.

[0048] Cr (chromium, 0.15-0.35%): The addition of chromium can enhance the corrosion resistance of weld metal. At the same time, during the welding cooling process, chromium helps to form a stable ferrite structure, improving the oxidation resistance and toughness of the weld.

[0049] Ni (nickel, 3.5–4.0%): Nickel is an important element for enhancing the low-temperature toughness and strength of welds, exhibiting significant performance advantages in low-temperature environments. The addition of nickel can also refine the weld microstructure, reduce grain boundary segregation, and improve the weld's plasticity and impact resistance.

[0050] Cu (copper, 0.008–0.01%): Trace amounts of copper can enhance the crack resistance of weld metal and help improve the corrosion resistance of the weld. The copper content should be controlled within a low range to prevent the formation of harmful low-melting-point phases during high-temperature welding.

[0051] Mo (Molybdenum, 0.007–0.01%): The addition of molybdenum can refine the grains and enhance the strength and wear resistance of the weld. At the same time, molybdenum can also reduce grain growth at high temperatures, thereby improving the thermal stability of the weld.

[0052] Vanadium (V, 0.0005–0.001%): Trace amounts of vanadium can significantly refine the weld microstructure and improve the yield strength and impact resistance of the weld metal. Vanadium can also combine with carbon to form vanadium carbide, further enhancing the strength and toughness of the weld.

[0053] Ti (titanium, 0.02–0.07%): Titanium is a highly efficient grain-refining element that can significantly improve the low-temperature impact resistance of welds. Titanium can also combine with oxygen during welding to form TiO2, further enhancing the crack resistance of the weld metal.

[0054] Al (aluminum, 0.01–0.018%): Aluminum is an important deoxidizing element that can improve the purity of weld metal and refine grains. Trace amounts of aluminum can also stabilize the weld microstructure and enhance its toughness and impact resistance.

[0055] Synergistic effect:

[0056] Microstructure optimization: The interaction of Ni, Mo, and Ti in the welding wire promotes the formation of acicular ferrite and bainite in the weld, improving the strength and toughness of the weld; Crack resistance enhancement: The synergistic effect of Mn, V, and Cr can significantly reduce the tendency of hot cracking and porosity during welding, making the weld denser and more uniform; Low-temperature performance: Ni, Ti, and Al jointly optimize the microstructure of the weld, giving it better impact resistance in low-temperature environments; Grain refinement: Ti and V can form stable carbides or nitrides during welding, effectively refining the weld metal grains and improving the overall performance of the weld; Corrosion resistance: The synergistic effect of Cr and Cu can significantly enhance the corrosion resistance of the weld metal in harsh environments.

[0057] Through precise control and synergistic effect of the above elements, the welding wire can form a uniform and fine ferrite and bainite structure during the welding process, giving the weld metal high strength, high toughness and excellent low-temperature performance, meeting the application requirements of P690QL2 steel under extreme working conditions.

[0058] Specifically, the method for preparing the welding wire is as follows:

[0059] S21 Ultra-Pure Smelting: Raw materials are initially weighed and mixed according to a pre-set formula. A double-melting technology is employed, firstly in an electric arc furnace for initial melting, and then in an induction furnace for refining to further reduce the phosphorus (P) and sulfur (S) content. The refining temperature is controlled at 1600–1700℃ and maintained for 1.5–3 hours to ensure impurities are fully floated out. During the refining process, a vacuum of at least 10^-3 Torr is maintained to prevent component oxidation or the introduction of gaseous impurities.

[0060] S22 Forging and Rolling:

[0061] A gradual cooling technique is employed during forging to control the cooling rate and prevent stress concentration within the material. Each forging operation applies a pressure ≥120MPa to ensure material uniformity and density.

[0062] Specifically, the gradual cooling technique ensures uniform microstructure and grain refinement of the alloy across different temperature ranges by precisely controlling the cooling rate during forging and rolling. The forging process begins with preheating the alloy to 850–1050°C, followed by forging at an initial forging temperature of 1150–1250°C, using gradual cooling for temperature control. In the initial stage, a slower cooling rate (30–50°C / h) is employed to ensure the alloy's plasticity and prevent cracking. In the intermediate and low-temperature stages, the cooling rate is further reduced (20–40°C / h and 10–20°C / h) to prevent grain coarsening and stress concentration. After forging, the alloy should not be rapidly cooled immediately but rather slowly, down to approximately 900°C. During this process, direct exposure to air should be avoided to prevent stress concentration or cracking due to excessively high or low temperatures. To ensure material uniformity, cooling should be carried out under controlled conditions to avoid stress unevenness caused by environmental differences.

[0063] Specifically, the rolling process begins after forging. In the rough rolling stage, the billet is first heated to 1100–1150℃ for initial deformation and gradual thinning. During this process, the reduction in each rough rolling pass should be controlled between 25% and 35%, with a cooling rate of 40–60℃ / h to prevent surface cracking caused by excessively rapid cooling. The temperature control in the finish rolling stage is more stringent, requiring the temperature to be maintained between 850 and 900℃, with a cooling rate of 10–20℃ / h to ensure grain refinement and a uniform microstructure. During finish rolling, the reduction in each finish rolling pass should be controlled between 10% and 15% to maintain the alloy surface quality and optimize mechanical properties. After rolling, the alloy should continue to undergo uniform cooling to avoid excessive temperature differences that could cause internal stress or cracks, ultimately obtaining a high-quality, uniformly structured alloy material. This optimized gradual cooling method effectively avoids problems such as cracking, grain coarsening, and internal stress concentration caused by excessively rapid cooling, improving the mechanical properties and structural uniformity of the alloy.

[0064] S23 Bright Drawing: Lubricant is added during the drawing process to reduce friction and improve surface finish. A stable synthetic lubricant suitable for high temperature and high tensile conditions should be selected. The drawing speed should be controlled at 5–10 m / min to ensure the stability of the material during the drawing process.

[0065] S24 Surface Wiping and Cleaning: Mechanical wiping is performed using a highly absorbent microfiber cloth to improve cleaning efficiency. Ultrasonic cleaning is then performed after mechanical wiping to remove microscopic impurities. The ultrasonic frequency is 35–45 kHz, and the cleaning time is 10–20 minutes to thoroughly remove all attached impurities.

[0066] The welding wire is prepared using ultra-pure smelting technology, strictly controlling the content of impurity elements (such as P and S). Vacuum melting and refining deoxidation processes reduce the intrusion of gaseous elements (H, O, and N), ensuring the purity of the welding wire. Simultaneously, alloying elements (such as C, Ni, Mn, Ti, and Mo) are precisely added according to the designed proportions to achieve compositional homogenization, further improving the low-temperature toughness and crack resistance of the welding wire. Forging, rolling, and bright drawing processes refine the grain size, combined with surface pickling and cleaning treatments, ensuring the welding wire surface is free of impurities and contaminants, providing an excellent material foundation for high-strength welding.

[0067] Furthermore, after the welding wire is prepared, vacuum annealing technology is used to reduce the diffusible hydrogen content to prevent cold cracking, and it is stored in a dry or vacuum environment to ensure the long-term stability of the welding wire. The welding wire prepared by this method has high strength, high toughness, and low impurity content, which can meet the usage requirements of high-pressure and low-temperature equipment such as liquefied carbon dioxide storage tanks, and effectively improve the purity, impact resistance, and structural reliability of the weld.

[0068] It is worth emphasizing that the performance of welding wire is primarily determined by its composition design. While the manufacturing process can help improve its performance, its impact is relatively limited. This is because the welding wire actually undergoes another melting and solidification process during welding, ultimately forming a "new weld metal." The performance of this "new weld metal" plays a decisive role in the final welding performance. In other words, the most crucial role of the welding wire manufacturing process is to maximize the purity and uniformity of the wire, removing impurities, thereby resulting in a superior "new weld metal."

[0069] Specifically, the welding wire is vacuum-sealed before welding to prevent oxidation from affecting the welding quality.

[0070] The welding method includes the following steps:

[0071] S1: According to the material and specifications of the workpiece to be welded, and in accordance with implementation needs and relevant standards, bevels are made;

[0072] S2: Gas metal arc welding (MAG) is used for layer-by-layer stacking. After each layer is completed, the welding direction is adjusted. The weld directions of adjacent layers are opposite, while the weld directions of the same layer are the same.

[0073] S3: After welding is completed, post-weld treatment is performed.

[0074] Specifically, the workpieces to be welded undergo preheating treatment at a temperature of 100–120°C; during the welding process, the interpass temperature is controlled at 100–120°C. This ensures temperature consistency during the welding process, prevents grain coarsening or stress concentration caused by overheating or undercooling in the weld area, and ensures excellent performance of the welded joint.

[0075] Specifically, the shielding gas composition for MAG welding in step S2 is 95% argon + 5% carbon dioxide (by volume). Argon, as an inert gas, effectively inhibits oxidation reactions during welding, reduces grain boundary inclusions, and improves the purity of the weld metal, thereby improving the low-temperature toughness of the weld. The lower proportion of carbon dioxide provides a certain degree of arc stability while reducing the impact of oxygen content on the microstructure, resulting in a fine and uniform grain structure.

[0076] Furthermore, the weld microstructure using a shielding gas ratio of 95% argon and 5% carbon dioxide mainly consists of acicular ferrite, lamellar ferrite, and granular bainite, with a uniform distribution and fine grains. The high proportion of argon, as an inert gas, effectively suppresses oxidation reactions during welding, reduces the oxygen content in the weld metal, and minimizes the formation of oxide inclusions at grain boundaries. This gas ratio promotes a more ideal microstructure transformation during cooling. The high proportion of argon provides a moderate cooling rate, initially forming acicular and lamellar ferrite, followed by the precipitation of granular bainite at lower temperatures. This moderate cooling rate helps form a stable bainite phase, ensuring uniform dispersion of carbon within the ferrite, thus imparting high strength to the weld. The high proportion of bainite and the uniformly distributed ferrite together enhance the weld's strength and toughness, resulting in excellent performance in low-temperature impact tests. This fine-grained, multiphase microstructure effectively disperses stress concentration, reduces the risk of brittle fracture, and significantly improves the overall mechanical properties of the weld joint.

[0077] Specifically, the flow rate of the protective gas is controlled at 18–22 L / min, for example, 18, 19, 20, 21, or 22 L / min, to ensure a stable flow rate and uniform coverage of the welding area. A stable gas flow rate can create a uniform protective atmosphere and reduce defects caused by air entering the welding area.

[0078] Specifically, the specific process parameters for step S2 are as follows: welding current is 310-340A, for example, 310, 315, 320, 325, 330, 335, 340A; welding voltage is 31-33V; welding speed is 40-44cm / min; and heat input is controlled at 14-16KJ / cm.

[0079] The process parameters in step S2 are designed based on the welding wire composition and welding material characteristics to optimize welding quality and performance. Welding current and voltage directly determine the molten pool temperature and penetration depth. Matching the melting characteristics of alloy components such as Ni, Mn, and Cr in the welding wire ensures a stable arc and sufficient penetration depth. Welding speed is controlled at 40–44 cm / min to optimize the molten pool cooling rate. Combined with fine-graining strengthening elements such as Ti and V in the welding wire, this suppresses grain coarsening and improves weld formation quality and toughness. Controlling the heat input within the range of 14–16 KJ / cm is a key parameter, satisfying the need for sufficient melting of the base material and welding wire while avoiding grain coarsening and cracking problems caused by excessively high molten pool temperatures. Combined with Ni and Mo in the welding wire composition, this heat input range promotes the formation of fine and uniform ferrite and bainite structures within the weld, significantly improving the low-temperature toughness and impact resistance of the weld. Furthermore, reasonable heat input and cooling rates reduce welding stress concentration, decreasing the probability of cold and hot cracking. By optimizing the above parameters and ensuring close integration with the welding wire composition, the performance advantages of the alloying elements in the welding wire can be fully utilized, resulting in a stable weld structure with excellent strength, toughness, and impact resistance. The entire process parameter design can meet the special requirements of cryogenic and high-pressure equipment such as liquefied carbon dioxide storage tanks, ensuring the safety and reliability of the welded joint in extreme environments.

[0080] Specifically, a heat preservation and slow cooling method is used for post-weld treatment. For example, after welding, the weld is immediately kept at a high temperature for 2 to 6 hours, such as 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, or 6 hours, and then air-cooled. No post-weld heat treatment is performed; instead, a slow cooling method is used to control the cooling rate. Slow cooling can prevent the weld microstructure from becoming brittle and maintain a finer grain structure, thereby improving the low-temperature toughness of the welded joint.

[0081] Specifically, the welding method is applicable to the welding of P690QL2 steel.

[0082] Furthermore, the welding method is applicable to the manufacture and repair of cryogenic and high-pressure equipment such as liquefied carbon dioxide storage tanks and LNG storage tanks, and is particularly suitable for use in fully pressurized C-type double-eared liquid cargo tanks in ship transportation.

[0083] Specifically, the weld metal microstructure formed by the welding method consists of ferrite and bainite, with a volume ratio (bainite:ferrite) of 0.80–0.85:0.15–0.20, and a grain size of 1.70–1.80 μm. More specifically, it is composed of acicular ferrite, lath ferrite, and granular bainite, with a uniform distribution and fine grains. The high proportion of bainite and the uniformly distributed ferrite together improve the strength and toughness of the weld, resulting in excellent performance in low-temperature impact tests. This fine-grained, multiphase microstructure effectively disperses stress concentration, reduces the risk of brittle fracture, and significantly improves the overall mechanical properties of the weld joint.

[0084] Furthermore, the welded joint formed by the welding method has a yield strength ≥792MPa, a tensile strength ≥894MPa, an elongation ≥13.5%; an average energy absorbed in the -60℃ impact test ≥130.4J; and a joint hardness of 315~367HV.

[0085] Examples and Comparative Examples

[0086] The specific composition of the welding wires in each embodiment and comparative example is shown in Table 1.

[0087] Table 1. Specific composition of welding wires in each embodiment and comparative example.

[0088] C Si Mn Cr Ni Cu Mo V Ti Al Example 1 0.051 0.39 1.81 0.28 3.77 0.0097 0.0092 0.0006 0.055 0.013 Example 2 0.056 0.35 1.85 0.26 3.78 0.0089 0.0089 0.0007 0.054 0.014 Example 3 0.072 0.37 1.82 0.28 3.72 0.0092 0.0095 0.0006 0.055 0.015 Example 4 0.043 0.44 1.86 0.25 3.74 0.0094 0.0084 0.0007 0.054 0.014 Example 5 0.065 0.42 1.75 0.28 3.76 0.0092 0.0092 0.0007 0.056 0.015 Comparative Example 1 0.051 0.39 1.81 0.28 3.77 0.0097 0.0092 0.0006 0.055 0.013 Comparative Example 2 0.051 0.39 1.81 0.28 3.77 0.0097 0.0092 0.0006 0.055 0.013 Comparative Example 3 0.019 0.39 1.80 0.26 3.78 0.0092 0.0090 0.0005 0.051 0.012 Comparative Example 4 0.032 0.28 1.81 0.1 2.43 0.022 0.52 / 0.0064 /

[0089] The preparation processes for each embodiment and comparative example are as follows:

[0090] S21 Ultra-Pure Smelting: Employing a double-melting technology, initial melting takes place in an electric arc furnace, followed by refining in an induction furnace to further reduce phosphorus (P) and sulfur (S) content. The refining temperature is controlled at 1650℃ for 2 hours, with the vacuum level maintained at no less than 10^-3 Torr during the refining process.

[0091] S22 Forging and Rolling:

[0092] Forging process: First, the alloy is preheated to 950℃, then forged at an initial forging temperature of 1200℃, using a gradual cooling technique for temperature control. In the initial stage, a slow cooling rate (40℃ / h) is adopted to ensure the plasticity of the alloy and avoid crack formation; in the medium and low temperature stages, the cooling rate is further reduced (30℃ / h and 15℃ / h) to prevent grain coarsening and internal stress concentration. After forging, slow cooling is used until approximately 900℃ is reached.

[0093] Rolling process:

[0094] Roughing section: heating temperature 1120℃, the reduction in each roughing roll should be controlled between 25% and 35%, and the cooling rate is 50℃ / h;

[0095] Finishing rolling section: The temperature is maintained between 880±10℃, and the cooling rate is controlled at 15℃ / h to ensure grain refinement and uniform microstructure. During the finishing rolling process, the reduction amount of each finishing roll should be controlled between 10% and 15% to maintain the alloy surface quality and optimize mechanical properties. The finished product is used for welding wire rods.

[0096] S33 Bright Drawing: The welding wire rod is drawn, and a lubricant is added during the drawing process (common commercially available high temperature and high tensile strength lubricants are sufficient, no special requirements are needed), and the drawing speed is controlled at 6-8 m / min.

[0097] S34 Surface Wiping Cleaning: Mechanical wiping is performed using a highly absorbent microfiber cloth to improve cleaning efficiency. Following mechanical wiping, ultrasonic cleaning is performed at a frequency of 40kHz for 15 minutes.

[0098] The test plate material was P690QL2 steel plate, with dimensions of 500mm long × 150mm wide × 20mm thick. A V-shaped bevel was cut along the rolling direction of the test plate, with a bevel gap of 16mm. Welding tests were conducted on the deposited metal, and the specific welding process parameters are shown in Table 2.

[0099] Table 2 Welding process parameters for each embodiment and comparative example

[0100]

[0101]

[0102] The mechanical properties and low-temperature toughness of the welds / weld joints in each embodiment and comparative example were tested, and the test results are shown in Table 3.

[0103] Table 3. Mechanical properties and low-temperature toughness test results of each embodiment and comparative example.

[0104]

[0105] In a detailed analysis of the embodiments and comparative examples, the mechanical properties of the embodiments were significantly superior to those of the comparative examples by precisely controlling the chemical composition and welding process parameters. The embodiments optimized welding performance by adjusting the proportions of key elements such as carbon, silicon, nickel, and chromium, ensuring high strength and good low-temperature toughness. These adjustments directly affected the microstructure and mechanical properties of the weld. Examples 3 and 5 particularly demonstrate how increasing the carbon and aluminum content refines the grain structure, thereby improving overall performance, especially in applications under extreme environments. It is worth noting that the welding wire formulation design ensures the lower limit of the welding performance of the described welding method, while more optimized welding process parameters help to further fully utilize the performance of the welding wire.

[0106] In contrast, the comparative examples demonstrate the performance limitations under non-optimized conditions. Although Comparative Examples 1 and 2 used the same basic alloy composition as the examples, differences in the shielding gas ratio or increased heat input power show how these variations negatively impact weld quality and performance. In Comparative Example 1, the poor shielding gas ratio resulted in a relatively high content of inclusions in the weld metal, leading to decreased mechanical properties. In Comparative Example 2, the higher heat input caused overheating in the weld zone, potentially causing grain coarsening, which reduced the weld's impact toughness, highlighting the importance of controlling heat input.

[0107] Overall, the comparison between the examples and the comparative examples highlights the crucial role of optimizing chemical composition and welding process parameters in achieving excellent welding performance. The composition optimization and welding parameter control in the examples ensured high-performance welded joints, while the performance limitations of the comparative examples underscore the necessity and effectiveness of optimizing welding wire formulations and process parameters.

[0108] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for MAG welding of P690QL2 steel without post-weld heat treatment, characterized in that, The welding method employs a bevel welding process, with adjacent weld layers having opposite weld bead directions and the same weld bead direction within the same weld layer. The welding method employs a low-carbon, low-alloy welding wire, the components of which, by mass percentage, are: C 0.051–0.056%, Si 0.30–0.50%, Mn 1.81–1.85%, Cr 0.26–0.28%, Ni 3.77–4.0%, Cu 0.008–0.0097%, Mo 0.007–0.01%, V 0.0005–0.001%, Ti 0.054–0.055%, Al 0.013–0.018%. The welding wire is prepared using ultra-pure smelting, forging, rolling, bright drawing, and surface cleaning processes, and is then vacuum annealed after preparation. The welding wire is vacuum-stored before welding. The welding method includes the following specific steps: S1: Cut the bevel according to the material and specifications of the workpiece to be welded; S2: Gas metal arc welding (GMAW) is used for layer-by-layer stacking welding. After each layer is welded, the welding direction is adjusted. The weld directions of adjacent weld layers are opposite, while the weld directions of the same weld layer are the same. The volumetric composition of the shielding gas for the MAG welding is 95% argon + 5% carbon dioxide. The interpass temperature is controlled at 100-120℃ during the welding process. The specific process parameters are: welding current of 310-340A, welding voltage of 31-33V, welding speed of 40-44cm / min, and heat input controlled at 14-16KJ / cm. S3: After welding, post-weld treatment is carried out by heat preservation and slow cooling; heat preservation is carried out directly after welding for 2 to 6 hours, followed by air cooling; The weld metal microstructure formed by the welding method consists of ferrite and bainite, with a ratio of bainite:ferrite of 0.80–0.85:0.15–0.20, and a grain size of 1.70–1.80 μm.

2. The welding method according to claim 1, characterized in that, The workpiece to be welded is preheated before welding at a temperature of 100-120°C.

3. The welding method according to claim 1, characterized in that, The flow rate of the protective gas is controlled at 18–22 L / min.

4. The welding method according to claim 1, characterized in that, The specific process parameters for step S2 are as follows: welding current is 325±5A, welding voltage is 31~33V, welding speed is 42cm / min, and heat input is controlled at 14~16KJ / cm.

5. The welding method according to claim 1, characterized in that, The weld metal microstructure formed by the welding method consists of ferrite and bainite, with a ratio of bainite:ferrite = 0.83:0.17 and a grain size of 1.74 μm.

6. The welding method according to claim 1, characterized in that, The welded joint formed by the welding method has a yield strength ≥792MPa, tensile strength ≥894MPa, elongation ≥13.5%; average energy absorbed in the -60℃ impact test ≥130.4J, and joint hardness of 315~367HV.

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