A high-toughness nickel-based flux-cored wire for LNG ships and a preparation method and application thereof

Abstract

The present application relates to the technical field of welding material, in particular to a kind of LNG ship high toughness nickel-based flux-cored wire and its preparation method and application.The flux-cored wire is composed of nickel-based steel strip and the flux powder filled therein, and the filling rate of flux powder is 20-25%.According to weight percentage, the flux powder includes mineral powder and metal powder, wherein the mineral powder includes: 20-25% rutile, 2-5% sodium feldspar, 5-7% sodium fluoroaluminate, 2-5% alumina, 5-10% silicon dioxide, 5-10% sodium silicotitanate, 3-8% potassium silicotitanate;The metal powder includes 10-16% chromium powder, 3-5% molybdenum powder, 3-5% tungsten powder, 4-8% titanium iron, 17-20% nickel powder.The welding wire of the present application uses 100% CO2 shielding gas, can meet 9Ni steel in 2F, 3F, 3G multi-position welding, deslagging rate >91%, X-ray flaw detection I level, weld metal tensile strength ≥690MPa, yield strength ≥430MPa elongation ≥35%, impact absorption power ≥50J at-196℃, 180 unilateral lateral bending tensile surface no crack, effectively solve the problem that traditional welding wire welding process and low temperature mechanical property are difficult to take into account.

Description

Technical Field

[0001] This invention relates to the field of welding materials technology, and more specifically, to a high-toughness nickel-based flux-cored welding wire for LNG ships, its preparation method, and its application. Background Technology

[0002] The storage tanks of liquefied natural gas (LNG) ocean-going vessels operate under ultra-low temperature conditions of -162℃ for extended periods. The main body of the tank is made of 9Ni steel, which imposes stringent requirements on the low-temperature toughness, weldability, and weld formation quality of the welding materials used. Nickel-based flux-cored welding wire, with its high deposition efficiency and ability to perform gas-shielded welding at multiple locations, has become the preferred welding material for welding 9Ni steel in LNG vessel storage tanks. Currently, most mainstream nickel-based flux-cored welding wires on the market use ENiCrMo-3 and ENiCrMo-4 technologies. The flux core generally adopts an acidic slag system, and the formulation contains a large amount of alloy powders such as chromium and molybdenum. This inherent formulation structure makes it difficult to simultaneously achieve weld slag removal performance, arc stability, and low-temperature mechanical properties. There is a common problem that welding process and mechanical properties cannot be optimized simultaneously, making it difficult to meet the stringent multi-condition welding requirements of LNG vessel storage tanks.

[0003] To address these issues, existing technologies have made numerous attempts. For example, existing patent CN114669911A discloses a flux-cored gas-shielded welding wire for 9% Ni storage tank steel. This wire uses Fe-based low-Ni steel strip with a Ni content of only 10.15-12.03% and a Cr content of 3.15-5.03%. The flux powder contains rutile, zircon sand, potassium titanate, and a relatively high content of molybdenum powder (10.40-12.68%). While this approach improves welding efficiency, the Fe-based steel strip results in insufficient matching between the alloy system and the desired high-Ni weld metal. Furthermore, the higher molybdenum content, while increasing strength, may negatively impact the low-temperature toughness of the weld. The required impact energy at -196℃ ≥47J is also relatively low, making it difficult to fully meet the stringent high toughness requirements of LNG ship storage tanks for welds.

[0004] Furthermore, existing patent CN120306883A proposes a nickel-based alloy flux-cored welding wire for welding 9Ni steel. The nickel-based steel strip is a Ni-Cr-Mo alloy, and the flux core contains components such as potassium titanate and ferrotitanium, which improves deoxidation and arc stabilization performance to some extent. However, the flux-cored welding wire prepared by this method has a high alloy content, making it prone to forming brittle intermetallic compound precipitates in the weld metal, potentially negatively impacting low-temperature toughness and plasticity. Moreover, this method mainly relies on complex fluoride compounding for dehydrogenation and auxiliary methods such as mineral powder arc melting pretreatment to reduce porosity and crack sensitivity, without actively suppressing harmful precipitates from the source of alloy composition design. Its technical focus is on solving crack resistance and porosity sensitivity, without quantitatively emphasizing the key process indicator of achieving high slag removal rate, which may still lead to slag adhesion problems in actual multi-position welding.

[0005] Furthermore, existing patent CN121199455A discloses a high-Ni alloy flux-cored welding wire for welding 9Ni steel. This welding wire uses a nickel-based steel strip with a composition close to that of C-276 Hastelloy. The flux core contains rutile, quartz, albite, potassium feldspar, mica, cryolite, and a self-made raw material. The metal powder includes nickel, chromium (5-8%), molybdenum (5-9%), and tungsten (1-2%) powder. This design exhibits a low-temperature impact toughness ≥50J and tensile strength >700MPa at -196℃, and utilizes a 100% CO2 shielding gas. However, the flux core of this welding wire does not contain alumina and sodium / potassium titanate, and it does not use ferrotitanium as a deoxidizing and alloying component, resulting in limited ability to suppress harmful second phases in the weld metal. Its alloy ratio is medium chromium and medium-high molybdenum, making it difficult to achieve excellent elongation and bending performance while pursuing high strength, which is why its low-temperature elongation is only 27-31%. In addition, the document does not conduct quantitative testing on the weld slag removal rate, nor does it clarify whether the welding wire is suitable for key welding positions in the construction of LNG ship storage tanks, such as flat fillet welds, vertical fillet welds, and vertical butt welds, resulting in insufficient comprehensive evaluation of the welding process. Summary of the Invention

[0006] In view of this, the present invention aims to propose a high-toughness nickel-based flux-cored welding wire for LNG ships, its preparation method, and its application, to solve the problems of insufficient low-temperature toughness, poor welding processability, unstable mechanical properties, and difficulty in synergistically improving welding processability and comprehensive mechanical properties in existing nickel-based flux-cored welding wires. Specifically, it aims to overcome the problems of insufficient impact toughness of welds in ultra-low temperature environments, easy generation of porosity and slag inclusions during welding, difficulty in slag removal, and large fluctuations in elongation and bending performance, thereby providing a flux-cored welding wire with high strength, high and low temperature toughness, high slag removal rate, and excellent welding process stability to meet the stringent welding requirements of LNG ship storage tanks.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0008] This invention discloses a high-toughness nickel-based flux-cored welding wire for LNG ships. The flux-cored welding wire is composed of nickel-based steel strip and flux powder filled therein. By weight percentage, the flux powder contains 47%~62% mineral powder and 38%~53% metal powder. The weight fractions of each component in the mineral powder and metal powder are as follows:

[0009] Mineral powder: rutile 20-25%, albite 2-5%, sodium fluoroaluminate 5-7%, alumina 2-5%, silicon dioxide 5-10%, sodium titanate 5-10%, potassium titanate 3-8%;

[0010] Metal powders: 10-16% chromium powder, 3-5% molybdenum powder, 3-5% tungsten powder, 4-8% ferrotitanium, and 17-20% nickel powder.

[0011] The functions of each mineral powder in the core powder are as follows:

[0012] Rutile: The core slag-forming agent of the slag system in this invention, and the main source of TiO2 in the molten pool. During the welding process, it can stably form molten slag, effectively covering the surface of the molten pool and protecting the weld metal from air contamination; at the same time, it forms a ternary composite system with alumina and silica, synergistically improving the physical properties of the molten slag, making the slag shell easy to separate from the weld metal after welding, and significantly improving the slag removal performance.

[0013] Sodium feldspar: As an arc stabilizer and slag-forming agent, its Na2O content can reduce the ionization potential of the electric arc and enhance the stability of the electric arc; at the same time, after its decomposition, it can provide a certain amount of Al2O3 and SiO2, which can be used to adjust the viscosity and fluidity of the molten slag, improve the weld formation quality, and adapt to the application requirements of different welding positions.

[0014] Sodium fluoroaluminate: It can effectively enhance arc stability, reduce the surface tension and viscosity of molten slag, and improve the fluidity of molten slag. At the same time, sodium fluoroaluminate can enhance the permeability of molten slag, promote the smooth escape of gases generated during welding from the molten pool, thereby reducing porosity defects and improving the density and purity of the weld.

[0015] Alumina: As an auxiliary slag-forming agent, it can increase the high-temperature viscosity of molten slag, enhance the slag adhesion ability of molten slag in vertical welding positions, prevent molten slag from flowing down, and thus improve the weld formation quality of multi-position welding.

[0016] Silica: In synergy with rutile and alumina, it modulates the physical properties of slag, reduces the interfacial bonding force between slag and weld metal, and promotes automatic slag shell peeling. Simultaneously, silica can optimize the surface finish of the weld, resulting in a uniform, delicate, and smooth weld scale pattern.

[0017] Sodium titanate: As a composite arc stabilizer, it can effectively stabilize the welding arc and reduce spatter; at the same time, it participates in regulating the viscosity and surface tension of the slag, improving weld formation and slag removal performance.

[0018] Potassium titanate: Similar to sodium titanate, it enhances arc stability and reduces spatter; its K₂O content has a lower ionization potential, resulting in a more significant arc stabilization effect. It also helps refine droplet transfer and improve welding process performance. The synergistic use of sodium titanate and potassium titanate can maintain stable arc combustion over a wide current range, solving the problems of arc vibration and arc interruption under different welding parameters.

[0019] The functions of the various metal powders in the core powder are as follows:

[0020] Nickel powder: As an austenite stabilizing element, it can ensure that the weld metal maintains a stable face-centered cubic austenite structure at an ultra-low temperature of -196℃, avoids brittle martensitic phase transformation, and thus obtains excellent low-temperature impact toughness, ensuring the performance reliability of the weld metal during long-term service.

[0021] Chromium powder is an important solid solution strengthening element that can significantly improve the strength and oxidation resistance of weld metal. During welding, chromium enters the weld metal, promoting the formation of a dense passivation film and enhancing corrosion resistance.

[0022] Molybdenum powder: Molybdenum is one of the key strengthening elements in nickel-based alloys, which can significantly improve the strength and pitting resistance of weld metal. This invention controls the molybdenum powder content at a low level of 3-5%, which can exert a moderate solid solution strengthening effect to improve weld strength, and effectively inhibit the formation of harmful brittle precipitates, thereby avoiding the deterioration of weld plasticity and low-temperature toughness.

[0023] Tungsten powder: It works synergistically with chromium powder and molybdenum powder to strengthen the weld metal through solid solution, further enhancing its strength and corrosion resistance. Furthermore, the synergistic effect of chromium powder and tungsten powder can compensate for the strength loss that may result from the reduction in Mo content under low Mo conditions, ensuring the tensile strength of the weld metal.

[0024] Titanium iron: It plays a role in both deoxidation and impurity removal and grain refinement. On the one hand, titanium preferentially reacts with oxygen and nitrogen, inhibiting the formation of CO and N2 pores, reducing oxide inclusions, and improving weld purity; on the other hand, the fine particles formed by titanium and nitrogen can serve as heterogeneous nucleation sites, refining weld grains, thereby improving low-temperature toughness and elongation.

[0025] By limiting the formulation of mineral and metal powders in the core powder and the proportion of each component in the formulation, the industry pain point of traditional nickel-based welding wires, which struggles to balance welding processability and low-temperature mechanical properties, is solved from the source of the formulation.

[0026] Furthermore, by weight percentage, the weight fractions of each component in the nickel-based steel strip are: C≤0.02%, Si≤0.2%, Mn≤1.0%, Cr:14.5~16.5%, Mo:15.0~17.0%, W:3.0~4.5%, Fe:4.0~7.0%, with the balance being Ni and unavoidable impurities.

[0027] The functions of each component in the nickel-based steel strip are as follows:

[0028] Ni: As the matrix element of the alloy, it provides a stable face-centered cubic austenitic structure, ensuring that the weld metal has good structural stability and impact toughness at ultra-low temperatures of -196℃.

[0029] Cr: Improves the oxidation and corrosion resistance of weld metal, and enhances strength through solid solution strengthening; too low a Cr content will result in insufficient strengthening effect, while too high a Cr content may promote the precipitation of brittle phases.

[0030] Mo is one of the most important solid solution strengthening elements in C-276 alloy. It can significantly improve the strength of weld metal and its resistance to pitting and crevice corrosion. At the same time, Mo works synergistically with Ni to stabilize the austenitic structure.

[0031] W works synergistically with Mo to strengthen through solid solution, further improving the high-temperature strength and corrosion resistance of the weld metal, while also contributing to grain refinement.

[0032] Fe: A moderate amount can improve the processing performance of the alloy and reduce material costs, but excessive content will have an adverse effect on low-temperature toughness.

[0033] Mn has the dual functions of deoxidation and sulfur fixation. It preferentially combines with sulfur to form high-melting-point sulfides, inhibits the formation of low-melting-point grain boundary phases, and reduces the risk of welding cracks.

[0034] C and Si are unavoidable impurity elements, and their contents must be strictly controlled. Excessive C content can easily form carbides, reducing toughness, while excessive Si content can increase susceptibility to hot cracking.

[0035] By limiting the alloy composition range of the nickel-based steel strip, the outer sheath and the core powder are well matched in the metallurgical reaction, providing a stable austenitic matrix and the necessary solid solution strengthening basis for the weld metal.

[0036] Furthermore, the amount of flux-cored powder filling accounts for 20% to 25% of the total weight of the flux-cored wire.

[0037] By limiting the flux-cored powder filling rate to 20%-25%, the appropriate proportion of flux-cored powder in the welding wire is ensured, which not only meets the requirements of alloy transition and slag formation, but also facilitates the drawing process and guarantees the forming quality of the flux-cored welding wire.

[0038] Furthermore, the flux-cored welding wire uses 100% CO2 as the shielding gas for welding.

[0039] By limiting the use of 100% CO2 as the shielding gas, the cost of welding is significantly reduced without the need for expensive argon-rich mixed gases, while still achieving excellent welding process performance.

[0040] The present invention also discloses a preparation method for preparing the above-mentioned flux-cored welding wire, comprising the following steps:

[0041] Preparation of S1 core powder;

[0042] S2 welding wire rolling;

[0043] S3 drawing reduction;

[0044] S4 winding tray packaging.

[0045] The preparation method of this flux-cored welding wire is clear, easy to operate, highly repeatable, and suitable for industrial mass production.

[0046] Furthermore, in step S1, the preparation of the core powder includes:

[0047] S11 Weigh the mineral powder and metal powder raw materials according to the formula ratio of the core powder;

[0048] S12. The weighed mineral powder is dried at 300~500℃ for 1.5~2.5h;

[0049] S13 then puts the dried mineral powder and metal powder into a mixer and stirs for 20-50 minutes until they are evenly mixed;

[0050] After S14 is mixed, the mixture is kept at 120~200℃ for 1.5~3 hours.

[0051] In the preparation of the core powder, a two-stage drying process of drying mineral powder and heat preservation of the mixture is set up to effectively remove adsorbed water and crystal water in the powder, prevent porosity defects during welding, and ensure the tightness of the weld.

[0052] Furthermore, in step S2, the rolling of the flux-cored welding wire includes:

[0053] S21 rolls nickel-based steel strip into a grooved strip;

[0054] S22 is filled with core powder in the grooved strip, and the filling rate is controlled between 20% and 25%.

[0055] After S23 filling is completed, the grooved strip is rolled into a closed wire to ensure that the core powder does not leak.

[0056] By controlling the filling rate during the rolling of flux-cored welding wire and rolling the grooved section into a closed wire, it is ensured that the flux-cored powder is uniformly and tightly wrapped in the steel strip, thus avoiding leakage or uneven distribution of the flux-cored powder during the drawing process.

[0057] Furthermore, in step S3, the drawing and diameter reduction of the flux-cored wire includes:

[0058] The rolled closed wire is drawn in multiple passes to gradually reduce its diameter to the target diameter, which is 1.0~1.6mm.

[0059] The diameter of the finished welding wire is controlled within the commonly used specification range of 1.0~1.6mm, which is compatible with most flux-cored arc welding equipment on the market. The wire feeding is smooth, the welding process is stable, and it is convenient for on-site construction and use.

[0060] The present invention also discloses an application of flux-cored welding wire, which is used for welding 9Ni steel for LNG ship storage tanks, and the welding positions include flat fillet welds, vertical fillet welds and vertical butt welds.

[0061] By applying flux-cored welding wire to fillet welds, vertical fillet welds, and vertical butt welds of 9Ni steel used in LNG ship storage tanks, the process adaptability of the welding wire in multi-position welding was verified, meeting the construction requirements of different welding positions in the construction of storage tanks.

[0062] Compared with existing technologies, the high-toughness nickel-based flux-cored welding wire for LNG ships, its preparation method, and its application described in this invention have the following advantages:

[0063] (1) This invention optimizes the ratio of mineral components in the flux-cored powder and utilizes the synergistic effect of rutile, alumina, and silica to achieve a slag removal rate of over 91% and easy slag shell removal. Simultaneously, sodium fluoroaluminate, sodium titanate, and potassium titanate effectively enhance arc stability, resulting in a stable welding process. The prepared flux-cored welding wire is suitable for 2F, 3F, and 3G multi-position welding required for LNG ship storage tank manufacturing. X-ray flaw detection can reach the highest level I standard, effectively solving the problems of difficult slag removal and unstable arc that easily occur during multi-position welding in existing technologies.

[0064] (2) By precisely controlling the content of key alloying elements (especially chromium and molybdenum) in the flux core, the formation of harmful brittle precipitates in the weld metal is effectively suppressed, so that the weld can obtain high strength while also having excellent plasticity and low temperature toughness. It can stably achieve comprehensive mechanical performance indicators such as tensile strength ≥690MPa, yield strength ≥430MPa, elongation ≥35%, impact absorption energy ≥50J at -196℃, and no cracks on the tensile surface under 180° lateral bending, which fully meets the usage requirements of the cryogenic service environment of LNG ships.

[0065] (3) This invention clearly specifies the complete preparation process and key parameters, from the strict drying treatment of the core powder and the precise filling rate control during rolling, to the multi-pass drawing and sizing, and finally to vacuum packaging. The process flow is clear, the quality control points are well-defined, and it can effectively avoid the performance degradation caused by moisture in the powder or nickel-based steel strip, ensuring the performance stability of the flux-cored welding wire. It is suitable for industrial mass production and facilitates large-scale promotion and application.

[0066] (4) The flux-cored welding wire of the present invention is specifically designed for the welding requirements of 9Ni steel for LNG ship storage tanks. It is compatible with flux-cored arc welding (FCAW) and can be widely used in various key welding positions such as flat fillet welding (2F), vertical fillet welding (3F) and vertical butt welding (3G) for LNG ship storage tanks. Moreover, by optimizing the specific ratio of mineral powder and metal powder in the flux-cored powder, the generation of harmful precipitated phases is effectively suppressed, while the slag removal and arc stability are improved. This effectively overcomes the technical defects of existing welding wires in LNG ship welding, which make it difficult to balance welding processability and mechanical properties, and significantly improves the welding quality and service safety of LNG ship storage tanks. Attached Figure Description

[0067] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0068] Figure 1 This is a weld morphology diagram at position 2F obtained in Embodiment 1 of the present invention;

[0069] Figure 2 This is a weld morphology diagram at position 3F obtained in Embodiment 1 of the present invention;

[0070] Figure 3 This is a weld morphology diagram of position 3G obtained in Embodiment 1 of the present invention;

[0071] Figure 4 The metallographic structure of the weld at position 3G obtained in Example 1 of the present invention is shown in the figure (scale bar is 20 μm).

[0072] Figure 5 The image shows the metallographic structure of the weld at position 3G obtained in Example 1 of the present invention (scale bar is 100μm).

[0073] Figure 6 This is a weld morphology diagram of position 3G obtained in Comparative Example 1 of the present invention. Detailed Implementation

[0074] The present invention will be further described below with reference to specific embodiments. First, it should be noted that the data in the following experimental examples were obtained by the inventors through numerous experiments. Due to space limitations, only a portion of these data is shown in the specification, and those skilled in the art can understand and implement the present invention based on this data. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various modifications or alterations to the invention, and these modifications or alterations also fall within the scope of protection of this application.

[0075] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0076] like Figures 1 to 6 As shown, the present invention provides a high-toughness nickel-based flux-cored welding wire for LNG ships, which is composed of nickel-based steel strip and flux powder filled inside the nickel-based steel strip. The amount of flux powder filling accounts for 20-25% of the total weight of the entire flux-cored welding wire.

[0077] The composition of the nickel-based steel strip conforms to the composition range of the C-276 alloy system. By weight percentage, the composition and weight fraction of each component of the nickel-based steel strip in this invention are as follows: C≤0.02%, Si≤0.2%, Mn≤1.0%, Cr:14.5~16.5%, Mo:15.0~17.0%, W:3.0~4.5%, Fe:4.0~7.0%, with the balance being Ni and unavoidable impurities.

[0078] The core powder of the present invention comprises mineral powder and metal powder by weight percentage, with the mineral powder content being 47% to 62% and the metal powder content being 38% to 53% by weight percentage.

[0079] The mineral powder includes: 20-25% rutile, 2-5% albite, 5-7% sodium fluoroaluminate, 2-5% alumina, 5-10% silicon dioxide, 5-10% sodium titanate, and 3-8% potassium titanate;

[0080] The metal powders include: 10-16% chromium powder, 3-5% molybdenum powder, 3-5% tungsten powder, 4-8% ferrotitanium, and 17-20% nickel powder.

[0081] The particle size and composition requirements of each component in mineral powder and metal powder are shown in Tables 1 and 2:

[0082] Table 1. Particle size and composition requirements of each component in mineral powder

[0083]

[0084] Table 2. Particle size and composition requirements of each component in metal powder

[0085]

[0086] In this invention, by adjusting the formulation of mineral powder and metal powder in the core powder and the ratio of each component in the formulation, the synergistic optimization of welding process and comprehensive performance of weld metal is achieved. The specific approach is as follows:

[0087] (1) This invention strictly limits the molybdenum powder content in the flux core to a low level of 3-5%, and simultaneously uses a rutile-alumina-silica ternary composite slag system. The key to this combination is that the low molybdenum design thermodynamically reduces the precipitation of harmful brittle phases in the weld metal, controlling the area fraction of precipitated phases to below 1%; while the ternary slag system adjusts the basicity, viscosity, and solidification shrinkage characteristics of the slag, so that the slag shell generates a difference in thermal expansion coefficient and interfacial stress that matches the weld metal during the cooling process, achieving a slag removal rate of over 91%. Both are indispensable. Without the low molybdenum design, simply optimizing the slag system cannot solve the toughness degradation caused by precipitated phases; if there is only low molybdenum without the specific slag system of this invention, a high slag removal rate cannot be achieved under pure CO2 protection.

[0088] (2) This invention reduces the molybdenum content in the core powder while correspondingly increasing the chromium content to 10-16% and the tungsten content to 3-5%. The solid solution strengthening effect of chromium and tungsten powder compensates for the strength loss caused by the reduction in molybdenum, ensuring that the tensile strength of the weld metal is ≥690MPa. At the same time, 4-8% ferrotitanium is introduced, and the fine particles generated by its reaction serve as heterogeneous nucleation cores to refine the solidification structure of the weld and simultaneously achieve deep deoxidation and denitrification, improving the purity of the weld. As a result, the weld metal achieves high strength while having an elongation of ≥35% and an impact absorption energy of ≥50J at -196℃.

[0089] (3) Sodium fluoroaluminate, sodium titanate, and potassium titanate ionize to release Na at high temperatures in an electric arc. + K + Alkali metal ions reduce the ionization potential of the arc space, enabling the arc to burn stably even in a pure CO2 atmosphere; at the same time, these components regulate the surface tension of the molten slag and improve the droplet transition morphology.

[0090] This invention suppresses precipitated phases through low-molybdenum design, optimizes slag removal performance through ternary composite slag system, enhances arc stability through composite arc stabilization system, and combines synergistic strengthening with chromium, tungsten, and ferrotitanium to enable flux-cored wire to maintain stable molten pool behavior and excellent slag removal in 2F, 3F, and 3G multi-position welding, achieving Class I standard in X-ray flaw detection.

[0091] The preparation method of the flux-cored welding wire in this invention includes:

[0092] S1 core powder preparation

[0093] S11 Weigh the mineral powder and metal powder raw materials according to the formula ratio of the core powder;

[0094] S12 The weighed mineral powder is dried at 300~500℃ for 1.5~2.5h to completely remove the moisture from the mineral powder; the preferred drying temperature is 400℃ and the drying time is 2h.

[0095] S13 Then, the dried mineral powder and metal powder are put into a mixer and stirred for 20-50 minutes until they are evenly mixed; the preferred stirring time is 30 minutes.

[0096] After S14 is mixed, the mixture is kept at 120~200℃ for 1.5~3 hours to further remove residual moisture and ensure the dryness of the core powder. The preferred holding temperature is 160℃ for 2 hours.

[0097] S2 welding wire rolling

[0098] S21 uses nickel-based steel strip that conforms to the composition range of C-276 alloy series, and the strip is gradually rolled into a C-shaped section by shaped rolls;

[0099] S22 simultaneously fills the prepared core powder into the C-shaped profile via a conveyor belt. By precisely coordinating and controlling the powder feeding speed of the conveyor belt and the rolling speed of the C-shaped profile, the core powder filling rate is strictly controlled between 21-25%.

[0100] After S23 filling is completed, the C-shaped profile is further rolled into a wire with a completely closed overlap to ensure that the core powder does not leak. At this time, the wire diameter is 2.5~3.5mm, preferably 2.8mm.

[0101] S3 Drawing and Reduction: The rolled wire is passed through multiple drawing dies and drawing cans in sequence to complete the rough drawing and fine drawing processes in steps, and finally drawn into a finished welding wire with a diameter of 1.0~1.6mm. The preferred diameter of the finished welding wire is 1.2mm.

[0102] S4 disc packaging

[0103] S41 Perform the wire winding operation according to the established wire winding standard, requiring the slack diameter of the welding wire to be ≥800mm and the warp distance to be ≤20mm. The finished welding wire is evenly wound on the wire winding spool, and the weight of a single spool of welding wire is controlled to be about 5kg.

[0104] S42 then vacuum-packs the completed welding wire to effectively prevent it from getting damp and oxidizing.

[0105] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0106] Example 1

[0107] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 1 of Table 3. The nickel-based steel strip is a C-276 alloy nickel-based steel strip with a thickness of 0.4 mm and a width of 10 mm.

[0108] The flux-cored welding wire in this embodiment is prepared using the aforementioned preparation method of the present invention. The diameter of the nickel-based steel strip after rolling is 2.8 mm. The wire passes through 10 drawing dies sequentially, with the die-side apertures being 2.72 mm, 2.42 mm, 2.20 mm, 1.98 mm, 1.83 mm, 1.65 mm, 1.42 mm, 1.35 mm, 1.25 mm, and 1.18 mm respectively. The diameter of the finished welding wire after drawing is 1.2 mm, the winding relaxation diameter is 950 mm, the warp distance is 12 mm, and the weight of a single coil is 5.5 kg. Finally, a finished flux-cored welding wire with a diameter of Φ1.2 mm is obtained, with the flux powder filling amount being 21% of the total weight of the flux-cored welding wire.

[0109] Welding Tests: The flux-cored welding wire prepared in Example 1 was used for welding 9Ni steel plates, and welding tests were conducted at positions 2F, 3F, and 3G. Welding parameters: DC reverse polarity (DCEP) was used, the shielding gas was 100% CO2, the gas flow rate was 18 L / min, and the interpass temperature was controlled below 80℃. Specifically, at position 2F: welding current 180 A, welding voltage 28 V, welding speed 32 cm / min; at positions 3F and 3G: welding current 148 A, welding voltage 24 V, welding speed 13 cm / min.

[0110] Sample preparation: After welding, weld metal samples were prepared in accordance with GB / T25774.1-2023 standard.

[0111] Weld morphology: The weld morphology at positions 2F, 3F, and 3G obtained after welding in this embodiment is as follows: Figures 1 to 3 As shown, the weld surface exhibits no slag adhesion, demonstrates good slag removal, and displays an aesthetically pleasing morphology, free from defects such as hot cracks and porosity. Its metallographic structure is as follows: Figure 4 and Figure 5 As shown, the structure is dominated by austenitic dendrites, and the precipitated phases are small in size and discontinuous.

[0112] Performance testing methods:

[0113] Welding process performance testing:

[0114] Slag removal performance test: The test was conducted using the falling ball method according to GB / T25776-2010 standard. Ten minutes after the 3G welding was completed, a 100g steel ball was dropped from 1m above the steel plate and impacted the weld in a free-fall state. The flux slag shells that fell off the weld metal sample each time were collected and measured. The slag shells that did not fall off were cleaned up and weighed, and the slag removal rate was calculated.

[0115] Radiographic testing: X-ray testing was performed on the weld metal sample at position 3G according to GB / T3323.1-2019 standard, and the results were divided into four levels: I, II, III, and IV, with level I being qualified; according to NB / T47013.5-2015 standard, penetrant dyeing was used to detect whether there were obvious defects such as porosity on the surface of the weld metal sample at position 2F, and the results were divided into two levels: I and II.

[0116] Microstructure observation: The metallographic structure of the weld metal sample at position 3G was observed using a metallographic microscope. Twelve fields of view were taken for each weld to count the area fraction of precipitated phases and the average value was taken.

[0117] Mechanical property testing:

[0118] Tensile properties: The tensile strength, yield strength and elongation of the weld metal at position 3G were determined in accordance with GB / T228.1-2021 standard.

[0119] Bending performance: In accordance with GB / T2653-2008 standard, lateral bending, positive bending and back bending tests were conducted on the weld metal at position 3G under the conditions of D=4t and 180°, and the presence of cracks on the tensile surface was observed.

[0120] Low-temperature impact toughness: The impact absorption energy (KV2) of the weld metal at position 3G was determined at -196℃ according to GB / T229-2020 standard.

[0121] The performance test results are shown in Tables 4, 5 and 6.

[0122] Example 2

[0123] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Table 3, Example 2. The nickel-based steel strip size and material are the same as in Example 1.

[0124] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 23% of the total weight of the flux-cored wire.

[0125] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0126] The performance test results are shown in Tables 4, 5 and 6.

[0127] Example 3

[0128] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Table 3, Example 3. The nickel-based steel strip size and material are the same as in Example 1.

[0129] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 21% of the total weight of the flux-cored wire.

[0130] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0131] The performance test results are shown in Tables 4, 5 and 6.

[0132] Example 4

[0133] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Table 3, Example 4. The nickel-based steel strip size and material are the same as in Example 1.

[0134] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 20% of the total weight of the flux-cored wire.

[0135] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0136] The performance test results are shown in Tables 4, 5 and 6.

[0137] Example 5

[0138] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 5 of Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0139] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 25% of the total weight of the flux-cored wire.

[0140] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0141] The performance test results are shown in Tables 4, 5 and 6.

[0142] Example 6

[0143] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 6 of Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0144] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 21% of the total weight of the flux-cored wire.

[0145] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0146] The performance test results are shown in Tables 4, 5 and 6.

[0147] Example 7

[0148] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 7 of Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0149] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 21% of the total weight of the flux-cored wire.

[0150] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0151] The performance test results are shown in Tables 4, 5 and 6.

[0152] Example 8

[0153] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 8 of Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0154] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 21% of the total weight of the flux-cored wire.

[0155] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0156] The performance test results are shown in Tables 4, 5 and 6.

[0157] Example 9

[0158] This embodiment provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Example 9 of Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0159] The preparation method of the flux-cored wire in this embodiment is the same as that in Embodiment 1. The difference is that the amount of flux-cored powder is 21% of the total weight of the flux-cored wire.

[0160] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0161] The performance test results are shown in Tables 4, 5 and 6.

[0162] Comparative Example 1

[0163] This comparative example provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Comparative Example 1 in Table 3. The nickel-based steel strip size and material are the same as those in Example 1.

[0164] The preparation method of the flux-cored welding wire in this comparative example is the same as that in Example 1, except that the amount of flux-cored powder filling is 21% of the total weight of the flux-cored welding wire.

[0165] The welding test and sample preparation methods were the same as in Example 1.

[0166] Weld morphology: The weld morphology at position 3G obtained after welding in this comparative example is as follows. Figure 6 As shown, the weld surface exhibits severe slag adhesion, poor slag removal, and deep fish-scale patterns.

[0167] Performance testing method: Same as in Example 1.

[0168] The performance test results are shown in Tables 4, 5 and 6.

[0169] Comparative Example 2

[0170] This comparative example provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Comparative Example 2 in Table 3. The nickel-based steel strip size and material are the same as those in Example 1.

[0171] The preparation method of the flux-cored welding wire in this comparative example is the same as that in Comparative Example 1.

[0172] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0173] The performance test results are shown in Tables 4, 5 and 6.

[0174] Comparative Example 3

[0175] This comparative example provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Comparative Example 3 in Table 3. The size and material of the nickel-based steel strip are the same as those in Example 1.

[0176] The preparation method of the flux-cored welding wire in this comparative example is the same as that in Comparative Example 1.

[0177] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0178] The performance test results are shown in Tables 4, 5 and 6.

[0179] Comparative Example 4

[0180] This comparative example provides a high-toughness nickel-based flux-cored welding wire for LNG ships. The specific composition of its flux powder is shown in Comparative Example 4 in Table 3. The nickel-based steel strip size and material are the same as those in Example 1.

[0181] The preparation method of the flux-cored welding wire in this comparative example is the same as that in Comparative Example 1.

[0182] The welding test, sample preparation and performance testing methods are the same as in Example 1.

[0183] The performance test results are shown in Tables 4, 5 and 6.

[0184] The results are analyzed as follows:

[0185] (1) Comparison of component differences

[0186] The composition of the flux core powder in Examples 1 to 9 is within the preferred range defined by this invention: the alumina content is 3-5%, the silica content is 5-8%, the molybdenum powder content is 3-5%, the chromium powder content is 10-16%, the tungsten powder content is 3-5%, and the ferrotitanium content is 4-8%. The comparative examples deviate from the preferred range of this invention in terms of the above key components: the alumina content in Comparative Example 1 is as high as 10%; the silica content in Comparative Example 2 is as high as 12%; the molybdenum powder content in Comparative Example 3 is as high as 10%; and the chromium powder content in Comparative Example 4 is as high as 18% and the molybdenum powder content is as high as 12%. These differences in composition directly lead to significant changes in welding process performance and weld metal mechanical properties.

[0187] (2) Welding process performance analysis

[0188] As shown in Table 4, the slag removal rate of the weld after welding at position 3G in all embodiments was higher than 91%, reaching a maximum of 97%, and the X-ray and 2F penetrant testing results of all welds were at the highest level I standard. This indicates that the present invention, through the synergistic effect of the ternary composite slag system composed of rutile, alumina, and silica, significantly reduced the interfacial bonding force between the molten slag and the weld metal, achieving excellent slag removal performance while ensuring the internal quality of the weld.

[0189] In comparison, the slag removal rates of each comparative example were significantly lower. Comparative Example 1, due to excessive alumina addition, had a slag removal rate of only 23%; Comparative Example 2, due to excessive silica addition, had a slag removal rate of only 45%; and Comparative Example 4, with excessively high chromium and molybdenum powder contents, saw its slag removal rate drop to 75%. The reason for this is that excessive alumina or silica disrupts the physical balance of the ternary slag system, causing the molten slag to form a dense spinel structure on the weld metal surface during cooling, making the slag shell difficult to remove. Furthermore, the excessive chromium and molybdenum powder in Comparative Example 4 alters the chemical composition and basicity of the molten slag, further deteriorating the slag removal performance. Incomplete slag removal leads to residual weld slag being drawn into subsequent weld pools during multiple welding passes, forming slag inclusion defects. Ultimately, this causes the flaw detection levels of Comparative Examples 1, 2, and 4 to drop to Level III, IV, and II, respectively, and the 2F penetrant testing to drop to Level II, failing to meet the welding quality requirements for LNG ship storage tanks.

[0190] (3) Precipitation phase control analysis

[0191] As shown in Table 5, the area fraction of precipitated phases in each comparative example is significantly higher than 1%. Comparative Example 1 is 1.45%, Comparative Example 2 is 1.2%, and Comparative Examples 3 and 4 are even higher at 2.27% and 2.36%, respectively. In contrast, the area fraction of precipitated phases in the weld metal of each embodiment is less than 1%, ranging from 0.64% to 1.00%.

[0192] This is because the molybdenum powder content in the flux cores of Comparative Examples 3 and 4 is as high as 10% to 12%, which far exceeds the strict limit of 3% to 5% in this invention, resulting in a sharp increase in the local molybdenum concentration in the weld pool and the precipitation of a large amount of harmful intermetallic compounds. Although the molybdenum content in the flux cores of Comparative Examples 1 and 2 is within the range of this invention, the excessive alumina or silica disrupts the metallurgical balance of the slag, indirectly leading to an increased tendency for precipitation during the solidification of the weld, and an abnormal increase in the content of precipitated phases also occurs.

[0193] However, this invention does not simply reduce the molybdenum content in the flux core. Instead, it effectively suppresses the driving force for the precipitation of harmful brittle intermetallic compounds by strictly controlling the molybdenum powder content in the flux core to a low level of 3-5%. At the same time, it relies on the uniformly distributed molybdenum element in the C-276 nickel-based steel strip to provide basic solid solution strengthening, and the optimized ternary composite slag system ensures the metallurgical purity of the weld. Ultimately, without sacrificing the weld strength, it achieves precise and efficient control of harmful precipitated phases.

[0194] (4) Mechanical property analysis

[0195] As shown in Table 6, the tensile strength of the 3G weld metal in all embodiments and comparative examples is higher than 690 MPa, and the yield strength is higher than 430 MPa, far exceeding the requirements of classification societies such as DNV and BV for tensile strength Rm > 600 MPa and yield strength Rp0.2 > 375 MPa. This indicates that the present invention can still ensure sufficient strength under the low-molybdenum design of the flux-cored powder through the solid solution strengthening effect of chromium powder and tungsten powder.

[0196] Regarding plasticity and low-temperature toughness, the weld metal elongation obtained in each embodiment was ≥35%, the impact absorption energy at -196℃ was 53~73J, and no cracks appeared after 180° bending. In contrast, the elongation of each comparative example was 21%~32%, all below 35%, and the impact absorption energy at -196℃ was 26~39J, all below 50J. Some comparative examples even failed to meet the minimum impact absorption energy at -196℃ (34J) required by the classification society. Comparative examples 3 and 4 showed cracks and even fractures during bending.

[0197] This is because in Comparative Examples 1 and 2, the excessive alumina or silica led to incomplete slag removal and the introduction of slag inclusions, which indirectly deteriorated the plasticity and toughness. In Comparative Examples 3 and 4, the molybdenum content was as high as 10% to 12%, which led to the formation of a large amount of harmful precipitates, directly causing serious deterioration of plasticity and low-temperature toughness.

[0198] The significant differences in the aforementioned mechanical properties indicate that this invention, through the synergistic effect of the triple mechanism of "low molybdenum inhibiting precipitates + chromium-tungsten composite solid solution strengthening + titanium-iron fine grain purification," enables the weld metal to simultaneously achieve high strength, high plasticity, and excellent low-temperature toughness, thus solving the common industry contradiction that "improvement in mechanical properties inevitably leads to a decline in welding processability" in traditional nickel-based welding wires.

[0199] Table 3. Composition ratio of the herbal core powder in each embodiment and comparative example (wt.%)

[0200]

[0201] Table 4. Slag removal rate and weld flaw detection results for each embodiment and comparative example.

[0202]

[0203] Table 5. Statistics of precipitate area fraction in 3G welds of various embodiments and comparative examples.

[0204]

[0205] Table 6. Test results of mechanical properties of 3G weld metals in each embodiment and comparative example.

[0206]

[0207] In summary, this invention provides a high-toughness nickel-based flux-cored welding wire for LNG ships and its preparation method. The flux-cored powder of this welding wire utilizes a low-molybdenum design to suppress harmful precipitates, high-chromium and tungsten solid solution strengthening to compensate for strength loss, and titanium-iron microalloying to refine grains and improve purity. Simultaneously, it incorporates a rutile-alumina-silica ternary slag system to optimize slag removal performance and a sodium fluoroaluminate-sodium titanate-potassium titanate composite arc-stabilizing system to enhance arc stability. This allows the welding wire to be stably applied to 2F, 3F, and 3G multi-position welding under pure CO2 shielding gas, achieving a weld slag removal rate exceeding 91%, X-ray and penetrant testing meeting Class I standards, and a weld metal tensile strength of not less than 690 MPa, elongation of not less than 35%, and impact absorption energy at -196℃ of not less than 50 J, fully meeting the stringent welding requirements of 9Ni steel for LNG ship storage tanks.

[0208] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various alterations and modifications without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A high toughness nickel-based flux cored wire for LNG carriers, characterized by, The flux-cored welding wire is composed of nickel-based steel strip and flux powder filled therein. By weight percentage, the flux powder contains 47%~62% mineral powder and 38%~53% metal powder. The weight fraction of each component in the mineral powder and metal powder is as follows: Mineral powder: rutile 20-25%, albite 2-5%, sodium fluoroaluminate 5-7%, alumina 2-5%, silicon dioxide 5-10%, sodium titanate 5-10%, potassium titanate 3-8%; Metal powders: 10-16% chromium powder, 3-5% molybdenum powder, 3-5% tungsten powder, 4-8% ferrotitanium, and 17-20% nickel powder.

2. The flux-cored welding wire according to claim 1, characterized in that, The weight percentages of each component in the nickel-based steel strip are as follows: C≤0.02%, Si≤0.2%, Mn≤1.0%, Cr:14.5~16.5%, Mo:15.0~17.0%, W:3.0~4.5%, Fe:4.0~7.0%, with the balance being Ni and unavoidable impurities.

3. The flux-cored welding wire according to claim 2, characterized in that, The amount of the flux-cored powder is 20% to 25% of the total weight of the flux-cored welding wire.

4. The flux-cored welding wire according to claim 3, characterized in that, The flux-cored welding wire uses 100% CO2 as the shielding gas for welding.

5. A preparation method for preparing the flux-cored welding wire as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Preparation of S1 core powder; S2 welding wire rolling; S3 drawing reduction; S4 swivel packaging.

6. The preparation method according to claim 5, characterized in that, In step S1, the preparation of the core powder includes: S11 Weigh the mineral powder and metal powder raw materials according to the formula ratio of the core powder; S12. The weighed mineral powder is dried at 300~500℃ for 1.5~2.5h; S13 then puts the dried mineral powder and metal powder into a mixer and stirs for 20-50 minutes until they are evenly mixed; After S14 is mixed, the mixture is kept at 120~200℃ for 1.5~3 hours.

7. The preparation method according to claim 5, characterized in that, In step S2, the rolling of the flux-cored welding wire includes: S21 rolls nickel-based steel strip into a grooved strip; S22 is filled with core powder in the grooved strip, and the filling rate is controlled between 20% and 25%. After S23 filling is completed, the grooved strip is rolled into a closed wire to ensure that the core powder does not leak.

8. The preparation method according to claim 5, characterized in that, In step S3, the drawing reduction of the flux-cored wire includes: The rolled closed wire is drawn in multiple passes to gradually reduce its diameter to the target diameter, which is 1.0~1.6mm.

9. The application of a flux-cored welding wire as described in any one of claims 1 to 4, characterized in that, The flux-cored welding wire is used for welding 9Ni steel for LNG ship storage tanks, and the welding positions include flat fillet welds, vertical fillet welds, and vertical butt welds.

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