Welding wire suitable for 9Ni steel submerged-arc welding and welding method

Abstract

The invention relates to a welding wire suitable for 9Ni steel submerged-arc welding and a welding method. The welding wire comprises the following chemical components: 0.14-0.32% of C, 12-20% of Mn, 6.5-14% of Ni, 10-18% of Cr, 0.03-0.25% of V, 3-6.5% of Mo, 0.15-0.5% of N, less than or equal to 0.01% of P, less than or equal to 0.008% of S and the balance of iron and inevitable impurities. According to the welding method, a 2.4 mm welding wire is adopted, submerged-arc welding is selected as the welding method, a groove is in a K type, the welding current is 320-360 A, and the arc voltage is 26-30 V; the yield strength of cladding metal produced through the welding wire is larger than or equal to 400 MPa, the tensile strength is larger than or equal to 640 MPa, and the ductility is larger than or equal to 25%; the tensile strength of a welded joint is larger than or equal to 640 MPa, and the low-temperature impact energy KV2 at-196 DEG C is larger than or equal to 34

Description

Technical Field

[0001] The present invention relates to a welding wire and a welding method suitable for submerged arc welding of 9Ni steel, belonging to the field of arc welding. Background Art

[0002] With the continuous adjustment and optimization of the global energy structure, natural gas, as a clean and efficient energy resource, has gradually emerged and shown a continuous growth trend. In recent years, energy research institutions at home and abroad have predicted that within the next few decades, the proportion of natural gas in global primary energy consumption will continue to rise. By 2030, the production and consumption of natural gas are expected to exceed those of oil and become the main energy form. This development trend is mainly due to the significant advantages of natural gas in environmental protection, energy security, and efficient utilization. Therefore, to meet the huge demand for natural gas storage, transportation, and reception, the liquefied natural gas (LNG) industry has entered a period of rapid development.

[0003] The core links of the LNG industrial chain include liquefaction, transportation, storage, and regasification. Especially during the storage process of LNG, the application of low-temperature steel is crucial. The safety, durability, and economy of LNG storage tanks highly depend on the properties of the selected steel. Currently, 9Ni steel has become the preferred material for constructing equipment such as LNG storage tanks and tankers due to its excellent toughness, high strength, and low thermal expansion coefficient at low temperatures. However, as a submerged arc welding material for supporting welding, its cost has become the main bottleneck in the industry.

[0004] The existing industry mainly uses ERNiCrMo-4 and ERNiCrMo-3 submerged arc welding wires with a Ni content of about 65%. However, due to the high raw material cost of such welding materials, the price is as high as 600,000 - 650,000 yuan per ton, which severely restricts the cost optimization of the industry. The market demand for welding materials supporting 9Ni steel has been steadily increasing, with an annual demand of approximately 4,500 tons. All domestic welding materials of this type are dependent on imports. Most of the welding materials in the domestic R & D trial stage are Ni-Cr-Mo systems with a Ni content of about 65% similar to imports. Their performance is unstable and the cost is also very high, making it difficult to achieve true domestic substitution.

[0005] In recent years, there have also been patents specifically designed for welding materials made of 9Ni steel. Patent CN201310432774.7 discloses a fully automatic submerged arc welding wire for 9Ni steel, with a Mo content of 15-25%, a W content of 2-6%, an Al content of 0.1-0.5%, and the balance being Ni. Patent CN201310433946.2 discloses a flux-cored welding wire suitable for welding LNG storage tanks, with a Mo content of 5-10%, a Ti content of 0.05-0.5%, a rare earth element content of 0.005-0.05%, and the balance being Ni. These patented welding materials adopt a design concept of reducing Cr content and incorporating microalloying and rare earth elements based on traditional nickel-based alloy welding materials. However, because the Ni content is still above 60%, the production cost remains high, and the cost reduction effect is very limited. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention proposes a welding wire and welding method suitable for submerged arc welding of 9Ni steel.

[0007] The technical solution of this invention to solve the above technical problems is as follows: a welding wire suitable for submerged arc welding of 9Ni steel is provided, the chemical composition of which is as follows: C: 0.14-0.32%, Mn: 12-20%, Ni: 6.5-14%, Cr: 10-18%, V: 0.03-0.25%, Mo: 3-6.5%, N: 0.15-0.5%, P≤0.01%, S≤0.008%, with the balance being iron and unavoidable impurities.

[0008] The technical solution further defined in this invention is as follows:

[0009] Preferably, its chemical composition, by weight percentage, further includes: B: 0.008–0.12%, Nb: 0.03–0.25%, and Ti: 0.03–0.2%.

[0010] Preferably, its chemical composition is as follows: C: 0.31%, Mn: 17%, Ni: 10.78%, Cr: 14.3%, V: 0.2%, Mo: 5.7%, N: 0.32%, P≤0.01%, S≤0.008%, with the balance being Fe and unavoidable impurities.

[0011] This invention provides a welding method for a welding wire suitable for submerged arc welding of 9Ni steel, the welding method specifically including:

[0012] A 2.4mm diameter welding wire was used, and submerged arc welding was selected as the welding method.

[0013] Welding 9Ni steel plates with a thickness of 15-50mm, with a K-type bevel;

[0014] The welding current is 320-360A, the arc voltage is 26-30V, the bevel angle on one side is 45°, the other side is a straight edge, the welding speed is 30-50cm / min, and the welding heat input is 10-20kJ / cm.

[0015] Preferably, after the submerged arc welding is completed, the yield strength of the cladding metal is ≥400MPa, the tensile strength is ≥640MPa, and the elongation is ≥25%; the tensile strength of the welded joint is ≥640MPa, the low-temperature impact energy at -196℃ is ≥34J, and the side bend of the welded joint is d=4a, 180°, all four of which are qualified.

[0016] Preferably, the 9Ni steel comprises the following components in weight percentages: C: ≤0.08%, Mn: 0.3%–0.8%, Si: 0.15%–0.35%, S: ≤0.003%, P: ≤0.008%, Ni: 8.50%–10%, Mo: ≤0.1%, Cr: ≤0.25%, Cu: ≤0.35%, V: ≤0.01%, Nb: ≤0.08%, with the balance being Fe and unavoidable impurities.

[0017] Preferably, the 9Ni steel has a yield strength ≥550MPa, a tensile strength of 680~820MPa, an elongation ≥18%, and an impact energy KV2 ≥80J at -196℃.

[0018] In this invention, carbon (C) has a significant interstitial solid solution strengthening effect, which can greatly improve the strength of the weld metal. In addition, carbon can lower the martensite transformation temperature, thereby improving the stability of austenite and ensuring low-temperature impact toughness at -196℃. However, if the carbon content is too low, it cannot provide sufficient solid solution strengthening effect, causing the weld strength to fail to meet the standard requirements. If the carbon content is too high, it will lead to a decrease in the steel plasticity of the weld metal, thereby reducing the impact toughness and affecting the bending performance. Excessive carbon content will also cause carbon to diffuse into the heat-affected zone during welding, increasing the hardness and brittleness of the heat-affected zone and increasing the tendency of the weld to crack. Therefore, this invention controls the carbon content within the optimal range of 0.14% to 0.32%.

[0019] To further compensate for the reduced austenite stability caused by the decrease in alloying elements, this invention requires the addition of other austenite stabilizing elements. Mn, as an efficient and inexpensive Ni substitute, has been widely used in the production of various nickel-saving stainless steels to stabilize the austenite phase in the weld metal. Simultaneously, the addition of Mn can provide solid solution strengthening, improving the strength of the austenitic weld metal. Too low a Mn level cannot achieve the desired austenite phase stabilization and provides sufficient solid solution strengthening, while too high a Mn level can easily lead to Mn vapor volatilization during welding, severely impacting stability. This process harms welders' health and causes environmental pollution. Experiments show that when the Mn content is higher than 22%, the volatilization of Mn becomes significant after degassing the molten weld metal under a vacuum of 3 mbar, with its mass percentage decreasing by more than 1%. In addition, excessively high Mn content reduces the rigidity and ductility of the weld metal, affecting the weld's bending performance. Therefore, this invention controls the Mn content at 12-20%, which can significantly reduce Mn vapor volatilization during the welding process, improve the field applicability of the welding wire, and provide the required austenitic weld stability and strength.

[0020] This invention employs an alloy design concept that substitutes Ni with Mn to achieve a stable austenitic structure. Ni is beneficial for the activation of the low-temperature glide system and can increase the strength of cleavage planes, thereby improving low-temperature toughness. Experiments show that in the Cr-Mn-N austenitic system, the complete absence of nickel will decrease the low-temperature toughness of the weld. When the nickel content reaches 6% or more, the impact toughness at -196℃ begins to show a significant improvement. However, Ni is extremely expensive, and adding a large amount of Ni will significantly increase the smelting and production cost of the welding wire. On the other hand, the addition of Ni will affect the solubility of N. Therefore, in this invention, the Ni content is controlled at 6.5% to 14%. Compared with traditional imported nickel-based alloy welding materials, the Ni content of the welding wire of this invention is reduced by more than 80%, significantly reducing the smelting and production cost of the welding wire.

[0021] In this invention, the addition of Cr can lower the martensitic transformation temperature and stabilize austenite at low temperatures. In addition, the addition of Cr can increase the solubility of N, which is beneficial when combined with the addition of N in the welding wire. However, excessive Cr content will increase the tendency of coarse grain boundary carbonitride precipitation and intermetallic compound precipitation, resulting in a decrease in the impact toughness of the weld metal. Therefore, the Cr content is controlled at 10-18% in this invention.

[0022] In this invention, the addition of vanadium (V) promotes the formation of vanadium carbides and vanadium nitrides. These fine precipitates, distributed at grain boundaries and within grains, can hinder dislocation movement and increase strength. Furthermore, the addition of vanadium can prevent grain coarsening, refining austenite grains and thus improving both the strength and toughness of the weld metal. Moreover, V is a powerful substitution solid solution strengthening element, enhancing the strength of the austenite structure. A V content of 0.03% or higher can be effective in this invention; however, excessively high V content can lead to the precipitation of large amounts of carbonitrides, which can easily form crack initiations, impairing the low-temperature toughness of the weld metal and negatively impacting its resistance to cracking under stress. Therefore, the V content in this invention is 0.03–0.25%.

[0023] The addition of Mo in this invention can achieve a significant solid solution strengthening effect. Mo is the most effective substitutional metal element for solid solution strengthening of austenite in commonly used alloys, with a noticeable effect at concentrations above 3%. Simultaneously, Mo can lower the martensitic transformation temperature and stabilize the crystal structure of austenite at low temperatures. However, excessive Mo content can easily form intermetallic compounds with Cr during the welding thermal cycle cooling process, especially when Mo + Cr ≥ 23%. Furthermore, excessive Mo content can induce the precipitation of coarse carbides at grain boundaries, reducing grain boundary strength and low-temperature impact toughness. Therefore, the Mo content in this invention is controlled at 3–6.5%, resulting in a cost reduction of over 70% compared to imported conventional nickel alloys.

[0024] In this invention, the addition of nitrogen (N) significantly enhances the strength of austenitic welds. Nitrogen is the non-metallic element with the best interstitial solid solution strengthening effect in austenitic structures; the addition of more than 0.1% N can achieve a significant solid solution strengthening effect. Unlike carbon (C), N increases the strength of austenite without affecting its low-temperature toughness or the rigidity and ductility of the weld metal. Another function of N is to lower the martensitic transformation temperature, thus stabilizing the austenitic structure. In addition, N can form fine nitrides with microalloys (V, Nb, Ti), refining austenite grains and promoting precipitation strengthening. However, excessive N addition can cause nitrogen gas to be released in gaseous form at high temperatures during welding. Incomplete gas removal or excessively rapid gas generation during welding can lead to porosity, affecting weld quality. Therefore, the N content in this invention is controlled at 0.15–0.5%.

[0025] This invention requires strict control of the content of sulfur (S) and phosphorus (P). Excessive content of these two elements can easily lead to the segregation of low-melting-point eutectics at grain boundaries, resulting in a high susceptibility to hot cracking in the weld. Furthermore, P and S can cause grain boundary embrittlement, severely affecting the plasticity and low-temperature toughness of the weld metal. Therefore, this invention requires P ≤ 0.01% and S ≤ 0.008% to reduce the impact of P and S on weld formation and mechanical properties.

[0026] In this invention, 0.03–0.2% Ti can be added to refine the austenite grains and increase strength through the precipitation of fine precipitates; 0.03–0.25% Nb can also be added to refine the austenite grains and increase strength through the precipitation of fine precipitates; in addition, 0.008–0.12% B can be added to improve the strength of the weld metal through the precipitation of borides.

[0027] Beneficial effects:

[0028] This invention employs an alloy design that incorporates carbon, nitrogen, manganese, and vanadium elements in combination with chromium, molybdenum, and nickel, significantly reducing the content of heavy metals such as nickel, molybdenum, and tungsten in the welding wire, thereby substantially lowering the production cost of the welding wire. The C-Mn-Cr-Ni-Mo-N composition design exhibits excellent compatibility with 9Ni steel base material, providing sufficient strength while ensuring a stable austenitic weld structure at low temperatures. Compared to traditional imported nickel-based alloy welding materials, it has a simpler composition and lower smelting difficulty, making it suitable for large-scale welding wire manufacturing. Furthermore, the welded joint formed by this submerged arc welding wire achieves excellent comprehensive mechanical properties, with strength and bending performance at room temperature superior to imported traditional nickel-based alloy welding materials. Nickel costs are reduced by more than 80%, resulting in significant cost reduction. This can substantially reduce the welding cost of 9Ni steel used in the construction of equipment such as liquefied natural gas storage tanks, while its weld impact toughness meets standard specifications at -196℃. Attached Figure Description

[0029] Figure 1 This is a diagram of the welded joint bending test specimen of Embodiment 1 of the present invention;

[0030] Figure 2 The image shows a sample of a welded joint bending test specimen for ERNiCrMo-4.

[0031] Figure 3 This is a sample image of an ERNiCrMo-3 welded joint bending test specimen. Detailed Implementation

[0032] Example 1

[0033] This embodiment provides a welding wire suitable for submerged arc welding of 9Ni steel, with the following chemical composition: C: 0.31%, Mn: 17%, Ni: 10.78%, Cr: 14.3%, V: 0.2%, Mo: 5.7%, N: 0.32%, P≤0.004%, S≤0.002%, with the balance being Fe and unavoidable impurities.

[0034] The welding method is as follows: The diameter of the welding wire used is 2.4mm, and the welding is carried out by submerged arc welding. The welding plate is a 32mm thick 9Ni steel plate. The bevel of the 9Ni steel plate is K type, with a bevel angle of 45° on one side and a straight edge on the other side. AvestaC 806 flux is used, the welding current is 320-360A, the welding voltage is 26-30V, the welding speed is 30-50cm / min, and the welding heat input is 10-20kJ / cm.

[0035] The aforementioned 9Ni steel comprises the following components in weight percentages: C: ≤0.08%, Mn: 0.3%–0.8%, Si: 0.15%–0.35%, S: ≤0.003%, P: ≤0.008%, Ni: 8.50%–10%, Mo: ≤0.1%, Cr: ≤0.25%, Cu: ≤0.35%, V: ≤0.01%, Nb: ≤0.08%, with the balance being Fe and unavoidable impurities.

[0036] To compare the performance of the welding wire of this invention with that of traditional imported welding wire, two types of welding wires, ERNiCrMo-3 and ERNiCrMo-4, produced by ESAB, were selected and welded using the same welding process.

[0037] The microstructure of the weld metal is entirely austenitic, with no hot cracks, reheat cracks, or cold cracks observed. Tensile tests on the weld wire cladding metal and weld joints were conducted according to GB / T 2652-2008; Charpy pendulum impact tests at -196℃ were conducted according to GB / T229-2020; side bending tests were conducted according to GB / T2653-2008; detailed mechanical properties are shown in Tables 1-3.

[0038] Table 1: Impact Performance of Welded Joints

[0039]

[0040]

[0041] Table 2: Tensile and Lateral Bending Properties of Welded Joints

[0042]

[0043] Table 3: Tensile Properties of Welding Wire Cladding Metal

[0044]

[0045] As can be seen, the welded joint obtained in the embodiment exhibits excellent comprehensive mechanical properties. The yield strength of the cladding metal is ≥400MPa, tensile strength ≥640MPa, elongation ≥25%, impact energy (KV2) of the weld and heat-affected zone at -196℃ is ≥34J, tensile strength (Rm) of the welded joint is ≥640MPa, and the side bending (d=4a) of the welded joint at 180° is also satisfactory. All four samples passed the tests, meeting the requirements of the specifications and standards. The welding wire provided by this patent meets the mechanical properties of the specifications and standards, and its tensile properties of the cladding metal, tensile properties of the welded joint, and bending properties are superior to those of traditional nickel-based alloy welding materials, while significantly reducing production costs.

[0046] This invention provides a low-cost, high-performance, and highly practical submerged arc welding wire for 9Ni steel. The weld joint exhibits excellent strength, bending performance, and low-temperature impact resistance, meeting the welding requirements for 9Ni steel used in the construction of equipment such as liquefied natural gas storage tanks. In addition to the above embodiments, this invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by this invention.

Claims

1. A welding wire suitable for submerged arc welding of 9Ni steel, characterized in that, Its chemical composition is as follows: C: 0.14-0.32%, Mn: 12-20%, Ni: 6.5-14%, Cr: 10-18%, V: 0.03-0.25%, Mo: 3-6.5%, N: 0.15-0.5%, P≤0.01%, S≤0.008%, with the balance being iron and unavoidable impurities.

2. The welding wire suitable for submerged arc welding of 9Ni steel according to claim 1, characterized in that: Its chemical composition, by weight percentage, also includes: B: 0.008–0.12%, Nb: 0.03–0.25%, and Ti: 0.03–0.2%.

3. The welding wire suitable for submerged arc welding of 9Ni steel according to claim 1, characterized in that: Its chemical composition is as follows: C: 0.31%, Mn: 17%, Ni: 10.78%, Cr: 14.3%, V: 0.2%, Mo: 5.7%, N: 0.32%, P ≤ 0.01%, S ≤ 0.008%, with the balance being Fe and unavoidable impurities.

4. A welding method for a welding wire suitable for submerged arc welding of 9Ni steel according to any one of claims 1-3, characterized in that: The welding methods specifically include: A 2.4mm diameter welding wire was used, and submerged arc welding was selected as the welding method. Welding 9Ni steel plates with a thickness of 15-50mm, with a K-type bevel; The welding current is 320-360A, the arc voltage is 26-30V, the bevel angle on one side is 45°, the other side is a straight edge, the welding speed is 30-50cm / min, and the welding heat input is 10-20kJ / cm.

5. The welding method for a welding wire suitable for submerged arc welding of 9Ni steel according to claim 4, characterized in that: After the submerged arc welding is completed, the yield strength of the cladding metal is ≥400MPa, the tensile strength is ≥640MPa, and the elongation is ≥25%; the tensile strength of the welded joint is ≥640MPa, the low-temperature impact energy at -196℃ is ≥34J, and the side bend of the welded joint is d=4a, 180°, all four of which are qualified.

6. The welding method for a welding wire suitable for submerged arc welding of 9Ni steel according to claim 4, characterized in that: The 9Ni steel comprises the following components in weight percentages: C: ≤0.08%, Mn: 0.3%~0.8%, Si: 0.15%~0.35%, S: ≤0.003%, P: ≤0.008%, Ni: 8.50%~10%, Mo: ≤0.1%, Cr≤0.25%, Cu≤0.35%, V≤0.01%, Nb≤0.08%, with the balance being Fe and unavoidable impurities.

7. The welding method for a welding wire suitable for submerged arc welding of 9Ni steel according to claim 4, characterized in that: The 9Ni steel has a yield strength ≥550MPa, tensile strength 680~820MPa, elongation ≥18%, and impact energy KV2≥80J at -196℃.

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