Welding wire for invar steel die welding and preparation method and welding method thereof

Abstract

The invention discloses a welding wire for invar steel die welding and a preparation method and a welding method of the welding wire, and belongs to the technical field of metal welding materials and preparation of the metal welding materials. The problem that hot cracks exist in weld joints of existing invar steel welding materials is solved. According to the invar steel welding wire of the Fe-Ni alloy system, NbC and TiC are added in the vacuum induction smelting process of the welding wire, fine dispersed distribution of carbides is guaranteed, meanwhile, the mole number ratio of Nb to Ti to C is set to be (Nb + Ti): C = 1: 1, and the content of Nb in the welding wire is 1.4%-1.8%, and the content of Ti in the welding wire is 0.10%-0.25%. In the high-temperature stage of the welding process, the dispersively-distributed carbides have a pinning effect on the grain boundary, sliding of the grain boundary can be effectively hindered, the strength of the grain boundary at the high temperature is improved, generation of hot cracks is reduced, and therefore the hot crack sensitivity of a weld joint is reduced.

Description

Technical Field

[0001] This invention relates to a welding wire for welding Invar steel molds, its preparation method, and welding method, belonging to the field of metal welding materials and their preparation technology. Background Technology

[0002] Invar steel is an iron-nickel alloy with a Ni content of 36%, widely used in LNG (liquefied natural gas) carriers and aerospace composite material molding dies. The manufacture of composite parts typically involves hot pressing on molds, requiring extremely high mold quality. Invar steel has an extremely low coefficient of thermal expansion, similar to that of composite materials, and its coefficient remains essentially unchanged with temperature variations within a range of -250°C to 250°C. This ensures the dimensional accuracy of the molded composite parts, making Invar steel the optimal mold material for manufacturing high-quality, complex, and precision composite parts.

[0003] With the increasing demand for large-size composite material parts, the demand for Invar steel molds is also growing. Large-size composite material parts require large-structure molding dies, and large Invar steel molds are difficult to form in a single process. Therefore, welded structures are necessary for manufacturing. Invar steel molds are thick-plate welds, requiring multi-layer, multi-pass welding. Currently, the most common welding method is non-consumable inert gas welding (TIG welding), using Invar steel welding wire of the same material as the welding consumable. However, due to the low thermal conductivity of Invar steel and the poor fluidity and wettability of the liquid weld metal, welding defects such as hot cracking and porosity often occur during the welding process. These defects severely affect the mechanical properties of the weld joint and the reliability of the welded structure. Furthermore, the surface quality and airtightness of the mold are significantly reduced due to these defects. Therefore, given the urgent need for welding Invar steel molds, it is crucial to optimize Invar steel welding consumables to improve the hot crack resistance of the weld, thereby improving the welding quality of Invar steel molds. Summary of the Invention

[0004] To address the problem of hot cracking in the weld seams of existing Invar steel welding materials, this invention provides a welding wire for welding Invar steel molds, its preparation method, and welding method.

[0005] The technical solution of this invention: One objective of this invention is to provide a welding wire for welding Invar steel molds, the chemical composition of which is expressed as a percentage by weight: C 0.20~0.25%, 0<Si≤0.1%, Mn 0.4~0.6%, S≤0.001%, P≤0.002%, Ni 35.5~36.5%, Nb 1.4~1.8%, Ti 0.10~0.25%, with the balance being Fe and impurities; and the ratio of the total molar number of Nb and Ti to the molar number of C is 1:1.

[0006] Further specifying, the chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.24%, Si 0.08%, Mn 0.53%, S 0.0007%, P 0.0018%, Ni 35.95%, Nb 1.41%, Ti 0.23%, with the balance being Fe and impurities.

[0007] Further specifying, the chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.24%, Si 0.07%, Mn 0.52%, S 0.0008%, P 0.0017%, Ni 36.19%, Nb 1.53%, Ti 0.16%, with the balance being Fe and impurities.

[0008] Further specifying, the chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.25%, Si 0.08%, Mn 0.45%, S 0.0008%, P 0.0016%, Ni 36.01%, Nb 1.67%, Ti 0.14%, with the balance being Fe and impurities.

[0009] A second objective of this invention is to provide a method for preparing the above-mentioned welding wire, the method comprising the following steps: (1) Vacuum induction smelting is carried out using electrolytic nickel, low-phosphorus pure iron and crystalline silicon as raw materials; (2) Refining is carried out after all the raw materials have been melted and cleared; (3) After refining, add niobium carbide and titanium carbide powder, stir for 15 minutes, add remelted manganese, and pour after melting to obtain steel ingots; (4) After removing the oxide scale from the surface of the steel ingot, it is forged and rolled to obtain wire rod; (5) The wire rod is subjected to rough drawing, bright annealing, fine drawing, wire cutting, cleaning and packaging in sequence to obtain welding wire.

[0010] Further specified, (2) the refining temperature is 1560~1580℃ and the time is 40~60min.

[0011] Further specified, the pouring temperature in (3) is 1510~1490℃.

[0012] Further specifying, (4) the wire rod specification is Φ5.5mm.

[0013] Further specified, (5) the welding wire specification is Φ2.4×1000mm.

[0014] The third objective of this invention is to provide an application of the above-mentioned welding wire, specifically for TIG welding of Invar steel molds.

[0015] The fourth objective of this invention is to provide a welding method for Invar steel molds. Specifically, the above-mentioned welding wire is used for welding with non-consumable electrode inert gas shielded arc welding. The current type is DCEN, the welding current is 160~200A, the shielding gas is Ar, the gas flow rate during welding is 15~20L / min, and the interpass temperature is less than 120℃.

[0016] Beneficial effects: (1) This invention provides Fe-Ni alloy-based Invar steel welding wire. The microstructure of the weld is austenitic, which is the same material as the base metal. This ensures that the tensile strength of the weld metal not only meets the requirements of the base metal but also has high plasticity and toughness. Test results show that the tensile strength of the weld metal is ≥400MPa, the yield strength is ≥300MPa, the elongation after fracture is ≥25%, and the room temperature impact absorption energy is ≥60J. It can be used to weld Invar steel molds, and the weld metal has excellent crack resistance, enabling high-quality welding. It is suitable for TIG welding, the welding process is stable, and the weld formation is aesthetically pleasing.

[0017] (2) The welding wire provided by the present invention strictly controls S+P+Si≤0.103%, which reduces the tendency of S, P and Si elements to form low melting point eutectic, and greatly reduces the hot cracking sensitivity of the weld.

[0018] (3) In the vacuum induction smelting process, NbC and TiC are added to the welding wire of the present invention. While ensuring the fine and dispersed distribution of carbides, the molar ratio of Nb, Ti and C is set to (Nb+Ti):C=1:1, and the Nb and Ti contents in the welding wire are controlled to be 1.4~1.8% and 0.10~0.25%, respectively. In the high-temperature stage of the welding process, these dispersed carbides pin the grain boundaries, which can effectively hinder grain boundary slip, improve the strength of the grain boundaries at high temperature, reduce the generation of hot cracks, and thus reduce the hot crack sensitivity of the weld. According to standard GB / T 13817-1992 "Test Method for Cracks in Rigidly Restrained Welded Butt Joints" and standard GB / T 41107.2-2021 "Destructive Testing of Welds in Metallic Materials - Hot Cracking Tests of Welded Parts - Arc Welding Methods - Part 2: Self-Restraint Tests", crack tests were conducted on the welds of T-joints to evaluate the hot cracking sensitivity of the welding wire. The results showed that the welding wire provided by this invention did not exhibit hot cracking after welding. Attached Figure Description

[0019] Figure 1 The image shows the metallographic structure of the Invar steel welding wire obtained in Example 1. Figure 2 The image shows the metallographic structure of the Invar steel welding wire obtained in Example 2. Figure 3 The image shows the metallographic structure of the Invar steel welding wire obtained in Example 3. Figure 4 The metallographic structure of the Invar steel welding wire obtained in Comparative Example 1 is shown in the diagram. Figure 5 The metallographic structure of the Invar steel welding wire obtained in Comparative Example 2 is shown in the figure. Figure 6 The image shows the metallographic structure of the Invar steel welding wire obtained in Comparative Example 3. Detailed Implementation

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0021] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art may make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0022] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0024] Specific Implementation Method 1: This invention provides a welding wire for welding Invar steel molds, the chemical composition of which is expressed as a percentage by weight: C 0.20~0.25%, 0<Si≤0.1%, Mn 0.4~0.6%, S≤0.001%, P≤0.002%, Ni 35.5~36.5%, Nb 1.4~1.8%, Ti 0.10~0.25%, with the balance being Fe and impurities; and the ratio of the total moles of Nb and Ti to the moles of C is 1:1.

[0025] The reasons for the design of the composition range of the welding wire used for welding Invar steel molds in this invention are as follows: Ni is the most crucial element for achieving the Invar effect. When the Ni content is approximately 36%, the magnetism of the iron-nickel alloy weakens with increasing temperature, a phenomenon known as magnetostriction, which precisely cancels out the thermal expansion effect, resulting in an extremely low coefficient of thermal expansion. Simultaneously, Ni is an austenite stabilizing element, ensuring the formation of an austenitic structure in Invar steel, which is beneficial for improving the steel's plasticity and toughness. Preferably, the Ni content is 35.5% to 36.5%.

[0026] Fe is the basic element of Invar steel. As a solvent for Ni, it can form a solid solution with Ni.

[0027] C can form carbide precipitates with Nb, Ti, etc., thus strengthening Invar steel. However, excessive C content will significantly increase the coefficient of thermal expansion of Invar steel. Therefore, the C content is controlled between 0.20% and 0.25%.

[0028] Si is a deoxidizer that reduces the oxygen content in weld metal. Simultaneously, Si increases the fluidity of the molten weld metal, resulting in better weld bead formation and improved welding processability. However, Si tends to form low-melting-point eutectics, increasing susceptibility to hot cracking. Preferably, 0 < Si ≤ 0.1%.

[0029] Mn typically combines with sulfur to form high-melting-point MnS, with a melting point as high as 1610℃, avoiding the formation of low-melting-point eutectic Ni-Ni3S2. This helps eliminate the harmful effects of sulfur and reduces the material's tendency to hot crack. Simultaneously, Mn, as an austenitizing element, can stabilize the austenitic structure. Preferably, the Mn content is 0.4~0.6%.

[0030] S is a harmful impurity element that tends to agglomerate at grain boundaries, forming a low-melting-point eutectic Ni-Ni2S2 with a melting point of only 645℃. This leads to a decrease in the intergranular bonding force and cracking under stress. Therefore, the content of S must be strictly controlled. In this invention, the S content is controlled to be ≤0.001%.

[0031] The effect of phosphorus (P) is similar to that of sulfur (S). Ni is very sensitive to P; even a small amount of P can cause the formation of a low-melting-point eutectic, Ni-Ni3P, at the grain boundaries. This eutectic has a melting point of only 880°C, which can easily lead to hot cracking in the weld. Preferably, P ≤ 0.002%.

[0032] The combined effect of Nb, Ti, and C can effectively reduce the hot cracking susceptibility of Invar steel welds. Controlling the Nb and Ti content leads to the precipitation of carbides at grain boundaries, which pins the grain boundaries, hindering grain sliding and migration at high temperatures, thus increasing grain boundary strength and reducing the weld's hot cracking susceptibility. This reduces the formation of high-temperature hot cracks, thereby improving the weld metal's resistance to hot cracking. Preferably, Nb: 1.4~1.8%, Ti: 0.10~0.25%.

[0033] This invention provides a method for preparing the welding wire for welding Invar steel molds, the method comprising the following steps: (1) Powder preparation: Electrolytic nickel, low-phosphorus pure iron, crystalline silicon, niobium carbide powder, titanium carbide powder, and remelted manganese are used as raw materials and prepared according to the above formula; (2) Vacuum induction smelting is carried out using electrolytic nickel, low-phosphorus pure iron, and crystalline silicon as raw materials; (2) Refining is carried out after all the raw materials have been melted and cleared; the refining temperature is controlled at 1560~1580℃ and the refining time is controlled at 40~60min; (3) After refining, add niobium carbide and titanium carbide powder, stir for 15 minutes, add remelted manganese, and pour after melting. The pouring temperature is controlled at 1510~1490℃ to obtain steel ingots. (4) After removing the oxide scale from the surface of the steel ingot, it is forged and rolled to produce wire rod with a diameter of 5.5mm; (5) The wire rod is subjected to rough drawing, bright annealing, fine drawing, wire cutting, cleaning and packaging in sequence to obtain welding wire with a specification of Φ2.4×1000mm.

[0034] The welding method of the welding wire provided by the present invention is as follows: welding is performed using TIG, the current type is DCEN, the welding current is 160~200A, the shielding gas is Ar, the gas flow rate during welding is 15~20L / min, and the interpass temperature is less than 120℃.

[0035] Example 1: The chemical composition of the welding wire in this embodiment is expressed as a percentage by weight: C: 0.24%, Si: 0.08%, Mn: 0.53%, S: 0.0007%, P: 0.0018%, Ni: 35.95%, Nb: 1.41%, Ti: 0.23%, with the balance being Fe and impurities.

[0036] The method for preparing the welding wire in this embodiment includes the following steps: (1) Powder preparation: Prepare the ingredients according to the above formula, using 69kg electrolytic nickel, 120kg low phosphorus pure iron, 0.1kg crystalline silicon, 3.2kg niobium carbide powder, 0.6kg titanium carbide powder, and 0.9kg remelted manganese as raw materials; (2) Vacuum induction smelting is carried out using electrolytic nickel, low-phosphorus pure iron, and crystalline silicon as raw materials; (2) Refining is carried out after all the raw materials have been melted and cleared; the refining temperature is controlled at 1560~1580℃ and the refining time is controlled at 40~60min; (3) After refining, add niobium carbide and titanium carbide powder, stir for 15 minutes, add remelted manganese, and pour after melting. The pouring temperature is controlled at 1510~1490℃ to obtain steel ingots. (4) After removing the oxide scale from the surface of the steel ingot, it is forged and rolled to produce wire rod with a diameter of 5.5mm; (5) The wire rod is sequentially subjected to rough drawing, bright annealing, fine drawing, wire cutting, cleaning, and packaging to obtain welding wire with a specification of Φ2.4×1000mm. The metallographic structure of the welding wire is as follows: Figure 1 As shown.

[0037] The weld metal of the welding wire prepared in this embodiment was subjected to performance testing. Specifically, TIG welding weld metal was prepared using DCEN welding current of 160A, Ar shielding gas, a gas flow rate of 15L / min, and an interpass temperature of less than 120℃. The tensile and impact properties of the weld metal are shown in Table 2 below.

[0038] The above-mentioned welding wire was used to weld Invar steel welded joint test plates. T-joint weld crack tests were conducted according to standard GB / T 13817-1992 "Test Method for Rigidly Restrained Welding Cracks in Butt Joints" and standard GB / T 41107.2-2021 "Destructive Testing of Welds in Metallic Materials - Hot Cracking Tests of Welded Parts - Arc Welding Methods - Part 2: Self-Restraint Tests". The hot cracking sensitivity of the welding wire was evaluated, and the results are shown in Table 3.

[0039] Example 2: The difference between this embodiment and Embodiment 1 is that the chemical composition of the welding wire, expressed as a weight percentage, is: C: 0.24%, Si: 0.07%, Mn: 0.52%, S: 0.0008%, P: 0.0017%, Ni: 36.19%, Nb: 1.53%, Ti: 0.16%, with the balance being Fe and impurities. The remaining operating steps and parameter settings are the same as in Embodiment 1. The metallographic structure of the welding wire is as follows: Figure 2 As shown in Table 2, the tensile and impact properties of the weld metal are tested. The results of the rigid restraint test and the T-joint weld crack test are shown in Table 3.

[0040] Example 3: The difference between this embodiment and Embodiment 1 is that the chemical composition of the welding wire, expressed as a weight percentage, is: C 0.25%, Si 0.08%, Mn 0.45%, S 0.0008%, P 0.0016%, Ni 36.01%, Nb 1.67%, Ti 0.14%, with the balance being Fe and impurities. The remaining operating steps and parameter settings are the same as in Embodiment 1. The metallographic structure of the welding wire is as follows: Figure 3 As shown in Table 2, the tensile and impact properties of the weld metal are tested. The results of the rigid restraint test and the T-joint weld crack test are shown in Table 3.

[0041] Comparative Example 1 The difference between this comparative example and Example 1 is that the chemical composition of the welding wire, expressed as a weight percentage, is: C 0.01%, Si 0.07%, Mn 0.47%, S 0.0008%, P 0.0016%, Ni 36.14%, with the balance being Fe and impurities. The remaining operating steps and parameter settings are the same as in Example 1. The metallographic structure of the welding wire is as follows: Figure 4 As shown in Table 2, the tensile and impact properties of the weld metal are tested. The results of the rigid restraint test and the T-joint weld crack test are shown in Table 3.

[0042] Comparative Example 2 The difference between this comparative example and Example 1 is that niobium strips, sponge titanium, and graphite were used to replace titanium carbide and niobium carbide during the vacuum induction smelting process to achieve the addition of Nb, Ti, and C elements. The chemical composition of the welding wire, expressed as a weight percentage, is: C 0.24%, Si 0.06%, Mn 0.48%, S 0.0007%, P 0.0018%, Ni 35.89%, Nb: 1.45%, Ti: 0.22%, with the balance being Fe and impurities. The remaining operating steps and parameter settings are the same as in Example 1. The metallographic structure of the welding wire is as follows: Figure 5 As shown in Table 2, the tensile and impact properties of the weld metal are tested. The results of the rigid restraint test and the T-joint weld crack test are shown in Table 3.

[0043] Comparative Example 3 The difference between this comparative example and Example 1 is that the chemical composition of the welding wire, expressed as a weight percentage, is: C: 0.24%, Si: 0.07%, Mn: 0.51%, S: 0.0054%, P: 0.0078%, Ni: 36.07%, Nb: 1.43%, Ti: 0.23%, with the balance being Fe and impurities. The remaining operating steps and parameter settings are the same as in Example 1. The metallographic structure of the welding wire is as follows: Figure 6 As shown in Table 2, the tensile and impact properties of the weld metal are tested. The results of the rigid restraint test and the T-joint weld crack test are shown in Table 3.

[0044] Table 1 Chemical composition (weight percentage, %) of the welding wires used in the examples and comparative examples

[0045] Table 2 Mechanical Properties Test of Weld Metal

[0046] Table 3 Evaluation of the hot crack resistance of welded joints

[0047] From Table 1 to Table 3 above and Figures 1-6It can be seen that the weld metals of Examples 1, 2, and 3 do not exhibit hot cracking, while the weld metals of Comparative Examples 1, 2, and 3 do. Due to the more severe cracking in Comparative Examples 2 and 3, the performance of the weld metal deteriorates, leading to tensile failure. Comparative Example 1 did not contain Nb or Ti elements in its welding wire, resulting in no grain boundary carbide precipitation. Therefore, it did not pin the grain boundaries at high temperatures, leading to a tendency for hot cracking in the weld metal. Furthermore, the lack of carbide precipitation strengthening effect resulted in a certain reduction in the strength of the weld metal. Although Comparative Example 2 formed grain boundary carbides, the improper alloy addition method caused the carbides to aggregate and grow, preventing them from dispersing and reducing their pinning effect on the grain boundaries. Comparative Example 3 did not strictly control the content of S and P elements, making it easy for the weld metal to form low-melting-point eutectics, increasing its susceptibility to hot cracking.

[0048] The Invar steel welding wire provided by this invention incorporates NbC and TiC powders, which are dispersed at the grain boundaries, effectively pinning the grains and thus improving the crack resistance of the weld metal. The tensile strength of the weld metal not only meets the requirements of the base metal but also exhibits high plasticity and toughness. The weld metal has a tensile strength ≥400MPa, a yield strength ≥300MPa, an elongation after fracture ≥25%, and a room temperature impact absorption energy ≥60J. In summary, the Invar steel welding wire provided by this invention possesses excellent thermal crack resistance and mechanical properties, resulting in superior weld quality.

[0049] The above description is only a preferred embodiment of the present invention. Given that those skilled in the art can make appropriate changes and modifications to the above embodiments, the present invention is not limited to the specific embodiments described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention.

Claims

1. A welding wire for welding Invar steel molds, characterized in that, The chemical composition of the welding wire, expressed as a weight percentage, is as follows: C 0.20~0.25%, 0<Si≤0.1%, Mn 0.4~0.6%, S≤0.001%, P≤0.002%, Ni 35.5~36.5%, Nb 1.4~1.8%, Ti 0.10~0.25%, with the balance being Fe and impurities; and the ratio of the total moles of Nb and Ti to the moles of C is 1:

1.

2. The welding wire according to claim 1, characterized in that, The chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.24%, Si 0.08%, Mn 0.53%, S 0.0007%, P 0.0018%, Ni 35.95%, Nb 1.41%, Ti 0.23%, with the balance being Fe and impurities.

3. The welding wire according to claim 1, characterized in that, The chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.24%, Si 0.07%, Mn 0.52%, S 0.0008%, P 0.0017%, Ni 36.19%, Nb 1.53%, Ti 0.16%, with the balance being Fe and impurities.

4. The welding wire according to claim 1, characterized in that, The chemical composition of the welding wire, expressed as a percentage by weight, is: C 0.25%, Si 0.08%, Mn 0.45%, S 0.0008%, P 0.0016%, Ni 36.01%, Nb 1.67%, Ti 0.14%, with the balance being Fe and impurities.

5. A method for preparing the welding wire according to any one of claims 1 to 4, characterized in that, include: (1) Vacuum induction smelting is carried out using electrolytic nickel, low-phosphorus pure iron and crystalline silicon as raw materials; (2) Refining is carried out after all the raw materials have been melted and cleared; (3) After refining, add niobium carbide and titanium carbide powder, stir for 15 minutes, add remelted manganese, and pour after melting to obtain steel ingots; (4) After removing the oxide scale from the surface of the steel ingot, it is forged and rolled to obtain wire rod; (5) The wire rod is subjected to rough drawing, bright annealing, fine drawing, wire cutting, cleaning and packaging in sequence to obtain welding wire.

6. The preparation method according to claim 5, characterized in that, (2) The refining temperature is 1560~1580℃ and the time is 40~60min.

7. The preparation method according to claim 5, characterized in that, (3) The pouring temperature is 1510~1490℃.

8. The preparation method according to claim 5, characterized in that, (4) The wire rod is Φ5.5mm. (5) The welding wire is Φ2.4×1000mm.

9. An application of the welding wire according to any one of claims 1 to 4, characterized in that, TIG welding for Invar steel molds.

10. A welding method for Invar steel molds, characterized in that, Using the welding wire described in any one of claims 1 to 4, TIG welding is performed with DCEN current type, welding current of 160 to 200A, shielding gas of Ar, gas flow rate of 15 to 20L / min during welding, and interpass temperature of less than 120℃.

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