A stainless steel welding wire for repair welding of martensitic stainless steel turbine runners, its preparation method and application

Abstract

This invention discloses a stainless steel welding wire for repair welding of martensitic stainless steel turbine runners, its preparation method, and its application, belonging to the field of metal welding material preparation technology. The chemical composition of the welding wire provided by this invention, expressed as a percentage by weight, is: C≤0.03%, Si: 0%~0.6%, Mn: 5.0%~8.0%, S≤0.010%, P≤0.020%, Cr: 12.5%~15.5%, Ni: 5.0%~7.0%, Mo: 1.0%~2.0%, with the balance being Fe and impurities. The welding wire provided by this invention is suitable for repair welding of Cr13Ni4~6 type martensitic stainless steel, and post-weld heat treatment is not required. The weld metal produced by this welding wire exhibits good strength, plasticity, and toughness, with a tensile strength ≥750MPa, elongation after fracture ≥35%, and impact absorption energy at -40℃ ≥120J.

Description

Technical Field

[0001] This invention relates to a stainless steel welding wire for repair welding of martensitic stainless steel turbine runners, its preparation method and application, belonging to the field of metal welding material preparation technology. Background Technology

[0002] Super martensitic stainless steel boasts high strength and excellent resistance to cavitation, silt abrasion, and underwater fatigue, making it widely used in the manufacture of large hydroelectric turbine runners. During the runner manufacturing process, super martensitic stainless steel welding materials of the same material are used for welding. The weld joint exhibits a significant tendency to harden. When the weld joint has high restraint or high hydrogen content, cold cracking is easily caused, requiring preheating at above 100°C before welding and tempering at 580°C after welding to improve its ductility and toughness.

[0003] Defects such as cracks, sand holes, porosity, and shrinkage cavities are unavoidable during the casting of turbine runners. Poor welding quality can also occur during the runner welding process, mainly manifested in slag inclusions, impurities, porosity, microcracks, over-melting, or under-melting. Furthermore, significant residual stress and stress concentration exist at the weld joints, with residual tensile stress approaching or even exceeding the material's yield strength, inducing cracks in the runner. Defects generated during casting and welding reduce the material's strength and toughness, accelerating crack formation and propagation. Hydropower units operating long-term in rivers containing silt suffer varying degrees of damage to their runners, guide vanes, and other flow-through components due to cavitation and silt abrasion, such as chipping, spalling, perforation, and cracking, and even runner blade breakage. These problems not only reduce the turbine's power generation efficiency and output, significantly impacting the power plant's economic benefits, but also seriously threaten the safe and stable operation of the hydropower unit. Therefore, repairing turbine runners is of great importance.

[0004] The welding methods currently used for repairing turbine runners include GTAW, GMAW, and SMAW. When using equally matched super martensitic stainless steel welding materials, the weld metal, being martensitic, has a high hardening tendency. Under thermal imbalance conditions, it will form a large number of lattice defects, making it prone to cold cracking under welding stress. To reduce the tendency to embrittle and prevent cold cracking, preheating of the repair area is required before welding, generally to above 100°C. When using low-matching austenitic stainless steel welding materials, the weld metal, being austenitic, has a large coefficient of linear expansion and a low thermal conductivity. Welding repair will lead to increased thermal strain and thermal stress, resulting in high residual stress in the repair area and near the heat-affected zone, which can cause cracking. Furthermore, the significant difference in the coefficients of linear expansion between austenite and martensite will generate large residual stress during welding, accelerating the propagation of fatigue cracks and the formation of cavitation in the runner. Furthermore, the chemical composition and microstructure of the weld metal differ significantly from those of the base metal. This can easily lead to an imbalance of electrode potential at the weld defect, resulting in electrochemical corrosion.

[0005] Residual tensile stress accelerates the formation and propagation of fatigue cracks, severely reducing the fatigue performance of materials. To improve the welding stress generated during repair welding, post-weld annealing heat treatment is required. The entire rotor is subjected to annealing heat treatment at (575±15)℃ for 7-8 hours.

[0006] Regular repairs of the turbine during on-site installation and operation are difficult to perform due to site conditions, such as preheating or post-weld heat treatment. Therefore, it is essential to provide a repair welding material and process that uses cold welding and does not require post-weld heat treatment. Summary of the Invention

[0007] In order to solve the above-mentioned technical problems existing in the prior art, the present invention provides a stainless steel welding wire for repair welding of martensitic stainless steel turbine runners without preheating and post-weld heat treatment, as well as its preparation method and application.

[0008] The technical solution of this invention:

[0009] One objective of this invention is to provide a stainless steel welding wire whose chemical composition, expressed as a percentage by weight, is: C≤0.03%, Si 0~0.6%, Mn 5.0~8.0%, S≤0.010%, P≤0.020%, Cr 12.5~15.5%, Ni 5.0~7.0%, Mo 1.0~2.0%, with the balance being Fe and impurities.

[0010] The functions of each component in the welding wire of the present invention are briefly described below:

[0011] C is an austenite-forming element, and after quenching, it forms a martensitic structure. Excessive C content reduces the toughness of the weld metal and increases the hardening tendency of martensitic stainless steel, thus worsening the weldability of the material. C combines with Cr to form Cr₂. 23 C6 precipitates along grain boundaries, causing a decrease in the Cr content at the grain boundaries and resulting in intergranular corrosion. Therefore, the C content is controlled below 0.03%. Preferably, the C content is ≤0.015%.

[0012] Si (silicon) can be used as an alloying element, but it also has detrimental effects. Firstly, Si can act as a deoxidizer, participating in deoxidation reactions and reducing the content of oxygen (O) impurities in metals. Small amounts of Si can improve the processability of TIG welding, increase the wettability of liquid metal, improve weld bead formation, and reduce the incidence of defects such as inclusions and lack of fusion. Additionally, Si has solid solution strengthening and temper brittleness suppression effects. However, on the other hand, Si can form low-melting-point eutectics, increasing susceptibility to hot cracking. Therefore, the Si factor must be considered in alloy design. In this invention, the Si content is controlled between 0% and 0.6%.

[0013] The addition of manganese (Mn) is beneficial to the resistance of austenitic stainless steel to crystallization cracking. On the one hand, Mn preferentially combines with sulfur (S) to form MnS (melting point 1610℃), reducing the tendency of S to form low-melting-point eutectics (such as Ni-Ni3S2, melting point 645℃), thus increasing the austenite-sulfide eutectic temperature. On the other hand, it increases the surface energy of the solid and liquid phases, reducing the possibility of low-melting-point eutectic liquid film formation at grain boundaries, suppressing the adverse effects of S and P, thereby reducing the tendency of solidification crack formation in the weld metal. Simultaneously, Mn is also a deoxidizer, which can reduce the oxygen content in the weld metal and reduce the conditions for silicate formation. It is generally believed that adding 1% Mn to the weld metal can achieve the purpose of desulfurization. However, Mn has a strong tendency to segregate in the weld metal, resulting in Mn-depleted and Mn-rich regions within the weld. When the Mn content in the weld is insufficient, the Mn-depleted zone will develop high-temperature cracks due to the inability to achieve sulfur removal. Therefore, the weld should contain sufficient Mn to ensure adequate Mn removal in the Mn-depleted zone, even with segregation, thereby reducing the harmful effects of sulfur and improving the material's resistance to cracking. In general, Mn exists as an oxide on grain boundaries in the weld metal, acting as a grain boundary pinning agent and preventing grain boundary slip. Furthermore, the addition of Mn reduces the harmful effects of impurities such as sulfur and phosphorus, improving the material's resistance to intergranular cracking. Preferably, the Mn content is 5.0%–8.0%.

[0014] The effect of phosphorus (P) on stainless steel is similar to that of sulfur (S). Although its content in the alloy is very small, its harmful effect cannot be underestimated. In the alloy, P mainly forms low-melting-point eutectics with Ni, segregates at grain boundaries, increases the width of the half-melted zone, and promotes cracking. Therefore, the content of P in stainless steel must be controlled. In this invention, the P content is controlled to be ≤0.020%. Preferably, the P content is ≤0.010%.

[0015] Ni's solid solution strengthening effect can improve the strength of steel. As an austenite-forming element, Ni can expand the austenite phase region, resulting in a martensitic structure after quenching. Ni helps improve the plasticity and toughness of steel, especially significantly improving its low-temperature toughness. However, excessive Ni content can cause the steel to lose its hardenability. Preferably, Ni content is 5.0% to 7.0%.

[0016] Cr is the primary element responsible for the rust prevention of stainless steel. Stainless steel generally contains at least 10.5% Cr by mass. It forms a protective layer on the surface of the base material, resisting oxidation and corrosion, thus giving stainless steel basic corrosion resistance. As a ferrite-forming element, Cr promotes the formation and retention of ferrite in martensitic stainless steel, shrinking the austenite phase region in the phase diagram. Excessive Cr content can even prevent the steel from forming a martensitic structure after cooling from high temperatures. As a carbide-forming element, Cr has a strong affinity for C, forming chromium-rich carbide precipitation, which reduces the corrosion resistance of martensitic stainless steel. Additionally, Cr can form nitrides with N, causing mid-temperature temper brittleness in steels with high N content. Preferably, the Cr content is 12.5%–15.5%.

[0017] Mo (Mo) is primarily used in stainless steel for its antioxidant and corrosion-resistant properties. Adding Mo to stainless steel improves its resistance to pitting corrosion, crevice corrosion, and sulfide stress cracking. Additionally, the formation of carbides by Mo in martensitic stainless steel induces secondary hardening, increasing its strength. Mo inhibits the intergranular precipitation of carbon and nitrogen compounds in weld metal, increasing resistance to temper brittleness and thus improving weld toughness. However, the addition of Mo is detrimental to the hot working of the material. As a ferrite-forming element, Mo promotes the formation and retention of ferrite in martensitic stainless steel, impairing its plasticity and toughness. In this invention, the Mo content is controlled at 1.0%–2.0%, with a preferred Mo content of 1.5%.

[0018] Further specified, the chemical composition of the welding wire is expressed as a percentage by weight: C≤0.03%, Si 0.2~0.6%, Mn5.0~8.0%, S≤0.010%, P≤0.020%, Cr 12.5~15.5%, Ni 5.0~7.0%, Mo 1.0~2.0%, with the balance being Fe and impurities.

[0019] Further specifying, the chemical composition of the welding wire is expressed as a weight percentage: Mn: 5.0%–8.0%, Cr: 12.5%–15.5%, Ni: 5.0%–7.0%, Mo: 1.0%–2.0%, with the balance being Fe and impurities.

[0020] Further specifying, the chemical composition of the welding wire is expressed as a percentage by weight: C 0.0084%, Si 0.36%, Mn 6.36%, S 0.0042%, P 0.009%, Cr 13.6%, Ni 6.2%, Mo 1.5%, with the balance being Fe and impurities.

[0021] Further specifying, the chemical composition of the welding wire is expressed as a percentage by weight: C 0.015%, Si 0.46%, Mn 7.3%, S 0.0035%, P 0.011%, Cr 13.2%, Ni 6.0%, Mo 1.4%, with the balance being Fe and impurities.

[0022] Further specifying, the chemical composition of the welding wire is expressed as a percentage by weight: C 0.013%, Si 0.52%, Mn 5.4%, S 0.0053%, P 0.0094%, Cr 13.8%, Ni 6.6%, Mo 1.3%, with the balance being Fe and impurities.

[0023] The second objective of this invention is to provide a method for preparing the above-mentioned stainless steel welding wire, which includes smelting, electroslag remelting, forging, rolling and drawing processes.

[0024] The third objective of this invention is to provide an application of the above-mentioned stainless steel welding wire, specifically, the welding wire is used for repair welding of Cr13Ni4-6 type martensitic stainless steel turbine runners.

[0025] Furthermore, no heat treatment process is required during the repair welding.

[0026] The fourth objective of this invention is to provide a welding process for the above-mentioned stainless steel welding wire, specifically using tungsten inert gas (TIG) welding with DCEN current type, welding current of 140-180A, shielding gas of Ar, gas flow rate of 15-20L / min during welding, and interpass temperature of less than 150℃.

[0027] Further specified, the tensile strength of the weld metal of the above-mentioned stainless steel welding wire is ≥750MPa, the elongation after fracture is ≥35%, and the impact absorption energy at -40℃ is ≥120J.

[0028] The present invention has the following beneficial effects:

[0029] (1) The stainless steel welding wire provided by this invention adopts the Fe-Cr-Ni alloy system. On the basis of strictly controlling the content of S and P elements, Mn and Mo alloying elements are added, so that the weld metal structure of the welding wire is martensite + ferrite + austenite. The tensile strength of the joint containing the three-phase structure can not only meet the requirements of the base metal strength, but also has a high plasticity reserve. The cold cracking sensitivity is much lower than that of martensitic welding materials, and the comprehensive mechanical properties are between those of martensite and austenite. Iron research tests were carried out according to the standard GB4675.1-1984 "Test Method for Cracks in Welding with Oblique Y-type Groove", and pin tests were carried out according to the standard GB 9446-1988 "Test Method for Cold Cracks in Welding Pins" to evaluate the cold cracking sensitivity of the welding materials. The results show that no cracks were produced after welding martensitic stainless steel joints with the welding wire provided by this invention without heat treatment.

[0030] (2) The welding wire of the present invention contains 5.0% to 8.0% Mn element, and a certain amount of austenite structure appears in the weld, which makes the weld metal have high plasticity and toughness, can relax welding stress, and helps to reduce the tendency of cold cracking of the joint. At the same time, Mn element also increases the work hardening rate of the material, and can absorb more energy during deformation, thus absorbing more of the energy caused by cavitation and mud and sand wear during deformation, and has excellent resistance to cavitation erosion and mud and sand wear.

[0031] (3) The stainless steel welding wire provided by the present invention has a tensile strength of ≥750MPa, an elongation after fracture of ≥35%, and an impact absorption energy of ≥120J at -40℃. It can be used for repair welding of martensitic stainless steel turbine runners in large hydropower stations. Moreover, the weld metal has excellent crack resistance, which can eliminate the preheating and post-weld heat treatment procedures, improve welding production efficiency and reduce production costs.

[0032] (4) The stainless steel welding wire provided by the present invention is applicable to the TIG welding method, the welding process is stable, and the weld formation is beautiful. Attached Figure Description

[0033] Figure 1 The image shows the metallographic structure of the weld metal of the stainless steel welding wire obtained in Example 1.

[0034] Figure 2 The image shows the metallographic structure of the weld metal obtained in Example 2 of the stainless steel welding wire.

[0035] Figure 3 The image shows the metallographic structure of the weld metal obtained in Example 3 of the stainless steel welding wire.

[0036] Figure 4 Metallographic diagram of the weld metal of the stainless steel welding wire provided for Comparative Example 1.

[0037] Figure 5 Metallographic diagram of the weld metal of the stainless steel welding wire provided for Comparative Example 2. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0039] 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.

[0040] Example 1:

[0041] The chemical composition of the stainless steel welding wire for repairing turbine runners provided in this embodiment is expressed as a percentage by weight: C: 0.0084%, Si: 0.36%, Mn: 6.36%, S: 0.0042%, P: 0.009%, Cr: 13.6%, Ni: 6.2%, Mo: 1.59%, with the balance being Fe and impurities, as detailed in Table 1.

[0042] The stainless steel welding wire used for repairing the turbine runner was prepared through a process of smelting → electroslag remelting → forging → rolling → drawing, resulting in a welding wire with a specification of Φ2.4×1000mm.

[0043] The above-mentioned stainless steel welding wire was used to prepare TIG welding deposited metal. The welding current type was DCEN, the welding current was 160A, the shielding gas was Ar, the gas flow rate during welding was 20L / min, and the interpass temperature was less than 150℃. Tensile properties, impact properties, and metallographic structure of the deposited metal were tested, and the results are shown in Table 2. The metallographic structure is as follows: Figure 1 As shown.

[0044] Test plates of 0Cr13Ni5Mo low-carbon martensitic stainless steel welded joints were welded using the above-mentioned welding wire. Iron grinding tests were carried out according to standard GB4675.1-1984 "Test Method for Cracks in Welded Y-groove", and pin tests were carried out according to standard GB 9446-1988 "Test Method for Cold Cracks in Welding Pins". Cold crack sensitivity evaluation tests were conducted, and the results are shown in Table 3.

[0045] Example 2:

[0046] The difference between this embodiment and Embodiment 1 is that the chemical composition of the stainless steel welding wire, expressed as a percentage by weight, is: C: 0.015%, Si: 0.46%, Mn: 7.3%, S: 0.0035%, P: 0.011%, Cr: 13.2%, Ni: 6.0%, Mo: 1.4%, with the balance being Fe and impurities, as detailed in Table 1. The remaining operating steps and parameter settings are the same as in Embodiment 1. The metallographic structure of the deposited metal is as follows: Figure 2 As shown.

[0047] Example 3:

[0048] The difference between this embodiment and Embodiment 1 is that the chemical composition of the stainless steel welding wire, expressed as a weight percentage, is: C 0.013%, Si 0.52%, Mn 5.4%, S 0.0053%, P 0.0094%, Cr 13.8%, Ni 6.6%, Mo 1.3%, with the balance being Fe and impurities, as detailed in Table 1. The remaining operating steps and parameter settings are the same as in Embodiment 1. The metallographic structure of the deposited metal is as follows: Figure 3 As shown.

[0049] Comparative Example 1:

[0050] The difference between this comparative example and Example 1 is that the stainless steel welding wire used is ER410NiMo, and its chemical composition by weight percentage is: C 0.012%, Si 0.45%, Mn 0.54%, S 0.011%, P 0.014%, Cr 12.2%, Ni 4.7%, Mo 0.45%, with the balance being Fe and impurities (see Table 1 for details). The remaining operating steps and parameter settings are the same as in Example 1. The metallographic structure of the deposited metal is as follows: Figure 4 As shown.

[0051] Comparative Example 2:

[0052] The difference between this comparative example and Example 1 is that the stainless steel welding wire used is ER308L, and its chemical composition by weight percentage is: C 0.013%, Si 0.5%, Mn 2.1%, S 0.01%, P 0.018%, Cr 19.8%, Ni 9.86%, Mo 0.01%, with the balance being Fe and impurities, as detailed in Table 1. The remaining operating steps and parameter settings are the same as in Example 1. The metallographic structure of the deposited metal is as follows: Figure 5 As shown.

[0053] Table 1. Chemical composition (weight percentage, %) of the welding wires used in the examples and comparative examples.

[0054]

[0055] Table 2 Results of mechanical property testing and microstructure observation of the deposited metal

[0056]

[0057]

[0058] Table 3 Evaluation of Cold Cracking Resistance of Welded Joints

[0059] Cold crack sensitivity test Example 1 No cracks Example 2 No cracks Example 3 No cracks Comparative Example 1 Cracks Comparative Example 2 No cracks

[0060] As shown in Tables 1-3 above, the weld metal of the stainless steel welding wire in Comparative Example 2, which is mainly austenitic, has a relatively low tensile strength. Stress and strain are concentrated at the weld joint, promoting crack formation and reducing the fatigue life of the weld joint. Furthermore, the low strength and hardness of austenitic stainless steel result in poor cavitation erosion resistance and wear resistance of the weld joint. The weld metal of the stainless steel welding wire in Comparative Example 1, which is mainly super-martensitic, has poor plasticity and toughness, exhibiting high cold cracking sensitivity and easily developing cold cracks during welding. Simultaneously, the weld joint shows a significant tendency to embrittle. The stainless steel welding wire provided by this invention has a weld metal tensile strength ≥750MPa, elongation after fracture ≥35%, and impact absorption energy ≥120J at -40℃. It retains excellent comprehensive mechanical properties in the weld state, combining the high strength and hardness of martensitic stainless steel with the good plasticity and toughness of austenitic stainless steel. In summary, the stainless steel welding wire provided by this invention exhibits excellent cold crack resistance, cavitation erosion resistance, and wear resistance.

[0061] Finally, it should be noted that the above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A stainless steel welding wire, characterized in that, The chemical composition of the welding wire is expressed as a percentage by weight: C≤0.03%, Si 0~0.6%, Mn 5.0~8.0%, S≤0.010%, P≤0.020%, Cr 12.5~15.5%, Ni 5.0~7.0%, Mo 1.0~2.0%, with the balance being Fe and impurities; The weld metal of the stainless steel welding wire has a tensile strength ≥750MPa, an elongation after fracture ≥35%, and an impact absorption energy of ≥120J at -40℃.

2. The stainless steel welding wire according to claim 1, characterized in that, The chemical composition of the welding wire is expressed as a percentage by weight: Mn: 5.0%~8.0%, Cr: 12.5%~15.5%, Ni: 5.0%~7.0%, Mo: 1.0%~2.0%, with the balance being Fe and impurities.

3. The stainless steel welding wire according to claim 1, characterized in that, The chemical composition of the welding wire is expressed as a percentage by weight: C 0.0084%, Si 0.36%, Mn 6.36%, S 0.0042%, P 0.009%, Cr 13.6%, Ni 6.2%, Mo 1.5%, with the balance being Fe and impurities.

4. The stainless steel welding wire according to claim 1, characterized in that, The chemical composition of the welding wire is expressed as a percentage by weight: C 0.015%, Si 0.46%, Mn 7.3%, S 0.0035%, P 0.011%, Cr 13.2%, Ni 6.0%, Mo 1.4%, with the balance being Fe and impurities.

5. The stainless steel welding wire according to claim 1, characterized in that, The chemical composition of the welding wire is expressed as a percentage by weight: C 0.013%, Si 0.52%, Mn 5.4%, S 0.0053%, P 0.0094%, Cr 13.8%, Ni 6.6%, Mo 1.3%, with the balance being Fe and impurities.

6. A method for preparing the stainless steel welding wire according to claim 1, characterized in that, This includes smelting, electroslag remelting, forging, rolling, and drawing processes.

7. An application of the stainless steel welding wire according to claim 1, characterized in that, Used for repair welding of Cr13Ni4~6 type martensitic stainless steel turbine runners.

8. The application of the stainless steel welding wire according to claim 7, characterized in that, No heat treatment process is required when repairing the weld.

9. A welding process for the stainless steel welding wire according to claim 1, characterized in that, The welding is performed using tungsten inert gas (TIG) welding with DCEN current, a welding current of 140~180A, Ar shielding gas, a gas flow rate of 15-20L / min during welding, and an interpass temperature of less than 150℃.

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