A welding process for dissimilar steel of different strength grades of low alloy steel

Abstract

The present application relates to the technical field of welding, and provides a welding process for dissimilar steel of low alloy steel with different strength grades, which comprises the following steps: step one, welding pre-treatment: processing double-sided X-type groove on the to-be-welded part of the first low alloy steel and the second low alloy steel, and adopting a partition preheating mode; step two, base welding: adopting ferrite type welding material and completing by TIG welding; step three, filling welding: adopting ferrite type welding material and completing by MAG welding, and adopting a multi-layer and multi-pass welding process; step four, surface welding: adopting ferrite type welding material and completing by MAG welding; and step five, post-welding graded tempering treatment.The welding process for dissimilar steel of low alloy steel with different strength grades can realize joint organization homogenization, significantly improve crack resistance, and greatly reduce residual stress, so that the welding joint has high strength, high toughness and low stress characteristics, and can meet the long-term and severe service requirements of high-end equipment.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and more specifically, to a welding process for dissimilar low-alloy steels of different strength grades. Background Technology

[0002] Low-alloy high-strength steel, due to its advantages such as high strength, high toughness, lightweight, and low cost, has become the preferred material for core components in fields such as pressure vessels, ships, marine engineering equipment, and high-end mechanical components. In actual engineering manufacturing, to achieve structural lightweighting and functional integration, it is often necessary to perform dissimilar welding (such as R...) on low-alloy steels of different strength grades. p0.2 =785MPa high-strength steel and R p0.2 (590MPa medium-strength steel butt joint).

[0003] At present, there are many technical problems in the dissimilar welding process of low alloy steels with different strengths: (1) The hardening tendency of the base materials on both sides is large, and the heat-affected zone of the weld is prone to forming a high-hardness martensite structure. Under the high restraint of the thick plate, the incidence of cold cracks and lamellar tearing defects is high, and the crack rate can reach 3% to 5%; (2) If austenitic welding materials are used for welding, the thermal expansion coefficient is much different from that of low alloy steel, the residual stress after welding is seriously concentrated and the deformation is large. Long-term service is prone to fatigue cracking. At the same time, the austenitic weld is prone to precipitate σ phase in the high temperature, high pressure and corrosive environment, which leads to joint embrittlement and a significant reduction in service life; (3) The toughness of existing conventional welding materials decreases significantly after stress relief heat treatment, which does not meet the service requirements of equipment; (4) There is a lack of targeted post-weld heat treatment processes, which cannot effectively eliminate welding residual stress, further aggravate the deterioration of joint performance, and make it difficult to meet the long-term and stringent service requirements of high-end equipment.

[0004] In summary, the existing welding processes for dissimilar low-alloy steels of different strength grades suffer from insufficient microstructure uniformity, poor cold crack control, and incomplete stress relief, resulting in welded joints with low strength, low toughness, and high residual stress, which makes it difficult to meet the long-term and stringent service requirements of high-end equipment.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to propose a welding process for dissimilar low-alloy steels of different strength grades, in order to solve the problems of insufficient microstructure uniformity, poor cold crack control, and incomplete stress relief in the existing welding processes for dissimilar low-alloy steels of different strength grades, which result in low strength, low toughness, and high residual stress in the welded joints, making it difficult to meet the long-term and stringent service requirements of high-end equipment.

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

[0008] A welding process for dissimilar low-alloy steels of different strength grades, the welding process comprising the following steps:

[0009] Step 1: Pre-welding treatment: Process double-sided X-shaped grooves on the parts to be welded of the first low alloy steel and the second low alloy steel, and adopt a zoned preheating method. The first low alloy steel side is preheated to 100-150℃, and the second low alloy steel side is preheated to 60-100℃. The strength grade of the first low alloy steel is higher than that of the second low alloy steel.

[0010] Step 2, Root pass welding: Ferritic welding material is used, and the welding is completed by TIG welding;

[0011] Step 3, Filler Welding: Ferritic welding material is used, and MAG welding is performed using a multi-layer, multi-pass welding process.

[0012] Step 4, Cover welding: Ferritic welding material is used, and the welding is completed by MAG welding;

[0013] Step 5: Post-weld graded tempering treatment.

[0014] Furthermore, in step five, the post-weld graded tempering treatment specifically involves: first, holding at 200–250℃ for ≥2 hours for low-temperature tempering, then raising the temperature to 560–580℃ at 50–60℃ / h and holding for 4–6 hours for high-temperature tempering, followed by furnace cooling to below 100℃ and then air cooling.

[0015] Furthermore, the preheating width of the preheating zone is ≥200mm, and the preheating and heat preservation time is 30-60min.

[0016] Furthermore, in steps two through four, the ferritic welding material used for the root pass, fill pass, and cap pass is ER59-Ni5.

[0017] Furthermore, the root pass welding uses ER59-Ni5 welding wire with a diameter of 2.0 to 3.0 mm; the filler weld and cover weld use ER59-Ni5 welding wire with a diameter of 1.0 to 1.2 mm.

[0018] Furthermore, the shielding gas for the root pass welding is pure argon, and the process parameters for the root pass welding are: welding current 160-200A, arc voltage 14-16V.

[0019] Furthermore, the shielding gas for filler welding is a mixture of argon and carbon dioxide; the process parameters for filler welding are: welding current 180-220A, arc voltage 24-27V, heat input 12-15kJ / cm; interpass temperature controlled at 100-150℃; single-pass weld thickness 4-5mm, width 6-8mm, and overlap of adjacent welds 2-4mm.

[0020] Furthermore, the process parameters for cover welding are as follows: welding current 200-240A, arc voltage 26-29V, heat input 13-16kJ / cm; the width of the cover weld bead is 2-3mm wider than the bevel width, and the excess height is controlled at 2-4mm.

[0021] Furthermore, the chemical composition of the ER59-Ni5 welding wire, by mass percentage, includes C≤0.10, Si≤0.5, Mn:0.4~0.9, Ni:4~6, Cr≤0.6, Ti≤0.10, with the remainder being Fe and impurity elements.

[0022] Furthermore, the bevel angle of the double-sided X-shaped bevel is 55° to 65°, the blunt edge is 2 to 3 mm, and the root gap is 3 to 4 mm.

[0023] This invention proposes a welding process for dissimilar low-alloy steels of different strength grades. Compared with the prior art, the welding process for dissimilar low-alloy steels of different strength grades described in this invention has the following advantages:

[0024] (1) Significantly improved crack resistance: Ferritic welding materials are used for the root pass, fill pass and cover pass welding. The deposited metal has no martensitic or austenitic phase transformation structure and has a very low hardening tendency. Combined with precise preheating in zones and low heat input control, cold cracks and lamellar tearing defects are completely solved. The joint crack rate is ≤0.3%, which is far lower than the existing process.

[0025] (2) Excellent and balanced joint performance: The ferritic welding material is matched to achieve the strength and toughness of the weld and the base material on both sides. The tensile strength of the joint is ≥590MPa, the impact absorption energy at -20℃ is ≥64J, the impact absorption energy at -50℃ is ≥34J, and the maximum hardness of the heat-affected zone is ≤410HV. It takes into account both high strength and high toughness, and avoids the performance degradation caused by equal strength matching or low strength matching.

[0026] (3) Stable structure without embrittlement: The overall structure of the joint is uniform ferrite with no σ phase precipitation, which avoids the risk of embrittlement of austenitic welding material joints. It has strong long-term service stability and is suitable for harsh environments such as high temperature, high pressure and corrosion.

[0027] (4) Significantly reduced residual stress: The post-weld graded tempering treatment eliminates stress in stages, combined with the filling stage hammer stress elimination process, the welding residual stress is ≤150MPa, which effectively improves the fatigue resistance of the joint and extends its service life.

[0028] (5) Strong process adaptability: It can be flexibly adapted to R p0.2Dissimilar welding of low alloy steels with different strength grades from 390 to 785 MPa; bevel, welding materials, and parameters can be flexibly adjusted according to plate thickness and joint type; applicable to pressure vessels, ships, marine engineering equipment and other fields; high process stability; can be mass-produced and applied. Detailed Implementation

[0029] To make the technical means and the objectives and effects of the present invention easier to understand, the embodiments of the present invention will be described in detail below.

[0030] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0031] The present invention will now be described in detail with reference to embodiments.

[0032] Existing technologies for welding dissimilar low-alloy steels of different strength grades suffer from problems such as insufficient microstructure uniformity, poor control of cold cracking, and incomplete stress relief. This results in welded joints with low strength, low toughness, and high residual stress, making it difficult to meet the long-term and stringent service requirements of high-end equipment.

[0033] To address the aforementioned technical problems, this invention proposes a welding process for dissimilar low-alloy steels of different strength grades, wherein the R of the first low-alloy steel... p0.2 ≥785MPa, the first low-alloy steel is a high-strength steel, and the second low-alloy steel has R p0.2 ≥590MPa, the second low alloy steel is a medium-low strength steel; the strength grade of the first low alloy steel is higher than that of the second low alloy steel.

[0034] The welding process includes the following steps:

[0035] Step 1: Pre-welding treatment: Process double-sided X-shaped bevels on the parts to be welded of the first and second low-alloy steels to balance the welding heat input on both sides. The bevel angle is 55° to 65°, the blunt edge is 2 to 3 mm, and the root gap is 3 to 4 mm to avoid excessive heat input or incomplete fusion defects due to unreasonable bevel angle. Beveling is done by flame cutting or mechanical processing. After cutting, mechanically grind the bevel surface and 100 to 150 mm on both sides to remove oxide scale, cutting heat-affected layer and impurities, ensuring that the surface is free of oil, rust and moisture. The surface roughness is controlled at Ra25 to 50 μm to improve the welding fusion quality. Zoned preheating is a crucial preliminary step: based on the differences in hardening tendency of low-alloy steels with different strengths, infrared or high-frequency sensors are used for precise temperature control. The preheating temperature of the first low-alloy steel side is 100-150℃, and the preheating temperature of the second low-alloy steel side is 60-100℃. The preheating holding time of the zoned preheating is 30-60 minutes, and the preheating width is ≥200mm to ensure uniform temperature in the preheating area, reduce the cooling rate of the joint, and reduce the risk of martensite formation.

[0036] Step 2, Root Pass Welding: Root pass welding is the core step to ensure the quality of the joint root. It is performed using ER59-Ni5 welding wire with a diameter of 2.0–3.0 mm via TIG welding. ER59-Ni5 weld metal has a low carbon content and low hardening tendency, exhibiting excellent crack resistance and toughness. Pure Ar gas is used as the shielding gas for root pass welding to avoid porosity and oxidation defects at the root. The process parameters for root pass welding are: welding current 160–200 A, arc voltage 14–16 V, and root pass weld thickness 3–5 mm, achieving single-sided welding with double-sided forming, laying a low-defect foundation for subsequent filler welding.

[0037] The chemical composition of the ER59-Ni5 welding wire, by mass percentage, includes C≤0.10, Si≤0.5, Mn: 0.4~0.9, Ni: 4~6, Cr≤0.6, Ti≤0.10, with the remainder being Fe and impurity elements. The deposited metal R of the ER59-Ni5 welding wire... p0.2 ≥590MPa, impact absorption energy at -50℃ ≥34J.

[0038] Step 3, Filler Welding: Filler welding uses ER59-Ni5 welding wire with a diameter of 1.0–1.2 mm and is completed using MAG welding. The shielding gas for filler welding is an Ar + 18–20% CO2 mixture, balancing arc stability and deposition efficiency while reducing welding spatter. The process parameters for filler welding are: welding current 180–220 A, arc voltage 24–27 V, heat input 12–15 kJ / cm, with strict control of heat input to avoid grain coarsening. Before each filler layer, interpass slag and spatter must be thoroughly cleaned to prevent slag inclusions. The interpass temperature is controlled at 100–150℃ to avoid excessive temperature leading to grain coarsening in the heat-affected zone. Each weld pass is 4–5 mm thick and 6–8 mm wide, with an overlap of 2–4 mm between adjacent passes. A multi-layer, multi-pass welding process is used, and each pass can be lightly tapped with a hammer to further eliminate residual stress and improve joint density.

[0039] Step 4, Cover Welding: Cover welding is performed using 1.0–1.2 mm ER59-Ni5 welding wire via MAG welding. The process parameters for cover welding are: welding current 200–240 A, arc voltage 26–29 V, heat input 13–16 kJ / cm; the width of the cover weld bead is 2–3 mm wider than the bevel width, and the excess weld height is controlled at 2–4 mm to ensure a smooth transition of the joint surface, eliminate stress concentration caused by excessive excess weld height, and improve the appearance quality and service reliability of the joint.

[0040] Step 5, Post-weld graded tempering treatment: Post-weld heat treatment is a key step to eliminate residual stress and optimize joint microstructure. Graded tempering treatment is adopted to avoid microstructural distortion caused by one-time high-temperature treatment.

[0041] First, perform low-temperature tempering at 200–250℃ for ≥2 hours to effectively remove hydrogen introduced during welding, inhibit cold cracking, stabilize the ferrite structure, and prevent microstructural transformation. Then, perform high-temperature tempering at 50–60℃ / h to 560–580℃ for 4–6 hours. This stage refines the grains in the heat-affected zone, softens the high-hardness martensite structure, eliminates residual welding stress, and improves the toughness and fatigue resistance of the joint. Subsequently, cool the joint in the furnace to below 100℃ and then air-cool it. Slowly cool the joint out of the furnace to below 100℃, and then air-cool it. This slow cooling avoids the generation of secondary thermal stress and ensures the stability of the joint structure.

[0042] During the heat treatment process, thermocouples are used to measure the temperature at multiple points in real time to ensure that the temperature measurement error of the joint center, heat-affected zone, and base material area is ≤±3℃. If the temperature deviates, the heating is paused and the process is continued after the temperature is uniform to ensure the stability of the process.

[0043] This invention discloses a welding process for dissimilar low-alloy steels of different strength grades. The process employs ferritic welding materials and achieves uniform joint microstructure through the combined effects of stress-relieving ferritic welding material matching, zoned preheating, low heat input control, and post-weld graded tempering. This process completely solves problems such as cold cracking, embrittlement, and residual stress concentration, resulting in welded joints with high strength, high toughness, and low stress characteristics, meeting the stringent requirements of high-end equipment manufacturing.

[0044] Example 1

[0045] This embodiment proposes a welding process for dissimilar low-alloy steels of different strength grades. In this embodiment, the first low-alloy steel is 10CrNi5MoV, with a yield strength of 785 MPa, and its strength grade is high-strength steel. The second low-alloy steel is 10CrNi3MoV, with a yield strength of 590 MPa, and its strength grade is medium-strength steel. The strength grade of the first low-alloy steel is higher than that of the second low-alloy steel. This embodiment focuses on a butt joint between 10CrNi5MoV low-alloy high-strength steel and 10CrNi3MoV low-alloy medium-strength steel, with a plate thickness of 35 mm.

[0046] The welding process includes the following steps:

[0047] Step 1: Pre-welding treatment: Machin double-sided X-shaped bevels on the parts to be welded of the first and second low-alloy steels, with a bevel angle of 60°, a blunt edge of 2.5mm, and a root gap of 3.5mm; after beveling, mechanically grind until the surface is free of oxide scale, remove oil with acetone, and achieve a surface roughness of Ra32μm; use an infrared preheater for zoned preheating, with the first low-alloy steel side preheated at 130℃ and the second low-alloy steel side preheated at 80℃, preheating and holding for 30 minutes, and a preheating width of 220mm.

[0048] Step 2, Root Pass Welding: The root pass welding uses φ2.4mm ER59-Ni5 welding wire and is completed via TIG welding. The shielding gas for the root pass welding is pure Ar gas with a flow rate of 13L / min. The process parameters for the root pass welding are: welding current 160A, arc voltage 15V, heat input 9kJ / cm, root pass weld thickness 3mm, single-sided welding with double-sided forming, and no surface defects.

[0049] Step 3, Filler Welding: Filler welding uses φ1.0mm ER59-Ni5 welding wire and is completed by MAG welding. The shielding gas for filler welding is Ar + 20% CO2 with a flow rate of 20L / min. The process parameters for filler welding are: welding current 200A, arc voltage 25V, heat input 13kJ / cm. Filler welding is performed in 3 layers. Before each layer, impurities between layers are cleaned. The interpass temperature is 120℃. The thickness of a single weld bead is 4.5mm, the width of a single weld bead is 7mm, and the overlap between adjacent weld beads is 2mm. After each weld bead is completed, it is lightly hammered to relieve stress.

[0050] Step 4, Cover Welding: Cover welding is performed using φ1.2mm ER59-Ni5 welding wire via MAG welding. The process parameters for cover welding are: welding current 220A, arc voltage 27V, heat input 14kJ / cm; cover weld width 23mm (groove width 20mm), reinforcement height 2mm, and smooth surface transition.

[0051] Step 5, post-weld graded tempering: First, perform low-temperature tempering by holding at 220℃ for 2.5h; then, perform high-temperature tempering by raising the temperature to 570℃ at 60℃ / h and holding for 5h; finally, cool with the furnace to 90℃ and air-cool.

[0052] Performance testing

[0053] 1. After the welded joint obtained in Example 1 cooled to room temperature, it underwent visual inspection (VT), ultrasonic testing (UT), and radiographic testing (RT) in sequence. The test results are shown in Table 1. VT inspection focused on surface defects such as cracks, undercut, porosity, and slag inclusions. UT inspection used an ultrasonic flaw detector to detect volumetric defects such as incomplete fusion, incomplete penetration, and cracks inside the joint. RT inspection used an X-ray flaw detector to detect internal defects such as porosity and slag inclusions. Visual inspection was conducted according to NB / T47013.7-2012 "Non-destructive testing of pressure equipment - Part 7: Visual inspection (VT)"; UT inspection was conducted according to NB / T 47013.3-2023 "Non-destructive testing of pressure equipment - Part 3: Ultrasonic testing"; and RT inspection was conducted according to NB / T 47013.2-2015 "Non-destructive testing of pressure equipment - Part 2: Radiographic testing".

[0054] 2. After the welded joint obtained in Example 1 was cooled to room temperature, mechanical properties were tested, and the test results are shown in Table 1. The tensile strength of the welded joint was tested according to GB / T 2651-2023 "Destructive Testing of Welds in Metallic Materials - Transverse Tensile Test"; the impact absorption energy at -50℃ and -20℃ was tested according to GB / T 2650-2022 "Destructive Testing of Welds in Metallic Materials - Impact Test"; the residual stress was tested according to GB / T 24179-2023 "Determination of Residual Stress in Metallic Materials - Indentation Strain Method"; and the highest hardness (HV) of the heat-affected zone was tested according to GB / T 2654-2008 "Test Method for Hardness of Welded Joints".

[0055] Table 1. Joint Test Results

[0056]

[0057] As shown in Table 1, the welded joint VT obtained in Example 1 of the present invention is qualified, UT I grade, RT I grade, and has no internal or surface defects.

[0058] As shown in Table 1, the tensile strength of the welded joint is 690 MPa, the impact absorption energy at -20℃ is 105 J, the impact absorption energy at -50℃ is 86 J, the maximum hardness of the heat-affected zone is 385 HV, and the welding residual stress is ≤150 MPa. The welded joint combines high strength, high toughness, high hardness, and low stress characteristics, avoiding performance degradation caused by equal strength matching or low strength matching, effectively improving the fatigue resistance of the joint, extending its service life, and meeting the long-term and stringent service requirements of high-end equipment.

[0059] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations 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 welding process for dissimilar low-alloy steels of different strength grades, characterized in that, The welding process includes the following steps: Step 1: Pre-welding treatment: Process double-sided X-shaped grooves on the parts to be welded of the first low alloy steel and the second low alloy steel. The Rp0.2 of the first low alloy steel is ≥785MPa and the Rp0.2 of the second low alloy steel is ≥590MPa. Use a zoned preheating method, preheating the first low alloy steel side to 100~150℃ and the second low alloy steel side to 60~100℃. The strength grade of the first low alloy steel is higher than that of the second low alloy steel. Step 2, Root pass welding: Ferritic welding material is used, and the welding is completed by TIG welding; Step 3, Filler Welding: Ferritic welding material is used, and MAG welding is performed using a multi-layer, multi-pass welding process. Step 4, Cover welding: Ferritic welding material is used, and the welding is completed by MAG welding; Step 5: Post-weld graded tempering treatment; In step five, the post-weld graded tempering treatment is as follows: first, low-temperature tempering is performed by holding at 200-250℃ for ≥2h, then high-temperature tempering is performed by raising the temperature to 560-580℃ at 50-60℃ / h and holding for 4-6h, followed by furnace cooling to below 100℃ and then air cooling. In steps two through four, the ferritic welding material used for the root pass, fill pass, and cap pass is ER59-Ni5. The chemical composition of ER59-Ni5 welding wire, by mass percentage, includes C≤0.10, Si≤0.5, Mn:0.4~0.9, Ni:4~6, Cr≤0.6, Ti≤0.10, with the remainder being Fe and impurity elements.

2. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The preheating width of the preheating zone is ≥200mm, and the preheating and heat preservation time is 30-60min.

3. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The root pass welding uses ER59-Ni5 welding wire with a diameter of 2.0 to 3.0 mm; the filler weld and cover weld use ER59-Ni5 welding wire with a diameter of 1.0 to 1.2 mm.

4. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The shielding gas for the root pass welding is pure argon. The process parameters for the root pass welding are: welding current 160-200A, arc voltage 14-16V, and root pass weld thickness 3-5mm.

5. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The shielding gas for filler welding is a mixture of argon and carbon dioxide. The process parameters for filler welding are: welding current 180-220A, arc voltage 24-27V, heat input 12-15kJ / cm; interpass temperature controlled at 100-150℃; single pass weld thickness 4-5mm, width 6-8mm, and overlap of adjacent passes 2-4mm.

6. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The process parameters for cover welding are: welding current 200-240A, arc voltage 26-29V, heat input 13-16kJ / cm; the width of the cover weld bead is 2-3mm wider than the bevel width, and the reinforcement height is controlled at 2-4mm.

7. The welding process for dissimilar low-alloy steels of different strength grades according to claim 1, characterized in that, The bevel angle of the double-sided X-shaped bevel is 55° to 65°, the blunt edge is 2 to 3 mm, and the root gap is 3 to 4 mm.

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