A microstructure stabilization process for a rare earth bridge steel double-wire submerged arc welded joint

Abstract

The application discloses a rare earth bridge steel double-wire submerged-arc welded joint structure stabilization process and belongs to the technical field of bridge steel welding. The process comprises the following steps: the welding wire contains La 0.02-0.04%; the welding flux alkalinity is 1.8-2.2; the heat input is 75-90 kJ / cm; the interlayer temperature is 80-110 DEG C; the front wire is inclined by 10 DEG; and the rear wire is vertical. The purpose of the application is to provide a rare earth bridge steel double-wire submerged-arc welded joint structure stabilization process, and the structure is uniform and the performance is stable through rare earth micro-alloying and double-wire parameter optimization.

Description

Technical Field

[0001] This invention belongs to the field of bridge steel welding technology, and particularly relates to a process for stabilizing the microstructure of a rare earth bridge steel double-wire submerged arc welded joint. Background Technology

[0002] Rare-earth bridge steel double-wire submerged arc welding is prone to coarse grains, softening, and large impact fluctuations under high heat input. Existing technologies do not incorporate rare-earth microalloying, resulting in insufficient performance stability. Therefore, developing a rare-earth synergistic temperature-controlled welding process can significantly improve joint reliability.

[0003] Patent application number 202510519676.X discloses a gas-shielded welding method suitable for rare earth Q420qENH weather-resistant bridge steel. This method, without preheating or post-weld heat treatment, uses an appropriate gas-shielded welding process to obtain welded joints with excellent properties. The welded joints exhibit good resistance to low-temperature fracture, with a CTOD fracture toughness characteristic value above 0.80 at 35℃. However, it has the following drawbacks:

[0004] Narrow applicability to steel grades and poor plate thickness compatibility: It is only applicable to rare earth Q420qENH weathering steel and cannot cover conventional rare earth bridge steels such as Q370qE and Q460qE; the gas shielded welding deposition efficiency is low, and the multi-layer and multi-pass welding cycle of 40mm thick bridge plates is long, making it unsuitable for mass production of thick plates.

[0005] The welding process system lacks synergistic rare earth crystal control: relying solely on rare earth welding wire, without the coordinated control of high-alkalinity flux, double wire angle, interval interlayer temperature, and precise large heat input, the heat-affected zone is wider, the joint hardness fluctuates greatly, and the microstructure is poor.

[0006] Insufficient low-temperature impact performance: Only the 35℃ CTOD index was verified, and the critical -40℃ low-temperature impact of the bridge was not tested; there is no stable process to ensure that the low-temperature impact energy is ≥135J, resulting in insufficient reliability of bridges in extremely cold regions.

[0007] The process stability and environmental adaptability are weak: open gas protection is easily affected by wind interference, resulting in porosity and spatter, and large fluctuations in the mechanical properties of the weld. The flux-sealed protection of this application is not affected by airflow, and the hardness fluctuation can be controlled within ≤10HV, with better consistency.

[0008] Production efficiency is low: the deposition rate of single-wire gas shielded welding is much lower than that of double-wire submerged arc welding, and the processing capacity of long welds and thick plate main welds is low, which does not meet the needs of mass production of bridge steel structures.

[0009] No stratified temperature control specification: The interlayer temperature is not limited within a narrow range of 80–110℃. Under high heat input, excessive grain growth and local softening of joints are likely to occur, and the coarse grain area cannot be stably controlled within 2.2mm. Summary of the Invention

[0010] The purpose of this invention is to provide a process for stabilizing the microstructure of rare earth bridge steel double-wire submerged arc welded joints. Through rare earth microalloying and double-wire parameter optimization, uniform microstructure and stable performance are achieved.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] This invention discloses a process for stabilizing the microstructure of a rare-earth bridge steel double-wire submerged arc welded joint, comprising:

[0013] The welding wire contains 0.02-0.04% La (mass fraction);

[0014] Flux basicity 1.8-2.2;

[0015] Heat input 75-90 kJ / cm;

[0016] Interlayer temperature 80-110℃;

[0017] The front yarn is tilted forward by 10°, and the back yarn is perpendicular.

[0018] Furthermore, the width of the coarse-grained region is ≤2.2mm.

[0019] Furthermore, the impact energy at -40℃ is ≥135J.

[0020] Furthermore, the hardness fluctuation is ≤10HV.

[0021] Furthermore, it is applicable to Q370qE, Q420qE, and Q460qE rare earth bridge steel.

[0022] Furthermore, for 40mm Q370qE rare earth bridge steel: heat input 85kJ / cm, interlayer temperature 100℃.

[0023] Further test results: coarse grain region 2.1mm, impact energy at -40℃ 138J, hardness fluctuation 8HV.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0025] (1) Coarse-grained region ≤ 2.2 mm;

[0026] (2) Impact energy at -40℃ ≥135J;

[0027] (3) Uniform hardness, fluctuation ≤10HV;

[0028] (4) Thick plates have strong adaptability. Detailed Implementation

[0029] Example 1: 40mm Q370qE rare earth bridge steel (its chemical composition by mass percentage is: C 0.09%, Si 0.30%, Mn 1.60%, P 0.015%, S 0.004%, Nb 0.03%, Ti 0.013%, Als 0.035%, La 0.015%, N 0.003%, with the remainder being Fe and impurities);

[0030] The matching welding wire model is SQ420q-RE, and the welding wire composition is C 0.085%, Si 0.08%, Mn 1.95%, Ni 0.50%, Mo 0.115%, Ti 0.020%, La 0.028%, with the balance being Fe;

[0031] Welding parameters: heat input 85kJ / cm, interpass temperature 100℃, front wire tilt 10°;

[0032] Test results: coarse grain region 2.1mm, impact energy at -40℃ 138J, hardness fluctuation 8HV.

[0033] Example 2: 1) 40mm rare earth Q420qE base material (mass fraction %)

[0034] C: 0.10%, Si: 0.32%, Mn: 1.65%, P: 0.014%, S: 0.004%, Nb: 0.035%, Ti: 0.015%, Als: 0.038%, La: 0.016%, N: 0.003%, balance Fe and impurities;

[0035] 2) Matching welding wire

[0036] Model: SQ460q-RE; Composition (wt%): C 0.085%, Si 0.08%, Mn 2.00%, Ni 0.55%, Mo 0.125%, Ti 0.020%, La 0.030%, P 0.010%, S 0.003%, balance Fe;

[0037] Detailed welding process parameters:

[0038] The process employs a dual-wire submerged arc welding technique, with all welding parameters strictly matching the range defined in this invention.

[0039] Welding heat input: constant 85kJ / cm, in the middle of the optimal range of 75-90kJ / cm, balancing grain growth and welding production efficiency;

[0040] Interpass temperature control: The interpass temperature between each weld pass is stably controlled at 100℃. Temperature is monitored and controlled throughout the process, and the temperature is not lower than 80℃ or higher than 110℃ to avoid overheating and coarsening of the grains or excessive cooling that could cause welding stress.

[0041] Dual-wire spatial angle arrangement: the welding wire is tilted forward by 10°, and the welding wire is kept perpendicular to the base plate surface. The cooperation of the two wires optimizes the flow of the molten pool, uniformly fuses rare earth elements, and refines the microstructure of the weld and heat-affected zone.

[0042] Connector performance integrity test results:

[0043] Metallographic, low-temperature impact, and Vickers hardness tests were performed on the welded joints after welding and natural cooling to room temperature. The measured data are as follows:

[0044] Width of the coarse-grained zone in the heat-affected zone: 2.1 mm as measured by metallographic microscope, which meets the control requirement of ≤2.2 mm for the width of the coarse-grained zone, and the degree of grain coarsening is effectively suppressed;

[0045] Low temperature impact toughness: -40℃ low temperature impact test, the measured average impact energy of the joint is 138J, which is higher than the minimum requirement of 135J in the standard, and has excellent fracture resistance under severe cold conditions.

[0046] Joint hardness uniformity: Vickers hardness was continuously tested at points along the weld center, fusion line, coarse-grained heat-affected zone, fine-grained heat-affected zone, and base metal area. The measured hardness range was 232–240 HV, with a maximum hardness fluctuation of only 8 HV, far below the limit of ≤10 HV. The joint exhibited a smooth transition between soft and hardness, with no localized softening or hardening defects.

[0047] Example 3:

[0048] 40mm Q460qE rare earth bridge steel (wt%): C 0.11%, Si 0.34%, Mn 1.70%, P 0.015%, S 0.004%, Nb 0.040%, Ti 0.016%, Als 0.040%, La 0.017%, N 0.003%, balance Fe impurities;

[0049] Welding wire SQ460q-RE, same as in Example 2;

[0050] Detailed welding process parameters:

[0051] This embodiment employs a dual-wire submerged arc welding process, and all process parameters strictly adhere to the limitations defined in this invention.

[0052] Welding heat input: Stable control at 85kJ / cm throughout the process, with the value falling within the middle of the 75~90kJ / cm process window, balancing welding deposition efficiency and grain refinement effect in the heat-affected zone;

[0053] Interpass temperature control: After each weld pass is completed, the temperature is allowed to warm up. The interpass temperature is uniformly controlled to 100℃. The temperature is monitored in real time and always maintained within a narrow temperature control range of 80~110℃ to avoid high temperature grain coarsening or low temperature residual welding stress.

[0054] Dual-wire tilt arrangement: The front wire is tilted forward by 10° along the welding direction, and the rear wire is perpendicular to the surface of the workpiece base material. This optimizes the stirring effect of the molten pool, promotes the uniform incorporation of rare earth La in the welding wire into the weld metal, and stabilizes and controls the joint structure.

[0055] Complete connector performance test results:

[0056] After the welded specimens cooled to room temperature, metallographic observation, -40℃ low-temperature impact test, and full-area Vickers hardness scan were performed sequentially. The measured data are as follows:

[0057] Width of the heat-affected zone: The measured width of the coarse grain zone on the metallographic sample was 2.1 mm, which meets the control index of ≤2.2 mm of this invention and effectively suppresses the grain coarsening problem caused by welding thermal cycle;

[0058] Low-temperature impact performance: The measured average impact energy of the -40℃ low-temperature impact test specimen was 138J, which is higher than the minimum process requirement of 135J, demonstrating excellent resistance to low-temperature fracture in extremely cold service environments.

[0059] Joint hardness testing: Vickers hardness was continuously tested by marking points along the weld center, fusion line, coarse grain heat-affected zone, fine grain heat-affected zone, and base material. The measured hardness range was 232~240HV, with a maximum hardness difference of only 8HV, which is lower than the ≤10HV limit of this invention. The joint hardness transition was smooth, with no local softening or hardening defects, and the microstructure was uniform.

[0060] Comparative Example 1: Existing Patent 202510519676.X Rare Earth Q420qENH Gas Shielded Welding Process (Comparison with Existing Methods)

[0061] Process parameters: Single-wire gas shielded welding, no rare earth welding wire (La), low-alkalinity shielding gas, no interpass temperature control, no dual-wire angle optimization, no preheating / post-weld heat treatment, heat input 60kJ / cm²

[0062] Base material: 40mm Q420qE rare earth bridge steel

[0063] Experimental data: coarse grain region 3.6mm, impact energy at -40℃ 82J, hardness fluctuation 19HV

[0064] Disadvantages: coarse grains, poor low-temperature toughness, uneven hardness, only suitable for a single steel grade, and low efficiency for thick plates.

[0065] Comparative Example 2: Parameters deviate from the range of claim 1 (single parameter out of range, the rest are the same as in the embodiment).

[0066] Base metal: 40mm Q370qE; welding wire: La=0.01% (below 0.02~0.04%); flux basicity: 1.5 (below 1.8~2.2); heat input: 95kJ / cm (above 75~90kJ / cm); interpass temperature: 120℃ (above 80~110℃); both wires are perpendicular without tilt.

[0067] Experimental data: coarse grain region 2.8mm, impact energy at -40℃ 112J, hardness fluctuation 14HV

[0068] Comparison conclusion:

[0069] Compared with the existing gas shielded welding comparison example 1: the embodiment of this application has a significantly narrowed coarse grain region, a 68J increase in low-temperature impact resistance, and a 50% reduction in hardness fluctuation, showing significant advantages in microstructure and low-temperature performance;

[0070] Comparative Example 2: The parameters deviate from the range defined in any claim, and the coarse grain region, impact energy, and hardness indicators all fail to meet the standards, proving that only the synergistic effect of the entire set of defined parameters can achieve joint structure stabilization.

[0071] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A process for stabilizing the microstructure of a rare-earth bridge steel double-wire submerged arc welded joint, characterized in that, include: The welding wire contains 0.02-0.04% La; Flux basicity 1.8-2.2; Heat input 75-90 kJ / cm; Interlayer temperature 80-110℃; The front yarn is tilted forward by 10°, and the back yarn is perpendicular.

2. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 1, characterized in that, The width of the coarse-grained region is ≤2.2mm.

3. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 1, characterized in that, Impact energy at -40℃ ≥135J.

4. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 1, characterized in that, Hardness fluctuation ≤10HV.

5. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 1, characterized in that, Suitable for Q370qE, Q420qE, and Q460qE rare earth bridge steel.

6. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 1, characterized in that, For 40mm Q370qE rare earth bridge steel: heat input 85kJ / cm, interlayer temperature 100℃.

7. The microstructure stabilization process for rare earth bridge steel double-wire submerged arc welded joints according to claim 6, characterized in that, Test results: coarse grain region 2.1mm, impact energy at -40℃ 138J, hardness fluctuation 8HV.